Common Mistakes made by candidates in A-level Chemistry Examination
1.
Failure in following the standard conventions in Chemistry
(a)
Failure to follow the IUPAC conventions
1
incorrect names for organic compounds and inorganic complexes;
O
4-oxopentanoic acid
2 +
O
C
CH2CH2C
CH3
OH
2
Cu(NH3)4
principal functional
group – carboxylic
acid (not ketone)
When to/ not to include an “e”
.
3-
Fe(CN)6
2-Methylpentane-2,4-diol;
Tetraamminecopper(II) ion
ammine, not amine
Hexacyanoferrate(III) ion
characteristic ending for
anionic complexes
3,7-dimethyloct-6-enal
3
CF3CH2F – 1,1,1,2-tetrafluoroethane;
4
missing out the stereochemical prefix cis-/trans- in the naming of geometrical isomers;
CH3
H
C
Cl
Cl
Cl
Br
H
CH3
Cl
NH3
Cl
NH3
Cl
Cr
or
Cr
C
NH3
Trans-trans-hexa-2,4-diene trans-1,2-dibromocyclohexane
5
-
Cl
H
C
Cl
-
Br
C
H
3-phenylpropan-1-amine, not 3-phenylpropanamine
NH3
Underlining used
to indicate
positions of
substituents or
describe
stereoisomers.
Cis-tetrachlorodiamminechromate(III) ion
Unable to write IUPAC cell diagrams.
the hydrogen electrode half-cell should be 2H+(aq, 1M) /H2(g, 1atm) /Pt(s)
the MnO4-/Mn2+ half cell should be [MnO4-(aq) + 8H+ (aq)], [Mn2+ (aq) + 4H2O(l)] /Pt(s)
the cell diagram of an electrochemical cell formed by hydrogen electrode half-cell and Cu2+/Cu half cell should be:
Pt(s) /H2(g, 1atm) /2H+(aq, 1M) ¦ ¦ Cu2+(aq, 1M) /Cu(s)
E o = +0.34V
6
using [A] for partial pressure
[ ] – concentration expressed in terms of mol dm-3
7
it is not necessary to include the subscript n when expressing the structure of the repeating unit of a
polymer, (-CH2-CH2-) is the repeating unit of polythene;
8
Ionization enthalpy of an element, M, is the molar enthalpy change involved in the process: M(g) → M+(g) + e-
(b) Incorrect symbols for elements and/or incorrect formulae for ions/compounds
1
CH3I instead of CHI3 for triiodomethane (iodoform);
2
MO4-, MnO42-, MnO3- instead of MnO4- for manganate(VII) ion;
3
SO42- instead of SO32- for sulphate(IV) ion; PbO4 instead of PbO2 for lead(IV) oxide;
4
[Co(H2O)6]2+ Cl2- instead of
5
Ti instead of Sn for tin; Mn for magnesium; Mg for manganese; Ru instead of Rb for rubidium.
(c)
1
[Co(H2O)6]2+ 2Cl-
Incorrect representation of oxidation states of elements
Using N5+, Mn7+, Si4+ instead of N(V)/N(+5), Mn(VII)/Mn(+7), Si(IV)/Si(+4) to represent
oxidation states; giving oxidation state of S in SO42- as 6+ instead of +6.
(d) Use of “→” or “” instead of “
to represent inductive effect, and of “
(e)
”to represent reversible reactions, of “→” instead of “ ”
” instead of “” to represent resonance
Use of “
full arrow“ instead of “
fish-hook arrow/ half arrow“ to represent flow of an
odd electron.
Curly arrows flow from +ve charge center instead of –ve charge center
Page 1
correct mechanism for nucleophilic
substitution should be
R
HO-
I
O
correct mechanism for homolytic
fission (chain initiation) should be
Cl
O
C
CF2Cl
O
C
O
Dichlorodifluoromethane
Benzoyl peroxide
H
correct mechanism for free radical
attack should be
Cl
H
Cl
C
H + CH3
H
H
(f)
+ve/-ve sign are not included in the H values of endothermic/exothermic enthalpy changes
(g) Physical states are not included in thermochemical equations, cell diagram.
(h) Electronic configuration should be in ascending order of principal quantum no:
Cu atom at ground state should be 1s2 2s2 2p6 3s2 3p6 3d10 4s1
2.
Imprecise answers
(a)
Imprecision/incorrectness in reaction conditions
1
using AgNO3(aq) instead of AgNO3(aq) + HNO3(aq) in the test of halide ions;
2
using NaOH(aq) followed by AgNO3(aq) instead of NaOH(aq), then acidified by HNO3(aq),
and AgNO3(aq) in the test of halide ions after hydrolysis of alkyl halide
3
using NaOH(aq) instead of NaOH(aq) and heating to detect NH4+(aq)
4
RCONH2 can be detected by heating with NaOH(aq) and testing the pH of the gas evolved;
5
using cobalt chloride instead of anhydrous cobalt chloride in the test of water;
6
using HCl(aq) instead of HNO3(aq) to acidify AgNO3(aq);
7
using HCl(aq) instead of H2SO4(aq) to acidify KMnO4(aq)/K2Cr2O7(aq);
8
using BaCl2(aq) instead of BaCl2(aq) + HCl(aq) in the test of sulphate ion;
9
using LiAlH4, water instead of LiAlH4, ether in the reduction of carbonyl compounds;
10
using NaBH4, water instead of NaBH4, alcohol in the reduction of carbonyl compounds;
11
using conc. H2SO4 instead of alcoholic KOH & heating in the dehydrohalogenation of RX;
12
using NaCl(aq) or KCl(aq) in the salt bridge without realizing that Cl-(aq) reacts with Ag+(aq);
13
using direct heating instead of hot water bath in crystallization;
14
using displacement of water instead of syringe/downward delivery in the collection of SO2(g);
15
using glowing splint instead of burning splint in the test of hydrogen;
16
using NaNO3(s) + HCl(aq) instead of NaNO2(s) + HCl(aq) in the azotization of aromatic amines;
17
no indication of temperature (<5oC) in the azotization of aromatic amines;
18
alkali should be used in coupling reaction (formation of azo dye) – phenoxide ion is a stronger
nucleophile than phenol;
19
Acid should be used in condensation reaction of carbonyl compounds – nucleophilic addition is
catalysed by acid;
20
using HCN instead of KCN in converting alkyl bromide to a nitrile; (CN- is a stronger nucleophile than HCN)
21
using bacteria instead of yeast in the fermentation of glucose;
22
using starch instead of starch solution in the titration between iodine and thiosulphate;
Page 2
23
no indication of H3O+/H+(aq) in acid hydrolysis reactions:
1 LiAlH4 , ether
CH3CHO
2
24
CH3CH2OH
+
H (aq)
no indication of temperature (0o – 10oC) in the hydration of alkene;
CH3
H
C
H
C
H
CH3
H
C
C
conc H2SO4
0 - 10
o
H
H2O
H
H
CH3
H
C
C
OH
H
H
C
OSO3H H
25
In the chlorination of CH4, excess CH4 is needed to prepare chloromethane as the major product;
26
In the preparation of RNH2 from RX and NH3, excess NH3 is bubbled through an alcoholic
solution of RX so as to prevent the formation of 2o and 3o amine;
27
In the dehydration of C2H5OH, excess conc. H2SO4 is needed to give C2H4 as the major product;
28
Heating under reflux is necessary to convert nitrile to acid/ acid anion in acid/alkaline
hydrolysis; RCN + 2H2O + H+  RCOOH + NH4+; RCN + H2O + -OH  RCOO- + NH3
29
Hydrolysis of ester is irreversible in alkaline medium;
30
Catalytic hydrogenation under pressure is needed to reduce aldehyde/ alkanone
31
When proposing conversion of organic compounds containing more than one functional groups,
candidates should consider the molecule as a whole in each step and not just focus on the
reactions of only one of the functional groups
32.
In the catalytic converter, Rh catalyses the reaction:
33.
The reagents for Hofmann degradation are Br2 and NaOH. Some candidates wrongly gave I2 and
NaOH, which are in fact reagents for the iodoform reaction.
34.
Simply stated the Markovnikoff’s rule in explaining the nucelophilic addition of alkene.
