GENERAL CERTIFICATE OF EDUCATION
TYSTYSGRIF ADDYSG GYFFREDINOL
EXAMINERS' REPORTS
AS/ADVANCED
CHEMISTRY
SUMMER 2007
Unit
CH1
CH2
CH3a
CH3b/c
CH4
CH5
CH6a
CH6b/c
Page
3
7
12
14
17
20
28
32
Statistical Information
GCSE
The Examiners' Report may refer in general terms to statistical outcomes. Statistical
information on candidates' performances in all examination components (whether internally
or externally assessed) is provided when results are issued. As well as the marks achieved
by individual candidates, the following information can be obtained from these printouts:
For each component: the maximum mark, aggregation factor, mean mark and standard
deviation of marks obtained by all candidates entered for the examination.
For the subject or option: the total entry and the lowest mark needed for the award of each
grade.
Annual Statistical Report
Other information on a centre basis is provided when results are issued. The annual
Statistical Report (issued in the second half of the Autumn Term) gives overall outcomes of
all examinations administered by WJEC.
CHEMISTRY
General Certificate of Education
2007
Advanced Subsidiary/Advanced
Statistical Information
The following information is included in this report in order to provide centres with as full a
picture as possible of the examination in each component. The statistics include all
candidates entered for the unit, whether or not they 'cashed in' for an AS/A level award. The
attention of centres is drawn to the fact that the statistics listed should be viewed strictly
within the context of the paper and that differences will undoubtedly occur between one year
and the next and also between subjects in the same year. Moreover, information is
provisional in the sense that it does not take account of changes resulting from appeals.
Component Entry
Max. Mark
Mean Mark
Grade
Raw Mark
Cumulative %
CH1
2037
66
31.0
A
B
C
D
E
41
36
31
26
21
26.0
40.0
52.8
65.1
76.5
CH2
2591
66
32.3
A
B
C
D
E
43
37
32
27
22
26.4
40.7
53.0
65.9
75.8
CH3a
2479
30
18.7
A
E
23
13
28.2
83.7
CH3b
108
103
86.8
A
E
89
67
46.3
98.1
CH3c
2362
103
86.4
A
E
89
67
49.9
95.8
1
Advanced Level
Component Entry
Max. Mark
Mean Mark
Grade
Raw Mark
Cumulative %
CH4
908
75
38.7
A
B
C
D
E
52
46
40
34
28
20.5
33.7
47.5
64.4
77.6
CH5
1375
75
38.9
A
B
C
D
E
53
47
41
35
29
21.5
34.8
46.7
60.4
73.1
CH6a
1370
50
26.4
A
E
35
22
15.8
72.6
CH6b
53
103
81.9
A
E
89
61
28.3
96.2
CH6c
1322
103
86.2
A
E
89
61
49.2
97.1
N.B. The marks given above are raw marks and not uniform marks.
2
CHEMISTRY
General Certificate of Education June 2007
Advanced Subsidiary
Paper CH1
Chief Examiner:
M. E. Anthoney Ph.D.
General Comments
2037 candidates sat this paper, a 5% decrease on June 2006, with 1259 sitting the paper for
the first time. 189 sat through the medium of Welsh.
The mean mark was 31.0, very similar to that for January 2007, but 4.6 below the mean for
the previous summer. The highest mark was 63 (out of 66 maximum) and the lowest 1.
The first page of Section A was well answered, with the majority of candidates scoring full
marks, but the second page on moles and redox proved a disaster area for many. Section B
was more of a mixed bag, with the worst answers to be found in the questions on bonding
(7d, 8c and 10d). A significant number of weaker candidates seemed to have learned the
words but not the context in which to apply them. Sadly, the following answer was by no
means atypical:
“Sodium chloride bonds by ionic bonding, the molecules of NaCl having strong
intermolecular forces due to the molecules being polar”.
The paper showed up large differences between centres in the preparedness of their
candidates for this paper, a comment which has been made in previous reports. With some
centres, none of the candidates had progressed much beyond GCSE level, while other
centres had all their candidates scoring high marks across the whole syllabus. There
seemed to be a number of weaker students who were very ill-prepared for AS level.
Section A
Q.1
There was a high proportion of correct answers with, as expected, containing the
same number of neutrons being the commonest error.
Q.2
Well answered, with only a few candidates incorrectly using the 3d level or giving
2,8,8,1.
Q.3
Again well known, though there was evidence of guesswork from some weaker
candidates.
Q.4
Generally well answered, though too many of the diagrams were scruffy. “Dot and
cross” diagrams are not acceptable for electron density distributions.
Q.5
(a)
Very few candidates were able to correctly define the mole, with much
confusion between this definition and those for relative molecular and atomic
masses.
(b)
Only the best candidates were able to do the calculation.
3
Q.6
(a)
Generally well answered, although a few candidates seemed confused as to
which boxes were unshaded.
(b)
Many candidates did not gain the mark because either they did not
understand oxidation numbers or they did not answer the question and
named a compound and not an atom.
Section B
Q.7
Q.8
(a)
(i)
Many students simply stated the name “gamma radiation” which
gained no credit as it is not an explanation.
(ii)
Generally well answered, although a few students gave 12 hours and
not 8 p.m.
(b)
All three parts were well known and most candidates gained full marks,
though several were given benefit of the doubt over some imprecise and
poorly worded answers.
(c)
Generally well answered, although a small number of candidates used
incorrect Ar values. A significant number calculated the empirical formula
correctly but then failed to derive the correct molecular formula. The
saturated solution concentration was well done, although a number of
candidates lost ½ mark through inappropriate truncation.
(d)
Very few candidates gained full marks here; there was a definite lack of
understanding within centres about metallic bonding. As emphasised in the
introduction, there was confusion between atoms, molecules, ions and van
der Waals forces in the metallic bond. Some answers contained all these and
hydrogen bonding as well!
(a)
(i)
A small number of candidates showed transfer of electrons or HO2,
but most diagrams were clear and correct.
(ii)
The VSEPR explanation of the shape was usually satisfactory, and in
some cases over-detailed, although some candidates forgot to
actually give the shape of the water molecule.
(b)
Both parts were well answered. A few candidates failed to specify the
covalent nature of a co-ordinate bond.
(c)
Very poor, with few structured answers and only a very small number of
students gaining 3 marks. Very often, the answers did not refer to water but
were vague, general answers to do with hydrogen bonding. Only a few
students mentioned the relevance to hydrogen bonding of the electron lone
pairs on the oxygen atom.
(d)
(i)
Although most students gained the mark for stating that molecules
gain more energy as temperature increases, very few were able to
follow this through and explain the vapour pressure increase.
(ii)&(iii) Most candidates gained both these marks. In (ii), any curve showing
a higher vapour pressure at all temperatures was allowed.
(e)
A significant number of candidates could not handle these calculations.
4
Q.9
Q.10
(a)
Most candidates gained the flame test mark for calcium. Red is not
acceptable as the colour of a calcium flame, and precipitation of carbonate is
not an acceptable test for calcium. Presumably through confusion with the
hydrolysis of bromoalkanes, a large number of candidates added alkali at the
start of the silver nitrate test. Though totally unnecessary, they were not
penalised provided the solution had clearly been acidified before the addition
of silver nitrate. Only the best candidates included the effect of ammonia on
the silver bromide precipitate.
(b)
Surprisingly, many candidates could not (or did not) give appropriate
observations yet gained credit for correct equations.
(c)
This calculation was done well, with even weaker candidates who had scored
poorly on earlier recall questions getting the correct answer.
(d)
Perhaps the worst answered question on the whole paper. Only the very best
candidates knew about carbonate and hydrogencarbonate.
(a)
Most candidates gained the majority of marks available on this part question.
As expected, weaker candidates resorted to guesswork, and for some
unknown reason argon often seemed to be first choice as a guess.
(b)
Not as well answered as one would expect. Indeed, it was depressing to find
that 30% of all candidates gave NaCl as an example of a covalent chloride.
Bad enough on its own account, but the question on the opposite page is
about the ionic bonding in NaCl!
