GCE A2 CHEMISTRY Teachers' Guide 1
Contents
GCE A2 CHEMISTRY
Teachers' Guide
Page
1.
Introduction
1.1 - Overview of New Specification
1.2 - Rationale
1.3 - Changes for teaching from September 2008
4
5
6
2.
Internal Assessment of practical skills, CH6
13
3.
Contributors to the Teachers' Guide
21
GCE A2 CHEMISTRY Teachers' Guide 3
1.
INTRODUCTION
The WJEC A2 CHEMISTRY specification has been modified and updated for delivery
from September 2009. The first AS awards will be made in Summer 2009 and the
first A level awards in summer 2010. For the first availability of units, see page 2 of
the specification. The specification can be delivered and assessed in centres
throughout the UK.
The revised QCA subject criteria for GCE CHEMISTRY require 3 units for A2, one of
which will take the form of internal assessment of practical skills.
This Guide is one of a number of ways in which the WJEC provides assistance to
teachers delivering the new specification. Also essential to its introduction are the
Specimen Assessment Materials (question papers and marking schemes) and
professional development (CPD) conferences.
Other useful provision:
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Examiners' reports on each examinations series
Free access to past question papers via the WJEC secure website
Easy access to specification and other key documents on main website
On-line examination review
Easy access to both the Subject Officer and to administrative sections
Contact Points for GCE Chemistry are as follows:
Jonathan Owen
(Subject Officer)
jonathan.owen@wjec.co.uk
Matthew Roberts
(Subject Support Officer)
matthew.roberts@wjec.co.uk
Subject page
www.wjec.co.uk
GCE A2 CHEMISTRY Teachers' Guide 4
1.1
Overview of the A2 Specification
UNIT CH4 – Spectroscopy and Organic Chemistry (Analysing and building
molecules)
TOPIC 9
Spectroscopy
TOPIC 10
Isomerism and aromaticity
TOPIC 11
11.1
11.2
11.3
Organic compounds containing oxygen
Alcohols and phenol
Aldehydes and ketones
Carboxylic acid and derivatives
TOPIC 12
Organic compounds containing nitrogen
TOPIC 13
Organic synthesis and analysis
TOPIC 14
The process of how science works
UNIT CH5 – Physical and Inorganic Chemistry
TOPIC 15
15.1
15.2
15.3
Redox
Redox and standard electrode potential
Redox reactions
Applications
TOPIC 16
16.1
16.2
16.3
16.4
Chemistry of the p block
General
Group 3 (13)
Group 4 (14)
Group 7 (17)
TOPIC 17
d Block transition elements
TOPIC 18
Chemical kinetics
TOPIC 19
19.1
19.2
Energy changes
Enthalpy changes for solids and solutions
Entropy and feasibility of reactions
TOPIC 20
20.1
20.2
Equilibria
General Equilibria
Acid-Base Equilibria
UNIT CH6 – Assessment of Practical Skills
GCE A2 CHEMISTRY Teachers' Guide 5
1.2
Rationale
The division of content in the A2 units has remained essentially the same as in the
previous specification.
The CH4 unit contains advanced level organic chemistry and associated
spectroscopy.
The CH5 unit contains advanced level inorganic and physical chemistry.
The CH6 unit assesses practical skills acquired throughout the course via the
completion of two exercises.
One must be based on inorganic chemistry and its associated physical chemistry.
The other must be based on organic chemistry and its associated physical
chemistry.
One must involve preparation and some analysis of a compound. The other may
involve the identification of some unknowns through a planned procedure.
Thus, students will be exposed to a wide range of meaningful practical techniques.
A CH4 paper was set for the January series from 2010 to 2014 but this will not be the
case for the remainder of the lifetime of this specification. From the academic year
2014-15 all A2 units will be available for summer entry only.