35.
Unlike ammonia or amine, acid amides are weak nucleophile and will not react with alkyl halides.
36.
In the preparation of soluble salt (e.g. CuSO4, CaCl2), excess Cu should be added to concentrated
sulphuric acid/ excess CaCO3 should be added to dilute hydrochloric acid.
2NO + 2CO  N2 + 2CO2
(b) Imprecise/incorrect record of observation/product of reaction
1
unable to give a precise record of changes in test-tube experiments, such as wrong colour for
CHI3(s)(CHI3 is yellow precipitate), iodine vapour (iodine vapour is purple), AgCl(s) (AgCl is
white precipitate and is soluble in aqueous ammonia);
2
Ag2O as the product for the ‘silver mirror test’ (Ag(NH3)2+ is reduced to Ag), CuO as the product
for the Fehling’s Test (red precipitate Cu2O is formed) etc;
3
failure to recognize elimination is a side reaction of substitution and vice versa;
4
indiscriminate use of the terms ‘precipitate’ and ‘turbidity’; in Lucas Test (to distinguish 1o, 2o &
3o alcohol), 3o alcohol turns turbid readily as alkyl halide is immiscible with water;
5
aqueous soapy detergent turns turbid with the addition of acid; forms white precipitate
(scum) with the addition of hard water;
6
Iron(II) hydroxide – green, iron(III) hydroxide – reddish brown, silver(I) oxide – brown, copper(II)
hydroxide – blue, Cu(NH3)42+ - deep blue, CuCl42- - yellow, K2Cr2O7 – orange, Na2CrO4 – yellow,
[Fe(H2O)5NO]2+ - blood red,
7
H2O2  O2 + 2H+ + 2e- (acidic medium);
8
CCl3F cleaves upon absorption of electromagnetic radiation instead of ultra-violet radiation;
H2O2 + 2e-  2-OH (alkaline medium)
Frequency of the associated radiation = bond energy/(L h)
where L – Avogadro constant; h – Planck’s constant
9
using the term ‘no reaction’ for a test with ‘no observable change’;
Page 3
10
both Na2SO3(aq) and Na2S2O3 (aq) can decolorize I2(aq);
11
unable to sketch titration curves (strong acid/ strong base, strong acid/ weak base, strong base/
weak acid/ polybasic acid and sodium hydroxide);
12
Ammonia gas gives a white fume with hydrogen chloride: NH3(g) + HCl(g) 
The white fume is suspended particles of solid ammonium chloride.
13
Conc. HCl fumes in moist air. The white fume is acid mist (droplets of hydrochloric acid).
14
Oily droplet (aniline) is formed when aq. NaOH is added to a solution formed by mixing aniline
with hydrochloric acid. C6H5NH3+ + -OH  C6H5NH2 + H2O
(c)
1
NH4Cl(s)
Incorrect structure for reaction product
OH
Wrong position of coupling for naphthalen-2-ol
Wrong answer (C3 is attacked)
Correct answer (C1 is attacked)
OH
N
N
N
N
OH
2
H
H
There are four dipeptides coming from
C
H
N
H
H
N
C
H
H
CH3
C
*
C
O
COOH
CH3
*
C
H
C
NH2
NH2
2-aminoethanoic acid
&
2-aminopropanoic
acid
HOOC
O
H
C
C
H
O
H
H
H
O
CH3
N
*
C
H
NH2
CH3
*
C
C
C
H
N
COOH
COOH
3
Cl
NH2
Cl
Cl
H
Cl
CH2CH3
CH2CH3
The following pairs are identical:
Cl
Cl
Py
Py
Py
Fe
Py
Py
Wrong representation of free radicals
Cl
Py
Py
4
Cl
Fe
Py
ClO
O
Oxygen atom does not possess any unpaired/odd electron. The dot in ClO. should be on the
oxygen atom.
5
+
positive charge of the diazonium ion
6
bonding in ClO-, –CN and CO
N
Cl
Ox
-
C
N
N
C
O
The following (7 – 12) are INCORRECT equations
Page 4
7
KCN
RCH2OH
8
C
RCH2CH2Cl
H
O
R
conc. H2SO4
RCH2CN
+
CN
-
R
O
C
OH
R
+ NH3
O
C
R
R
C
OH
10
C
NH2
O
O
+ HCl
R
OH
CN
R
O
C
OH
9
RCH=CH2
O
C
LiAlH4
+
R
C
OH
Cl
O
H
O
O
+
H (aq)
R
C
+ NaBH4
R
RCH2OH
OH
11
R
C
+
+
LiAlH4, H 3O (aq)
I2, NaOH
RCH2OH
C
NH3
OH
NH2
O
R
C
CH3 CH2OH
HCOOH + CHI3
OH
12
COOCH2CH3
COOCH2CH3
Sn, conc. HCl
fuming H2SO 4
R
OSO 3H
R
heat
NH2
NO 2
13
aliphatic diazonium ions are unstable, while aromatic diazonium salt is stable;
14
oxidative rancidity of oils and fats involve alkylperoxyl radical (ROO.) but not the carboxyl radical (RCOO.);
15
1o & 2o alcohols, but not tertiary alcohols, can be oxidized by acidified K2Cr2O7(aq);
16
failure to recognize that alanine synthesized from ethanal is a racemic mixture, while that from the
hydrolysis of protein is optically pure;
17
wrong position of electrophilic addition of alkene (HBr is bubbled through a solution of propene in CCl3CH3);
Correct answer: follow Markownikoff’s rule
Stability of 2o carbocation > 1o carbocation; associated
activation energy is lower; reaction rate is faster.
18
H
CH3
CH3CH2Br
minor
HBr
C
C
H
H
CH3CHBrCH3
major
Intramolecular reaction occurs more readily than intermolecular reaction:
(CH2)2Br
CH2NH2
H
N
O
O
(CH2)2Br
K 2CO 3
RNH2
R'
X
instead of
X
N
H
R'
R
CH2NH(CH2)2O(CH2)2Br
or
CH2NH(CH2)2O(CH2)2NHCH2
H
R
HX
N
R'
CH2CH2CH2Cl
CH2CH2CH2Cl
AlCl3
instead of
heat
19
Alkyl group undergoes free radical substitution more readily than benzene ring (C-H bond in the
methyl group is weaker than the C-H bond in benzene):
Page 5
CH3
+ Br2
benzoyl
peroxide
CHBr2
CH2Br
+
heat/ h 
bromomethylbenzene
dibromomethylbenzene
In reactions involving homolysis, the weaker bond normally breaks first.
20
Butyl group (C4H9-) could be CH3CH2CH2CH2- or (CH3)2CHCH2-.
(d) Imprecise definition of chemical term
1
Equations should be included in your explanation.
2
Actual examples should be used to illustrate your answer.
3
Critical temperature
temperature above which isothermal compression of the substance, in its
gaseous state, would not result in condensation to liquid
4
Bond dissociation
enthalpy
Bond dissociation enthalpy (BDE) of a diatomic molecule (H-X) is the
enthalpy change when 1 mole of H-X bond is ruptured.
H-X(g)  H(g) + X(g)
5
Elementary reaction
a chemical reaction in which one or more of the chemical species react
directly to form products in a single reaction step and with a single
transition state.
In a unimolecular elementary reaction a molecule, A, dissociates to form the products(s); the rate
of such a reaction, at constant temperature, is proportional to the concentration of the species A
In a bimolecular elementary reaction, two atoms, molecules, ions or radicals, A and B, react
together to form the product(s); the rate of such a reaction, at constant temperature, is proportional
to the product of the concentrations of the species A and B.