(c)
Quite well answered, with most errors being due to incorrect balancing of
equations, though a few candidates insisted hydrogen gas was evolved in
both cases.
(d)
As with previous bonding questions, many answers were spoiled by
interchangeable use of ion, atom and molecule. Many students seemed to
believe ionic, covalent and intermolecular bonds are all present in NaCl.
Other common errors were failing to identify Na+ and Cl- ions in the structure
and failing to draw sufficient of the face-centred cube to show the
coordination. More than any other, this question discriminated between
centres, with often either all the candidates from a centre scoring well or all
the candidates giving poor answers. In the case of a few centres, every single
candidate gave the co-ordination number as just 6 rather than 6,6.
5
CYMRAEG
Roedd safonau’r atebion a welwyd yn y papurau cyfrwng Cymraeg a chyfrwng Saesneg yn
debyg eleni. Nid oedd unrhyw arwydd fod cyfrwng y cwestiynau wedi achosi problemau
ychwanegol i unrhyw ymgeiswyr. Roedd cysondeb yn y termau a ddefnyddiwyd a gwelwyd
iaith raenus mewn nifer o’r atebion hirach. Yn anffodus, yn yr atebion ar radicalau rhydd, yng
nghwestiwn 8 ac yng nghwestiwn 11, gwelwyd amrywiaeth sylweddol yn y termau. Roedd y
termeg yn anghyson, ac yn aml yn aneglur neu’n anghywir, gyda disgrifiadau cywir o
radicalau yn ronynnau gydag electronau di-bâr neu ddigymar, ond hefyd disgrifiadau
anghywir yn trafod electronau rhydd, electronau nad ydynt yn bondio neu electronau
ychwanegol. Dylid sicrhau bod y termeg a ddefnyddiwyd yn disgrifio radicalau rhydd yn unig,
ac nid yn berthnasol hefyd i electronau mewn graffit neu fetel (electronau rhydd), mewn
parau unig (electronau nad ydynt yn bondio) neu ïonau negatif (electron ychwanegol).
The standard of written language seen in both English and Welsh medium scripts was
similar this year. There was no indication that the medium of the questions affected the
standard of the answers produced. In general, there was consistency in the use of scientific
terms and the standard of the language used in longer answers was generally good.
Answers discussing free radicals, both in question 8 and 11, produced a wide variety of
correct terminology discussing particles with unpaired or ‘un-partnered’ electrons, but also a
variety of unacceptable descriptions including ‘free’ electrons, ‘unbonded’ electrons and
‘extra’ electrons. It should be ensured that the terminology used when discussing free
radicals is specific to these particles, and not applicable also to electrons in metals and
graphite (‘free’ electrons), lone pairs (‘unbonded’ electrons) or negative ions (extra
electrons).
6
CHEMISTRY
General Certificate of Education June 2007
Advanced Subsidiary
Paper CH2
Principal Examiner:
Mr. Elfed Charles
General Comments
For about four fifths of candidates, this was their first attempt at the CH2 module. There was
no evidence that the paper was too long, and it was felt that the paper gave plenty of
opportunity for weaker candidates to show positive achievement. However, there was an
increase in the number of very weak candidates with 58 candidates scoring 10% or less and
only 17 candidates gaining 90% or more. Many of these weaker scripts had whole questions
unattempted.
The highest mark was 62 and the lowest 0. As expected, Section A was answered fairly
well. In Section B, the most successfully answered question as a whole was Q.8, with Q.11
being the least successfully answered. The easiest parts on the entire paper proved to be
Q.8 (d)(iii), Q.6 and Q.8 (a), while the hardest parts were Q.7, Q.9 (d)(iii) and Q.11 (a)(i) in
that order.
The standard of presentation of answers for many candidates was quite high and these
papers were a pleasure to mark. However, as always for a significant number of candidates,
answers that required detailed responses often lacked depth. This was particularly true
when writing about homologous series (Question 11(a)(i)) and boiling temperatures
(Question 10 (b)).
Some physical chemistry concepts, such as equilibrium constants and Le Chatelier’s
principle, are well understood and the Haber process was well known. Although many
candidates generally did well in the calculation questions, most had difficulty when required
to change kg into g (Question 8e) or J into kJ (Question 9d). Explaining organic terms
(Question 11 (a)), naming organic compounds (Question 1 and Question 10 (a)(iii)) and
writing balanced equations (Question 3) were poorly done and give cause for concern.
Atebion Cyfrwng Cymraeg
Safodd tua 9% o’r ymgeiswyr y papur trwy gyfrwng y Gymraeg. Gwelwyd aeddfedrwydd yn
yr atebion a sylw i fanylder a chywirdeb y disgrifiadau cemegol. Nid oedd iaith y cwestiynau
wedi achosi unrhyw anhawster ac ni welwyd diffygion mawr yn ansawdd yr iaith. Roedd y
camgymeriadau yn debyg iawn i’r rhai a welwyd gan y myfyrwyr Saesneg ond roedd canran
y papurau gwan iawn llawer yn uwch ac o’r herwydd roedd cymedr y papurau Cymraeg tua
3.5 yn llai na’r rhai Saesneg.
7
Section A
The candidates generally scored fairly well in this section, with the average mark being
about 5.5 out of 10.
Q.1
This should have been a fairly easy start for the candidates but fewer than half gave
the correct name of 2,2-dimethylbutane. The commonest mistakes were to omit one
of the ‘2s’ or the ‘di’.
Q.2
(a)
Generally well answered, although a number of candidates used the words
‘atoms’ or ‘ions’ incorrectly or just wrote ‘nucleus loving’ without qualifying
their answer.
(b)
The majority could correctly give a suitable example.
Q.3
Only fairly well answered. Just under half could write a correct balanced equation for
the combustion of octane. A number of candidates did not know that carbon dioxide
and water are the products of this combustion while others did not include oxygen as
a reactant.
Q.4
Part (i), writing an expression for the equilibrium constant, Kp, was well answered.
Part (iii) was fairly well answered. Part (ii), giving a method for following the kinetics,
was not so well answered. Many candidates wanted to measure temperature or they
simply stated apparatus without qualifying why this apparatus was used.
Q.5
Poorly answered. Only about 1 in 4 candidates could correctly define ΔHθ. Many
omitted ‘1 mole’ from the definition.
Q.6
Very well answered. The vast majority correctly chose C as the answer.
Q.7
The worst answered part in the whole paper. Only about 1 in 10 candidates gave a
correct example of a homogeneous catalyst and the process in which it is used. The
commonest answer was vanadium(V) oxide in the Contact Process.
Section B
Q.8
This was the most successfully answered question in this section.
(a)
Very well done, although some candidates thought that nitrogen came from
natural gas.
(b)
Both parts (i) and (ii) were well answered. Le Chatelier’s Principle is well
understood. However, many candidates lost marks by not answering the
question and failing to state what happened to the yield of ammonia or by not
giving a full explanation, merely stating that equilibrium shifts to the left or
right.
(c)
Part (i) was fairly well answered, with over half the candidates giving a correct
answer in terms of activation energy. Part (ii) was poorly answered. Most
candidates failed to give an answer in terms of successful collisions.
8
Q.9
Q.10
(d)
Part (i) was well answered, with around 2 candidates in 5 getting full marks
and only 1 in 10 failing to score. Vague answers such as ‘these were the
optimum conditions for the best yield’ gained no credit. Parts (ii) and (iii) were
very well answered. Obviously, the Haber process is well known.
(e)
Only about 1 in 3 gave the correct answer in part (i). Most candidates failed
to convert kg into g in order to correctly calculate the number of moles. In
part (ii), candidates were not penalised again for failing to convert g into kg
and so over half obtained the mark. Surprisingly, many lost the mark
because they used the molar mass of nitric acid rather than that of
ammonium nitrate.
(a)
Generally very well answered. However a few candidates still do not use
square brackets and others persist in using a ‘+’ sign instead of multiplying
the concentrations.
(b)
Adequately answered but many candidates either failed to give the required
explanation in terms of complete dissociation or gave vague references to the
concentration of H+ ions in solution.