GCE A2 CHEMISTRY Teachers' Guide 6
1.3
Changes to the specification for delivery in September 2009
Changes in Assessment Structure for A2
(a)
Changes to Assessment Objectives
The assessment objectives are listed on page 9 of the specification. A
simplified interpretation of these is as follows:
•
•
•
(b)
AO1 – k + u
AO2 – applying k + u
AO3 – prac skills (How Science Works)
Changes to Assessment units
There are 3 units for A2. Each assessment unit must now test all three AOs
UNIT
%of AS
UM
AO1
AO2
AO3
120
Raw
mark
80
CH4
40
25
45
10
CH5
40
120
80
25
45
10
CH6
20
60
60
6
6
48
Note that AO2 forms a greater proportion of the total raw mark for the CH4 and CH5
papers compared to CH1 and CH2.
No split units are allowed, so that the CH6a paper (the so-called synoptic paper) of
the previous specification will cease to exist in the new specification.
Changes in Content
These have arisen in the following two ways:
(a)
60% is specified by QCA criteria and this has resulted in
•
•
•
(b)
some topics moving from AS to A2
some topics moving from A2 to AS
some new topics
The opportunity to delete unnecessary recall from the previous specification
at the discretion of the examining team.
GCE A2 CHEMISTRY Teachers' Guide 7
Details of changes in content
The following indicates new learning outcomes and some deletions.
UNIT CH4 – Spectroscopy and Organic Chemistry (Analysing and building molecules)
TOPIC 9
Spectroscopy
All statements referring to atomic emission spectra have been removed to AS.
TOPIC 10
(b)
Isomerism and aromaticity
understand the term stereoisomerism as embracing both E–Z and optical isomerism;
As stated in the AS Teacher Guidance booklet, E is from the German entgegen (opposite)
and replaces trans; Z is from the German zusammen (together) and replaces cis.
(g)
describe and classify the nitration and halogenation reactions of benzene as
electrophilic substitution, and recall the mechanism for these reactions †*;
(The equation for the formation of NO2+ is not required.)
Note that the equation for the formation of the NO2+ is not required.
(h)
describe the Friedel-Crafts alkylation of benzene †*;
This is a new statement, introduced due to its continuing importance in organic synthesis.
Note that several statements on halogenoalkane chemistry have been moved to AS.
TOPIC 11
Organic compounds containing oxygen
11.1
Alcohols and phenol
Simpler alcohol work – physical properties / social effects / use as a fuel – has been
removed to AS.
Alkaline hydrolysis of halogenoalkanes is now more generalised and not restricted to 1bromobutane
11.2
Aldehydes and ketones
11.3
Carboxylic acid and derivatives
TOPIC 12
(g)
Organic compounds containing nitrogen
understand the formation of polypeptides and proteins and have an
understanding of primary, secondary and tertiary protein structure;
outline
This statement has been modified to include simple ideas on primary, secondary and tertiary
structure of proteins.
GCE A2 CHEMISTRY Teachers' Guide 8
TOPIC 13
(b)
Organic synthesis and analysis
use given mass spectral data to elucidate the structure of simple organic molecules
(up to and including C8 molecules, with one chlorine atom);
The number of carbon atoms has increased from 5 to 8.
(d)
understand that n.m.r. spectra can give information regarding the environment and
number of equivalent hydrogen atoms in organic molecules and use such supplied
information in structure determination *;
NMR will include spin-spin splitting or unsplit spectra at low resolution
(l)
outline the uses of thin layer chromatography (TLC), gas chromatography and highperformance liquid chromatography (HPLC) in analysis (details of the theory of
chromatography and of the methods used are not required) and be able to find the
composition of mixtures by use of retardation factor (Rf), retention time and peak area.
The new criteria have necessitated the introduction of this statement.
TOPIC 14
The process of how science works
(a)
understand and use the methods scientists employ in determining the accuracy,
reliability and validity of their own and others' work;
(b)
appreciate the methods scientists employ in developing their own and others' work to
produce new materials and applications, for example in natural product synthesis,
using computational chemistry and in aspects of nanotechnology.
Topic 14, the process of how science works, has been interpreted by some as largely the
practical side of chemistry, and this is obviously an essential part of the subject. However,
the way in which scientists work and how the ideas that they generate are further developed
is an essential part of the scientific process. New ideas and procedures are subject to
professional scrutiny as others try to replicate and assess the implications and possible
developments from the initial work of both themselves and others. Questions will be asked,
on a fairly basic level, that will attempt to explore this aspect of scientific thinking. The
examiners will not expect detailed responses to questions but will be looking for answers
that show that candidates have thought about modern developments in chemical science
and how these might be further applied in a way to benefit mankind in general. Candidates
will not be expected to have studied particular applications in any depth and indeed,
questions may be phrased with the word 'suggest' so that candidates can have the
opportunity to express their own thoughts in a meaningful way.
