你問我答模擬香港中學文憑考試
Hong Kong Diploma of Secondary Education Examination (Mock)
Chemistry
2021
Question Papers
(with marking schemes and comments on candidates’ performance)
你問我答
Chem 2021
The Examination
Assessment Objectives
The objectives of the public assessment of Chemistry are to evaluate the abilities of candidates to:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
recall and show understanding of chemical facts, patterns, principles, terminology and conventions;
show an understanding of the use of apparatus and materials in performing experiments;
handle materials, manipulate apparatus, carry out experiments safely and make accurate observations;
demonstrate an understanding of the method used in chemical investigation;
analyse and interpret data from various sources, and draw relevant conclusions;
manipulate and translate chemical data and to perform calculations;
apply chemical knowledge to explain observations and to solve problems which may involve unfamiliar
situations;
select and organise scientific information from appropriate sources and to communicate this information in an
appropriate and logical manner;
understand and evaluate the social, economic, environmental and technological implications of the applications
of chemistry; and
make decisions based on the examination of evidence and arguments.
Synopsis of curriculum contents
The topics in the curriculum of Chemistry are listed below. For details, please refer to the Chemistry Curriculum
and Assessment Guide (Secondary 4-6) jointly prepared by the Curriculum Development Council and the Hong Kong
Examinations and Assessment Authority.
Compulsory Part
I.
Planet Earth
II. Microscopic World I
III. Metals
IV. Acids and Bases
V.
Fossil Fuels and Carbon Compounds
VI. Microscopic World II
VII. Redox Reactions, Chemical Cells and Electrolysis
VIII. Chemical Reactions and Energy
IX. Rate of Reaction
X. Chemical Equilibrium
XI. Chemistry of Carbon Compounds
XII. Patterns in the Chemical World
Elective Part
XIII. Industrial Chemistry
XIV. Materials Chemistry
XV. Analytical Chemistry
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Mode of assessment
The public assessment of Chemistry consists of a public examination component as outlined in the following table:
Component
Public
Examination
Weighting
Duration
Paper 1
Compulsory part of the curriculum
75%
2 hours
30 minutes
Paper 2
Elective part of the curriculum
25%
1 hour
Paper 1 comprises two sections: A and B. Section A consists of multiple-choice questions and carries 22.5% of the
subject mark. Section B includes short questions, structured questions and essay questions, and carries 52.5% of
the subject mark. In each of the sections A and B, Part I will set questions mainly on topics I to VIII of the curriculum,
while Part II mainly on topics IX to XII. Candidates have to attempt all questions in this paper.
Paper 2 consists of structured questions and carries 25% of the subject mark. Candidates are required to answer the
questions on the 2 electives selected.
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3
2021-DSE
CHEM
PAPER 1A
你 問 我 答
HONG KONG DIPLOMA OF SECONDARY EDUCATION EXAMINATION 2021
MOCK 1
CHEMISTRY PAPER 1
(2 hours 30 minutes)
This paper must be answered in English
GENERAL INSTRUCTIONS
1.
There are TWO sections, A and B, in this Paper. You are advised to finish Section A in about 45
minutes.
2.
Section A consists of multiple-choice questions in this question paper, while Section B contains
conventional questions printed separately in Question-Answer Book B.
3.
Answers to Section A should be marked on the Multiple-choice Answer Sheet while answers to
Section B should be written in the spaces provided in Question-Answer Book B. The Answer
Sheet for Section A and the Question-Answer Book B for Section B will be collected
separately at the end of the examination.
4.
A Periodic Table is printed on page 22 of Question-Answer Book B. Atomic numbers and relative
atomic masses of elements can be obtained from the Periodic Table
INSTRUCTIONS FOR SECTION A (MULTIPLE-CHOICE QUESTIONS)
1.
Read carefully the instructions on the Answer Sheet. After the announcement of the start of the
examination, you should first insert the information required in the spaces provided.
2.
When told to open this book, you should check that all the questions are there. Look for the words
‘END OF SECTION A’ after the last question.
3.
All questions carry equal marks.
4.
ANSWER ALL QUESTIONS. You are advised to use an HB pencil to mark all the answers on the
Answer Sheet, so that wrong marks can be completely erased with a clean rubber. You must mark
the answers clearly; otherwise you will lose marks if the answers cannot be captured.
5.
You should mark only ONE answer for each question. If you mark more than one answer, you will
receive NO MARKS for that question.
6.
No marks will be deducted for wrong answers.
Not to be taken away before the
end of the examination session
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This section consists of two parts. There are 24 questions in PART I and 12 questions in PART II.
Choose the best answer for each question.
Candidates may refer to the Periodic Table printed on page 20 of Question-Answer Book B.
PART I
1.
Which of the following statements about oxygen and its compounds is / are correct ?
(1)
(2)
(3)
Silicon dioxide is an electrical conductor.
Oxygen contributes to about one-fifth of the gas volume in the atmosphere.
Oxygen can only exist in oxidation states of 0 and −2.
A.
B.
C.
D.
2.
Which of the following species does not have the same number of electrons with other species ?
A.
B.
C.
D.
3.
P3− ion
Ar atom
S atom
K+ ion
Which of the following statements about anodisation is correct ?
A.
B.
C.
D.
4.
(1) only
(2) only
(1) and (3) only
(2) and (3) only
The target object is connected to the anode of a battery and concentrated sulphuric acid is used as
the electrolyte in anodisation.
The target object is connected to the cathode of a battery and concentrated sulphuric acid is used
as the electrolyte in anodisation.
The target object is connected to the anode of a battery and dilute sulphuric acid is used as the
electrolyte in anodisation.
The target object is connected to the cathode of a battery and dilute sulphuric acid is used as the
electrolyte in anodisation.
For which of the following species should back titration be adopted when titrated against standard HCl ?
(1)
(2)
(3)
NaOH(aq)
NH3(g)
CaSO4(s)
A.
B.
C.
D.
(2) only
(3) only
(1) and (3) only
(2) and (3) only
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5.
Which of the following is the condensed structural formula of 3-ethyl-2,4-dimethylhexane ?
A.
B.
C.
D.
6.
7.
Which of the following graphs sketches the relationship between boiling points of group IV (solid line) and V
(dotted line) hydrides ?
A.
B.
C.
D.
When hydrated barium hydroxide is mixed with ammonium chloride at room conditions, ΔH has a positive
value. Which of the following statements about the reaction must be true ?
(1)
(2)
(3)
The total enthalpy of products is greater than that of reactants.
The surrounding environment loses heat to the system under constant volume conditions.
The reaction is said to be endothermic.
A.
B.
C.
D.
8.
CH3CH2CH(CH3)CH(CH2CH3)CH(CH3)2
CH3CH2CH(CH2CH3)CH(CH3)CH(CH3)2
CH3CH(CH3)CH(CH2CH3)CH2CH(CH3)2
CH3CH(CH2CH3)CH2CH(CH3)CH(CH3)2
(1) and (2) only
(1) and (3) only
(2) and (3) only
(1), (2) and (3)
The silver nitrate solution used in the test for chloride ions has to be acidified first. Which of the following
acids is suitable for such acidification ?
A.
B.
C.
D.
dilute nitric acid
concentrated sulphuric acid
dilute hydrochloric acid
concentrated hydrochloric acid
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Direction: Questions 9 and 10 refer to the following chemical equation.
xAuCl4−(aq) + ySO2(aq) + zH2O(l) → aAu(s) + bH+(aq) + cSO42−(aq) + dCl−(aq)
9.
Which of the following statements about the above chemical reaction are correct ?
(1)
(2)
(3)
Au is the oxidizing agent in the above chemical reaction.
Au does not react with HCl or H2SO4.
The resultant solution is acidic.
A.
B.
C.
D.
10.
Which of the following combinations shows the correct stoichiometric coefficients of the above chemical
equation ?
A.
B.
C.
D.
11.
(1) and (2) only
(1) and (3) only
(2) and (3) only
(1), (2) and (3)
x
2
2
3
3
z
3
6
4
6
b
6
12
8
9
A sample of strontium (Sr) contains four naturally occurring isotopes, 84Sr, 86Sr, 87Sr and 88Sr. It is known that
the relative abundances of 84Sr and 87Sr are 0.56% and 7.00% respectively. What is the relative abundance of
86
Sr in nature ?
(Relative atomic mass : Sr = 87.6)
A.
B.
C.
D.
12.
Which of the following fractions obtained in the fractional distillation of petroleum is used as a fuel for
aeroplanes ?
A.
B.
C.
D.
13.
15.4%
36.0%
56.5%
77.1%
petrol
naphtha
kerosene
diesel oil
Which of the following combinations of acid and alkali will give the greatest temperature rise upon mixing ?
A.
B.
C.
D.
25.0 cm3 of 0.1 M hydrochloric acid and 25.0 cm3 of 0.1 M potassium hydroxide solution
20.0 cm3 of 0.1 M sulphuric acid and 30.0 cm3 of 0.1 M potassium hydroxide solution
30.0 cm3 of 0.1 M sulphuric acid and 20.0 cm3 of 0.1 M potassium hydroxide solution
25.0 cm3 of 0.1 M ethanoic acid and 25.0 cm3 of 0.1 M sodium hydroxide solution
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14.
The following graph shows the change in mass that occurs when 1.0 g of powdered magnesium carbonate is
heated with a non-luminous Bunsen flame :
Which of the following graphs would be obtained when 1.0 g of powdered calcium carbonate is heated under
the same condition ?
(Relative atomic masses : C = 12.0, O = 16.0, Mg = 24.3, Ca = 40.1)
A.
B.
C.
D.
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15.
A spirit burner containing propan-1-ol (CH3CH2CH2OH) is used to heat up a beaker of water (400 cm3) from
20°C to 70°C. The spirit burner can give out 450 J of energy per second. Given that the time required for the
water to heat up is 4 minutes, what is the combustion efficiency of the spirit burner in heating the water ?
(Specific heat capacity of water = 4.2 J g−1 K−1;
density of water = 1.0 g cm−3;
enthalpy change of combustion : propan-1-ol = −2010 kJ mol−1)
A.
B.
C.
D.
16.
72.3%
76.4%
75.9%
77.8%
Consider the following hazard warning label :
Which of the following chemicals need to be stored in a reagent bottle with the above label ?
(1)
(2)
(3)
formaldehyde
chloroform
acetone
A.
B.
C.
D.
17.
(1) and (2) only
(1) and (3) only
(2) and (3) only
(1), (2) and (3)
Refer to the following reaction :
C2H4(g) + H2(g) → C2H6(g)
Which of the following statements regarding the reaction are correct ?
(1)
(2)
(3)
If ethene is converted to ethane via four stages instead of one as shown above, the enthalpy change of
the reaction will be unchanged.
The standard enthalpy change of the reaction is −85 kJ mol−1.
The reaction can be catalysed by palladium.
(Standard enthalpy changes of formation : C2H4(g) = +52 kJ mol−1, C2H6(g) = −85 kJ mol−1)
A.
B.
C.
D.
(1) and (2) only
(1) and (3) only
(2) and (3) only
(1), (2) and (3)
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18.
The percentage by mass of oxygen in a mixture of zinc phosphate and ammonium sulphate is found to be
43.8%. What is the mass of zinc phosphate present in 150 g of the mixture ?
(Relative atomic masses : H = 1.0, N = 14.0, O = 16.0, P = 31.0, S = 32.1, Zn = 65.4)
A.
B.
C.
D.
19.
29 g
46 g
75 g
104 g
0.100 mol dm−3 CH3COOH is added dropwise to 10.0 cm3 of 0.100 mol dm−3 NaOH. The change in pH is
recorded by a pH meter throughout the process and the results are plotted on a graph below :
At which point on the graph, W, X, Y or Z, is the concentration of the acid equal to that of the salt in the
reaction mixture ?