6
Na(s)  Na(g),
½ Cl2(g)  Cl(g)
Enthalpy change of
atomization
The bond dissociation enthalpy of chlorine is two times its enthalpy change of atomization
7
1st / 2nd ionization enthalpy
e.g. A(g)  A+(g) + e-,
8
1st & 2nd electron affinity
9
Homolytic fission
Covalent bond is broken and the bonding electrons are shared
Cl-CH3  Cl. + .CH3
10
Isotope
Use 35Cl and 37Cl to illustrate the answer
11
Lattice enthalpy
M+(g) + X-(g)  MX(s)
U  Q1Q2/(r+ + r-) (symbols used must be defined clearly)
12
Relative atomic mass
Use 35Cl (75%) and 37Cl (25%) to illustrate the answer
13
Standard electrode
potential
A+(g)  A2+(g) + 2e-
O(g) + e-  O-(g), O-(g) + e-  O2-(g)
Eo
(Zn2+(aq)/Zn(s)) = -0.76 V
0.76 V – e.m.f of the electrochemical cell formed by coupling standard Zn half cell and
standard hydrogen electrode;
-ve sign indicates Zn half-cell is the negative electrode in the captioned electrochemical
cell;
electrode potential of standard hydrogen electrode is arbitrarily assigned 0.00 V;
construction of standard hydrogen electrode involves the use of platinum black (platinum
coated with finely divide Pt) and the Pt electrode should be just below the liquid surface;
14
Standard enthalpy
change of formation
Enthalpy change when one mole of the compound is formed from its
elements under standard conditions. (pressure = 1 atm., temp.= 298K).
15
Stereoisomer
Include geometric and optical isomers
16
Structural isomer
Same molecular formula, but different structure
Use propan-1-ol, propan-2-ol, methoxyethane as example
17
BOD5
biological oxygen demand in 5 days
18
Unit cell
the smallest basic portion of the crystal lattice that, when repeatedly stacked
together in three dimensions, can generate the entire crystal structure.
Page 6
19
Dipole moment of a
diatomic molecule
Dipole moment of a diatomic molecule is the product of the charge and
the separation between +ve and –ve centers.
20
Disproportionation
The process in which the same element undergoes oxidation and
reduction at the same time. Cl2 + 2NaOH  NaCl + NaOCl + H2O
3HNO2  HNO3 + 2NO + H2O; 2Cu(I)
 Cu(II) + Cu(0)
21
Octet rule
the atom tends to attain a stable electronic configuration with eight
electrons in the outermost shell (exception: BCl3, NO2, PCl5, Fe2+, Fe3+)
22
Aufbau principle
Electrons always fill in the first available orbital of lowest energy.
(building-up principle)
23
Pauli exclusion Electrons in the same orbital must take up opposite spins/ no two electrons in an
principle
atom can have the same four quantum numbers.
24
Hund’s rule
When electrons fill in a set of orbitals of equivalent energy, they will distribute
themselves so as to occupy as many orbitals as possible before any pairing or
coupling of electrons with opposite spins takes place.
25
Primary
standard
A standard solution of the substance can be prepared by dissolving a known
mass of the substance in a solvent & making up the solution to a known volume.
26
Orbital
The regions of space around the nucleus where an electron is likely to be found.
The shape of an atomic orbital is visualized as a boundary surface within which
the electron spends 95% of its existence.
s-orbitals are spherical symmetrical about the nucleus.
p-orbitals have two opposing lobes, one on each side of the nucleus, with a
region of zero electron density (a node) between them centred on the nucleus.
Any single orbital can hold a maximum of two electrons.
27
To predict the feasibility of a redox reaction, it is necessary to determine the overall e.m.f. for the
reaction and see whether it has a +ve value (balanced equation should also be included).
(e)
Imprecise/incorrect explanation for chemical phenomenon
1
Reactivity of inert gases increases down the group – atomic size increases down the group, IE
decreases down the group; ease of distortion of electron cloud and subsequent bond formation
increase down the group;
2
Sodium carbonate solution is alkaline:
(Carbonate ion is a conjugate base of carbonic acid.)
Hydrolysis of carbonate ion
CO32- (aq) + H2O(l)
HCO3- (aq) + -OH(aq)
Imbalance of H+(aq) and OH-(aq) results H2O(l)
H+(aq) + -OH(aq)
3
Sodium hydrognesulphate solution is acidic:
HSO4- + H2O(l)
H3O+ (aq) + SO42-(aq)
4
Benzene aromaticity is lost during substitution, accounting for high activation energy;
5
NH3 & amine are soluble in water because intermolecular hydrogen bonds are formed with water;
6
Iodine is soluble in KI(aq). Brown colour of iodine in KI solution is due to I3-;
7
Alkene is more reactive than alkane (C-C pi bond is weaker than C-C sigma bond).
8
In methanol, the O-H bond is stronger than the C-H bond but it breaks first in the following reaction
H
H
H
C
H
O
+
Na
H
C
H
-
O Na
+
1 2
H2
H
The reaction takes place in a polar medium, in which the methoxide ion is stablized by solvation
effect. (Consider the total energy change in the above two possible reactions.)
(f)
1
Imprecise answers for chemical hazard/ side effect
“Chromium is harmful” is not specific enough. (Chromium compounds are toxic to aquatic life.)
Page 7
2
Hazards of ethoxyethane: higher vapour pressure & causes an explosion if pressure is not released
during shaking; low ignition temperature and catches fire easily; has an anaesthetic effect;
3
“Hydrogen is dangerous” is not specific enough. (Hydrogen is explosive/ flammable.)
4
“Aspirin could cause damage to stomach lining.”
(g) Imprecise expression/ answer for chemical equilibrium
1
[H2O(l)]eqm is omitted in the expression of Kc of esterification.
2
[CaCO3(s)]eqm is included in the expression of Kc of thermal decomposition of CaCO3(s).
3
Ka is confused with Kc for the ionization of weak acid.
HA(aq) + H2O(l)
H3O+(aq) + A-(aq)
4
Liquefied butane is stored in steel cylinder at room temperature
Critical temperature of butane is above room temperature. Once liquefied, the intermolecular
forces between the molecules are greater than the average molecular kinetic energy;
5
Rf value of a compound is related to both its solubility in the mobile phase and its binding strength
to the stationary phase
(h) Failure to provide explanation to support deduction
1
3.
In the elucidation of unknown organic compound, name and formula are given, but without any explanation.
Carelessness in reading instructions/in answering questions
(a)
Carelessness in reading instructions
1
Failure to give/balance chemical equation(s)/half equation(s) even if question requires so;
2
reactions of lithium instead of those of lithium compounds despite the question asks for lithium compounds;
3
Giving answers on chemical equilibrium despite the question asks for rate constant;
4
Giving Kp despite the question asks for Kc;
5
Giving molar mass despite the question asks for RAM;
6
describe reactions of phenol, aniline despite the question asks for aromatic hydrocarbon;
7
the question asked for name, but formula is given;
8
the question asked for shape, but drawing is given;
9
the question asked for acyclic compound, but cyclic structure is given;
10
the question asked for ascending order, but descending order is given (bond strength: C-F > C-Cl
> C-Br, C-I; bond length: C-I > C-Br >, C-Cl > C-F);
11
Charge of an electron is 1.6 x 10-19 C, but relative charge of an electron is –1;
12
Drawing of electrochemical cell instead of giving the required cell diagram;
13
H+(aq) is wrongly included in half-equation for reaction in alkaline medium;
14
include nylon, polyester despite the question asks for poly(alkene).
15
the question asked for ‘single step reactions’, but energy profiles showing an intermediate is given.
16
the question asked for physical tests, but chemical tests are given.
17
The careless candidates omitted the dilution factor of 10 in their calculations.
(b)
1
Carelessness in answering questions involving chemical calculations
Wrong/no unit in chemical calculations; relative atomic mass – no unit; molar mass – with unit;
rate constant: 0th order – mol dm-3 s-1; 1st order – s-1; 2nd order – mol-1 dm3 s-1; equilibrium constant –
depends on the given equation; activation energy – kJ mol-1, Rf is dimensionless;
2
solving an energetic problem without drawing appropriate energy cycle/enthalpy level diagrams;
3
Failure to use limiting reactant in the calculation of percentage yield;
Page 8
4
Failure to use compatible units in calculations (e.g. general gas equation, activation energy,
energy of photon);
5
Failed to convert the concentration unit from mol dm-3 to g m-3 and to ppm
6
failure to work out answers for those questions involving natural log (k = Ae-Ea/RT, Nt = Noe-kt)
(c)
Failure to use the Periodic Table in the paper to find the symbols/relative atomic masses of elements
or identify elements.