GCE A2 CHEMISTRY Teachers' Guide 9
UNIT CH5 – Physical and Inorganic Chemistry
The examiners have removed items regarded as unnecessary recall from this unit.
TOPIC 15
Redox
15.1
Redox and standard electrode potential
(d)
recall and use the redox systems specified below, including the appropriate colour
change and ion/electron half-equations
Cu2+(aq)|Cu(s);
Zn2+(aq)|Zn(s);
H+(aq)|H2(g) Pt;
Fe3+(aq), Fe2+(aq)|Pt;
MnO4– (aq), Mn2+(aq)|Pt;
X2(g)|2X  (aq) (X = Cl  , Br  , I  );
(e)
use redox systems in addition to those in (d), for which all relevant information is
supplied;
Statement (e) gives examiners scope to use other redox systems provided all relevant
information is supplied.
15.2
Redox reactions
15.3
Applications
(m)
explain the principles underlying the operation of the hydrogen fuel cell in terms of
the electrode half reactions 2H+ + 2e–  H2 and O2 + 4H+ + 4e–  2H2O in the
presence of a platinum catalyst, its potential use for storing energy and generating
electricity and heat, and the benefits and drawbacks of its use (details of cell
construction not required).
The hydrogen fuel cell is essentially an electrochemical cell.
Overall reaction 2H2 + O2 → 2H2O
EMF + 1.23V
Forward (exothermic) reaction used to release energy. A fuel cell works like a battery but
does not run down or need recharging. it will produce electricity and heat as long as fuel
(hydrogen) is supplied.
Reverse (endothermic) reaction used to store energy.
Examples of advantages:
"Clean" technology with water as only product;
Convenient method of storing energy;
More efficient than internal combustion energy at releasing energy from fuel.
Examples of disadvantages:
Energy is lost as storage cycle not 100% reversible;
Problems of storing gases;
Run at relatively low temperatures (~ 80 °C) so an expensive catalyst needed.
GCE A2 CHEMISTRY Teachers' Guide 10
TOPIC 16
Chemistry of the p – block
16.1
General
16.2
Group 3 (13)
(e)
understand the electron deficient nature of Group 3 systems such as BF3, BCl3 and
monomeric AlCl3 and their electron acceptor properties;
(f)
explain the ready formation of the Al2Cl6 dimer, its structure and its bonding;
The statements are modifications and one is an extension of the previous specification
(g)
understand the formation of donor-acceptor compounds such as NH3.BF3;
(h)
recall that the affinity of AlCl3 for chlorine species results in industrially important
catalysts such as
(i)
(halogen carriers in) the chlorination of benzene and
(ii)
low melting temperature ionic liquids, containing the chloroaluminate(III) ion,
AlCl4–, which are being developed as "clean technology" solvents and
catalysts for processes such as the polymerisation of alkenes;
Ionic liquids are organic salts that generally have melting points below 100 °C, unlike
conventional salts; and the melts are liquid over a wide temperature range. Ionic liquids with
melting points below or near room temperature (RTILs) are especially interesting. For
example, reactions and extractions may be performed under mild conditions, promoting
lower likelihood of product thermal degradation and reducing energy costs. The organic
products are immiscible in the ionic liquids, separate out and can be tapped off. The ionic
liquid is recyclable.
The archetypal ionic liquids are the N butylpyridinium chloride-aluminium(III) chloride,
[N Bupy]Cl-AlCl3, and 1-ethyl-3-methylimidazolium chloride-aluminium(III) chloride,
[emim]Cl-AlCl3, systems. In each case, the organic Cl dative bonds to AlCl3 followed by
ionisation.
There are approximately one trillion (1018) accessible room temperature ionic liquids. Room
temperature ionic liquids have developed, in less than 20 years, from an adjunct to the US
'Star Wars' research on battery electrolytes, into an industrial reality as media for catalytic
chemical processes.