(c)
The ionic equation for the neutralisation of acid with alkali was fairly well
answered. The most common error was to omit the state symbols.
(d)
Part (i), calculating the number of moles, was well done. Part (ii) was poorly
answered, with about half the candidates failing to get any mark. About a
quarter gained both marks. A common error was to use 50 g rather than 100
g as the mass of the solution. In part (iii), only about a half of the candidates
who obtained both marks in part (ii) scored both marks again. The main
errors were not changing to kJ mol-1 or failing to give a negative sign.
(e)
Generally well answered. Some candidates lost the mark by not comparing
the acids.
(f)
In part (i), only about half the candidates gave a satisfactory definition of
Hess’s Law. A lack of clear expression cost many candidates the mark, while
others just wrote H = H1 - H2 without any further explanation. In part (ii),
about 1 in 5 could correctly explain the connection. A significant number
could state the principle of conservation of energy and so gained some credit.
(g)
Well answered. About 60% obtained both marks, while a significant number
gained 1 mark for giving an enthalpy change with the correct magnitude but
the wrong sign.
(a)
(i)
Generally well answered. However, some candidates thought that
cracking was the breaking of ‘long polymer chains’.
(ii)
Generally well done. Some candidates lost a mark by drawing the
structure of the cis-isomer which was given in the stem of the question
and others by sloppily linking the methyl group via the hydrogen to the
alkene chain.
(iii)
Only about 1 in 5 of candidates gained both marks. Naming the
compound was poorly answered; the commonest mistakes were
giving ‘2-bromobutane’ or ‘1,2-dibromobutane’ instead of 2,3dibromobutane.
The reaction type was better known, although a significant number
gave ‘addition’ instead of electrophilic addition.
9
(b)
This question was about boiling temperatures in alkenes and only about half
scored any mark. Many candidates were confused about which bonds were
broken and consequently gave vague answers such as ‘less bonds to break
in ethene therefore it has a lower boiling point’ which gained no credit.
(c)
(i)
Well answered. About three quarters of the candidates gave the
correct structures to A and B. Weaker candidates gave
H
H
H
C
C
H
Ni
H
as the structure to B.
(d)
Q.11
(ii)
Fairly well done. Of the candidates who gave the correct reagent, the
main error was to omit either heating or acidic conditions.
(iii)
Generally well answered, with about 60% of the candidates giving the
correct type of reaction.
Disappointingly answered, with only about a third gaining both marks, while
an equal number failed to obtain a mark. Either many candidates failed to
read the question properly or they did not understand the word 'substituted'
and, too frequently, examples such as ‘polypropene’ and ‘polybutene’ were
seen.
This was the least successfully answered question in this section.
(a)
Many candidates did not express themselves clearly, failing to explain the
terms correctly and losing marks as a result. Also, poor question reading
skills meant that incorrect examples were given, which again resulted in
marks being lost.
(i)
This part of the question was about homologous series. It was poorly
answered, with only a handful of candidates gaining all 3 marks and
about 40% failing to gain any mark. The most common mark was for
correctly giving the general formula for alkanes. Only a small number
gained the mark for stating that alkanes show similar chemical
properties or that there is a gradation in physical properties along the
series.
(ii)
This was about structural isomers and was easily the best answered
part of the question with over 60% getting at least 2 marks. The main
reasons for losing marks were for not giving alkanes as examples or
for failing to state ‘… the same molecular formula’.
(iii)
Again poorly answered. Only about 1 in 10 scored both marks. Most
candidates failed to explain the term ‘substitution reaction’, simply
reiterating the word ‘substituted’. Again, many lost the other mark by
giving an example of nucleophilic substitution rather than using an
alkane.
10
(b)
This part was about the greenhouse effect and it was generally well done with
less than 1 in 5 failing to obtain at least 3 marks out of 6. It seems that the
planet is safe in the future since most candidates knew about the problems
caused by carbon dioxide and how to reduce its build up in the atmosphere.
Unfortunately for the examiners, many wrote at length about this topic, some
using all of the rough work page in addition to the given space! The main
errors were simply to state how carbon dioxide forms in the atmosphere
instead of explaining why there is a build up and failing to explain in detail
why carbon dioxide stops heat energy from escaping.
11
CHEMISTRY
General Certificate of Education June 2007
Advanced Subsidiary
Paper CH3a
Chief Examiner:
P. G. Blake, B.Sc., D. Phil., Formerly Senior Lecturer in Chemistry
University of Wales College of Cardiff.
This penultimate running of the paper appears to have passed off successfully with a
generally good standard of performance by the candidates and a reasonable uniformity
across the questions, with the organic question in particular being better answered than
usual.
Q1
(a)
Plotting of the points in (i) was usually accurate and curve drawing generally
good. In (ii), the question required the plot to be used to calculate the initial
rate. This was not always done and some tangent drawing and slope
calculation was unrealistic with the concept of c/t not always being
understood. Most candidates however scored well here.
(b)
There was some uncertainty here. Any error in (a)(ii) was marked
consequentially. Quite a few candidates were too vague to gain full marks,
using phrases such as “the rate increases”; what was needed was “ the rate
is directly proportional to concentration” in each case or "the rate doubles as
the concentration doubles”.
(c)
Parts (i) to (iii) were generally done well with some vagueness in (i), sound
knowledge in (ii) and a very clear understanding and statement in (iii).
Section (iv) was generally not well-answered, the key point being the 1:1
stoichiometry of reaction as shown by the equation, so that ester and KOH
concentrations are always equal.
Vagueness in answers is a general cause of partial mark loss. This is of
course sometimes due to the candidate making the best of an imperfect
knowledge but, in many cases, it is felt that the candidate does know enough
to gain the marks but fails to put the knowledge onto the paper. Training in
exam technique, careful reading of what the question actually says and an
attitude of regarding the examiner to be an idiot who needs everything spelled
out can all help in this. There are no penalties for writing too much, provided
that statements are not mutually exclusive.
Q2
(a)
An amazingly large number of candidates were unable to write the correct
ionic equation for this complex and esoteric reaction!
(b)
These parts were reasonably well done except for (ii), where acidification is to
prevent the precipitation of silver salts other than the chloride. In (iv), it is
necessary to specify that the impurities removed must be soluble impurities.
12
(c)
Q.3
This short calculation exposed the usual lack of understanding of mole
matters in many candidates, although some were quite happy. The problem,
which is an intractable one, is not mathematical but stems from the difficult
concept of amount of substance that causes trouble at all levels. The
mechanical application of m = nM and triangular diagrams based on this
ought to work but often does not. The general approach is to calculate the
moles of AgCl and thus (Cl) in 25 cm3 and thus the concentration and then
the mass, alternatively the mass of chloride ion can be found first. One
common error was to omit the scaling up from 25 cm3 to 1 dm3.
The marks here were generally good, aided in some cases by the fact that the
compound and observation columns were marked independently. Many candidates
gained full marks on this question. Reasons for mark loss included a failure to specify
a cream precipitate in the A observation and to say for the D observation that a gas is
evolved that turns lime water milky (carbon dioxide evolved is not an observation).
In conclusion, a very satisfactory result with some very high marks and a feeling that
the paper has served a useful purpose in helping in a small way to integrate the
subject.
13
CHEMISTRY
General Certificate of Education June 2007
Advanced Subsidiary
Paper CH3b/c
Chief Examiner:
P. G. Blake, B.Sc., D. Phil., Formerly Senior Lecturer in Chemistry
University of Wales College of Cardiff.
At the end of a successful year in AS coursework it is tempting to reissue last year's report
changing only the date, since most of the points made then are still relevant. Once again
most centers used the 3c route. Perhaps this is not surprising so far as the candidates are
concerned, since each new cohort encounters the same problems, but it also applies to
centre methods. While most centres select and present the work in an excellent manner, in a
few cases the deficiencies highlighted in past reports continue to occur. Either the reports
are not seen and INSETS not attended or they are not acted upon.