(d)
Carelessness in drawing diagrams (without labels)/sketching graphs
1
n= 5
n= 4
n= 3
wrong
Cl
Cl
C
C
H
H
IE
n= 2
Cl instead of Cl
1st IE decreases
n= 1
energy gap between n =2& n = 1
should be greater than that between n = infinity & n =2
2
Cl
Cl
Cl
Cl
Al
Cl
(e)
Et3P
Cl
Cl
H
Cl
Cl
F
N
B
H
F
F
instead of
F
Cl
F
H
Be
Be
Be
PEt3
Br
Cl
Cl
Ni
Cl
Be
H
Cl
instead of
Br
Al
H
Cl
Cl
instead of
instead of
Cl
PEt3
Br
C
H
Be
Be
Cl
C
11
Cl
Cl
Ni
Cl
Cl
Cl
Br
Al
Al
atomic no.
3
Et3P
down the group
N
Cl
H
H
B
F
Carelessness in reading organic structures
1
Failure to recognize (CH3)3COH as a 3o alcohol; (3o alcohol needs a vigorous condition for oxidation)
2
Failure to recognize (CH3)3CCH2Cl as a 1o alkyl halide; (For Sn2 reactions, 1o > 2o > 3o RX)
Many failed to show that Sn2 reaction involves the inversion of configuration of the substrate.
Few recognized the bulkiness of attacking agent causes the steric hindrance in the Sn2 reaction.
3
Failure to recognize the asymmetric carbons in glucose (ring and chain form);
4
The concept of good leaving group was not well mastered.
Cl
Cl
Cl
NH2
O
C
C
C
CH2CH3
C 6H5NH2
CH2CH3
O
C 6H5NH
HO
CH2CH3
Cl- is a good leaving group.
NHC 6H5
O
C
CH2CH3
NHC 6H5
-H
O
H
C
+C l
CH2CH3
Page 9
NH2
C
C
C
O
H
H
H
C 6H5NH2
CH2CH3
C 6H5NH
HO
CH2CH3
O
H- is a poor leaving group
H is very small in size,
the two electrons experience
strong repulsion.
H
H
-H
CH2CH3
CH2CH3
5
+ OH
C
C 6H5N
H
C
C 6H5N
CH2CH3
Failure to recognize the planar structure (sp2 hybridized carbon) of carbocation;
CH3
Sn1 reaction
-
CH3
C2H5
CH3
Nn
C
X
C
Nu
H
-
Nu
CH3
+
H
C2H5
H
Unimolecular
nucleophilic
substitution
C
C2H5
C
trigonal planar structure
A racemic mixture
may be formed
C2H5
H
Nu
racemic mixture
CH3
CH3
CH3
C
Electrophilic addition
of HBr on but-2-ene
A racemic mixture
may be formed
C
C2H5
H
-
Br
C+
Br
CH3
H
H
H
C
C2H5
H
Br
6
Failure to recognize the possible attacking sites of strong base in dehydrohalogenation
-
B
-
B
7
H
H
C
C
C
C
X
H
H
C
C
C
X
H
C
C
C
C
X
H
A mixture of several
alkenes is formed
C
H
-
B
Failure to recognize the enantiomeric relationship of the following two structures. (The two
structures are not superimposable.)
CH3
CH3
C
C
H
4.
H
H
CH3
CH3
C
C
H
C
H
C
H
Confusion in fundamental concept of chemistry
(a)
1
Indiscriminate use of terms
transition state
‘transition state’ and ‘intermediate’
potential
energy
intermediate
reaction coordinate
2
functional group is confused with ‘homologous series. ‘Alkanoic acid’ instead of ‘carboxylic acid’ is used
to denote the functional group of benzoic acid. ‘Alkanol’ instead of ‘hydroxyl group’ is used to denote the
functional group of phenol.
Page 10
3
‘electron affinity’ and ‘electronegativity’; ‘polar bond’, ‘bond dipole’ and ‘dipole moment’;
4
using ‘face-centred cubic structure’ and ‘body-centred cubic structure’ in describing lattice
structure of ionic compound;
(In NaCl, both Na+ and Cl- ions adopt face-centred cubic lattices, which interpenetrate each
other. The structure shows 6:6 coordination.
In CsCl, both Cs+ and Cl- ions adopt simple cubic lattices, which interpenetrate each other.
The structure shows 8:8 coordination.)
5
‘homolytic’ and ‘heterolytic’ fission of covalent bond; ‘electrophile’ and ‘nucleophile’;
6
‘+ve’ and ‘-ve’ inductive effect; (CH3- exerts +ve inductive effect; Cl- exerts –ve inductive effect)
7
‘screening’ and ‘effective nuclear charge’; (screening : electron-electron repulsion within an atom;
effective nuclear charge experienced by the outermost electron: nuclear charge – screening effect)
8
‘vapour pressure-composition curve’ and ‘boiling point-composition curve’;
liquid
-2
p/kNm
vapour
XA' = PA/ PT
X B' = PB/ PT
40
30
PA
20
PT
10
PB
0
0
0.2
XA 0.4
0.6
XA' 0.8
o
PA = PA XA
o
PB= PB X B
1.0
XB
XA
mole fraction of A
PT= PA+PB
Vapour pressure against liquid composition (liquid) & vapour pressure against vapour (vapour) composition
- ve deviation from Raoult's Law
T /o C
-2
p/kNm
o
PB
P
30
o
TB
A
PT
20
PB
TA
40
10
PA
0
0
0.2
0.4
0.6
0.8
1.0
0
moe fraction of A
Vapour pressure-composition curve for a binary
mixture showing –ve deviation from Raoult’s Law
0.2
0.4
0.6
0.8
mole fraction of A
1.0
Boiling point-composition curve for a binary mixture
showing +ve deviation from Raoult’s Law
9
‘rate constant (k)’ and ‘equilibrium constant (K)’; ‘absorb’ and adsorb’;
10
‘cyclic (ring)’ and ‘acyclic (non-cyclic)’; ‘structural isomer’ and ‘stereoisomer’;
11
‘end-point’ and ‘equivalent point’ in titration; ‘conjugate acid’ and ‘conjugate base’;
12
‘empirical formula’, ‘molecular formula’ and ‘structural formula’;
13
‘transition elements’ and ‘d-block elements’; ‘hydrolysis’ and ‘hydration’;
14
‘enthalpy change of hydration’ and ‘enthalpy change of solution’; ‘pKa’ , ‘pKb’ and ‘pKw’;
15
‘dissolve’ and ‘hydrolyse’; (NaCl dissolves in water; ester hydrolyses in alkaline medium.)
16
‘polarization’, ‘polarizing power’ and ‘polarizability’;
(Br-Br is polarized by adjacent electron rich C=C bond.
In anhydrous AlCl3, Al3+ ion is small in size and highly charged, it has high charge density and strong
polarizing power. Thus the electron cloud of the Cl- is distorted.
Iodine is bigger in size and has a higher molecular polarizability.)
17
mixing up the state symbols ‘aq’ and ‘l’; mixing up ‘polarimeter’ with ‘colorimeter’
18
nylon is an example of polyamide, not poly(alkene);
(b)
1
Oxidation states, redox
failed to recognize that in Fe3O4 (FeO + Fe2O3), iron exists in oxidation states of +2 and +3;
in Pb3O4 (2PbO + PbO2), lead exists in oxidation states of +2 and +4
Page 11
2
Did not recognize that the two S atoms in S2O32- have different bonding environments. The
oxidation state of the central S atom is +4 and the other S atom is 0.
3
chromate(VI)-dichromate conversion is wrongly considered as a redox reaction;
4
wrong product for the reduction of esters and amides by LiAlH4; (ester is reduced to alkanol;
amide to amine)
5
NaBH4 is used to reduce carboxylic acid; (acid, ester, amide should be reduced by LiAlH4.)
6
LiAlH4/NaBH4 is used to reduce alkene; (LiAlH4/NaBH4 is a nucleophilic reducing agent.
catalytic hydrogenation should be used to reduce alkene)
(c)
1
Structure, bonding and intermolecular forces
MgCl2 is a covalent compound; (MgCl2 is an ionic compound with small covalent character – Mg2+ has
high charge to size ratio and thus strong polarizing power, electron cloud of Cl- is distorted to a small extent.)