As an example, a wide range of acidic chloroaluminate(III) and alkylchloroaluminate(III) ionic
liquids catalyse the dimerisation and oligomerisation of olefins. In what is an exceptionally
simple system, the olefinic feedstock may be mixed with, or simply bubbled through, the
ionic liquid catalyst to produce oligomeric products which have low solubility in the ionic
liquid catalyst and separate as a less dense organic phase which is readily removed by
tapping off.
GCE A2 CHEMISTRY Teachers' Guide 11
(i)
explain how boron nitride, BN, forms hexagonal and cubic structures corresponding
to graphite and diamond respectively and, because of its hardness, chemical
inertness, high melting temperature and semiconductor properties, is finding
increasing use as lubricant, as wear-resistant coating and as nanotubes for wire
sleeving, catalyst support and semiconduction;
Boron nitride is isoelectronic to the elemental forms of carbon and isomorphism occurs
between the two species. That is boron nitride possesses three polymorphic forms; one
analogous to diamond, one analogous to graphite and ones analogous to the fullerenes.
Graphite-type Boron Nitride
B
B
B
B
B
N
N
N
N
N
B
B
B
B
B
N
N
N
N
N
B
B
B
B
N
N
N
B
B
N
B
N
N
These sheets (unlike those in graphite) are in register.
This means that layers are placed directly upon one
another.
The polar B-N bonds interfere with electron transfer so
that boron nitride in this form is not an electrical conductor
(in contrast to graphite).
N
N
B
B
N
Boron nitride nanotubes can also be constructed by
"bending over the layer" and joining the two meeting
edges.
Due to its excellent dielectric and insulating properties,
BN is used in electronics e.g. as a substrate for semiconductors, microwave-transparent
windows, structural material for seals, electrodes and catalyst carriers in fuel cells and
batteries.
It can be prepared in the form of fibres, structurally similar to carbon fibres, sometimes called
white carbon fibre.
The diamond-like allotrope of boron nitride is widely used as an abrasive for industrial
tools.
(j)
recognise that in none of the cases (f) to (i) above is the compound electron deficient.
This new statement is self explanatory.
16.3
16.4
Group 4 (14)
Group 7 (17)
TOPIC 17
d block transition elements
TOPIC 18
Chemical kinetics
GCE A2 CHEMISTRY Teachers' Guide 12
TOPIC 19
Energy changes
19.1
Enthalpy changes for solids and solutions
19.2
Entropy and feasibility of reactions
(f)
appreciate that the entropy, S, of a chemical system is to a large degree determined
by the freedom possessed by the molecules or atoms within the system, and that for
all natural changes, entropy increases towards a maximum (definitions not required);
(g)
recognise that molecules or atoms in a solid have much more restricted freedom than
in a gas and that, other factors being equal, entropy increases in the sequence
S(solid) < S(liquid) < S(gas);
(h)
apply the equation relating free energy change, ΔG, to the entropy change, ΔS,
ΔG = ΔH - TΔS
and understand that, for a reaction:
if ΔG is negative, the reaction occurs spontaneously;
if ΔG is positive, the reaction does not occur spontaneously;
(i)
understand that, because of the entropy change, endothermic processes such as the
boiling of liquids, the solution of some salts and the thermal decomposition of
oxysalts may still occur spontaneously.
The new QCA criteria have necessitated the introduction of this topic. The topic is treated in
such a way as to keep recall to a minimum.
There is a useful analogy with enthalpy which will help to explain the examiners' intentions
here. Students may not be able to offer a definition of enthalpy or fully grasp its meaning.
However, they may still be able to use the concept of enthalpy change and Hess' law to
calculate enthalpy changes. In a similar way, it is intended that students should understand
and be able to use and apply the above statements.
TOPIC 20
Equilibria
20.1
General Equilibria
(e)
recognise that for a reaction with ΔG negative (see 19.2 (h)) Kp and Kc will have large
values as the products predominate and that for a reaction with ΔG positive Kp and
Kc will have small values as the reactants predominate;
This is a new statement.
20.2
Acid-Base Equilibria
GCE A2 CHEMISTRY Teachers' Guide 13
2.