Some points are relatively trivial, such as the use of paper clips instead of treasury tags
(these always come off), not using plastic sacks (paper envelopes get ripped open in the
post) and not combining a candidate's two pieces of work and the front sheet (causes
problems for the markers). Old proformas and Teacher Results sheets are still being use to
cause difficulties and, worse, mixtures of old and new sheets.
Others can have a serious effect on the marks obtained. Some Teacher Result Sheets are
incomplete or contain incompatible data and on a few occasions the teacher's results do not
match those of the best candidates, which presents the examiners with a problem.
Magnesium metal is still being weighed on two-place balances in Expts 3A and 3B, giving an
intrinsic uncertainty of 10% on 0.10 g and Expt 4 is being performed with low temperature
changes using 0.5 degree thermometers, causing candidates to lose marks (see
Circular 67).
The most serious problem lies in the use of impure and even inhomogeneous solids in the
Thermochemistry in which 40% of the marks are allocated to the actual T values. While
examiners can allow to some extent for impure but homogeneous solids, nothing can be
done if there is inhomogeneity. The obvious answer is to use new solids (not expensive) or,
at the very least, to examine and test the solid beforehand. One teacher, having obtained a
T of less than 10% of the true value with a rise of 0.2 degrees per gram wrote “incredible
but true”. Incredible indeed!
Having completed its catharsis the report now turns to some general points.
General Points
Candidates are still being penalized for excessive use of pencil. Graphs and odd pages are
sometimes omitted from the package. Instruction sheets and rough work should not be
included.
14
Planning
One mark is allocated for a clear statement of the aim of the experiment at the beginning of
this section. Some plans are excessively long and contain unnecessary drawings of
standard equipment; this is not required and gains no marks.
Implementation and Analysis
These are considered under the specific experiment.
Evaluation
Improvement continues and more objective analysis is seen. Estimation of error need not be a
long process, the main source of error should be focused on and the percentage uncertainty in
this used for the estimate. The main weakness now is in applying this estimate to the results so
that if the error is 1% the result should be quoted to three significant figures, since a result of
1.10 could lie between 1.09 and 1.11 so that any additional numbers are meaningless.
The decimal point/significant figures battle is far from won but improving and is not discussed
yet again here (see Circular 67 for the marking of significant figure errors). Finally, neither the
difference between Class A and Class B burettes nor changes in temperature during titrations
are likely to be significant causes of error at this level.
Individual Experiments
Expts 1a,1b and 2
Results were generally good here, with occasional problems of drift and endpoint detection.
Once again marks were lost by not setting up a titration table with initial, final and difference
readings and not reading the burette to 0.05 cm3.
Care is needed to avoid inversion errors in calculating concentrations in the Analysis section by
simple checking. If more of solution A than B is needed to reach the endpoint then A must be
the weaker.
Expts 3 and 4
Some of the problems here were raised in the introduction. Wide ranges in T/g values between
centres were again found and it is worth repeating the typical good T/g values given last year.
Mg around 95, MgO around 15, MgCO3 2.6, Na2CO3 2.7 and NaHCO3 –3.0.
Values widely different from these suggest that impure materials are being used.
Temperature changes of 7-10 degrees are ideal, especially with a 0.2 degree thermometer and
quantities can be adjusted to ensure adequate precision. Extrapolation can cause difficulties;
these are reduced if insulation is good and mixing rapid when the extrapolated maximum should
not be very different from the temperature reached fairly soon after mixing.
There is still some confusion over signs in two respects: first some candidates state that T is
negative if the temperature rises and, secondly and more understandably, go wrong in relating
the signs of T and H as required in the evaluation. All that is needed is to say that if T is
positive then H is negative. Use of the words exothermic and endothermic appears only to
confuse matters. The full understanding is subtle and not of course needed. What is happening
is that the system of magnesium species and acid are reacting and losing energy to the
surrounding liquid which heats up so that H for the Mg system is negative while T is positive
for the surroundings.
15
Candidates should be very careful in measuring T from their graphs; there are not infrequent
errors. The examiners correct these if they find them, but responsibility lies with the candidate.
Finally, full use should be made of the graph paper, not starting from T = 0, weighing masses
within the range given and recording the masses to the precision of the balance. Even more
finally, bomb calorimeters are not quite the thing for acid-base thermochemistry!
Expts 5 and 6
There were not usually any serious problems here and good results were achieved.
Marks were lost through poor significant figures in the tables for volumes, concentrations or
1/time; in graph plotting poor use of space by starting at 0/0, inverting axes, failure to title graph
or label axes and in A5 not stating clearly that the rate is directly proportional to the
concentration. Too narrow a concentration range or too short a reaction time lost marks and
evaluation was often rather limited.
After this catalogue of problems and omissions, it is a pleasure to end by congratulating
candidates and staff alike on a generally very successful year.
16
CHEMISTRY
General Certificate of Education January 2007
Advanced Level
Paper CH4
Chief Examiner:
D.H.Ballard, B.Sc., Ph.D., C.Chem., F.R.S.C.
Lecturer in Science Education, Nottingham Trent University
General Comments
This paper is largely exclusive to those in the second year of the ‘A’ level course. The cohort
for this paper was similar to previous examinations.
The range of marks gained by the candidates was again very wide, with a few candidates
scoring 70 or more out of 75 and, sadly, around ten candidates gaining scores in single
figures.
Candidates are becoming more used to handling spectroscopy in analytical questions and
this was seen in this paper.
There was little evidence that the paper was too long, although a few candidates gave very
brief answers to questions 4 and 5, but the examiners thought that this was due to
insufficient knowledge rather than a shortage of time.
Many candidates dealt competently with calculations in questions, although some spoilt their
yield calculation by not using relative molecular masses in their response.
Section A
Q.1
(a)
(i)
The term ‘empirical formula’ seems to be poorly understood. Many
responses were seen where the molecular, and even the provided
structural, formula was given.
(ii)
The question stated that heat was given out during the reaction. A
number of candidates then drew the profile for an endothermic
process. The examiners were expecting a profile with a ‘peak’ but a
number of candidates did not give this in their response.
(iii)
I.
It is surprising how many candidates cannot describe a free
radical. Unacceptable descriptions included ‘a lone electron’
and even ‘a pair of unpaired electrons’.
II.
Many candidates described the presence of an O-H absorption
for benzoic acid but then neglected to state that this was not
present in the infrared spectrum of the ester. Candidates
usually described the reaction with a carbonate for the
chemical test.
17
(b)
Q.2
(i)
The chiral centre was usually identified correctly but a number of
candidates spoilt their answers by stating that light was being
polarised in both directions or light was being bent.
(ii)
This question asked for a reagent – either by name or correct formula.
Simply writing H+ / Cr2O72- represents the ions and not the reagents
used. The examiners expected all the aliphatic hydrogen atoms to be
shown on the formula of the intermediate alcohol.
(i)
There was a disappointing response to this recall question; many
candidates had clearly not learnt the industrial preparation of ethanol.
(ii)
Some candidates could not think of a benefit of alcohol! These were
two easy marks.
(iii)
There were some good responses to this question about hydrogen
bonding. Sometimes the lone pair(s) on the oxygen atom were not
given and references to bond strengths / forces were weak.
Candidates are urged to read their answers – it was not unusual to
see water written as O – H – O and the polarisation being given the
wrong way round.
(i)
The test for a phenol was well known.
(ii)
There seemed to be an awful lot of incorrect answers in both this
section and in (iii). The examiners wondered if candidates really knew
what was meant by ‘nucleophilic’ and ‘electrophilic’, even if they
recognised addition and substitution.
(c)
(i)
One of the examiners commented that in 70% of his papers the
graphic formula of ethanamide lacked an oxygen atom.
The
chemistry of carboxylic acid derivatives and of nitrogen compounds
seems to be a weak area, although some good response were seen
for the production of ethanamide from ethanoic acid. The several
different routes were equally acceptable.
(a)
(i)
Many candidates were able to calculate the number of moles of ester
present – but the question led them through the process stage by
stage!
(ii)
To find the relative molecular mass of the ester in (ii) and then to
suggest a graphic formula in (iii) relied on a correct answer to (i) and
many correct responses were seen but, sadly, some were uncertain of
the general structure of an ester.