2
SiCl4/SiH4 has a giant three dimensional structure; (SiCl4/SiH4 has simple molecular structure.)
3
The catenation of C and Si are related to the strengths of the C-C and Si-Si bonds.
4
boiling point of H2O > H2S because O is more electronegative and forms stronger hydrogen bonding;
(intermolecular hydrogen bond in H2O; van der Waals’ forces in H2S, more energy is needed to
overcome hydrogen bond, accounting for the higher boiling point)
5
boiling point of H2O > HF because water forms stronger intermolecular hydrogen bond;
(F is more electronegative than O, thus H-F forms stronger hydrogen bond. However, unlike H-F,
water is capable of forming two hydrogen bonds per molecule and the intermolecular force in water
is stronger than that in H-F.)
6
the intermolecular force between HCl molecules is dipole-dipole attraction (Cl is a period 3
element which is bigger than F, the lone pair electrons is too diffuse to form hydrogen bond);
7
H-bond has directional character, H2O in ice are arranged tetrahedrally to form an open structure.
8
boiling point of D2O > H2O because molar mass of D2O > H2O.
9
incomplete answer – b.p. of H2O > OF2, intermolecular hydrogen bond exists in water but did
not mention that the intermolecular attraction between OF2 is van der Waals’ force.
10
boiling point of H2S > SiH4 because H2S has a net dipole and SiH4 is non-polar;
11
due to strong interaction between sucrose molecules and water, an aqueous solution of sucrose has
a lower vapour pressure than pure water and hence its b.p. is higher than 100oC at 1 atm.
12
melting point of sulphur is higher than that of yellow phosphorus because molecular size of
sulphur (S8) is bigger and has stronger van der Waals’ forces;
13
the reason why N2 and silica can exist in nature for ages is because of the high bond energies in
these chemical species, resulting in high activation energy required for bond-breaking reactions.
14
The metallic bond in zinc metal is due to the attraction between the cations of zinc & the delocalized
electrons. Some candidates erroneously wrote ‘zinc atoms’ or ‘zinc nuclei’ instead of ‘cations of zinc’. The
boiling point of metal depends on the strength of metallic bond which in turn depends on its atomic size
and the number of valence electrons (b.p. of Na > K, Mg > Na)
15
carbon/nitrogen has no 3d orbital (they have no low-lying 3d orbitals)
16
NO2 is a v-shaped
molecule with and odd
electron. It has two
double bonds.
Wrong – because N does not possess
low-energy d orbital and can not
use d orbital in pi bond formation
N
O
O
N
O
O
The bond angle is around 130o.
17
NO2+ is linear; NO3- is trigonal planar; NH4+ is tetrahedral; NH3 is pyramindal;
O
-
+
O
N
O
N
N
N
O
+
H
H
O
H
H
H
H
H
Page 12
18
confusing the second and third periods of the Periodic Table (Na is a period 3 element)
19
increase in covalent character of compound does not imply a decrease in stability (most likely,
compounds with higher covalent character will have lower electrical conductivity in molten
state, lower melting point and higher solubility in non-polar solvent)
20
solubility depends on enthalpy change of solution, which in terms depends on the difference
between lattice enthalpy and enthalpy changes of hydration of cations & anions;
21
Hydrogen bonding does not occur only between molecules; (hydrogen bonding can be formed
between SO42- ion and water of crystallization in hydrated copper sulphate)
22
Apart from F2, bond strength decreases steadily down group VII. This is mainly because the size
of atom increases down the group (increase in number of electron shells and screening effect)
and the attraction between the nucleus & the bonding electrons weakens down the group.
23
H-Cl bond is stronger than H-Br bond because Cl is more electronegative than Br; (In fact,
covalent bond strength mainly depends on the attraction between the nucleus and the
bonding electrons. Thus the smaller the atomic size, the stronger the covalent bond.)
24
The short C-C bond distance within layers of atoms in graphite is explained in terms of electron
delocalisation rather than partial double bond character (1 + 1/3 bond);
25
 bond in C6H6 is formed by sideways or lateral overlapping of unhybridised p orbitals of the
carbon atoms; σ bond (C-H) is usually not mentioned in the discussion of the structure of C6H6;
26
The relatively long distance between layers of atoms in graphite is wrongly considered to have
originated from the repulsion of the delocalised -electrons rather than weak van der Waals’
force between hexagonal layers;
27
MnO4- , CrO42-, SO42-, ClO4have square planar structure;
(These oxyanions are
tetrahedral in shape.)
O
O
Mn
O
O
-
O
-
O
Cr
O
-O
-
O
Cl
S
O
-
O
O
O
-O
O
O
28
NH4+ - 4 bps; NH3 – 3 bps & 1 lp; NH2- - 2 bps & 2 lps
Since lp-lp > lp-bp > bp-bp  Bond angle of NH4+ > NH3 > NH2- (VSEPR theory)
29
ClF3 has a T-shaped structure; Pb(C2H5)4 is tetrahedral in shape. ClF5 has a square-based
pyramidal structure.
30
Cyclopropane is highly strained;
31
Vegetable oil contains C=C bonds which are rigid. The C=C bonds prevent molecules of oil from packing
close together. In hardening oils, interlocking of hydrocarbon chains restricts the relative motion of the oil
molecules. Therefore, vegetable oils become hardened upon hydrogenation.
32
Cis-isomer of N2F2 has a higher b.p. than the trans- isomer, as it has a net dipole moment;
33
Anhydrous calcium sulphate is harder than hydrated calcium sulphate
strong ionic bonds are to be overcome in anhydrous calcium sulphate, while only weak hydrogen
bonds are broken in hydrated calcium sulphate;
34
Bond order of O-O bond in ozone is 1½ (1 sigma + 1 pi bond); C-C bond in benzene is 1½ (1
sigma + 1 pi bond); C-O bond in alkanoate anion is 1½ (1 sigma + 1 pi bond); C-O bond in
carbonate ion is 1 1/3 (1 sigma + 1/3 pi bond);
35
Failed to produce a correct three-dimensional
structure of P4.
There are 6 P-P bonds in the P4 molecule.
P
P
P
P
36
Failure to recognize that the occurrence of optical isomerism is due to the lack of a plane of
symmetry in the structure/ absence of a non-superimposable mirror image.
37
Hydrogen bonds between polymeric chains should not be confused with cross links.
Page 13
38
H in methyl group undergoes free radical substitution more readily than that in benzene ring:
CH 2Br
CH 3
Br2
CH2
CH2
CH2
CH3
+ HBr
h
Benzylic radical is stabilized by
resonance with benzene ring.
(d)
1
Phenyl radical is not
stabilized by resonance.
Chemical kinetics, reaction mechanism and energy profile
Confusion in using integrated form of rate equation or half life;
1st order:
Nt = Noe-kt
2nd order:
k=
1
1
t
Nt
Or
1
-
k = 1/t ln(No/Nt)
Or
No
k =
1
t
Half-life =
x
a(a - x)
ln2/k
Half-life =
1
ka
2
The catalyst lowers the activation energy of the reaction instead of that it provides an alternate
pathway with lower activation energy;
3
Acid anhydride acting as an initiator in free radical polymerization; (organic peroxide/benzoyl
peroxide should be used as the initiator for free radical polymerization)
4
reaction mechanism: wrong direction of arrows to represent the movement of electron pairs,
indiscriminate use of ‘half arrows’ and ‘full arrows’;
5
incorrect labeling of the axes in an energy profile: x-axis as time, reaction coordination, reaction
progress and y-axis as reaction profile;
6
Incorrect representation of carbocation: no charge, too many charges, pentavalent cation etc.
7
Failure to show that AlCl3 functions as a Lewis acid (electron pair acceptor) in electrophilic
addition of aromatic compounds.
O
CH3
CH3
C
Cl
8
AlCl3
CH3
C
O
O
C
AlCl4
H
Cl
AlCl3
Failure to recognize the most probable mechanism for hydrolysis of tertiary alkyl halide
H
Br
-
OH(aq)
Br
NH
- Br-
+
NH
N
H2O
H
HO
N
HO
-
H
O
+
NH
Because of its abundance, H2O, instead of –OH is more likely to be the nucleophile attacking the
carbocation.