A.
B.
C.
D.
20.
W
X
Y
Z
Butenedioic acid exists as a pair of cis-trans isomers. Which of the following statements concerning the pair
of isomers are correct ?
(1)
(2)
(3)
In the solid state, both of them have the same density.
The cis-isomer is more polar than the trans-isomer.
The cis-isomer has a lower boiling point than the trans-isomer.
A.
B.
C.
D.
(1) and (2) only
(1) and (3) only
(2) and (3) only
(1), (2) and (3)
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21.
Cyclopentane has the following structure :
It is treated with bromine under sunlight and a reaction occurs. Which of the following compounds is a possible
by-product of the reaction ?
22.
A.
B.
C.
D.
Which of the following types of reactions must be redox reaction(s) ?
(1)
(2)
(3)
metal displacement reactions
combustion reactions
precipitation reactions
A.
B.
C.
D.
23.
(1) and (2) only
(1) and (3) only
(2) and (3) only
(1), (2) and (3)
Which of the following molecules has the highest solubility in water ?
A.
B.
C.
D.
CH3CHO
COF2
CH2S
trans-HFC=CFH
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24.
Consider the following statements and choose the best answer :
1st statement
Lead(II) chloride conducts electricity only in its
molten state.
A.
B.
C.
D.
2nd statement
There are mobile ions in molten lead(II) chloride.
Both statements are true and the 2nd statement is a correct explanation of the 1st statement.
Both statements are true but the 2nd statement is NOT a correct explanation of the 1st statement.
The 1st statement is false but the 2nd statement is true.
Both statements are false.
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PART II
25.
The reaction between potassium manganate(VII) and ethanedioic acid is an example of autocatalytic reactions
where one of the products catalyses the reaction. The chemical equation for the reaction is shown below :
2MnO4−(aq) + 5C2O4−(aq) + 16H+(aq) → 2Mn2+(aq) + 10CO2(g) + 8H2O(l)
Which of the following graphs correctly represents the kinetics of the reaction ?
26.
A.
B.
C.
D.
Benzylamine (C6H5CH2NH2) is a common precursor in organic synthesis. Which of the following statements
about benzylamine is correct ?
A.
B.
C.
D.
27.
Its aqueous solution is neutral.
It can be distinguished from CH3CH2NH2 by treating each of them with aqueous bromine.
Its boiling point is higher than that of C6H5CONH2.
It does not exhibit cis-trans isomerism or enantiomerism.
Which of the following shows the increasing order of resultant pH when the compounds are added to water ?
A.
B.
C.
D.
P4O10 < Na2O < MgO
Al2O3 < Na2O < SiO2
Cl2O7 < MgO < Al2O3
SO2 < Al2O3 < Na2O
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28.
Volcanic eruptions give off gases like H2S and CS2. In a post-eruption investigation, a 70 cm3 gaseous sample
of H2S and CS2 in the ratio of 3 : 2 was analysed by combustion using 200 cm3 of oxygen. It is known that any
sulphur present was converted to SO2 after combustion. After measuring the volume of gas remaining, the
product was treated with excess aqueous sodium hydroxide and the volume of gas was measured again. Given
that all volume measurements were made at 50°C, which of the following combinations is correct ?
A.
B.
C.
D.
29.
Volume of gas after adding sodium hydroxide /
cm3
0
28
53
81
The following reaction systems are at equilibrium in separate sealed containers. The volumes of the containers
are halved at constant temperature. Which reaction has the largest percentage change in the reaction quotient
immediately after the volume change ?
A.
B.
C.
D.
30.
Volume of gas after combustion /
cm3
126
126
179
179
N2O4(g) ⇌ 2NO2(g)
H2(g) + I2(g) ⇌ 2HI(g)
2CO2(g) ⇌ 2CO(g) + O2(g)
CO(g) + 2H2(g) ⇌ CH3OH(g)
Consider the information below :
Reaction
AgBr(s) ⇌ Ag+(aq) + Br−(aq)
Ag2CrO4(s) ⇌ 2Ag+(aq) + CrO42−(aq)
Ag2C2O4(s) ⇌ 2Ag+(aq) + C2O42−(aq)
Ag2CO3(s) ⇌ 2Ag+(aq) + CO32−(aq)
Numerical value of equilibrium constant at 25°C
5.35 × 10−13
1.12 × 10−12
5.40 × 10−12
8.46 × 10−12
Which of the following compounds has the highest concentration of Ag+(aq) in a saturated solution at room
temperature ?
A.
B.
C.
D.
31.
AgBr(s)
Ag2CrO4(s)
Ag2C2O4(s)
Ag2CO3(s)
Which of the following statements concerning the complete combustion of alkanes is INCORRECT ?
A.
B.
C.
D.
At 120°C, the volume of steam produced per mole of an alkane is more than the volume of carbon
dioxide produced.
More oxygen is required for the complete combustion of 1 mol of an alkane as compared to 1 mol
of an alkene with the same number of carbon atoms.
The volume of oxygen required is directly proportional to the number of carbon atoms present in
an alkane.
The reaction can be speeded up by raising the temperature of the system.
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32.
Which of the following statements concerning Group II elements and their compounds are correct ?
(1)
(2)
(3)
Marble is harder than chalk, while chalk is harder than limestone.
Strontium (Sr) has a higher electronegativity than barium.
Beryllium has a higher melting point than magnesium.
A.
B.
C.
D.
33.
A gaseous dimer, Z, is introduced to an empty gas syringe which has a movable and tight-fitting plunger. The
gas is allowed to expand until an equilibrium is reached at a constant temperature. When the dynamic
equilibrium is attained, 20% of Z has dissociated into its gaseous monomer, X. Which of the following
statements must be correct ?
A.
B.
C.
D.
34.
(1) and (2) only
(1) and (3) only
(2) and (3) only
(1), (2) and (3)
A decrease in the temperature favours the formation of Z.
The equilibrium constant of this reaction is 0.2 mol dm−3.
The pressure inside the syringe at the equilibrium is higher than the atmospheric pressure.
The formation of X is favoured when the plunger is pushed back into the equilibrium mixture.
Which of the following statements concerning the following two compounds is / are correct?
A
(1)
(2)
(3)
B
Compound A is a stronger acid than compound B.
They have identical solubilities in water.
Only compound B can react with LiAlH4 to form an alcohol.
A.
B.
C.
D.
(1) only
(2) only
(1) and (3) only
(2) and (3) only
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35.
Compound J has the following structure :
In an experiment, a sample of J is allowed to react with a limited supply of chlorine gas in the presence of
sunlight. Assume that only monosubstitution with chlorine takes place, which of the following statements are
correct ?
(1)
(2)
(3)
The maximum number of structural isomers of any chloroalkanes formed is 8.
The chloroalkanes produced rotate the plane of polarised light from a polarimeter.
In the step of propagation, the following reaction occurs :
C10H20 + Cl2 → C10H19· + Cl· + HCl
A.
B.
C.
D.
36.
(1) and (2) only
(1) and (3) only
(2) and (3) only
(1), (2) and (3)
Consider the following statements and choose the best answer :
1st statement
Boron is an electrical insulator under any
conditions.
A.
B.
C.
D.
2nd statement
Boron has a giant covalent structure with strong
covalent bonds between atoms.
Both statements are true and the 2nd statement is a correct explanation of the 1st statement.
Both statements are true but the 2nd statement is NOT a correct explanation of the 1st statement.
The 1st statement is false but the 2nd statement is true.
Both statements are false.
END OF SECTION A
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B
2021-DSE
CHEM
PAPER 1B
你 問 我 答
Telegram username
HONG KONG DIPLOMA OF SECONDARY EDUCATION EXAMINATION 2021
MOCK 1
CHEMISTRY PAPER 1
SECTION B : Question-Answer Book B
This paper must be answered in English
INSTRUCTIONS FOR SECTION B
(1)
After the announcement of the start of the
examination, you should first write your Telegram
username in the space provided on Page 1.
(2)
Refer to the general instructions on the cover of
the Question Paper for Section A.
(3)
This section consists of TWO parts, Parts I and II.
(4)
Answer ALL questions in both Parts I and II. Write your
answers in the spaces provided in this
Question-Answer Book. Do not write in the
margins. Answers written in the margins will not
be marked.
(5)
An asterisk (*) has been put next to the questions
where one mark will be awarded for effective
communication.
(6)
Supplementary answer sheets will be provided on
request. Write your Telegram username and mark the
question number box on each sheet, and fasten them
with string INSIDE this Question-Answer Book.
(7)
No extra time will be given to candidates for filling in
the question number boxes after the ‘Time is up’
announcement.
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*A140E01B*
PART I
Answer ALL questions. Write your answers in the spaces provided.
With the aid of chemical equations, explain the chemical weathering of rocks by rainwater.
Answers written in the margins will not be marked.
Answers written in the margins will not be marked.
1.
(4 marks)
Answers written in the margins will not be marked.
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Sulphur can form three fluorides, namely SF2, SF4 and SF6.
Answers written in the margins will not be marked.
(a)
For SF2,
(i)
draw its three-dimensional shape, showing clearly all electron pairs at the central atom.
(ii)
based on its molecular shape, explain whether it is polar or not.
(3 marks)
(b)
In the Periodic Table, oxygen and sulphur are in the same group but oxygen can only form one
fluoride (OF2) while sulphur can form three. Explain this phenomenon.
(1 mark)
(c)
Deduce and explain whether SF2 or OF2 has a higher boiling point.
(2 marks)
Answers written in the margins will not be marked.
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Answers written in the margins will not be marked.
2.
The empirical formula of an oxide of copper can be determined by the following set-up. Some reddish brown
solid is obtained after a period of time.
Answers written in the margins will not be marked.
(a)
(i)
Name the reddish brown solid.
(ii)
State a major component in town gas, and briefly explain its function in this experiment.
(2 marks)
(b)
After the experiment, a student wishes to remove the remaining copper oxide left in the mixture with
an acid and find the weight of the reddish brown solid produced. Describe the process, and give the
name of the acid in your answer.
(2 marks)
Answers written in the margins will not be marked.
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Answers written in the margins will not be marked.
3.
3.
(c)
The following masses are taken during the experiment :
The mass of the copper oxide initially = 10.21 g
The mass of the mixture after the experiment = 9.46 g
The mass of the reddish brown solid = 4.47 g
(2 marks)
(d)
A student subsequently discovered that the copper oxide sample was contaminated with magnesium
oxide. Explain whether this will have any effect on the experiment and the calculations in (c) for the
empirical formula.
(2 marks)
Answers written in the margins will not be marked.
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Answers written in the margins will not be marked.
Answers written in the margins will not be marked.
Determine the empirical formula of the copper oxide.
4.
Compound X is the organic compound used to produce Perspex. Its structure is shown below :
(a)
Give the systematic name of X.
(1 mark)
Perspex can be prepared in laboratories with the following set-up :
(i)
Name the type of reaction involved.
(ii)
Write a chemical equation for the reaction involved.
(iii)
This experiment is performed in a fume cupboard. Why ?
Answers written in the margins will not be marked.
Answers written in the margins will not be marked.
(b)
(3 marks)
Answers written in the margins will not be marked.
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4.
(c)
Perspex is usually used as a substitute for glass. Suggest ONE advantage of Perspex over glass.
(1 mark)
(2 marks)
Answers written in the margins will not be marked.
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Answers written in the margins will not be marked.
Perspex is a thermoplastic which can be softened or melted by heating and hardened by cooling. This
process can be repeated for many times. Explain this property with reference to the structure of
Perspex.
Answers written in the margins will not be marked.
(d)
5.
Enthalpy changes of a lot of reactions cannot be found directly. We often use Hess’s Law to determine the
enthalpy change of these reactions.
(a)
State Hess’s Law.