(a)
(b)
Q.3
(b)
The answer required butan-2-ol and all too often simply butanol was given.
Many candidates stated ‘iodoform’ instead of giving the reagents for the test
as required.
(c)
Some candidates did not realise that two moles of ethene had been produced
and gave C7H16 instead of C5H12. Many candidates, too, did not realise that a
branched isomer was required.
18
Section B
Q.4
(a)
Compound X was butane-2,3-dione (diacetyl) and only stronger candidates
were able to gain all five marks. Although many wrote the structure of the
groups obtained from the NMR data, it was rare to see a statement that all the
protons were equivalent. However, most candidates gained two or more
marks in this discriminating question.
(b)
(i)
The number of candidates who wished to heat the alcohol solution
directly was frightening. Simple applications of practical work seemed
to be a mystery for many.
(iii)
Recrystallisation seems to present a problem for some – for whom
their response seemed like guesswork. For others the steps were
clear and logical.
(i)
Although many candidates gained full credit, too many are still
suggesting that yellow light is emitted.
(ii)
The percentage yield calculation was spoilt for a number of candidates
who simple divided the masses of the two compounds without
reference to their Mr values. Not everybody noticed the need for the
answer to be given to two significant figures.
(c)
Q.5
(d)
Too many candidates confused this question with the atomic emission
spectrum of hydrogen. A clear understanding of the relationships between
energy, frequency and wavelength was not often seen.
(e)
The examiners were ‘generous’ here but still too many candidates did not
make it clear which type of spectroscopy they were using.
(a)
Too many candidates did not give a fully balanced equation and left out
hydrogen chloride.
(b)
This mechanism should have been straightforward but was often not. It was
obvious that some candidates did not understand what they were doing and
made glaring errors. Candidates must be more precise in their arrow
directions.
(c)
There were six marks here and candidates generally gained at least three.
Common errors were using hydrogen cyanide in place of potassium cyanide
and failing to name the nitrile correctly. Most candidates knew how to esterify
pentanoic acid but fewer gave concentrated sulphuric acid as the catalyst.
(d)
The aim of the question was to confirm that each bottle was correctly labelled.
Some candidates tried to identify the compounds as unknowns, which was
not required. A few candidates used the iodoform test to confirm pentan-3one, instead of 2,4-dinitrophenylhydrazine. The identification of the amine
was troublesome; some added sodium hydroxide and identified ammonia!
(e)
Many candidates gained full credit here for correctly identifying possible
fragments and deducing that the compound was likely to be
methoxymethane. The examiners were pleased with the connection between
mass spectroscopy and structure which many candidates were able to make
in this question.
19
CHEMISTRY
General Certificate of Education June 2007
Advanced Level
Paper CH5
Chief Examiner:
D.H.Ballard, B.Sc., Ph.D., C.Chem., F.R.S.C.
Lecturer in Science Education, Nottingham Trent University
General Comments
This paper is taken, almost exclusively, by candidates during the second year of the‘A’ level
course and the cohort for this paper was similar to previous examinations.
The range of marks gained by the candidates was again very wide with a few candidates
scoring 70 or more out of 75. Sadly, more than twenty candidates failed to gain double
figures.
Inorganic chemistry is a very broad field, and requires knowledge of the observations and
patterns apparent in chemical properties and reactions of elements and their compounds
from all parts of the Periodic Table, and the explanations for these. To master this broad
subject a detailed and intensive study is required, which is more than can be achieved within
the restrictions of the hours spent in class – significant independent effort and study is
required, and the lack of this was apparent in many candidates who did not attain the
expected standard.
An obvious weakness was in writing balanced chemical equations, and even more
worryingly in writing simple chemical formulae. This is an essential skill for chemistry and
candidates who had not mastered this skill were penalised throughout the paper.
Calculations were generally well performed, although many candidates did not give much
detail in their method, which led them to lose significant marks if their answers were
incorrect. It is sensible therefore to give a fair amount of detail in their working. There
seemed to be little appreciation of the feasibility of numerical answers, and whether a value
obtained was realistic. This did not allow candidates to notice errors in their calculations and
correct them. Few questions tested candidates handling of significant figures, although their
use of units was lacking in some questions, with the units absent or the incorrect units used.
Section A
Q.1
(i)
This question was answered well; a few weaker candidates were unable to
distinguish between the electronic configurations of an Fe atom and an Fe3+
ion.
(ii)
Most candidates were able to identify the arrangements of six ligands around
the central atom. However, the diagrammatic representations frequently
failed to indicate the three dimensional nature of the complex. A structure that
was indicative of a hexagonal planar structure was presented, even amongst
some of the better candidates. The examiners felt that the 3d dimensional
arrangement of the ions must be made clear.
20
Q.2
(iii)
Most candidates gained at least 2 marks on this question. However many did
not indicate the 2/3 split of d-orbitals and far too many candidates from some
centres thought that emission of light was the origin of colour in these
complexes. This is a continuing problem.
(iv)
The red-brown precipitate of Fe(OH)3 should have been very familiar to
candidates, but some candidates described the precipitate as white or green.
A number of candidates, surprisingly, considered Fe3+ to be amphoteric and
described the precipitate as dissolving in an excess of aqueous sodium
hydroxide. The required equation was not always balanced.
(v)
Most candidates gained 2 marks on this question, although it is apparent that
some candidates confused their CH4 and CH5 studies and thought that
hydrolysis with NaOH was required as a preliminary step in the reaction.
When the described test did not include neutralisation after adding sodium
hydroxide, no marks could be awarded as the response was ambiguous.
(a)
(i)
This was meant to be an easy question but proved to be less than
straightforward.
(ii)
Most candidates were able to gain some marks from this question,
and many were able to gain full credit. Apart from frequent
mathematical errors, particularly in the handling of scientific notation,
the most common errors were in the initial calculation of [H+], with
some candidates giving [H+] = 11.5 and others calculating [H+] = 1011.5
rather than [H+] = 10-11.5.
Where answers were incorrect, few candidates appreciated that
answers such as 3.16 x 10-26 or 3.16 x 103 were not possible as
values for the concentration of hydroxide ions.
(b)
(i)
It was pleasing to see so many correct answers to this question, with
few candidates falling for the usual error of including square brackets.
(ii)
This was, in general, a poorly done question. Many candidates could
identify the lack of units for the equilibrium constant, although this
needed to be specified to gain the mark.
Few could state that the partial pressure of carbon dioxide at
equilibrium was 1150 kPa. The calculation was not even attempted by
many, and many others simply gave an expression for Kp with
incorrect values inserted without indicating what each value was
supposed to represent, or how these values were obtained. Neither of
these gained any credit. Few candidates succeeded in gaining the
correct final answer of 132(.25), although those who showed all their
working on the way to an incorrect answer frequently managed to gain
some marks.
(iii)
A well answered question where the vast majority of candidates were
aware of the effects of pressure on the rate of the reaction. A few
spoilt their answer by stating that the yield was changed, even though
it denied this in the question stem.
21
(c)
Q.3
(i)
Candidates who correctly identified the rate equation generally
completed the question with full marks. A significant number believed
that the rate equation must follow the stoichiometric equation and
incorrectly gave the equation as rate = k[H2][2ICl] or rate = k[H2][ICl]2.
This is a fundamental error and shows a lack of basic knowledge in
reaction kinetics.
(ii)
This proved challenging for many candidates and again showed the
lack of understanding in this topic. Many candidates started the first
equation incorrectly, and even those that started correctly generally
gave equations which were not balanced in terms of charge. The
second equation, in (iii), often did not follow from the first.
(a)
This question provided many opportunities to obtain marks in both the
observations and explanations. Most candidates gave some observations and
could name some products, but few gained full marks for a sufficiently
complete answer. The examiners felt that words should complement any
equations and that candidates should not simply rely on formulae in equations
to gain marks.
(b)
(i)
This was generally well answered, with many candidates gaining full
credit. Weaker candidates did not calculate the number of moles of I2
correctly, although most identified the 1:4 ratio of arsenic(III) oxide to
iodine.