9
in nucleophilic substitution, the predominant reaction mechanism for 3o / 1o RX is SN1/ SN2;
as nucleophile is not involved in the rate determining step of SN1 reaction, the rate of SN1 reaction
is independent of the nature of the nucleophile;
a strong nucleophile (one with high nucleophilicity) favours SN2 reaction;
(–OH has a stronger tendency to donate electron and is a stronger nucleophile than H2O);
10
-OH group can be converted to a good leaving group after protonation.
(e)
Periodicity
Difference in the chemical/physical properties of the second and third row elements:
Page 14
1
CCl4 is inert towards hydrolytic reaction whereas SiCl4 is rapidly hydrolysed by water;
2
the diatomic O2 molecule is stable towards thermal decomposition whereas the S-S bond in
elemental sulphur is easily broken upon heating;
3
the presence of low-lying empty 3d orbitlas in the third row elements allows these elements to
expand their valence shell (e.g. NCl3 and PCl5; OF2 and SF6) ;
4
The small atomic radii in C, N, O and F favour the formation of pi bond by overlapping of p
orbitals;
5.
NCl3 donates a lone pair electron to water, NH3 and HOCl are formed;
NCl3 + 3H2O → NH3 + 3HOCl
PCl3 (Cl is more electronegative than P) accepts a lone pair electron from water, H3PO3 and HCl
are fromed. PCl3 + 3H2O → H3PO3 + 3HCl
6.
NH3 is highly soluble in water and gives an alkaline solution NH3 + H2O
PH3 is nearly insoluble in water and gives a neutral solution
5.
NH4+ + -OH
Others
(a)
Wrong spelling of terms commonly used in Chemistry
1
‘flame cupboard’, ‘anormal’, ‘vicosity’
(fume cupboard, abnormal, viscosity, goggles, ninhydrin, pasteurization, Le Chatelier’s
Principle, Markownikoff’s rule, homogeneous catalysis, heterogeneous catalysis, trigonal
planar, trigonal bipyramidal, carbocation, alkoxide, phenoxide, benzoate, chromatogram,
catenation, racemic mixture, malleability, pestle & mortar, Bunsen burner, Buchner funnel
and Buchner flask, stillhead, receiver adaptor, receiver adaptor with side-arm,
double-necked pear-shaped flask, desiccator, Liebig condenser, platinum)
2
Failure to give concise and precise definitions for chemical terms (e.g. order, rate equation, azeotropic mixture,
racemic mixture, electrophile, nucleophile, free radical, iodine value, chiral centre, DO, BOD5 etc.)
(b)
Weaknesses in answering essay questions
1
failure to cover different areas of chemistry (bonding and structure, preparations, physical and
chemical properties, uses, applications, separation, identification, safety measures etc.;
2
inclusion of a lot of irrelevant/superfluous materials;
3
essay is not focused, inclusion materials learnt in CE chemistry course;
4
Information presented NOT in the context of the current situation in HK;
5
essay with no equation, diagram and/or illustrative example
6
lack of attractive titles & illustrations (A picture is sometimes worth more than a thousand words.)
7
communication skills can be improved through reading more articles and watching more
television programmes on popular science.
(c)
Unable to state/describe practical procedure for an experiment
1
failed to use clock experiment (iodine clock experiment) to compare reaction rates;
In the determination of the initial reaction rate of the following reaction:
2I-(aq) + H2O2(aq) + 2H+(aq) → I2(aq) + 2H2O(l)
starch solution is used to detect the presence of iodine; a fixed and small amount of thiosulphate
is also added. The starch solution turns blue after a fixed amount of iodine is formed and all
thiosuphate consumed.
Rate = d[iodine]/dt; the amount of iodine is proportional to the amount of thiosulphate added
2
failed to use non-destructive methods to compare reaction rates (colorimeter, volume change,
pressure change, mass change, polarimeter);
Page 15
3
did not recognize that sucrose, glucose and fructose have different optical rotation and hence
polarimetry can be used to monitor the progress of the reaction:
C12H22O11 + H2O → C6H12O6 + C6H12O6
4
Tollen’s reagent should be prepared by adding dilute ammonia solution dropwise to silver nitrate
solution until the precipitate is just dissolved;
Ag+(aq) + 2OH-(aq) → Ag2O(s) + H2O(l); Ag2O(s) + 4NH3(aq) + H2O(l) → 2Ag(NH3)2(OH)2
5
in the determination of the amount of water of crystallization in a hydrated salt, the sample should
be heated in a crucible until there is no further change in mass.
6
incorrect direction of water flow in the condenser (flow of water should be against the gravity);
7
failed to use recrystallization, suction filtration, centrifugation in purification;
Recrystallization: add just enough hot solvent to completely dissolve the impure solid;
heat the solution with activated charcoal (if necessary);
filter hot solution with short stem funnel/ suction filtration
(insoluble impurities are removed);
allow hot solution to cool down (soluble impurities remain in the solvent); and
filter to isolate the pure crystals
8
failed to use fractional crystallization in separating KClO3 from KCl
Fractional crystallization: repeatedly crystallizing a salt so as to separate the substances of
different solubilities. KCl is more soluble and will stay in the aqueous solution on crystallization
of a mixture of KClO3 and KCl.
9
failed to state the organic solvent (ether) used in solvent extraction;
10
failed to use flame test, hydroxide test, chloride test, sulphate test to identify unknown cation;
11
failed to use brown ring test to identify nitrate(V) ion;
12
Failed to recognize that the reaction of a weak acid (CH3COOH(aq)) and a weak base
(NH3(aq))gives a salt and water in such a way that the salt (CH3COO- NH4+) dissociates to give a
greater of ions which are then responsible for a higher conductivity;
13
failed to use acid/base properties to separate a mixture of organic compounds (phenol, aniline
and hexan-1-ol);
14
failed to use double indicator (phenolphthalein and methyl orange) to find out the concentration of
Na2CO3 and NaHCO3 / NaOH and Na2CO3 in a mixture;
Phenolphthalein shows colour change when CO32- is changed to HCO3Methyl orange change colour when HCO3- is changed to H2CO3
15
Drying agent is confused with dehydrating agent;
A drying agent is a substance which readily takes up water to become hydrated.
drying agent
calcium chloride, CaCl2
calcium sulfate, CaSO4
magnesium sulfate, MgSO4
potassium carbonate, K2CO3
sodium sulfate, Na2SO4
capacity speed
applications
high
medium used for hydrocarbons
low
fast generally useful
high
fast not used for very acid-sensitive compounds
medium medium not for acidic compounds
high
slow generally useful
Add a small amount of drying agent to the liquid; if it is all clumped together, add more.
a desiccator with suitable drying agent could be used to dry a moist solid sample
16
the reaction between red P and bromine is highly exothermic, reflux condenser is necessary in
the bromination of alkanol;
17
it is necessary to shake the vessel vigorously so as to increase the contact area of the reactants in
two different phases;
18
proper procedure to determine Ka of weak acid should include the preparation of a buffer
solution with equal no. of moles of weak acid and conjugate base;
Page 16
19
In a flame test, the Pt wire used should be cleaned with conc. HCl, and the wire stuck with
the sample should be heated in a non-luminous Bunsen flame;
Conc. HCl is used to convert the salt to the chloride thus making it more volatile;
At temperature of a Bunsen flame, a compound will decompose to give gaseous atoms of its constituent
elements. When an electron absorbs energy from the flame, it will move from an orbital with lower energy
to an orbital with higher energy. When an electron returns from an orbital of higher energy to an orbital of
lower energy, a photon with energy equal to the difference of the two orbitlas will be emitted. An emission
line will be resulted.
Each element has a unique emission spectrum. A metal can be identified if it has a strong emission line in
the visible region of the electromagnetic radiation.