(1 mark)
(b)
To determine the enthalpy change of complete ionisation of ethanedioic acid in water, a student
measures the enthalpy change of neutralisation between ethanedioic acid and sodium hydroxide
solution. The complete ionisation of ethanedioic acid in water can be described by the following
equation :
(i)
Why can the enthalpy change of the above reaction not be measured directly ?
(ii)
A standard solution of ethanedioic acid can be made directly from its solid. Describe the
preparation process.
(3 marks)
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Answers written in the margins will not be marked.
Answers written in the margins will not be marked.
(COOH)2(aq) ⇌ (COO)22−(aq) + 2H+(aq)
(c)
A student has prepared some 1.32 M standard ethanedioic acid using the method in (b)(ii). She then
mixed 50.0 cm3 of 1.32 M ethanedioic acid with 100 cm3 of 1.5 M NaOH(aq). The maximum
temperature of the solution has been recorded to be 33.4°C. Find the enthalpy change of
neutralisation of ethanedioic acid.
(Specific heat capacity of the mixture = 4.2 J g−1 °C−1;
density of the reaction mixture = 1.05 g cm−3;
temperature of the solutions before reaction = 24.2°C)
(2 marks)
(d)
It is known that the actual value of the enthalpy change of neutralisation of ethanedioic acid with
NaOH(aq) is more negative than the calculated value in (c). Assuming that the heat loss to the
surroundings is small, briefly explain the difference.
(1 mark)
(e)
Assume that the enthalpy change of neutralisation between H+(aq) ions and NaOH(aq) is
−57.1 kJ mol−1. Using an enthalpy change cycle and the result in (c), find the enthalpy change of
complete ionisation of ethanedioic acid in water.
(2 marks)
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Go on to the next page
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5.
6.
Nitric acid is a typical acid found in laboratories.
(a)
Concentrated nitric acid is usually stored in a brown bottle. Explain this practice with the aid of a
chemical equation.
(b)
Concentrated nitric acid is a reagent commonly seen in redox reactions. In an experiment, a small
amount of concentrated nitric acid is added to iron(II) sulphate solution in a test tube.
(i)
Write an ionic equation for the reaction between concentrated nitric acid and iron(II)
sulphate solution.
(ii)
State ONE expected observation in the test tube.
(iii)
Does concentrated nitric acid undergo oxidation or reduction in the reaction in (i) ? Explain
in terms of the oxidation number of nitrogen.
(3 marks)
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Answers written in the margins will not be marked.
Answers written in the margins will not be marked.
(2 marks)
(c)
A student would like to perform a titration with nitric acid to determine the percentage by mass of
sodium hydrogencarbonate, which is an active ingredient in antacid, in an antacid tablet.
(i)
Is nitric acid suitable to be a ‘primary standard’ ? Explain.
(ii)
1.50 g of the antacid tablet was dissolved in 50.0 cm3 of 1.00 M nitric acid. The excess acid
required 16.50 cm3 of 2.00 M of potassium hydroxide solution for complete neutralisation.
Calculate the percentage by mass of sodium hydrogencarbonate in the antacid tablet.
(Assume that other ingredients of the tablet react with neither nitric acid nor potassium
hydroxide solution.)
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Answers written in the margins will not be marked.
6.
(4 marks)
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Go on to the next page
Lead-acid accumulators can be used as car batteries because they provide high current, which is required by
starter motors. The diagram below shows the changes inside a lead-acid accumulator when the battery is being
discharged.
Answers written in the margins will not be marked.
(a)
It is known that instead of H2SO4(aq), HSO4−(aq) reacts with PbO2 and Pb respectively to give
PbSO4. Write the half equations for the change that occurs at each of the following electrodes of the
lead-acid accumulator :
(i)
cathode
(ii)
anode
(2 marks)
(b)
Briefly explain why the reaction is exothermic in terms of bond breaking and forming.
(1 mark)
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Answers written in the margins will not be marked.
7.
(c)
Lead-acid accumulator is a secondary battery, meaning that it can be recharged after use.
(i)
Explain why the recharging of lead-acid accumulator can be regarded as an electrolysis
reaction.
(ii)
Suggest ONE reason why the efficiency of the battery decreases after multiple discharge
and recharge.
(iii)
Suggest ONE difference between a lead-acid accumulator and a zinc-carbon cell.
(3 marks)
(d)
Suggest ONE use of H2SO4 in the lead-acid accumulator.
(1 mark)
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Go on to the next page
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Answers written in the margins will not be marked.
7.
*8.
Sodium metal (Na) can react with iodine (I2) to form a salt, sodium iodide (NaI). It is known that sodium
iodide has the same structure as sodium chloride in solid state.
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Answers written in the margins will not be marked.
Illustrate the structures and bonding of sodium and sodium iodide with labelled diagrams. Hence, explain why
sodium metal is more malleable than sodium iodide, and explain ONE further difference in physical properties
between the two species.
(6 marks)
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PART II
Answer ALL questions. Write your answers in the spaces provided.
Outline a synthetic route, with NO MORE THAN THREE STEPS, to accomplish the following conversion.
For each step, give the reagent(s), reaction conditions (as appropriate) and structure of the organic product.
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Answers written in the margins will not be marked.
9.
(3 marks)
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Go on to the next page
10.
An ‘Elephant Toothpaste’ experiment is conducted to follow the change in rate of the decomposition of
hydrogen peroxide. 50 cm3 of 6% H2O2(aq), soap solution and 0.5 g of MnO2(s) are added into a measuring
cylinder in order. The reading, which represents the volume of foam produced, is recorded at regular time
intervals. The experimental results were tabulated as follows :
Time / s
10
40
70
100
130
160
190
220
250
Volume of foam / cm3
80
210
280
330
370
400
420
430
440
(a)
Why is the soap solution added ?
(b)
Answers written in the margins will not be marked.
Answers written in the margins will not be marked.
(1 mark)
Plot a graph of the volume of foam against time. Label the curve as ‘A’ and all axes.
(2 marks)
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10.
(c)
From your graph, determine the instantaneous rate at the 100th second, in cm3 s−1.
(d)
Sketch on your graph the curve that would have been obtained if the experiment was repeated in an
identical manner, except that 10 cm3 of 3% H2O2(aq) was added into the original H2O2(aq) before
the experiment. Label the curve as ‘B’.
(1 mark)
(e)
Suggest ONE disadvantage of this experiment.
(1 mark)
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Answers written in the margins will not be marked.
(2 marks)
11.
Lactic acid, HC3H5O3, is a common acid present in dairy products.
(a)
Lactic acid is a weak acid that ionises only partially when dissolved in water. The chemical equation
for the ionisation of lactic acid in water is shown below :
HC3H5O3(aq) + H2O(l) ⇌ C3H5O3−(aq) + H3O+(aq)
(i)
Write the expression for the equilibrium constant Kc for the reaction.
(ii)
Calculate the pH value and the percentage ionisation of 0.1 M lactic acid.
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Answers written in the margins will not be marked.
At a certain temperature, the equilibrium constant Kc for the ionisation of lactic acid is
1.67 × 10−4 mol dm−3.
(4 marks)
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11.
(b)
Explain how diluting a 0.1 M solution of lactic acid changes its pH value and percentage ionisation.
Answers written in the margins will not be marked.
(c)
A student is trying to distinguish a bottle of lactic acid and a bottle of hydrochloric acid, both with
unknown molarities. She has measured the pH of the two solutions but found that the values are the
same. Suggest a method for distinguishment.
(2 marks)
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(2 marks)
(a)
‘In the Periodic Table, both zinc and chromium are found in the area of transition metal. However,
zinc is usually not considered as a transition metal while chromium is usually considered as a
transition metal.’
Justify the statement by comparing TWO properties of zinc and chromium.
(2 marks)
(b)
With reference to the characteristics of transition metals, explain TWO industrial applications of
transition metals.
(2 marks)
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Answers written in the margins will not be marked.
Answers written in the margins will not be marked.
12.
Using 1,2-dibromoethene as an example, describe and briefly explain the term ‘cis-trans isomerism’ and the
physical properties of cis-trans isomers.
(6 marks)
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Answers written in the margins will not be marked.
*13.
END OF SECTION B
END OF PAPER
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2021-DSE
CHEM
PAPER 2
你 問 我 答
HONG KONG DIPLOMA OF SECONDARY EDUCATION EXAMINATION 2021
MOCK 1
CHEMISTRY PAPER 2
(1 hour)
This paper must be answered in English
INSTRUCTIONS
(1)
This paper consists of THREE sections, Section A, Section B and Section C. Attempt ALL
questions in any TWO sections.
(2)
Write your answers in the DSE(D) Answer Book provided. Start each question (not part of a
question) on a new page.
(3)
A Periodic Table is printed on page 8 of this Question Paper. Atomic numbers and relative
atomic masses of elements can be obtained from the Periodic Table.
Not to be taken away before the
end of the examination session
©你問我答
保留版權
All Rights Reserved 2021
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39
Section A
Industrial Chemistry
Answer ALL parts of the question.
1.
(a)
Answer the following short questions :
(i)
Suggest ONE disadvantage of a simple electrolytic setup using carbon electrodes in
chloroalkali industry.
(1 mark)
(ii)
Sketch a Maxwell-Boltzmann distribution curve of molecular kinetic energy to illustrate
the effect of a catalyst on a chemical reaction involving a gaseous species.
(2 marks)
(iii)
Vitamin C is an essential nutrient for humans. Its deficiency can cause scurvy. To meet
the market demand, some effective industrial processes are used in the production of
vitamin C.
(1)
State a progress of the industrial production of vitamin C.
(2)
Hence, explain why it is regarded as a progress.
(2 marks)
(b)
Methanol is an important feedstock for the production of acetic acid in the chemical industry. It
has to be manufactured industrially and massively to ensure a constant supply.
(i)
Methane is often converted to methanol in the industry partly because methanol can be
further turned into many useful organic compounds. Suggest another industrial benefit of
the conversion of methane into methanol.
(1 mark)
(ii)
Methanol can be used to produce acetic acid through its reaction with a component
of syngas. The reaction requires iridium (Ir) as the catalyst.
(iii)
(1)
Write a chemical equation for the reaction involved.
(2)
This way of manufacture is adopted widely in the industry since it saves capital
for being energy efficient. In terms of the steps involved, explain why this way is
deemed energy efficient.
(2 marks)
The reaction scheme below can also yield acetic acid :
(1)
Calculate the atom economy for the first step of this reaction scheme with respect
to bromomethane.
(2)
Is this reaction scheme greener than the reaction in (ii) ? Support your answer
with TWO reasons.
(4 marks)
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1.
(c)
To investigate the kinetics of the reaction between 2-chloro-2-methylbutane and sodium
hydroxide in an alcoholic medium at room temperature, two experiments with different initial
concentrations of 2-chloro-2-methylbutane were performed. The graph below shows the variation
of the molarity of sodium hydroxide with time :
(i)
It is known that the alcoholic medium refers to the use of ethanol as the solvent. Suggest
ONE advantage of using ethanol over water.
(1 mark)
(ii)
Deduce the orders of reaction each with respect to 2-chloro-2-methylbutane and sodium
hydroxide.
(2 marks)
(iii)
Hence, state the rate equation for this reaction.
(1 mark)
(iv)
Sketch a graph showing how the rate of reaction varies with the molarity of 2-chloro-2methylbutane (y-axis : rate of reaction; x-axis : molarity of 2-chloro-2-methylbutane).
(1 mark)
(v)
The same setup for reaction 2 is repeated again, but at 100°C. The rate of reaction of
sodium hydroxide is found to be −4.98 × 10−3 mol dm−3 s−1. Calculate the activation
energy for this reaction.