(ii)
Most candidates gained some marks here, but many did not give their
answer to the required number of significant figures and were
penalised for this. Some candidates failed to multiply their answer to
(i) by 10.
(c)
Answers to this question were frequently unclear and incomplete. Most
disappointing was the frequency of answers that described the structure of
arsine as trigonal planar or trigonal bipyramidal when the structure was
shown in the question. This question is really an application of the reasons
for the shape of gaseous ammonia but many candidates could not apply this
to the question.
Section B
Q.4
(a)
(i)
Nearly every candidate gained this mark.
(ii)
Whilst candidates that realised that a base was needed produced well
structured answers to this question, they frequently neglected to
describe the filtration required to finally separate the two compounds.
Candidates who did not suggest bases frequently produced extended
answers based on adding acid or on electrolysis, neither of which
gained any credit.
22
(b)
(c)
(d)
(e)
(i)
It was surprising that a substantial minority of the candidates did not
know the formula for aluminium oxide, and even some of those who
did could not balance the equation.
(ii)
Generally poorly answered. A comparison of the electronegativities of
aluminium and oxygen was required and fewer than half of the
candidates included this. Even those that did include the first point
frequently did not follow through to discuss electron transfer, merely
the relative attraction of electrons in a covalent bond. Candidates are
advised to read through the question very carefully and to answer
what is required rather than what they think should be answered.
(i)
In both parts (i) and (ii) the questions asked ‘State the name of…’
which seemed to be missed by the vast majority of candidates who
gave chemical formulae. On this occasion, correct formulae were
given credit but the examiners will be loathe to do this in future. Iodine
was a far too frequent incorrect answer here and appeared more often
than the expected answer of potassium iodide, although any soluble
iodide was accepted.
(ii)
Lead(II) chloride was the correct answer, however many candidates
lost marks by giving an answer of lead chloride without indicating
which chloride of lead was present.
(iii)
Most candidates gave the correct ionic equation and stated that
lead(II) hydroxide dissolved in excess aqueous sodium hydroxide but
few stated that a colourless solution resulted.
(iv)
Many candidates obtained the correct formula of the oxide of lead but
(correct) working was needed for full mark credit.
(v)
Some gave PbCl4 as the product.
(i)
This question was poorly answered, with many candidates producing
answers that were unrealistic without appreciating that their answers
were unlikely or impossible. Few candidates produced complete
correct answers but those candidates that gave their complete
working frequently gained some credit for correct steps.
(ii)
Almost all candidates could identify the oxidation states of the
elements involved in this reaction but some still record these as
charges (2+ or 5+) rather than oxidation states and were penalised for
this. Many candidates incorrectly gave carbon as the reducing agent.
Although carbon is the atom oxidised, the reducing agent is the
carbon monoxide molecule.
Surprisingly poorly answered, with a number of equations being unbalanced
or otherwise incorrect. The most common correct equation was the reaction
of iron(III) oxide with carbon monoxide. Very surprising was the number of
candidates that suggested carbon monoxide as a reducing agent for
aluminium oxide, calcium oxide or sodium oxide. The use of electrolysis for
the extraction of aluminium and the reasons for this should be familiar to
candidates at this level.
23
Q.5
(a)
(i)
Most candidates identified thymolphthalein as a suitable indicator but
the reasons were frequently poorly explained.
(ii)
A poorly answered question. Only the best candidates identified
sodium ethanoate as being the only species present upon addition of
25 cm3 of NaOH, therefore the pH is 9. Many explanations were based
upon the end point with little additional detail.
(iii)
I.
Long prose explanations were frequent here, relating the
enthalpy terms mentioned to ionic charge or size, but did not
relate them to each other. A simple expression such as:
enthalpy of solution = lattice breaking enthalpy + hydration enthalpy
was all that was required.
II.
(iv)
(b)
This was an example of a question of application set in a
different way. It revealed that a number of candidates do not
understand clearly the principles involved. Poor answers for
what should have been a straightforward question. Many
candidates attempted to use Energy = -m x C x T, when a
simple application of proportion was all that was required.
Very few candidates succeeded in gaining the full mark for this
question. Most candidates attempted answers based on hydrogen
bonding and received little credit for them. Even where answers were
relevant, the discussion of ‘forces between the atoms’ or ‘forces
between the molecules’, rather than ‘forces between the ions’,
rendered the answers incorrect. The expected discussion of the
relative sizes of the ions was found in only a few papers.
The thermal decomposition of the nitrates of sodium and calcium is one of the
significant differences between Group I and Group II compounds and has
been included in many of the CH5 papers over the last few years. As such, it
is surprising that many candidates still do not know the essential points
involved. Credit was given for expected observations, identification of the
products, explanation of the differences and balanced chemical equations,
with many more marks available than the five but only the very best
candidates attained full marks on the question.
Many candidates could not produce balanced equations for one or both of the
decompositions, (although these were not required to gain full credit) even if
they could identify the products of the reactions. A common incorrect product
suggested was N2 gas, which was disappointing in its frequency. It was also
very disappointing to see the numbers of candidates who could not write the
correct formulae for sodium nitrate, sodium nitrite, calcium oxide, calcium
nitrate and sodium oxide, although the last only appeared in incorrect
answers. Many candidates could not name NaNO2 correctly with sodium
nitride and sodium nitrate appearing in many scripts. Nitrate is considered to
be a reference to nitrate(V) rather than nitrate(III) unless otherwise stated.
Few candidates gave any significant observations, with the brown colour of
NO2 being unfamiliar to many. Where observations were given, it was obvious
that candidates were clutching at the simplest examples, such as ‘bubbling’ or
a ‘white precipitate’ which are more reminiscent of solution reactions.
24
The greater stability of NaNO3 was not commonly known, and explanations of
the order of stability were frequently based on the numbers of electrons lost to
form the cation, which is not relevant.
This was a question that required candidates to express themselves in writing
and giving a few equations and little writing was rewarded with few marks.
Overall, this question did not gain the standard of responses expected at the
end of a two year A-level course.
(c)
(i)
A well answered question and it was rare to find a candidate that did
not know that hydrogen was the standard for electrochemical
measurements.
(ii)
The great majority of candidates knew that zinc would be oxidised in
this reaction but a few left the 2e- on both sides of the equation.
Candidates who did not read the question, or had a surprisingly poor
understanding of chemical symbols, gave the reaction between zinc
and copper ions.
(iii)
Most candidates who attempted this question obtained the correct
answer, although a few obtained an answer of -0.36V by incorrectly
arranging the calculation; this did not obtain credit.
(iv)
It was uncommon to find candidates that did not predict that this
reaction was feasible and many gave an explanation based either on
a calculated EMF for the cell being positive or on the
oxidizing/reducing power of the half-cells.
Unfortunately, once again the concept of a cell EMF needing to be
above 0.3V or 0.4V for a reaction to take place and that reactions with
small positive EMF values will not occur. This idea was found for
candidates from more than one centre and was penalised.
(d)
The final question on the paper was a straightforward balancing of a chemical
equation. It was not answered as well as could be expected due to problems
identifying the second product of the reaction. The work studied in this
module, and the fact that no reduction or oxidation occurred, showed that the
second product must have been HCl. Unfortunately, weaker candidates
suggested Cl2 or even H2 or O2 as amongst the products, all of which would
require reduction or oxidation.
25
CEMEG CH5 HAF 2007
ADRODDIAD AR Y PAPURAU CYFRWNG CYMRAEG
Mae’r oes pan oedd pobl ifanc Cymru’n cael trafferthion wrth ymgodymu a’r ieithwedd a
ddefnyddir mewn papurau arholiad Cemeg UG ac A2 wedi hen ddiflannu a da o beth hynny.
Roedd y mwyafrif o ymgeiswyr y papur CH5 hwn yn deall y cwestiynau ac yn defnyddio iaith
goeth a chyfoethog wrth gynnig atebion. Diddorol fyddai gwybod faint o’r ymgeiswyr a fu’n
ymdrin â thestunau Cemegol astrus trwy’r Gymraeg a ddechreuodd eu gyrfa ysgol fel
Dysgwyr o’r Iaith. Clôd arbennig iddynt hwy ac i’w hathrawon gweithgar.