20
Coloured impurities can be removed by adsorption onto activated carbon;
21
sodium hydrogensulphate(IV) can be used to purify carbonyl compounds;
22
sharp melting point is a criterion for purity in the crystalline product and that comparing the
experimental melting point with the literature value would show whether it is the desired product
(mixed m.p. determination may also be used);
23
Procedure in investigating effect of temperature on reaction rate:
keep initial concentration and amounts of reactants constant,
find some ways to follow the rate of reaction,
repeat the experiment at different temperatures, indicate data treatment
24
glass electrode of the pH meter should be rinsed with distilled water and calibrated with
buffer solution before used;
25
in using a colorimeter to find out the concentration of coloured species in a sample, a suitable
filter should be so chosen that light ray strongly absorbed by the sample is used. A calibration
curve (absorbance against concentration of the coloured species) is then plotted;
26
in using the ‘continuous variation method’ to determine the stoichiometry of a complex, a
suitable filter should be so chosen that light ray strongly absorbed by the sample is used. The
absorbance of the solution (instead of transmittance) should be used in plotting the graph.
27
the titre which deviates significantly from the others, should be discarded in the calculation of the
mean titre;
28
NaOH(s) is not a primary standard. NaOH(s) is deliquescent and NaOH(s) reacts with CO2 in air.
29
H2SO4(aq) is not a primary standard. H2SO4(aq) is hygroscopic.
30
solid iodine is not used as a standard because it is volatile and can readily react with reducing
agents in the environment, and hence it is difficult to weigh a sample of solid iodine accurately;
31
fractionating column should not be included in the set-up of a simple distillation;
32
In the confirmatory test for carbonyl compounds, phenols and phenylamine, solid derivatives with
high molar mass should be prepared, e.g. 2,4-dinitrophenylhydrazine for carbonyl compounds;
bromine/p-bromobenzoyl chloride for phenols; & bromine/ethanoyl chloride for phenylamine
33
A control experiment is necessary in showing that the reaction of peroxodisulphate(VI) ions
(S2O82-) with iodide ions can be catalysed by iron(III) ion.
34
HCl is denser than air and should be collected by downward delivery;
35
Calcium carbonate is insoluble in water, thus the percentage by mass of calcium carbonate in a
sample of office paper should be determined by back titration;
36
A mixture of amino acids can be separated by chromatography or electrophoresis
(d)
Unable to apply chemical knowledge to an unfamiliar situation
1
questions involving reactions of bi-/multi-functional organic compounds or questions involving
cyclic organic compounds;
2
writing half equations and overall equations for unfamiliar redox reactions (electrons were always
included in an overall equation);
Page 17
3
failure to explain clearly how vitamin E works with reference to the function of BHT;
O
OH
(CH3)3C
C(CH3)3
(CH3)3C
C(CH3)3
CH3
CH3
4
ROOH
+
+ ROO
unable to answer questions involving unfamiliar oxidant/reductant.
(e)
Unable to produce correct drawings for electronic structures (‘dot and cross formula’ which follows
‘octet rule’), & 3-dimensioanl structures of molecules/ions (solid line for covalent bond & dotted
lines for outlining the shape of ions/molecules.
(f)
Misconceptions in acid base properties
1
explanation of the acidity of compounds such as phenols/carboxylic acids in terms of the strength
of O-H bond, instead of the relative stability of the acid and its conjugate base.
Acidity: HClO4 >HClO3 > HClO2 > HClO; HClO4 > H2SO4 > H3PO4
2
Para-nitrophenol is more acidic than ortho-nitrophenol.
Ortho-nitrophenol forms intramolecular hydrogen bonds. The nitro group cannot be coplanar with
the benzene ring and the OH group. Thus its electron-withdrawing effect becomes weaker.
3
Phenol is less acidic than H2CO3, but more acidic than HCO3-, so it does not react with NaHCO3,
and does not give off CO2 with carbonate but soluble in Na2CO3;
OH
O
2-
+ CO3
-
O-
OH
-
-
+ HCO3
+ H2CO3
+ HCO3
Phenol is more acidic than HCO3 , so the reaction proceeds to the RHS. Phenol is less acidic tha H2CO3, so the reaction proceeds to the LHS.
4
Carboxylic acid is a stronger acid than H2CO3
and phenol. The product formed in the following
reaction should be
OH
OH
COOH
NaHCO 3
COO
-
The carboxylate salt is soluble in water.
5
H-F(aq) is a weak acid. This is mainly because the hydronium ions, H3O+, are involved in
hydrogen bonding to F- ions to form tight ion pairs, so that the concentration of H3O+ ions is
lowered.
6
The basicity of an amine depends on: (1) the availability of electron pairs to donate to a proton,
and (2) the ease with which the protonated amine can undergo salvation with water molecules and
become stabilized. CH3CH2NH2 is a stronger base than (CH3)2NH
7
The basicity of a base, B, depends on the position of the equilibrium:
B(aq) + H2O(l)
HB+(aq) + OH-(aq)
which in turn depends on the relative stability of B(aq) and its conjugate acid, HB+(aq).
RNH2 is a stronger base than NH3 because +ve inductive effect of the R group in 1o amine
stabilizes RNH3+ , the conjugate acid to a greater extent.
8
-amino acids exist in form of dipolar ions (zwitterions) because –NH2 is a stronger base
than –COO- and –COOH is a stronger acid than –NH3+;
9
NaHSO4(aq) is acidic but NaHCO3(aq) is alkaline
HSO4-(aq)
H+(aq) + SO42-(aq);
+
HCO3 (aq) + H (aq)
H2CO3(aq); HCO3-(aq)
10
H+(aq) + CO32-(aq)
Acid strengths of HCl and HBr are comparable, whereas in acidic solvents such as ethanoic acid,
HBr demonstrates a stronger acidic property;
Page 18
(g)
Misconceptions in atomic structures
1
first emission line in the Lyman series involves electronic transition between n = 2 to n = 1
(quantum number can have values 1,2,3 etc; first emission line in Balmer series involves
electronic transition between n = 3 to 2; convergence limit in Balmer series involves electronic
transition between n = infinity to 2);
2
wrongly stated that the e- in a ground state hydrogen atom has a principal quantum number of 0;
3
convergence limit in Lyman series is needed to calculate the IE of hydrogen;
4
separation of emission lines within a particular series decreases with frequency;
5
a plot of log IE against no. of electrons removed reveals the existence of electron shell;
6
a plot of IE against atomic number reveals the presence of sub-shells (s, p-type orbital);
7
IE of a species depends on its size, its electronic configuration and the effective nuclear
charge on the electron to be removed;
8
A large amount of energy is required to break the stable electronic structure of a noble gas, thus
Na and K have large second IE; Mg and Ca have large third IE.
9
flame test – emission of EM radiation (electron is first excited by heat energy, electronic
transition from high to low energy levels),
aqueous solution of transition metal ion – absorption of EM radiation (Ni2+ has an incomplete
3d subshell while Zn2+ does not, and that the d-d transition in Ni2+(aq) is responsible for its colour).
(h)
Misconceptions in equilibrium
1
catalyst increases both reaction rate & yield; (catalyst increases the reaction rate but yield
remains unchanged);
2
a rise in temp increases the forward reaction rate and the equilibrium position shifts to the product
side. (A rise in temp. increases both the forward & backward reaction rates. For endothermic
reaction the extent of increase in forward reaction rate is greater than that of backward reaction
rate. Thus the position of equilibrium shifts to the product side. In case of endothermic/exothermic
reaction, Kc increases/decreases with temp.);
3
Ag+(aq) + Cl-(aq)
AgCl(s) the concentration of AgCl(s) is constant and should not be
included in the Kc expression. The solubility of silver chloride in water can be determined by the
equilibrium concentration of silver ion.