(Gas constant R = 8.31 J K−1 mol−1)
(3 marks)
END OF SECTION A
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Go on to the next page
Section B
Materials Chemistry
Answer ALL parts of the question.
2.
(a)
Answer the following short questions :
(i)
Suggest a reagent to obtain colloidal silver nanoparticles from silver nitrate solution. State
the colour change.
(2 marks)
(ii)
How should the alignment of liquid crystals be in order to avoid colour distortion in
curved displays ?
(1 mark)
(iii)
Melt blowing is a fabrication method where a polymer is melted and converted to
filaments with very small diameter. Which TWO of the following polymers are suitable
for melt blowing ?
nylon, urea-methanal, Kevlar, polypropene
(2 marks)
(b)
Poly(tetrafluoroethene) (PTFE) is a polymer widely used in daily life.
(i)
One common use of PTFE is in coating non-stick cooking ware.
(1)
Draw the repeating unit of PTFE.
(2)
With reference to its structure, explain why PTFE has a non-stick property.
(3 marks)
(ii)
Gore-Tex™ is a fabric membrane found in high-end waterproof and breathable clothing.
It is made from expanded PTFE, which is produced by rapid stretching of blocks of heated
PTFE. What structural features and properties make Gore-Tex™ both waterproof and
breathable ?
(2 marks)
(iii)
In terms of the production and the disposal of PTFE, explain why it is NOT considered
as a green material.
(2 marks)
(iv)
Suggest another use of PTFE.
(1 mark)
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2.
(c)
Nickel is a common metal that finds wide use in alloy.
(i)
(ii)
Nitinol is an alloy of nickel and titanium and it is a shape-memory alloy. After cooling to
a low temperature, Nitinol has a crystal structure with the following unit cell :
(1)
State the meaning of the term ‘shape-memory alloy’.
(2)
Deduce the respective number of Ni atoms and Ti atoms in the above unit cell.
(3)
Upon heating, Nitinol has a cubic open structure. Sketch a unit cell of the open
structure of Nitinol.
(5 marks)
Cupronickel, an alloy of nickel and copper, is a common coinage metal. In 1993, some
Hong Kong one-dollar coins were made by nickel-plated steel in a bid to phase out coins
made by cupronickel, but this was not well received and the production of nickel-plated
steel $1 coins was discontinued. Nowadays those $1 coins are rare and they are popular
among collectors as they are magnetic.
(1)
Suggest ONE reason why the alloy, cupronickel, is used to make coins.
(2)
Suggest ONE reason why the nickel-plated steel $1 coins were not well received.
(2 marks)
END OF SECTION B
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Go on to the next page
Section C
Analytical Chemistry
Answer ALL parts of the question.
3.
(a)
Answer the following short questions :
(i)
Briefly describe how the purity of a substance can be examined through melting point
measurement.
(1 mark)
(ii)
Suggest a chemical test to distinguish between CaCl2(aq) and Ca(OCl)2(aq).
(2 marks)
(iii)
Briefly describe how to purify C6H5COOH(s) using recrystallisation.
(2 marks)
(b)
The concentration of chloride ions in a river water sample was determined to investigate whether
a habitat is suitable for a certain aquatic species.
(i)
Mohr’s method can be used. It involves the following steps :
Step (1) :
Step (2) :
Step (3) :
250.0 cm3 of a river water sample was filtered. The filtrate was collected.
The pH of the river water sample was measured to be 5.8. Then, 10.0 cm3
of 1 M sodium carbonate solution was added to adjust the pH of the sample.
A standard 1.12 M silver nitrate solution was used to titrate the treated
sample using potassium chromate as an indicator. 15.0 cm3 of the solution
required 24.90 cm3 of the silver nitrate solution to reach the end point.
(1)
State the observation at the end point of the titration.
(2)
Why does the pH of the river water sample have to be adjusted in Step (2) ?
(3)
Calculate the concentration of chloride ions in the original river water sample.
(4 marks)
(ii)
The concentration of chloride ions can also be determined using colorimetry.
Hg(SCN)2(aq) and Fe3+(aq) are added to the solution. They react with chloride ions to
form dark red Fe(SCN)2+(aq). A calibration curve is then constructed as follows :
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3.
(b)
(c)
(ii)
(1)
Given that the mole ratio of SCN− to Cl− is 1 : 1, what would be the chloride
concentration of a sample if it gives an absorbance of 0.410 ?
(2)
Explain why the colorimetric method is more suitable for measuring chloride
concentration in freshwater than Mohr’s method.
(4 marks)
To obtain hexanoic acid from hexan-1-ol, acidified potassium dichromate solution was allowed
to react with hexan-1-ol under reflux. The chromatograms from thin-layer chromatography (TLC)
of the reaction mixture were obtained every hour and are shown below. The stationary phase was
alumina and the mobile phase was dichloromethane.
(i)
Explain whether the reaction has completed at time = 3 hours.
(1 mark)
(ii)
After 3 hours, the reflux setup was disassembled. The reaction mixture was subjected to
liquid-liquid extraction using hexane and sodium chloride solution, and the organic layer
was collected. Draw a labelled setup for the liquid-liquid extraction.
(2 marks)
(iii)
The solvent in the organic layer was evaporated away, and the remaining contents were
purified with column chromatography. The stationary phase and the mobile phase were
silica gel and dichloromethane respectively.
(1)
Briefly describe the principle of column chromatography.
(2)
Three fractions corresponding to spots A, B and C were collected. Which spot on
the chromatogram would give a peak at m/z = 100 upon an analysis by mass
spectrometry ? Explain your answer.
(4 marks)
END OF SECTION C
END OF PAPER
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Marking Schemes
This document was prepared for markers’ reference. It should not be regarded as a set of model answers.
Candidates and teachers who were not involved in the marking process are advised to interpret its contents with
care.
Chemistry
Paper 1
SECTION A
Question No.
Part I
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
*
Key
Question No.
Part II
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
B (80%)
C (88%)
D (35%)
D (39%)
A (71%)
B (35%)
B (31%)
A (88%)
C (12%)
B (71%)
A (92%)
C (80%)
B (55%)
D (16%)
D (78%)
A (61%)
B (82%)
B (57%)
C (27%)
C (16%)
D (53%)
A (73%)
A (41%)
*
Key
A (53%)
D (69%)
D (90%)
C (45%)
D (78%)
D (75%)
C (39%)
C (57%)
A (25%)
B (92%)
A (47%)
C (49%)
This item was deleted.
Note: Figures in brackets indicate the percentages of candidates choosing the correct answers.
General note on item deletion
It is normal for the Team to delete a small number of items from its multiple-choice question papers if they prove unsatisfactory.
In practice, there are a number of reasons why this is considered necessary. By far the most common reason for deleting an item
is that the item fails to discriminate between weak and able candidates – in other words, the majority of the candidates involved had
to rely on guesswork in answering that question. If such an item is retained, the measurement process is rendered less effective.
Where items have been deleted in the live papers, they are still included in this series of publications. They are indicated as deleted
items. Such items may be discussed in the corresponding examination reports.
©你問我答
47
General Marking Instructions
1.
In order to maintain a uniform standard in marking, markers should adhere to the marking scheme agreed
at the markers’ meeting.
2.
The marking scheme may not exhaust all possible answers for each question. Markers should exercise
their professional discretion and judgment in accepting alternative answers that are not in the marking
scheme but are correct and well reasoned.
3.
In questions asking for a specified number of reasons or examples etc. and a candidate gives more than
the required number, the extra answers should not be marked. For instance, in a question asking candidates
to provide two examples, and if a candidate gives three answers, only the first two should be marked.
4.
In cases where a candidate answers more questions than required, the answers to all questions should be
marked. However, the excess answer(s) receiving the lowest score(s) will be disregarded in the calculation
of the final mark.
5.
Award zero marks for answers which are contradictory.
6.
Chemical equations should be balanced except those in reaction schemes for organic synthesis. For
energetics, the chemical equations given should include the correct state symbols of the chemical species
involved.
7.
In the question paper, questions which assess candidates’ communication skills are marked with an asterisk
(*). For these questions, the mark for effective communication (1 mark per question) will be awarded if
candidates can produce answers which are easily understandable. No marks for effective communication
will be awarded if the answers produced by candidates contain a lot of irrelevant materials and/or wrong
concepts in chemistry.
©你 問 我 答
48
SECTION B
Part I
Marks
1.
•
Carbon dioxide in air dissolves in rainwater to form carbonic acid, making rainwater
slightly acidic.
CO2(g) + H2O(l) ⇌ H2CO3(aq)
The acid reacts with calcium carbonate in rocks to form soluble calcium
hydrogencarbonate and thus the rocks are slowly worn away.
H2CO3(aq) + CaCO3(s) → Ca(HCO3)2(aq)
•
•
•
2.
(a)
(i)
(ii)
1
1
1
1
•
S−F bonds are polar due to the different electronegativities of S and F. As they
are not arranged symmetrically in V-shaped SF2 molecules,
their polarities do not exactly cancel out each other, resulting in a net polarity.
Thus, SF2 is polar.
1
(b) Although O and S are in the same group, O is in period 2 while S is in period 3. The
outermost electron shell of O can only hold 8 electrons (= 2 × 22) so O can only form 2
covalent bonds to attain an octet, leaving only one fluoride, namely OF2. While the
outermost electron shell of S can hold 18 electrons (=2 × 32), S can form 2, 4 or 6 covalent
bonds with F to form SF2, SF4 or SF6.
1
(c)
Both SF2 and OF2 are V-shaped polar molecules and have simple molecular
structures where molecules are held together by weak van der Waals’ forces. SF2 is
larger in molecular size / The electronegativity difference between S and F is greater
than that between O and F according to their respective positions in the Periodic
Table. S—F bonds, so does SF2, are more polar,
resulting in stronger van der Waals’ forces between molecules / intermolecular
forces. More energy is required to overcome the van der Waals’ forces. Thus, SF2
has a higher boiling point.
1
(i)
Copper
1
(ii)
Hydrogen / Methane. To reduce the copper oxide into copper.
1
Add the mixture into excess dilute hydrochloric acid / dilute sulphuric acid. Filter
to obtain the residue.
Wash the residue with distilled / deionised water. Put the residue into a desiccator /
between filter papers to dry. Weigh the solid.
1
•
•
•
3.
1
(a)
(b) •
•
(c)
Mass of copper obtained = 4.47 g
Mass of copper oxide unreacted = 9.46 – 4.47 = 4.99 g
Mass of copper oxide reacted = 10.21 – 4.99 = 5.22 g
Mass of oxygen in the reacted copper oxide = 5.22 – 4.47 = 0.75 g
No. of moles of Cu = 4.47 / 63.5 = 0.0704 mol
No. of moles of O = 0.75 / 16.0 = 0.0469 mol
Ratio of Cu : O = 0.0704 : 0.0469 = 3 : 2
The empirical formula is Cu3O2.
©你 問 我 答
49
1
1
1
2
Marks
3.
(d) •
•
4.
(a)
Methyl methylpropenoate
(b) (i)
(c)
1
Addition polymerisation
1
(iii) Methyl methylpropenoate is harmful.
1
Perspex is less dense / lighter than glass.
1
Perspex consists of separate, long flexible polymer chains. These chains are tangled
and held by weak intermolecular forces / van der Waals’ forces.
When heated, the chains vibrate more vigorously, becoming farther apart. The
intermolecular forces are weakened / overcome, and the chains can slide over each
other easily. The plastic, thus, softens and melts.
1
Hess’s Law states that the enthalpy change of a reaction is independent of the reaction
pathway.
1
(b) (i)
1
Only a small amount of ethanedioic acid is ionised in water, so the temperature
change cannot be measured directly.
1
•
Dissolve a known mass of ethanedioic acid in a small amount of distilled /
deionised water in a beaker (and stir with a glass rod). Pour the solution into a
volumetric flask. Rinse the apparatus and pour all washings into the flask.