Y prif wersi i ddysgu i’r dyfodol fydd:
CW.1 Mae'n bwysig cofio bod Cemeg yn parhau’n bwnc ymarferol. Roedd llawer gormod o
bobl yn colli marciau ar y cwestiynau arsyllol (e.e. 1(iv); 3(a); 5(b)) ac roedd yn
amlwg o’r atebion fod yr ymgeiswyr erioed wedi gweld yr adweithiau dan sylw. Yn
anffodus,roedd y broblem hon yn fwy amlwg ymysg y canolfannau cyfrwng Cymraeg.
CW.2 Wrth wneud gwaith cyfrifo gwelwyd nifer fawr o ymgeiswyr yn hepgor iaith yn
gyfangwbl ac yn cyflwyno atebion ar ffurf rhifau yn unig. (e.e. 2(a)(ii); 2(b)(i); 3(b)(i) ).
Er mwyn i’r arholwyr ddeall yr atebion a gynigir rhaid esbonio’r camau a gymerwyd
ac yn hynny mae iaith yn holl bwysig.
CW.3 Gwelwyd ymysg rhai o’r atebion i’r cwestiynau ‘penagored’ (Cwestiynau 4 a 5) y
tueddiad gan rai ymgeiswyr i gyflwyno llithoedd hir gyda’r ateb cywir wedi’i gladdu
yng nghorff yr ysgrif. Dylid annog myfyrwyr i ganolbwyntio ar y prif bwyntiau pwysig
wrth lunio atebion ac i osgoi hir-hoedledd. Fe fyddai’n haws dyfarnu’r marciau ac fe
fyddai’r myfyrwyr eu hunain yn cael mwy o amser i ateb cwestiynau eraill yn llawn.
Gwelwyd ymysg rhai o’r ymgeiswyr cyfrwng Cymraeg fod rhannau o Gwestiwn 4 yn
gorchuddio tudalennau benbwygilydd o’r llyfryn ateb tra roedd Cwestiwn 5 wedi’i ateb
yn frysiog a hyn yn awgrymu fod y can munud yn brysur ddirwyn i ben a dim digon o
amser i orffen y papur.
Roedd y CH5 hwn yn ymestyn y myfyrwyr yn dda ac yn cynnig cyfleoedd iddynt i serennu.
Bydded i fyfyrwyr y dyfodol dalu sylw i’r sylwadau hyn wrth baratoi ar gyfer y papurau sydd i
ddod.
which translates to
CHEMISTRY CH5 SUMMER 2007
A REPORT ON THE WELSH LANGUAGE PAPERS
There was a time when the young people of Wales experienced difficulties with the
terminology used in the Welsh Language versions of their AS and A2 exam papers, but
thankfully those days are long past and this is certainly good news.. The majority of the
candidates who sat this CH5 paper showed a clear understanding of the questions as asked
and responded using a rich and mature standard of language. It would be of interest to know
how many of the young people, who tackled the complex concepts raised in this paper
through the medium of Welsh, actually started their school careers as Welsh Learners. They
are to be especially congratulated with full praise for their hard-working teachers.
The main lessons for the future are::
26
It is important to remember that Chemistry is still a practical subject. Far too many people
lost marks on the observation questions (e.g.. 1(iv); 3(a); 5(b)) where it was obvious that the
candidates had not seen the reactions they were being asked to describe. Strangely, and
unfortunately this problem seemed more marked in the Welsh Medium Centres.
During the questions involving calculations, an appreciable number of candidates seemed to
discard all aspects of language and to resort to the use of numbers only. (e.g. 2(a)(ii); 2(b)(i);
3(b)(i) ). In order that the examiners understand the points being made in answers, the steps
taken must be explained and this obviously involves making use of language skills.
Amongst the answers to the ‘open answer’ questions (Questions 4 and 5) there were far too
many examples of candidates producing long, extended prose with the required answer very
often buried within the verbage. Students should be encouraged to focus on the main points
of their argument and to avoid lengthy dissertations. Marks could then be more easily
awarded and the students themselves would have more time to concentrate fully on other
questions. It was noted that some Welsh Medium candidates were filling pages and pages
with their answers to Q4 and then obviously rushing through Q5 because their 100 minutes
was drawing to a close and they had not left themselves enough time to complete the paper.
This CH5 paper gave all candidates the opportunity to show their knowledge and ability. We
ask that the students of the future take note of these observations as they prepare for the
exam papers yet to come..
27
CHEMISTRY
General Certificate of Education June 2007
Advanced Subsidary
Paper CH6a
Chief Examiner:
M. E. Anthoney Ph.D.
General Comments
1370 candidates sat this paper, of whom only 36 were resitting. 108 sat through the medium
of Welsh.
The mean mark was 26.4, slightly above the mean for the previous summer. The lowest
mark was 4 (out of 50 maximum) and one individual scored full marks on the paper.
Many candidates seemed ill at ease with the synoptic nature of the paper, with questions
which might have appeared on one of the earlier individual papers generally attracting better
answers than those that cut across boundaries. Considering this paper came at the end of at
least two years of advanced chemistry for the candidates, the number of elementary errors
was quite disturbing.
Section A
Q.1
(a)
Generally well done, with most students able to correctly identify the two
yellow compounds, although a few incorrectly selected sodium iodide in place
of lead iodide.
(b)
Although iodoform was chosen by most, less than half the candidates
correctly named a second non-ionic compound, with the most popular
incorrect answer being aminoethanoic acid as candidates forgot the formation
of zwitterions.
(c)
Methyl ketone was well known as the group identified in the iodoform test,
with omission of the methyl component being the most frequent error.
(d)
This question was very poorly answered, with only the best candidates
recognising that sodium iodide solution should be mixed with a soluble
lead(II) salt to precipitate lead iodide. Many did not refer to solutions
whatsoever, whilst others selected lead compounds that were insoluble in
water. More fanciful suggestions, such as precipitating sodium metal from
aqueous solution by displacement with lead or fractional distillation of a lead
chloride / sodium iodide mixture, were not uncommon.
(e)
Only the best candidates correctly identified dehydration on heating as the
correct method. Some appreciated that water had to be lost but chose
inappropriate chemical dehydrating agents. Reduction with LiAlH4 was
another popular but incorrect answer.
(f)
Most candidates were able to identify two chemicals that reacted with sodium
hydroxide, but then often confused observations and naming products, e.g.,
there
were
references
to
ammonia
being
produced
from
ethanamide/ammonium ethanoate but unfortunately no description of any
kind of observation. Incorrect formulae and names lost many marks, and
though most candidates could offer a few correct answers, scores of three or
more marks out of four were few and far between.
28
Section B
Q.2
Generally well answered. Incorrect answers included a double bond between the
oxygens or omitting the lone pairs of electrons on the oxygen atoms.
(b)
(c)
(i)
Many correct answers, and even those who forgot to multiply each
enthalpy by two in the initial stages of the calculations still managed to
pick up a consequential mark.
(ii)
Most candidates were able to link enzyme catalysis and the
production of oxygen gas, though some just achieved ½ mark by only
mentioning one of the two points.
(i)
Not many obtained full marks here. Many were awarded 1 mark
consequentially for +288kJ mol-1 as they had forgotten to multiply the
O-O bond by 2 in their calculation. Other typical mistakes were using
o
“bonds formed – bonds broken” without utilizing the H value of -210
at all, and incorrect counting of the number of bonds either broken or
formed. In a few worst scenarios, the examiner could not award any
marks because it was impossible to follow the student’s attempt at
calculations.
(d)
(e)
(f)
(ii)
Despite wording that frequently could best be described as vague, a
majority of candidates managed to get the point across that the O-O
bond was broken in preference to the O-H as it was somehow weaker,
but a significant minority believed the production of OH radicals was
somehow the important factor.
(i)
Hydrogen peroxide was generally recognized as being the strongest
oxidizing agent because it had the most positive electrode potential,
but very few went on to secure the second mark by discussing the
ease with which electrons are gained.
