科
目
卷
Chemistry
考試日期
考試時間
20-6-2001
試題紙頁數
8:30-11:30 a.m.
備註
4

別
1.
Secondary Six
時限
3 hours
Periodic Table is printed on the last page
2000  2001 Pui Ching Middle School
Section A
級別
Second Examination
Total 8 questions, each carries 10 marks
Carbon dioxide in the atmosphere contains a trace amount of 14C.
14
C is radioactive and decays by
emitting -particles.
(a)
(b)
Write a nuclear equation for
(i)
the formation of 14C in the atmosphere, and
(ii)
the decay of 14C.
The 14C：12C ratio of an archaeological specimen decreases with time, but that of living things is
constant. Explain.
(c)
A wood sample from an ancient tomb has an activity of 9.6 counts per gram of carbon per
minute. A similar piece of modern wood has an activity of 15.3 counts per gram of carbon per
minute. Estimate the age of the wood sample from the tomb.
(Half life of the decay of 14C = 5730 years.)
2.
Glucose is a carbohydrate and tripalmitin is a fat. Their structures are shown below :
CHO
CH2 OCO(CH2 )14 CH3
CHOH
CHOH
CHOCO(CH2 )14 CH3
CHOH
CHOH
CH2 OCO(CH2 )14 CH3
CH2 OH
Glucose
(a)
Tripalmitin
The table below lists the standard enthalpy changes of formation of three compounds :
Compound
HfO,298 / kJ mol1
C6H12O6(s), glucose
 1274
CO2(g)
H2O(l)
 394
 286
(i)
Calculate the standard enthalpy change of combustion of glucose.
(ii)
Calculate the theoretical amount of energy released when 10.0 g of glucose undergoes
complete combustion.
(b)
The thermochemical equation for the combustion of tripalmitin (C51H98O6) is given below :
2 C51H98O6(s) + 145 O2(g)

102 CO2(g) + 98 H2O(l) HcO,298 =  62800 kJ mol1
Calculate the theoretical amount of energy released when 10.0 g of tripalmitin undergoes
complete combustion.
(c)
With reference to your answers in (a) and (b), suggest why plants store their energy mainly in
the form of carbohydrates, whereas animals store their energy mainly in the form of fats.
3.
A student remarked, ' The pH of pure water is 7.0. The range of pH in an acidic solution is within
0  7 and that in an alkaline solution is within 7  14. '
After the lessons in A-level Chemistry, do you agree with the student ? Explain.
1
4.
A current of 0.50 A was passed for 6.0 minutes through an electrolytic cell containing K[Au(CN) 2](aq)
and KCN(aq) to gold-plate a metal object with a surface area of 4.0 cm2.
(a)
Draw a labelled diagram to show how to carry out the above experiment.
(b)
Calculate the oxidation number of gold in the compound, K[Au(CN) 2].
(c)
Assuming 100% current efficiency, calculate the thickness of gold deposited.
(density of gold is 19.3 g cm3.)
(d)
Suggest one way to treat the cyanide ions in the spent electrolytic solution.
(e)
Explain why the gold ions in the spent electrolytic solution should be recovered before the
effluent is discharged.
5.
Consider the following data for the reaction at 700 K :
CO(g) + NO2(g)

CO2(g) + NO(g)
Initial concentration / mol dm3
[CO(g)]
[NO2(g)]
0.10
0.10
0.10
0.40
0.40
0.40
(a)
Initial rate
/ mol dm3 s1
0.013
0.052
0.208
For this reaction,
(i)
deduce the rate equation,
(ii)
calculate the rate constant at 700 K,
(iii) draw an energy profile.
(b)
The table below lists the rate constants for the above reaction at three other temperature, T.
T/K
k / dm3 mol1 s1
650
0.22
750
6.0
800
23.0
Determine the activation energy, in kJ mol1, for the reaction by plotting an appropriate
graph.
6.
Solid ammonium hydrogensulphide dissociates into hydrogen sulphide and ammonia as represented
by the following equation :
NH4HS(s)
NH3(g) + H2S(g)
At 295 K, when solid ammonium hydrogensulphide is introduced into an evacuated vessel, the
total pressure at equilibrium of the gaseous mixture is found to be 54.5 kPa.
(a)
Write an expression of the equilibrium constant, Kp, for the reaction.
(b)
Calculate the equilibrium constant, Kp, for the reaction at 295 K.
(c)
If the vessel initially contains hydrogen sulphide gas at a pressure of 33.8 kPa, solid ammonium
hydrogensulphide is then introduced. Calculate the partial pressure of ammonia when the
system is allowed to reach equilibrium at the same temperature ?
(d)
Explain whether the above reaction is exothermic or endothermic.
(e)
State, with explanation, the effect of each of the following changes on the percentage yield of
ammonia in the above reaction.
(i)
Increasing the temperature of the reaction vessel.
(ii)
Adding a small amount of solid ammonium hydrogensulphide while keeping the volume
of reaction vessel unchanged.
7.
A ketone X contains 69.7% carbon, 11.7% hydrogen and 18.6% oxygen. At 100kPa and 373K, the
vapour density of X is found to be 2.78 g dm3.
(a)
Calculate the relative molecular mass of X. State any assumption(s) you make.
(b)
Deduce the molecular formula of X.
(c)
Write all possible structural formulae of the ketone X.
2
8.
Consider the information below :
1
5
Nitric(V) acid is commercially available as 70% HNO 3(aq), since
70 % HNO3(aq) has the maximum boiling point. If a mixture of equal volume
of 70% HNO3(aq) and concentrated sulphuric(VI) acid is slowly distilled, pure
nitric(V) acid, with boiling point 86oC, may be obtained. Anhydrous nitric(V)
acid is a fuming yellow liquid which contains NO 2 gas. The yellow colour of
anhydrous nitric(V) acid may be removed either by passing a current of air
through it, or by cautiously adding urea, H2NCONH2, which converts NO2 gas
into nitrogen, also forming carbon dioxide.
(a)
With the help of balanced equations, describe how 70% HNO3(aq) is obtained in industry.
(b)
In line 3, what is the purpose of mixing the two acids ?
(c)
Draw a labelled diagram to show how a current of air is passed through nitric acid (line 6).
(d)
In the above information, two methods are suggested to remove the colour of anhydrous
nitric(V) acid. For each of the method, write balanced equations for all reactions involved.
End of Section A
Section B
Answer ONE question only and write your answers in a separate sheet.
Marks will be allocated approximately as follows :
Chemical knowledge
50 %
Organization
30 %
Presentation (including use of English)
20 %
Equations, suitable diagrams and examples are expected where appropriate.
The examiners are looking for the ability to analyze, to evaluate and to express ideas clearly.
9.
Many industrial manufacturing processes depend for their success upon a balance between the
equilibrium position and the speed of its attainment. With appropriate theory and with the synthesis of
sulphuric(VI) acid as an example, discuss how such reactions are controlled to give good yields.
(20 marks)
10.
Write an essay on iodimetry.
(20 marks)
END OF PAPER
Useful Constants
Gas constant, R = 8.31 J K-1 mol-1
Faraday constant, F = 9.65 x 104 C mol1
Avogadro constant, L = 6.02 x 1023 mol1
Ionic product of water at 298 K, Kw = 1.00 x 10 14 mol2 dm6
Specific heat capacity of water = 4.18 J g-1 K-1
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