Add distilled / deionised water into the volumetric flask until (the bottom of
the meniscus reaches) the graduation mark. Stopper the flask and invert the
flask for a few times.
1
No. of moles of ethanedioic acid = 0.05 × 1.32 = 0.066 mol
No. of moles of NaOH = 0.1 × 1.5 = 0.15 mol
Because 0.15 mol of NaOH reacts with 0.075 mol of ethanedioic acid, ethanedioic acid is
the limiting reagent.
Energy released = (100 + 50) × 1.05 × 4.2 × (33.4 – 24.2) = 6085.8 J
Enthalpy change of neutralisation = −6085.8 / 1000 / 0.066 / 2 = −46.1 kJ mol−1
2
(ii)
•
(c)
1
1
•
(a)
1
(ii)
(d) •
5.
Magnesium oxide is not reduced during the experiment. The decrease / change in
mass during the experiment only attributes to the reduction of copper oxide.
Magnesium oxide is reacted with the acid (similar to unreacted copper oxide) and
removed together with copper oxide / the copper obtained at the end is not
contaminated with magnesium oxide. Thus, it will have no effect on the experiment
and calculation of empirical formula.
(d) The ethanedioic acid solid (used for preparing the standard solution) is impure.
©你 問 我 答
50
1
1
Marks
5.
(e)
(COOH)2(aq) + 2NaOH(aq)
2
ΔH
(COO)22−(aq) + 2H+(aq) + 2NaOH(aq)
ΔH2
ΔH1
(COO)22−(aq) + 2Na+(aq) + 2H2O(l)
Enthalpy change of complete ionisation
= ΔH1 – ΔH2 = 2 × (−46.1) – 2 × (−57.1) = +22.0 kJ mol−1
6.
(a)
•
The decomposition of concentrated nitric acid that gives nitrogen dioxide, which
dissolves in water to form a yellow solution, is speeded up by light.
4HNO3(aq) → 2H2O(l) + 4NO2(g) + O2(g)
1
3Fe2+(aq) + NO3−(aq) + 4H+(aq) → 3Fe3+(aq) + NO(g) + 2H2O(l)
1
The solution turns from green to yellow. / Brown fumes are seen at the mouth of the
test tube.
1
(iii) It undergoes reduction in that reaction as the oxidation number of N decreases from
+5 in HNO3 to +2 in NO.
1
(i)
No as nitric acid decomposes continuously, its concentration cannot be determined
accurately / nitric acid always contains a little nitrous acid, HNO2, which has a
destructive action on many acid-base indicators / dilute / concentrated nitric acid is
a strong oxidising agent.
1
(ii)
No. of moles of HNO3 added at the start = 1.00 × 50 / 1000 = 0.05 mol
No. of moles of KOH used to neutralise unreacted HNO3
= 2.00 × 16.50 / 1000 = 0.033 mol
KOH(aq) + HNO3(aq) → KNO3(aq) + H2O(l)
From the equation, mole ratio of KOH to HNO3 = 1 : 1
Therefore, no. of moles of unreacted HNO3 = 0.033 mol
No. of moles of HNO3 reacted with NaHCO3 in the antacid tablet
= (0.05 – 0.033) mol = 0.017 mol
HNO3(aq) + NaHCO3(s) → NaNO3(aq) + H2O(l) + CO2(g)
From the equation, mole ratio of HNO3 to NaHCO3 = 1 : 1
No. of moles of NaHCO3 in the antacid tablet = 0.017 mol
Mass of NaHCO3 in the antacid tablet
= 0.017 × (23.0 + 1.0 + 12.0 + 3 × 16.0) g = 1.428 g
Percentage by mass of NaHCO3 in the antacid tablet = 1.428 / 1.5 × 100% = 95.2%
3
(i)
PbO2(s) + HSO4−(aq) + 3H+(aq) + 2e− → PbSO4(s) + 2H2O(l)
1
(ii)
Pb(s) + HSO4−(aq) → PbSO4(s) + H+(aq) + 2e−
1
•
(b) (i)
(ii)
(c)
7.
(a)
1
(b) The energy released during bond formation in the reaction is greater than the energy
required to break the bonds.
1
(c)
(i)
It converts electrical energy into chemical energy.
1
(ii)
The Pb plate is gradually corroded by sulphuric acid. / More lead(II) sulphate
deposits on the electrodes in recharging and fails to redissolve. / Leakage of
electrolyte may occur.
1
(iii) The zinc-carbon cell is a primary. / The zinc-carbon cell cannot provide continuous,
stable current over time. / The zinc-carbon cell uses ammonium chloride as the
electrolyte. / Zinc-carbon cell is a dry cell. / The zinc-carbon cell provides a lower
voltage.
1
©你 問 我 答
51
Marks
7.
(d) It is an electrolyte. / It provides H+ for the discharge reaction.
1
8.
Chemical knowledge
•
Labelled diagrams of both sodium metal (giant lattice of metal ions and sea of delocalised
electrons), and sodium iodide (NaCl-like lattice, with at least 3 layers of 3 × 3 ions array)
•
In sodium metal, metallic bonds exist between a giant lattice of sodium ions and the sea
of delocalised electrons. In sodium iodide, ionic bonds exist between oppositely charged
ions.
•
Upon hammering of both species, sodium metal ions are shifted / slides over one another
in the lattice, but the non-directional metallic bonds are not broken. Therefore, sodium
metal is malleable.
•
Yet, in sodium iodide, ions of the same charge come together. This results in a repelling
force between these ions. Thus, sodium iodide is brittle.
•
Sodium is a good conductor of electricity / heat because there are delocalised electrons in
its structure, while sodium iodide is a poor electrical / thermal conductor because the ions
are not mobile.
Communication mark
5
©你 問 我 答
52
1
Part II
Marks
9.
3
10. (a)
The soap solution is a foaming agent that facilitates the formation of foam / a surfactant /
wetting agent that reduces the surface tension of water. Foam is thus produced as oxygen
generated creates bubbles in soap.
(b)
©你 問 我 答
1
2
53
Marks
10. (c)
•
Correct tangent
1
•
Instantaneous rate at the 100th second = (450 – 350) / (180 – 110) = 1.43 cm3 s−1
1
(d)
(e)
11. (a)
1
Some oxygen produced might just escape from the measuring cylinder without being
trapped to form bubbles, making the rate of reaction underestimated.
(i)
Kc = [C3H5O3−(aq)][H3O+(aq)] / [HC3H5O3(aq)]
(ii)
HC3H5O3(aq) + H2O(l) ⇌ C3H5O3−(aq)
0.1 − x
x
x2 / (0.1 – x) = 1.67 × 10−4
x = −4.17 × 10−3 (rejected)
x = 4.00 × 10−3
pH = −log[H3O+(aq)] = −log x = 2.40
Percentage ionisation = x / 0.1 × 100% = 4.00%
©你 問 我 答
54
1
1
+
H3O+(aq)
x
3
Marks
11. (b) •
Even though the rightward shift in equilibrium position increases the percentage
ionisation, this increase in [H3O+(aq)] is smaller than the decrease due to dilution.
Overall, the pH value of the solution increases as [H3O+(aq)] decreases with the
concentration of lactic acid.
The percentage ionisation also increases as water is added during dilution, shifting
the equilibrium position to the right. A greater portion of lactic acid would thus be
in the ionised form.
1
She could dilute the two solutions with the same volume of distilled / deionised
water and measure their pH again respectively. The pH of lactic acid would
eventually be lower than that of hydrochloric acid.
As the extent of ionisation is lower in lactic acid (a weak acid), the decrease in [H+]
in lactic acid is smaller than that in hydrochloric acid. The increase in pH would
therefore be smaller in lactic acid.
1
Zinc can only show an oxidation state of +2 in its compounds. However, chromium
can show more than one oxidation states in its compounds.
Zinc ions in aqueous solution are colourless but chromium-containing ions in
aqueous solution are coloured.
1
As transition metals can exhibit variable oxidation states, they can be used as
catalysts in many industrial processes to shorten the production time. For example,
the contact process is speeded up by vanadium(V) oxide.
Transition metal can form coloured compounds which can be used to produce dyes
/ paints. For example, cobalt blue pigment is cobalt(II) oxide-aluminium oxide /
cobalt(II) aluminate / CoAl2O4.
1
13. Chemical knowledge
•
‘cis-trans isomerism’ is a type of stereoisomerism where isomers are with different spatial
arrangements of atoms due to rotation restriction by C=C group / C-C double bond.
•
Boiling point of cis-isomer is higher than trans-isomer with the same carbon skeleton.
Melting point of trans-isomer is higher than cis-isomer with the same carbon skeleton.
•
The intermolecular forces of cis-1,2-dibromoethene and trans-1,2-dibromoethene are
both weak van der Waals’ forces.
•
For boiling point, as cis-1,2-dibromoethene is polar while trans-1,2-dibromoethene is
non-polar, the strength of van der Waals’ force between molecules of
cis-1,2-dibromoethene
is
greater
than
that
between
molecules
of
trans-1,2-dibromoethene.
•
For melting point, as trans-1,2-dibromoethene has a better molecular symmetry and is
more regular in shape than cis-1,2-dibromoethene, the packing efficiency of trans-1,2dibromoethene is higher than that of cis-1,2-dibromoethene.
Communication mark
5
•
(c)
•
•
12. (a)
•
•
(b) •
•
©你 問 我 答
55
1
1
1
1
1
Paper 2
Marks
1. (a)
(i)
It is dangerous since mixture of hydrogen and chlorine is explosive under bright light.
/ Hydroxide ions are discharged to produce oxygen gas at anode, contaminating
gaseous chlorine formed.
(ii)
2
(iii) (1) Two-stage fermentation / Production using enzymes as the catalyst.
1
(b) (i)
(ii)
(2) It uses fewer toxic chemicals / feedstocks / raw materials / solvents. / It involves
a lower production cost / a higher yield.
1
It is easier / cheaper to store / transport methanol.
1
(1) CH3OH + CO → CH3COOH
1
(2) The reaction is exothermic, but the optimum temperature is relatively high. The
energy / heat released from the conversion is recycled to heat up the unreacted
gases / reaction mixture.
1
(iii) (1) CH3OH + HBr → CH3Br + H2O
Atom economy
= (12.0 + 1.0 × 3 + 79.9) / (12.0 + 1.0 × 3 + 79.9 + 1.0 × 2 + 16.0) × 100%
= 84.1%
(2) No
•
•
•
•
•
•
(c)
1
(i)
©你 問 我 答
2
2
Lower atom economy
More hazardous feedstocks / raw materials (HBr, KCN)
More hazardous products / intermediates (CH3Br, CH3CN)
No use of catalysts
More steps / derivatives
Use of a solvent (H2O)
2-chloro-2-methylbutane is much more miscible with ethanol than with water.
56
1
Marks
1. (c)
(ii)
•
•
Comparing the reactions, when [2-chloro-2-methylbutane] doubles from
0.25 M to 0.50 M, the slope of the graph doubles. This shows that the rate of
reaction doubles, hence the order of reaction with respect to 2-chloro-2methylbutane is 1.
For both reactions, when [NaOH] decreases, the slope of the graph remains
constant. The rate of reaction is independent of [NaOH], hence the order of
reaction with respect to NaOH is 0.
1
1
(iii) Rate = k [2-chloro-2-methylbutane]
1
(iv)
1
(v)
©你 問 我 答
Using reaction 1, at 298 K (25°C),
rate = slope of graph = (0.010 − 0.0052) / (29 × 60) = 2.76 × 10−6 mol dm−3 s−1
k298 = rate / [2-chloro-2-methylbutane] = 2.76 × 10−6 / 0.25 = 1.10 × 10−5 s−1
At 373 K (100°C),
k373 = rate / [2-chloro-2-methylbutane] = 4.98 × 10−3 / 0.50 = 9.96 × 10−3 s−1
By the Arrhenius equation, log k = constant − Ea / (2.3 RT)
Hence, log (k373 / k298) = (Ea / 2.3 R) × (1/298 − 1/373)
Ea = 837 kJ mol−1
57
3
Marks
2. (a)
(i)
•
•
Sodium borohydride solution / NaBH4(aq).