4
criteria in choosing an indicator – the pH range of colour change of the indicator falls into the
steepest part of the titration curve/ pKi is close to the pH of the solution at equivalent point;
5
the thin film of water on the chromatography paper acts as the stationary phase and that
partition is the underlying principle of paper chromatography;
(i)
Misconceptions in chemical kinetic
1
failure to recognize the autocatalytic effect of Mn2+ in redox reactions involving acidified MnO4-;
2
failure to explain a slow initial increase in volume of gaseous product collected in terms of its
solubility in reaction mixture; (Carbon dioxide/ chlorine is soluble in water)
3
failure to explain a slow initial reaction rate in terms of impurities on the surface of solid reactant;
(Mg ribbon is contaminated with magnesium oxide)
4
failure to recognize insoluble calcium sulphate/lead(II) sulphate coated on metal surface stop/slow
down the reaction between calcium/lead with sulphuric acid;
5
failure to account rapid increase in reaction rate in terms of exothermic nature of reaction;
(reaction between magnesium ribbon and hydrochloric acid is exothermic)
6
if the following reaction A + B  product, is first-order, it cannot be a single step reaction;
7
A redox reaction with a positive Eo value only indicates that the reaction is feasible and gives no
information about how fast the reaction will proceed;
Page 19
(j)
Misconceptions in spectroscopy
1
Mass spectrometer – negative ions instead of +ve ions are accelerated and deflected according
to mass/charge ratio (ion with smaller mass experiences greater deflection);
relative abundance of the isotope/ fragment can be obtained from the intensity of the peak in the
mass spectrum;
2
IR spectrometer – vibrational energy of molecule is quantized (only those molecules which
give rise a change in dipole moment in vibration can absorb IR);
a compound has an absorption at a certain wavenumber range in its infra-red spectrum;
stretching bands should be specified: 1680 – 1750 cm-1 and 3350-3500 cm-1 are due to C=O and
N-H stretches, respectively;
3
NMR spectrometer – H atoms in different environments shows different absorption peaks
(k)
1
2
(l)
1
Misconceptions in radioactive decay
The decay of carbon-14 starts after the death of living things instead of the ratio of carbon-12
and carbon-14 remains unchanged in a living organism, but decreases slowly after the death
of living things;
Radon is a noble gas and can be inhaled into the respiratory system;
Misconceptions in waste disposal
Landfilling is not regarded as an appropriate disposal method for PVC because PVC is
non-biodegradable;
The proper way is by incineration (with adequate supply of air at sufficient higher temperature)
and treat the flue gas in wet scrubber with Na2CO3(aq);
(m) Misconceptions in basic components of food and preservation of food
1
Starch - -glycosidic linkage
2
Cellulose -- glycosidic linkage
3
MSG (monosodium glutamate) is NOT a food preservative
4
Ninhydrin reacts with amino acids to give purple-coloured compounds
5
O
O
The correct structure of the diquinone formed
when apple turns brown is
R
(n)
Lack of general knowledge
1
length of the carbon chain in a typical natural fat or oil is 16 – 18 (even number);
2
stirring produces very small amount of heat energy and should not be considered as an important
source of error;
3
Dacron, terylene are common names of polyester;
4
Polymer is a mixture of polymeric chains with different molecular size;
5
Some plastics degrade much faster than others, e.g. hydrolysis of polyester (Dacron);
6
Carbon dioxide cannot be used to put out fire caused by burning sodium/magnesium;
2Mg + CO2  2MgO + C
(powder type extinguisher/ sand should be used)
7
sodium benzoate, benzoic acid are antimicrobial and act as food preservative
8
amber reagent bottle reduces the blue light which causes photodecomposition of conc. Nitric acid;
9
concentration of a solid/ pure liquid is independent its amount;
Page 20
10
polymers used in insulation foam and furniture stuffing are commonly made from methanal;
When combined with phenol or urea, methanal produces a hard thermoset resin. These resins are
commonly used in permanent adhesives, such as those used in plywood or carpeting. They are
also foamed to make insulation, or cast into moulded products.
11
ozone is formed in photocopying machines & in laser printers by the electric discharge of oxygen;
12
ozone can help kill bacteria and removal of pungent odour in the office environment
13
ozone is a pollutant derived from NO2.
UV
NO2
→
NO + O;
O + O2 →
O3
0
14
ozone is a polar molecule, while oxygen is nonpolar. Thus ozone is more soluble in water than
oxygen;
15
silica gel is a drying agent;
16
Sulphur stain can be oxidized by conc. H2SO4 or conc, HNO3, to soluble SO42- ions;
17
too many significant figures in the numerical answer
18
Banning the use of leaded petrol & CCl4 (CFCs) has been in force for a number of years in HK.
Lead compounds are highly toxic. Depletion of ozone layer is mainly due to the release of CFCs.
19
CCl4 and BCF are non-flammable; BCF is widely used in fire extinguisher;
20
fluorine is highly reactive and HF is corrosive;
21
data logger – able to monitor the progress of very fast reaction, recording of data at very short
intervals, more accurate recording of data, simplify the process of data treatment
22
microscale apparatus – more environmentally friendly, reduce the risk of chemical hazard
23
copper is below hydrogen in the electrochemical series and will not react with dilute acid
24
The conversion of fats & oils into biofuels involves transesterification of fats/oils into their
corresponding methyl esters.
25
Spent electrolytic solution can be treated by adding chlorine bleach which oxidizes the CN- ions
to form less toxic NCO- ions CN- + ClO- → NCO- + Cl-
26
TiO2 is a photocatalyst, which can be used to pave streets. It catalyses the oxidation of NOx
emitted from cars to give NO3- which is not harmful.
27
Use of MgSO4 – drying agent/ laxative/ making mineral water/ making ceramics/ as fertilizer
28
Use of BaSO4 – paint additive/ filler for textiles/ in X-ray diagnostic work/ opaque medium for
gastrointestinal radiography
29
Silicon dioxide is used as a filler in the sweetener. Many suggested that it acts as a desiccant.
30
0.5 M KI(aq) is pale yellow:
4I-(aq)+ O2(g) + 4H+(aq) → 2I2(s) + 2H2O(l); I2(s) + I-(aq) → I3-(aq)
Brown fume in a bottle of 14M HNO3(aq):
4HNO3(aq) → 4NO2(g) + 2H2O(l) + O2(g)
Brown stains within a bottle of 0.02M KMnO4(aq):
4MnO4-(aq) + 2H2O(l) → 4MnO2(s) + 3O2(g) + 4OH-(aq)
31
In real life situations, calorific values are often expressed in kJ kg-1 units rather than kJ mol-1
units. The difference in calorific value of PS and PVC is related to the difference in number of C
and H per unit mass of the two plastics. It is also due to the fact that oxidation of Cl is less
exothermic than the oxidation of C and H.
32
Many suggested ethanol as the active ingredient in rubbing alcohol without noticing that the
government imposes a high tax on ethanol.
33
Very few candidates realized that water vapour is a greenhouse gas.
Page 21
34
The hydrogen used in fuel cells is produced from hydrocarbons. As such, hydrogen is only an
energy carrier but not an alternative energy source.
35
The Continuously Regenerating Trap (CRT) is a newly-patented product commonly used in buses
and lorries that run on low sulphur diesel. CRT is a state-of-the-art invention and does not require
cleaning of particulate as in other diesel particulate filters. The most abundant element in
particulate matter is carbon. NO oxidizes the carbon particles to carbon dioxide.
36
Disinfection of drinking water is widely recognized for its significant role in reducing illness due
to waterborne pathogens that are responsible for numerous diseases. Although disinfection is
necessary for the elimination of these pathogenic organisms, it can also lead to the generation of a
variety of chemicals, known as disinfection byproducts (DBPs), for instance, trichloromethane,
1,1,1-trichloropropanone, which are formed as a result of reactions of the disinfectant with
organic matter in the water.
e.g. CH3COCH3 + 3Cl2 + H2O → CH3COOH + 3HCl + CHCl3
37
Dilution volume ratio of 1:99  100 cm3 solution prepared by diluting 1 cm3 conc. solution.
38
O
Lactam undergoes thermal
ring-opening polymerization
H
O
NH
H3O
n
+
N
heat
n
39
Aromatic compounds in natural products:
COOCH3
CHO
OH
O
Cinnamaldehyde in cinnamon
肉桂 (flavouring)
40
Methyl salicylate in oil of wintergreen Anethole in oil of anise 茴芹(used in
冬綠油
some curries and seafood dishes)
Drugs with aromatic rings
O
NHCCH3
COOH
O
CH2CH(CH3)NH2
OCCH3
OH
acetaminophen
41
Amphetamine
(synthetic stimulant)
aspirin
Common disinfectants
OH
OH
Cl
Cl
CH3
CH3
Cl
Cl
TCP
(o)
dettol
Poor English/language skill, illegible writing.
Page 22