(ii)
Well answered, with the only significant errors being incorrect
combination of the electrode potentials and putting both reduced
forms on the same side of the overall redox equation.
(i)
Chromophore was well known, with “double bonds”, “azo dyes” and
“pigments” being the most common wrong answers.
(ii)
Most candidates gained ½ mark for hydrogen peroxide breaking the
double bonds but it was necessary to link the chromophore and
double bond to get a full mark.
The concentration calculation was usually well done but some candidates
were penalised for not giving 3 significant figures.
29
Section C
Q.3
Q.4
(a)
(i)
Some very good answers here with many candidates obtaining 3 or
more marks by correctly identifying methylbenzene. Failure to utilise
ALL the information provided and incorrect reagents / conditions for
oxidation to benzenecarboxylic acid caused most problems. The latter
produced some strange suggestions along the lines of “using acidified
permanganate in alkali”
(ii)
Many good descriptions of recrystallisation and marks of 2½ and 3
were not uncommon. Loss of marks was mainly due to candidates
carelessly skipping one or two steps.
(b)
Most candidates got at least some parts of this section correct, but
decarboxylation was hardly known in some centres and phenol was a
frequent wrong answer for the product of the reduction of benzenecarboxylic
acid.
(c)
Many good answers but also the usual quota of mathematical howlers.
(a)
Most candidates scored at least one mark, but the common error of using the
term “lacking a full outer shell” in place of “lacking a full outer octet” of
electrons was penalised .
(b)
(i)
Although most candidates recognised dative bond formation, the two
atoms involved were not always identified.
(ii)
Quite well answered, with many of the mistakes deriving from
incorrect terminology, e.g., “trigonal biplanar” rather than lack of
understanding of the shapes.
(c)
(d)
(i)
Almost all candidates scored at least one mark, with “cheaper” and
“safer” predominant amongst the suggested advantages.
(ii)
Although “addition” would secure the mark, a number of candidates
spoiled their answer by attempting too much detail, e.g., “nucleophilic
addition”.
(iii)
Although the correct halogenation / chlorination of benzene was the
most popular answer, there was a wide variety of wrong preparations
quoted, from industrial processes such as Haber and Contact to
aspirin to polymerisation to nitration of benzene and beyond!
(i)
Many candidates mentioned endothermic rather than exothermic, and
even some of those that correctly wrote exothermic gained no credit
as they were unable to explain the reason why .
(ii)
Marked consequentially on (i) and generally better answered. Dropped
marks tended to come from candidates correctly stating the conditions
without explanation.
(iii)
The connection between pressure change and the number of moles of
gas was well known. The best answered of the three parts in (d).
30
Adroddiad CH6a 2007
Roedd safon y Gymraeg ysgrifenedig yn dda ac nid oedd tystiolaeth o unrhyw ddryswch
oherwydd y termau Cymraeg. Roedd y cryfderau a`r diffygion yn y papurau cyfrwng
Cymraeg yn debyg iawn i`r papurau cyfrwng Saesneg. Braf oedd gweld, ar y cyfan, y safon
dda mewn atebion mathemategol, gyda mwyafrif yr ymgeiswyr yn gweithio tuag at y nifer o
ffigurau arwyddocaol priodol.
Achosodd y cwestiwn cyntaf y mwyaf o anhawster, yn enwedig gyda pharatoi sampl pur o
blwm(II) iodid. Siomedig i weld bod rhai ymgeiswyr ar ddiwedd y cwrs dal heb sylweddoli
bod `arsylwadau` yn golygu disgrifiad o beth a welwyd, fel `eferwi` yn hytrach na `nwy yn
cael ei greu`. Dangoswyd dealltwriaeth dda o gynnwys y darn `Hydrogen Perocsid`yn yr
atebion, heblaw c) ii) ynglŷn â`r rheswm mai`r bond O-O sydd haws ei dorri pan fydd H2O2
yn adweithio . Roedd gwaith `galw i gof`` organig yn dda o safbwynt y math o adwaith, ond
prin oedd yr ymgeiswyr yn gallu enwi'r adweithyddion i drawsnewid bensen i asid
bensencarbocsilig. Ar ddiwedd y papur, roedd cymysgedd gan rhai ymgeiswyr wedi
cymysgu rhwng syniad `electron-ddiffygiol` ac `ehangu`r octawd`.
Does dim tystiolaeth nad oedd gan yr ymgeiswyr ddigon o amser i orffen y papur.
31
CHEMISTRY
General Certificate of Education June 2007
Advanced Level
Paper CH6b/c
Chief Examiner:
P. G. Blake, B.Sc., D. Phil., Formerly Senior Lecturer in Chemistry
University of Wales College of Cardiff.
This year’s A2 coursework appears to have proceeded very well with some excellent work;
once again most centres followed the 6c route.
General Comments
I3, O3, I1 and O1 were again the most popular experiments with I2, I4 and O2 and O7 also
figuring in the list.
In general the candidates work was very well-presented by the centres, making things much
easier for the examiners and Teacher Result sheets were usually unambiguous, although
not without arithmetical errors in a few cases. Both experiments should be secured together
along with the cover sheet.
Few centres are now restricted in choice by the non-availability of balances weighing to
three places or better.
Candidates should not all have the same unknowns in I3, I4, O3 and O4 in order to avoid the
risk of diffusion of information and it is of course crucial for the examiner to be informed
unambiguously as to who has which set.
Some oxidation numbers are recorded incorrectly, notably 2- or 3+, inferring an ionic charge,
instead of –2 or –II or +3 or +III. A plus sign is not essential for positive states but is probably
a good idea.
Some organic yields are still much lower than the norm and it is worth repeating last year’s
statement that probably too much solvent is used during recrystallisation in trying to dissolve
minor amounts of insoluble impurities. When most of the crude solid has dissolved and a
little more hot solvent has no effect, filtration and crystallization should be proceeded with.
Excessive truncation of data continues to lose marks for some candidates.
Individual Experiments
I1
Some very good work was seen in this quite testing experiment in which titration
skills are fully tested. Marks for the questions were often weak with answers to Qs1,
2, 4, and 6 often being too vague to gain full marks. It proved to be rather difficult to
gain the three marks in Q10.
I2
Results and the calculations and responses to questions were usually good. One
centre reported that it suspected that the calcium salt was inhomogeneous. This is
quite unsatisfactory from the standpoint of assessment by the examiner and should
have been checked in advance.
32
I3
Identifications were usually correct but marks were lost through failure to give a clear
plan of action and through an omission to provide equations. The synoptic
component here comprises writing equations for observable reactions and has 14
marks with penalties for writing equations that do not occur. Some candidates did not
give any equations and thus lost these marks.
I4
This was chosen by a fairly small number of candidates and gave no particular
problems.
I5
This quantitative experiment gave generally good results. It is important to maintain
the part A titration at 70 degrees to obtain consistent values, take care not to
overshoot because of the 5:1 stoichiometry and to add iodide to each sample
immediately before part B to avoid Mn(II)-catalysed air oxidation.
I6/I7
Little used
O1
A popular experiment with some use of the B experiment this year. In the A
experiment it is important to follow the procedure carefully, especially the paragraph
starting “At this stage patience is called for” in order of obtain a good yield. Weak
points in the questions were again the equation in Q1 and Q3 and in Q7 where both
tests must be both described and explained for full marks. Still problems with Test 4,
where the buff-coloured iron benzoate precipitate first produced dissolves in acid and
benzoic acid precipitates.
O2
No particular points to raise
O3
This good test of organic chemistry produced many excellent reports. As in I3 there
must be a real plan of action and not a list of organic reactions. Again, many marks
were lost in some cases by omitting equations and formulae for the reactions taking
place. These must be specific to the actual compound and not written for the group,
i.e., RCHO. Physical properties are useful supporting evidence and not primary
tests.
O4
Little used
O5/O6/O7
No specific comments
Conclusion
All in all a very good year’s work with most A2 candidates showing considerable ability in
their coursework.
Chemistry (GCE) Examiners Report (Summer 2007)ED
33
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