The colourless solution turns to yellow / brownish yellow / dark brown.
1
1
(ii)
The liquid crystals should be vertically / perpendicularly aligned to the glass
substrate. / The liquid crystals should not be aligned parallel / in-plane / horizontally
to the glass substrate.
1
(iii) •
•
(b) (i)
nylon
polypropene
(1)
1
(2) •
Molecules of PTFE are surrounded by fluorine atoms and F is highly
electronegative.
An evenly distributed layer of negative charge is developed on the
surface of PTFE and repel other materials.
1
•
•
Gore-Tex™ membrane is highly porous.
The size of the pores are small such that water droplets cannot penetrate them,
and the size of the pores are large enough forwater vapour due to sweating to
go through.
1
1
(iii) •
The production of PTFE is not green because the process involves toxic
materials such as HF / chloroform / fluorine / HCl / halogenated organic
compounds.
PTFE is not biodegradable.
1
•
(ii)
•
(c)
1
1
1
1
(iv) Insulation / Medical equipment / Dental floss / Burette stopcock.
1
(i)
1
1
(1) •
•
Shape-memory alloy can be deformed when it is cooled and
returns to its original shape when it is heated up.
(2) Ni :
1+2×"=2
1
8×#+2×"=2
1
Ti :
(ii)
©你 問 我 答
!
!
!
(3)
1
(1) Good corrosion resistance / Does not tarnish / Good wear resistance / Durable
/ Hard enough but not too hard for stamping / Low allergic risk.
1
(2) Higher allergic risk due to nickel on the surface / The magnetic coins may cause
problems in vending machines.
1
58
Marks
3. (a)
(i)
Pure substances have sharp melting points, while impure substances have a range of
melting points. / Impure substances have a lower melting point than pure substances.
1
(ii)
•
•
Add HCl to both solutions.
Only Ca(OCl)2(aq) reacts with HCl to form a greenish yellow gas (Cl2).
1
1
Dissolve the crude C6H5COOH(s) in a minimum amount of hot solvent.
Cool the filtrate for crystal formation, and collect the crystals by filtration.
1
1
(iii) •
•
(b) (i)
(ii)
(1) From yellow to reddish brown.
1
(2) To prevent CrO42− ions from changing to HCrO4− / Cr2O72− ions.
1
(3) Ag+ + Cl− → AgCl
Number of moles of Ag+ used = 1.12 × 24.9 / 1000 = 0.0279 mol
Concentration = 0.0279 × 260 / 15 / (250 / 1000) = 1.93 M
2
(1) •
•
•
(c)
(i)
From the graph, the concentration of Fe(SCN)2+ is 2.20 mg dm−3 =
0.0193 mM
Since SCN− to Cl− is 1 : 1 and each Hg(SCN)2 contains 2 SCN−,
the concentration of Cl− = 0.0193 × 2 = 0.0386 mM
1
1
(2) The colorimetric method is more sensitive.
1
The reaction has not completed because there is still hexan-1-ol in the reaction
mixture.
1
(ii)
2
(iii) (1) •
•
(2) •
•
©你 問 我 答
1
Different compounds have different solubilities in the mobile phase and
the stationary phase.
As the solvent moves down the column, different components of the
mixture come out of the column at different times.
1
Spot A.
m/z = 82 refers to hexanal, which has the lowest polarity of all three
compounds.
1
1
59
1
Candidates’ Performance
Paper 1
Paper 1 consisted of two sections, Section A (multiple-choice questions) and Section B (conventional questions).
Sections A and B each comprised two parts, Part I and Part II. Part I contained questions mainly on Topics I to VIII
of the curriculum, while Part II mainly on Topics IX to XII. All questions in both sections were compulsory.
Section A (multiple-choice questions)
This section consists of 36 multiple-choice questions. Without counting the deleted one, the mean score was 20.3.
Candidates’ performance was generally good. Some misconceptions of candidates were revealed from their
performance in the following items:
1.
For Q.3, it should be noted that the target object acts as an anode in anodisation. In other words, it is connected
to the cathode of a battery. Many candidates might have mistakenly chosen an incorrect option as they saw
the word ‘anode’ (mainly Option C). Besides, dilute sulphuric acid is used as the electrolyte instead of the
concentrated one, which is oxidising.
Q.3
Which of the following statements about anodisation is correct ?
A.
B.
C.
D.*
2.
The target object is connected to the anode of a battery and concentrated sulphuric acid
is used as the electrolyte in anodisation.
(12%)
The target object is connected to the cathode of a battery and concentrated sulphuric acid
is used as the electrolyte in anodisation.
(6%)
The target object is connected to the anode of a battery and dilute sulphuric acid is used
as the electrolyte in anodisation.
(47%)
The target object is connected to the cathode of a battery and dilute sulphuric acid is used
as the electrolyte in anodisation.
(35%)
For Q.4, the item examines the scenarios where back titration is needed. Many candidates might have
overlooked (2), which is ammonia gas. Indeed, if the sample is too volatile that the accuracy might be
sacrificed, back titration shall be carried out. Excess acid should be added to neutralise the base, and the
number of moles of excess acid shall be determined subsequently.
Q.4
©你 問 我 答
For which of the following species should back titration be adopted when titrated against standard
HCl ?
(1)
(2)
(3)
NaOH(aq)
NH3(g)
CaSO4(s)
A.
B.
C.
D.*
(2) only
(3) only
(1) and (3) only
(2) and (3) only
(10%)
(39%)
(12%)
(39%)
60
3.
For Q.6, the item examines the concept of hydrogen bonding. For group IV hydrides, such as methane, no
hydrogen bonding would occur. Hence, the boiling point down the group would increase almost linearly due
to the increase in molecular size. For group V hydrides, only nitrogen hydride (NH3) could exhibit hydrogen
bonding. Hence, the boiling point of NH3 is exceptionally higher than those of the other group V hydrides.
Thus, a ‘V’-shape curve is obtained in terms of the boiling point down the group.
Q.6
4.
Which of the following graphs sketches the relationship between boiling points of group IV (solid
line) and V (dotted line) hydrides ?
A.
(24%)
B.*
(35%)
C.
(16%)
D.
(25%)
For Q.7, it should be noted that enthalpy change deals with heat transfer under constant pressure conditions,
but not constant volume conditions. The change in internal energy would address the heat transfer under
constant volume conditions instead.
Q.7
©你 問 我 答
When hydrated barium hydroxide is mixed with ammonium chloride at room conditions, ΔH has a
positive value. Which of the following statements about the reaction must be true ?
(1)
(2)
(3)
The total enthalpy of products is greater than that of reactants.
The surrounding environment loses heat to the system under constant volume conditions.
The reaction is said to be endothermic.
A.
B.*
C.
D.
(1) and (2) only
(1) and (3) only
(2) and (3) only
(1), (2) and (3)
(2%)
(31%)
(6%)
(61%)
61
5.
For Q.9, most candidates might have failed to notice that the oxidising agent in the chemical reaction is
AuCl4−(aq), but not Au(s).
Q.9
Consider the following chemical equation :
xAuCl4−(aq) + ySO2(aq) + zH2O(l) → aAu(s) + bH+(aq) + cSO42−(aq) + dCl−(aq)
Which of the following statements about the above chemical reaction are correct ?
©你 問 我 答
(1)
(2)
(3)
Au is the oxidizing agent in the above chemical reaction.
Au does not react with HCl or H2SO4.
The resultant solution is acidic.
A.
B.
C.*
D.
(1) and (2) only
(1) and (3) only
(2) and (3) only
(1), (2) and (3)
(2%)
(14%)
(12%)
(73%)
62
6.
For Q.14, it should be noted that calcium carbonate undergoes thermal decomposition under a non-luminous
Bunsen flame with a relatively high temperature. In the reaction, one mole of calcium oxide and one mole
of carbon dioxide are formed. By calculations, the mass of the remaining solid calcium oxide should be
greater than 0.5 g. Moreover, calcium being more reactive than magnesium, calcium carbonate is more stable
than magnesium carbonate. The initial rate of decomposition is hence smaller in the case of calcium
carbonate.
Q.14
The following graph shows the change in mass that occurs when 1.0 g of powdered magnesium
carbonate is heated with a non-luminous Bunsen flame :
Which of the following graphs would be obtained when 1.0 g of powdered calcium carbonate is
heated under the same condition ?
(Relative atomic masses : C = 12.0, O = 16.0, Mg = 24.3, Ca = 40.1)
©你 問 我 答
A.
(12%)
B.
(10%)
C.
(63%)
D.*
(16%)
63
7.
For Q.19, it should be noted that point X (Option B) is the equivalence point of the reaction where the acid
has completely neutralised the alkali. At this point, the concentration of the acid equals zero and is in no way
equal to that of the salt produced in the neutralisation reaction. At point Y (Option C), the acid is in excess
and by calculations, the concentration of the excess acid is equal to that of the salt.
Q.19
0.100 mol dm−3 CH3COOH is added dropwise to 10.0 cm3 of 0.100 mol dm−3 NaOH. The change
in pH is recorded by a pH meter throughout the process and the results are plotted on a graph below :
At which point on the graph, W, X, Y or Z, is the concentration of the acid equal to that of the salt
in the reaction mixture ?
A.
B.
C.*
D.
8.
(14%)
(53%)
(27%)
(6%)
For Q.20, the item requires candidates to conclude that cis-trans isomers cannot share the same density in
their solid state, based on their knowledge of the difference in molecular packing due to the difference in
molecular polarities. It should be noted that the trans-isomer is more symmetrical in the molecular shape
than the cis-isomer, and therefore, packs more closely in the solid molecular crystal. (1) is hence incorrect.
Q.20
9.
W
X
Y
Z
Butenedioic acid exists as a pair of cis-trans isomers. Which of the following statements concerning
the pair of isomers are correct ?
(1)
(2)
(3)
In the solid state, both of them have the same density.
The cis-isomer is more polar than the trans-isomer.
The cis-isomer has a lower boiling point than the trans-isomer.
A.
B.
C.*
D.
(1) and (2) only
(1) and (3) only
(2) and (3) only
(1), (2) and (3)
(67%)
(10%)
(16%)
(8%)
For Q.23, it should be noted that CH3CHO (Option A) is an alkanal and can form intermolecular hydrogen
bonds with water, while other molecules in the item can only form weaker intermolecular van der Waals’
forces with water. This allows CH3CHO to be most soluble among the four.
Q.23
Which of the following molecules has the highest solubility in water ?
A.*
B.
C.
D.
©你 問 我 答
CH3CHO
COF2
CH2S
trans-HFC=CFH
(41%)
(39%)
(4%)
(16%)
64
10.
For Q.24, the item was deleted due to insufficient discrimination. This question tests candidates’ knowledge
of the insolubility of lead(II) chloride in water. Such a property allows the compound to conduct electricity
only in its molten state but not in its aqueous solution. It also requires candidates to recognise that the
presence of mobile ions in its molten state is insufficient to explain why lead(II) chloride conducts electricity
only in its molten state. Candidates might have overlooked that the 2nd statement lacks a comparative sense
and does not address the absence of mobile ions in all other states, and hence lowered the discrimination
power of the item.
Q.24
Consider the following statements and choose the best answer :
1st statement
Lead(II) chloride conducts electricity only in
its molten state.
A.
B.
C.
D.
11.
Both statements are true and the 2nd statement is a correct explanation of the 1st
statement.
Both statements are true but the 2nd statement is NOT a correct explanation of the 1st
statement.
The 1st statement is false but the 2nd statement is true.
Both statements are false.
For Q.28, the item examines the fact that under the same conditions, all gases share an equal molar volume.
Some candidates might have neglected that oxygen would be in excess after combustion and did not include
its volume in calculations. In the combustion process, gaseous carbon dioxide and sulphur dioxide are
produced. Both of them are acidic and react with sodium hydroxide in the presence of water. Consequently,
after adding sodium hydroxide, only the unreacted oxygen remains in the gaseous mixture.
Q.28
Volcanic eruptions give off gases like H2S and CS2. In a post-eruption investigation, a 70 cm3
gaseous sample of H2S and CS2 in the ratio of 3 : 2 was analysed by combustion using 200 cm3 of
oxygen. It is known that any sulphur present was converted to SO2 after combustion. After
measuring the volume of gas remaining, the product was treated with excess aqueous sodium
hydroxide and the volume of gas was measured again. Given that all volume measurements were
made at 50°C, which of the following combinations is correct ?
A.
B.
C.*
D.
12.
2nd statement
There are mobile ions in molten lead(II)
chloride.
Volume of gas after
combustion / cm3
126
126
179
179
Volume of gas after adding sodium
hydroxide / cm3
0
28
53
81
(12%)
(25%)
(45%)
(18%)
For Q.31, it should be noted that alkanes have the general formula of CnH2n+2. Every mole of an alkane can
undergo combustion to yield (n+1) moles of water and n moles of carbon dioxide. Hence, Option A is correct.
Since a molecule of an alkane has more hydrogen atoms than that of an alkene with the same number of
carbon atoms, more oxygen is required to convert the hydrogen atoms into water molecules. Hence, Option
B is correct. As the number of moles of oxygen depends on both the number of carbon atoms as well as that
of hydrogen atoms, it is not directly proportional to the former. Thus Option C is incorrect.
Q.31
Which of the following statements concerning the complete combustion of alkanes is
INCORRECT ?
A.
B.
C.*
D.
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At 120°C, the volume of steam produced per mole of an alkane is more than the volume
of carbon dioxide produced.
(22%)
More oxygen is required for the complete combustion of 1 mol of an alkane as compared
to 1 mol of an alkene with the same number of carbon atoms.
(24%)
The volume of oxygen required is directly proportional to the number of carbon atoms
present in an alkane.
(39%)
The reaction can be speeded up by raising the temperature of the system.
(16%)
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13.
For Q.33, the item requires candidates to recognise that the gas expands and pushes the plunger because the
pressure inside the syringe is initially higher than the atmospheric pressure. At the equilibrium, the plunger
stops moving outwards and reveals that the pressures have become equal. Moreover, candidates might have
overlooked the exothermic nature of the reaction where the dimer undergoes bond breaking, absorbing energy,
to form the monomer. A decrease in the temperature would therefore shift the equilibrium to the reactant side
and favours the formation of the dimer.
Q.33
A gaseous dimer, Z, is introduced to an empty gas syringe which has a movable and tight-fitting
plunger. The gas is allowed to expand until an equilibrium is reached at a constant temperature.
When the dynamic equilibrium is attained, 20% of Z has dissociated into its gaseous monomer, X.
Which of the following statements must be correct ?
A.*
B.
C.
D.
14.
A decrease in the temperature favours the formation of Z.
(25%)
The equilibrium constant of this reaction is 0.2 mol dm−3.
(18%)
The pressure inside the syringe at the equilibrium is higher than the atmospheric pressure.
(33%)
The formation of X is favoured when the plunger is pushed back into the equilibrium
mixture.
(24%)
For Q.35, the item tests candidates’ knowledge of the definition of structural isomerism, which is different
from stereoisomerism and excludes enantiomers and cis-trans isomers. Given the symmetrical nature of the
cyclohexyl ring, there are at most 8 structural isomers formed. Besides, some candidates may have been
mistaken about the step of propagation in a substitution reaction. In the step, a chlorine radical would react
with the alkane to produce an alkyl radical and a hydrogen chloride molecule. Then, the alkyl radical would
further react with a chlorine molecule to form a chloroalkane molecule and a chlorine radical.
Q.35
Compound J has the following structure :
In an experiment, a sample of J is allowed to react with a limited supply of chlorine gas in the
presence of sunlight. Assume that only monosubstitution with chlorine takes place, which of the
following statements are correct ?
©你 問 我 答
(1)
(2)
(3)
The maximum number of structural isomers of any chloroalkanes formed is 8.
The chloroalkanes produced rotate the plane of polarised light from a polarimeter.
In the step of propagation, the following reaction occurs :
C10H20 + Cl2 → C10H19· + Cl· + HCl
A.*
B.
C.
D.
(1) and (2) only
(1) and (3) only
(2) and (3) only
(1), (2) and (3)
(47%)
(6%)
(37%)
(10%)
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15.
For Q.36, it should be noted that boron is a metalloid. It conducts electricity when it is heated or slightly
impure. Moreover, metalloids, including boron and silicon, possess a giant covalent structure. There are
strong single covalent bonds between atoms.
Q.36
Consider the following statements and choose the best answer :
1st statement
Boron is an electrical insulator under any
conditions.
A.
B.
C.*
D.
©你 問 我 答
2nd statement
Boron has a giant covalent structure with
strong covalent bonds between atoms.
Both statements are true and the 2nd statement is a correct explanation of the 1st
statement.
(6%)
Both statements are true but the 2nd statement is NOT a correct explanation of the 1st
statement.
(14%)
The 1st statement is false but the 2nd statement is true.
(49%)
Both statements are false.
(31%)
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Section B (conventional questions)
Question
Number
Performance in General
1
The performance of candidates was poor. Only a small number of the candidates were able to
give a complete and correct answer. Some candidates only gave a very brief answer which is
inadequate for a four-mark question. The source of rainwater acidity as well as the corrosion or
weathering of rocks by acidic rainwater, which is not necessarily acid rain were expected to be
illustrated with proper explanation and chemical equations.
2
The performance of candidates was fair. In part (a)(i), most of the candidates were able to draw
the three-dimensional shape of SF2. In part (a)(ii), more than half of the candidates were able to
explain the fact that SF2 is polar based on its V-shaped structure. In part (b), only about twenty
percent of the candidates were able to give a complete and correct answer. Some candidates were
not able to clearly state the differences in the number of electron shells and the number of
outermost shell electrons between sulphur and oxygen and how these affect the formation of
compounds of S and O. In part (c), more than half of the candidates were able to deduce and
explain that SF2 has a higher boiling point.
3
The performance of candidates was poor. In part (a)(i), most of the candidates were able to
identify the reddish brown solid as copper. In part (a)(ii), about half of the candidates overlooked
that CO is only a minor component in town gas, while a few of the candidates did not recognise
the reduction reaction involved in the set-up. In part (b), about two thirds of the candidates failed
to describe the complete laboratory process. Some attempts on describing the process were
fragmented, whereas some were describing the expected observations of the reaction involved,
which is not required by the question. Only a handful of the candidates gave a complete answer
in this part. In part (c), more than half of the attempts were good. Some attempts showed
incompetent interpretation of the given mass measurements. In part (d), some candidates simply
stated magnesium oxide could not be reduced by town gas without stating the removal of the
oxide by a dilute acid before weighing the copper obtained. Some candidates gave wrong
predictions of the outcome. A low proportion of the candidates made no attempts at all.
4
The performance of candidates was fair. In part (a), about sixty percent of the candidates were
able to give the systematic name of X, which is the monomer of Perspex. In part (b)(i), just over
half of the candidates were able to name ‘addition polymerisation’ as the reaction involved. In
part (b)(ii), about forty percent of the candidates were able to write the chemical equation for the
reaction involved. In part (b)(iii), only a small number of candidates were able to explain why
the experiment is performed in a fume cupboard. Many candidates wrongly described peroxide
as volatile or toxic. In part (c), about half of the candidates were able to suggest an advantage of
Perspex over glass. Some candidates stated a property or an advantage of Perspex but failed to
show why that makes Perspex better than glass with a comparative sense. In part (d), only a small
number of the candidates were able to give a complete and correct answer. Quite a number of
the candidates were not able to relate the structure of Perspex to their knowledge of
intermolecular forces.
5
The performance of candidates was fair. In part (a), most of the candidates accurately stated
Hess’s Law. In part (b)(i), most attempts accurately recognised ethanedioic acid as a weak acid,
but some candidates failed to mention that only a small portion of the acid is ionised in water. In
part (b)(ii), almost all candidates gave a good understanding in the laboratory preparation
procedures of standard solution, in which half of the attempts were in lack of some details, such
as rinsing the apparatus and pouring the washings into the volumetric flask. In part (c), around a
third of the candidates showed a rigid knowledge in enthalpy change calculation. Some
candidates omitted the required calculation of ‘enthalpy change of neutralisation’ in the question,
while some candidates only calculated the energy released from the reaction. A small number of
the candidates did not include the minus sign in their final answer, and a low proportion of the
candidates failed to show any steps in calculations. In part (d), only a quarter of the candidates
recognised some sources of errors, such as the solid being impure and the exclusion of heat
capacities of apparatus. In part (e), only a low portion of the candidates included a multiple of
two to account for the release of two H+(aq) ions per ethanedioic acid molecule ionised. Some
attempts showed misunderstandings in drawing enthalpy change cycles.
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Question
Number
Performance in General
6
The performance of candidates was satisfactory. In part (a), about two thirds of the candidates
were able to relate the decomposition of concentrated nitric acid to its storage. In part (b)(i),
nearly no candidates were able to write a correct ionic equation with NO as the product. Most
candidates mistakenly wrote NO2 without recognising that the reaction between concentrated
nitric acid and iron(II) sulphate solution is an exceptional case. In part (b)(ii), most of the
candidates were able to state an expected observation for the reaction. In part (b)(iii), it should
be noted that if candidates gave a wrong answer in part (b)(i), they could still earn full marks in
this part as long as they were able to explain that concentrated nitric acid has undergone reduction
with the change in oxidation number of nitrogen in the answer they gave in part (b)(i) described
correctly. Thus, most of the candidates were able to give a correct answer in this part. In part
(c)(i), about seventy percent of the candidates were able to explain why nitric acid is not suitable
to be a primary standard. In part (c)(ii), most of the candidates were able to perform complete
and correct calculations.
7
The performance of candidates was fair. In part (a), only a third of the candidates were able to
write a balanced equation for the half-reactions taking place in both electrodes. In part (b), most
candidates were able to relate bond-forming and bond-breaking reactions with energy of the
reaction. In part (c)(i), half of the candidates were able to describe the property of electrolysis
reactions. In part (c)(ii), only a few of the candidates were able to point out why batteries wear
off in the long term. In part (c)(iii), most candidates were able to point out that zinc-carbon cells
are non-rechargeable. Other reasonable answers were also accepted. In part (d), most candidates
were able to point out that the acid could act as an electrolyte, or provide hydrogen ions for the
redox reaction to occur.
8
The performance of candidates was satisfactory. Most of the candidates correctly drew structures
of sodium and sodium iodide and described the bonding using accurate chemical terms such as
‘ions’, ‘delocalised electrons’ and ‘electrostatic attraction’. A few of the candidates were weaker
in such descriptions, with figures which were insufficient to show a NaCl-like lattice structure.
Around two thirds of the candidates correctly compared the malleability between a metal and an
ionic compound, with some of the candidates showing precise understanding in motions of ions
in sodium iodide. About half of the candidates gave a further difference in physical properties
such as electrical and thermal conductivities of the species, with some of the candidates
accounting for the properties correctly by mentioning terms like ‘delocalised electrons’ and
‘immobile ions’. In terms of communication, most candidates were able to write in complete
sentences, but some failed to communicate effectively in paragraph form while some presented
with inaccurate phrasings which may cause confusion.
9
The performance of candidates was very poor. Most candidates were not able to give a correct
and complete synthetic route and did not state the appropriate reagents and reaction conditions
in each step. Only a small number of the candidates were able to recognise that an amine is
alkaline in nature and would be ionised to form an alkylammonium ion in an acidic medium.
Some candidates correctly suggested that the cyclic amide could undergo alkaline hydrolysis to
open the ring and give a carboxylate ion with an amino group, but failed to give the appropriate
reagents for regenerating a carboxylic acid so that it could further undergo reduction. About two
thirds of the candidates were able to suggest the correct reagents for the reduction reaction of the
carboxylic acid involved in the second step. However, most candidates overlooked that the
acidification process after reduction with LiAlH4 would yield an alkylammonium ion, and hence
failed to suggest the correct intermediate as well as basification as the final step. Common
mistakes include stating inappropriate reagents and intermediates for the steps, and including
inappropriate steps in the proposed synthetic route.
10
The performance of candidates was fair. In part (a), two thirds of the candidates were able to
explain why the soap solution is added. In part (b), about two thirds of the candidates were able
to plot a correct graph with correct labels and axes. In part (c), more than half of the candidates
were able to calculate the instantaneous rate using the graph they plotted in part (b). In part (d),
only a small number of the candidates were able to plot the required curve. Some candidates
might have mistakenly regarded the new set-up as using 3% H2O2(aq) to replace the original 6%
H2O2(aq). In part (e), only one fifth of the candidates were able to suggest a disadvantage of the
experiment.
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69
Question
Number
Performance in General
11
The performance of candidates was poor. In part (a)(i), most of the candidates were able to write
the expression for the equilibrium constant Kc for the reaction. In part (a)(ii), only a small number
of the candidates were able to calculate the pH value and the percentage ionisation of 0.1 M
lactic acid. In particular, some candidates only calculated the pH value but did not give any
answer for the percentage ionisation. In part (b), just more than one third of the candidates were
able to explain how diluting a 0.1 M solution of lactic acid changes its pH value and percentage
ionisation. In part (c), less than one third of the candidates were able to suggest a method for
distinguishment.
12
The performance of candidates was poor. In part (a), about half of the candidates were able to
compare one property of zinc and chromium. However, only a small number of the candidates
were able to give a complete and correct answer to justify the statement. In part (b), only about
a quarter of the candidates were able to explain the industrial applications of transition metals
with reference to the characteristics of transition metals. Many candidates only suggested the
usage of transition metals industrially without correlating that to the properties of transition
metals.
13
The performance of candidates was fair. Many candidates showed some understanding on what
the physical properties of cis-trans isomers are. Some candidates could also briefly explain the
physical properties of cis-trans isomers, e.g. differences in boiling points and melting points,
with reference to the structure and intermolecular forces. However, about a quarter of the
candidates had mixed up the physical properties of cis-isomers with those of trans-isomers. A
few of the candidates could not show a comparative sense when comparing the physical
properties of cis-isomers and trans-isomers. It should be noted that the handwriting should be
clear and the paragraphs should be well organised for easier understanding.
©你 問 我 答
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Paper 2
Paper 2 consisted of three sections. Section A contained questions set on Topic XIII ‘Industrial Chemistry’, Section
B on Topic XIV ‘Materials Chemistry’ and Section C on Topic XV ‘Analytical Chemistry’. Candidates were required
to attempt all questions in two of the sections.
Question
Number
Popularity
(%)
Section A: 1
90
Performance in General
The performance of candidates in part (a) was poor. In part (a)(i), only about fifteen
percent of the candidates named a correct disadvantage of a simple setup for the
electrolysis of brine. Many candidates wrongly suggested that the carbon electrode
would react with the oxygen gas produced. Some candidates mistakenly proposed
that the reaction between chlorine gas and water or sodium hydroxide solution is
a disadvantage, which also occurs minimally in other electrolytic setups including
the membrane electrolytic cell. In part (a)(ii), about two thirds of the candidates
were able to sketch a correct distribution curve with molecular kinetic energy as
the x-axis and number of molecules as the y-axis. Some attempts represented the
distribution curve as the energy profiles for the catalysed and the uncatalysed
pathways of some reaction. Incorrect labels like ‘potential energy’ for the x-axis
and missing labelling for different activation energies before and after the addition
of a catalyst were common. In part (a)(iii)(1), about forty percent of the candidates
were able to suggest ‘fermentation’ as a progress, whereas a significant number of
candidates wrongly suggested the traditional Reichstein process. In part (a)(iii)(2),
about half of the candidates were able to explain why fermentation is a progress.
Some candidates might have interpreted the term ‘progress’ as ‘process’, thus
having given incorrect answers like ‘since it is used to produce vitamin C’.
The performance of candidates in part (b) was satisfactory. In part (b)(i), about
forty percent of the candidates gave the benefits of the conversion of methane into
methanol in terms of transport and storage. Quite a number of the candidates
suggested that methanol can be used as a fuel, which is incorrect since methane
can also be a fuel and such an answer lacked a comparative sense. In part (b)(ii)(1),
most candidates were able to write a chemical equation for the carbonylation of
methanol. In part (b)(ii)(2), more than half of the candidates were able to recognise
the exothermic nature of the reaction which makes the process energy efficient.
However, many candidates overlooked that the synthesis being one-step is green
in terms of reducing derivatives but is in no way related to energy efficiency. In
part (b)(iii)(1), three quarters of the candidates were able to calculate the atom
economy by writing a balanced equation with water as another product of the step.
In part (b)(iii)(2), most candidates were able to explain that the reaction scheme is
less green using the principles of green chemistry. However, some candidates
performed unfair comparisons or failed to show any comparative sense.
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Question
Number
Popularity
(%)
Section A: 1
90
The performance of the candidates in part (c) was fair. In part (c)(i), about half of
the candidates were able to suggest an advantage in terms of miscibility. A high
proportion of the candidates recognised the advantage, but failed to represent their
answers in a comparative sense. Some candidates misinterpreted the scenario and
wrongly suggested that the volatile nature of ethanol allows easier separation of
the solvent. In part (c)(ii), about sixty percent of the candidates were able to deduce
the required orders of reaction. However, some candidates drew a conclusion of
the order of reaction with respect to sodium hydroxide simply based on the graph
for being a straight line, and failed to show a logical deduction from the constant
slope of the graph regardless of the decrease in the concentration of sodium
hydroxide throughout the experiment. In part (c)(iii), most candidates who have
previously obtained the correct orders of reaction in part (c)(ii) were able to state
the rate equation for the reaction. However, some candidates wrote a power of 0
over [NaOH], which is not desirable. In part (c)(iv), most candidates who have
previously identified the reaction as first-order with respect to 2-chloro-2methylbutane in part (c)(ii) were able to sketch a graph of a first-order reaction,
showing a linear relationship between the molarity of 2-chloro-2-methylbutane
and the rate of reaction. Some candidates failed to sketch a straight line from the
origin. In part (c)(v), only a handful of the candidates were able to come up with
the correct activation energy for the reaction. Most candidates either overlooked
the unit of time in the graph, which is minute but not second as in the case of the
rate of reaction of sodium hydroxide provided, or neglected that the scale of the
graph is actually up to 29 minutes only. Some candidates failed to recall that the
rate constant is always positive and thus did not drop the minus sign of the value
of the given rate of reaction at 100°C. A number of the candidates failed to apply
the Arrhenius equation in a correct manner and hence did not convert the unit of
the temperature into Kelvins.
Section B: 2
2
The performance of candidates in part (a) was fair. In part (a)(i), the majority of
the candidates could correctly state the reducing agent for producing colloidal
silver nanoparticles and the respective colour change. In part (a)(ii), most
candidates did not recognise the alignment of liquid crystals in curved displays or
provided vague descriptions of their alignment, and lacked an awareness of
modern liquid crystal technology such as in-plane switching (IPS) and vertical
alignment (VA). In part (a)(iii), most candidates recognised the properties of
polymers suitable for melt blowing, which were not familiar to most candidates.
Performance in General
The performance of candidates in part (b) was fair. In part (b)(i)(1), the majority
of the candidates provided the correct repeating unit of PTFE. In part (b)(i)(2), the
majority of the candidates related the high electronegativity of fluorine atoms to
the non-stick property of PTFE. In part (b)(ii), most candidates were not able to
relate the properties of expanded PTFE to the characteristics of Gore-Tex™. Some
candidates provided explanations from the perspectives of other layers of
materials, which are not the essential materials of Gore-Tex™. In part (b)(iii), most
candidates recognised that the production of PTFE involves toxic materials and it
is not biodegradable. In part (b)(iv), most candidates correctly provided a common
usage of PTFE.
The performance of candidates in part (c) was poor. In part (c)(i)(1), the majority
of the candidates failed to provide a precise definition of shape-memory alloy.
Some candidates mixed up the temperature at which the alloys are deformed with
the temperature at which they return to their original shapes. In part (c)(i)(2), most
candidates correctly deduced the number of nickel and titanium atoms from an
unfamiliar unit cell. In part (c)(i)(3), although most candidates could draw a bodycentered cubic structure, many candidates failed to recognise the non-repeating
nature of the unit cell and provide two cells, which are not the smallest unit cell.
It should be noted that the unit cell in part (c)(i) is larger because of its inclined
structure. In part (c)(ii), most candidates correctly stated the properties of coinage
metals and related the properties to their practicality.
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Question
Number
Popularity
(%)
Section C: 3
90
Performance in General
The performance of candidates in part (a) was poor. Only some candidates were
able to describe the physical and chemical methods in chemical analysis
accurately. In part (a)(i), only fifty percent of the candidates were able to point out
the difference in melting points between pure and impure substances explicitly. In
part (a)(ii), only some candidates were able to suggest a chemical test to
distinguish both compounds. Some candidates did not realise that silver chloride
test would not be favourable since silver hypochlorite is unstable and subsequently
forms silver chloride under heat. In part (a)(iii), candidates were not able to
highlight the important steps for recrystallisation, such as using minimal solvent
to dissolve the solid. Some candidates specified the solvent used, but that
particular solvent would not dissolve the targeted solid well. Not specifying the
solvent used is recommended.
The performance of candidates in part (b) was poor. Only some candidates could
master the principle of Mohr’s method, and most calculations were erroneous. In
part (b)(i)(1), half of the candidates were able to point out the colour change at the
end point. In part (b)(i)(2), most candidates could not recognise the importance of
pH adjustment in Mohr’s method. In part (b)(i)(3), some candidates missed out the
10 cm3 of sodium carbonate solution added for pH adjustment. The total volume
should be 260 cm3, instead of 250 cm3. In part (b)(ii)(1), candidates did not
understand the colorimetric method, and were not able to calculate the chloride
concentration of the sample. Some candidates wrote down the numerical answer
without any steps, so they were not able to obtain the mark for correctly reading
off the calibration curve. In part (b)(ii)(2), some candidates were able to point out
the merits of the colorimetric method, i.e. higher sensitivity.
The performance of candidates in part (c) was poor. Candidates did not master the
use of thin layer chromatography (TLC), and did not understand the reaction fully.
In part (c)(i), candidates were not able to follow a reaction progress through TLC
chromatograms. If the reaction has completed, the original spot at time = 0 hour
should disappear. In part (c)(ii), candidates were generally able to draw and label
the set-up for liquid-liquid extraction. In part (c)(iii)(1), some candidates were able
to point out the working principle behind column chromatography. In part
(c)(iii)(2), some candidates did not realise that the reaction could produce both
aldehyde and carboxylic acid. Many candidates were not able to correlate polarity
with functional groups.
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