INTERNAL ASSESSMENT OF PRACTICAL SKILLS CH6
INTRODUCTION
The AS work in Unit Three was concerned with developing basic practical skills in general
and physical chemistry, especially in the fields of analysis, thermochemistry and kinetics,
and relating these to the associated theory units. In A2 the focus shifts to inorganic and
organic chemistry and the standard of practical skill required is increased to match the
maturing skills of the candidates.
As before, the ability to relate the practical work to the theory to their mutual benefit is an
important aim of the unit. In this connection, although the assessment is based mainly on
Assessment Objective Three - How Science Works (essentially the experimental aspect)
marks are also allocated for Assessment Objective One (knowledge and understanding) and
Assessment Objective Two (application of knowledge). To ensure full coverage, one
experiment will be in the field of inorganic chemistry and one organic, each selected from a
choice of four. Additionally there is a need to balance the type of experiment chosen: some
involve a substantial planning element while others involve the preparation and quantitative
analysis of a compound and there should therefore be no more than one experiment of
the planning type. To be specific, if 6.I2 is chosen 6.O2 cannot also be used and vice
versa.
All candidates must carry out their practical work independently. Pairing up candidates, e.g.
because of limited availability of equipment, is not permitted. The same applies of course to
the written elements of the task and candidates are required to sign a declaration stating that
all work is their own.
Each of these exercises is designed to be carried out in an ‘open-book’ examination
situation. Candidates should have free access throughout to their own notebooks, textbooks
and any other resources normally available to them in the laboratory, including the internet.
The work must however be carried out in the centre and under careful supervision.
Candidates must not be permitted to take the pro forma or the questions within them outside
of that environment.
Centres must inform WJEC when their candidates will be undertaking their practical
assessments so that random spot checks may take place. Completed work and
relevant teacher results sheets must be securely stored by the Exams Officer before it
is submitted to WJEC by the May 15 deadline. All candidates work will be marked by
WJEC.
GCE A2 CHEMISTRY Teachers' Guide 14
STRUCTURE OF THE UNIT
The internal assessment unit in A2 makes up 20% of the A2 mark and thus 10% of the total
A level mark. Each of the two pieces of work has a maximum of 30 marks, giving a total of
60 marks. Of these 30 marks, 24 are allocated to the overall practical exercise (called
Assessment Objective 3 - How Science Works), 3 to questions testing knowledge and
understanding (Assessment Objective 1) and three to questions on applying that knowledge
(Assessment Objective 2).
The 30 marks are divided into the four sections of Planning, Implementing, Analysing and
Evaluating with the actual mark distribution being dependent on the type of exercise.
The following describes the general nature of these sections and full details for each
experiment are given when these are described.
Planning
In experiments having a significant planning component. I2 and O2, nine marks are allocated
comprising six marks for the plan and three for AO1 and AO2 questions. In the remainder of
the experiments five marks are allocated, including three for AO1 and AO2.
Implementing
In I2 and O2 implementing is a relatively minor component and is awarded up to six marks
but comprises around fifteen marks in the other experiments where the actual conduct of the
exercise is of major importance.
Analysis
Analysing the observations made is clearly important in I2 and O2 and is allocated twelve
marks while being given around five marks in the other experiments.
Evaluation
This is given between three and five marks in all the exercises including three marks for AO1
and AO2 questions.
GCE A2 CHEMISTRY Teachers' Guide 15
PRESENTATION OF WORK
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Normally a pro forma will be used and any additional sheets must be labelled with the
candidate's name and securely attached to the pro forma at the correct place in the
work and not all at the end.
The two pieces of work must be connected together along with the Candidate Cover
Sheet.
The cover sheet must contain full details of the candidate and the experiments
chosen. The declaration by the candidate that this is their own work and
confirmation by the teacher must both be signed.
All the work, additional sheets and cover sheet must be bound together by using a
treasury tag in the top left-hand corner; paper clips should not be used, since they
become detached in the packets.
The work from the centre must be arranged in the candidate number order on the
invoice. In the case of large centres having several practical groups it is sufficient for
the scripts to be in invoice order within each group.
No rough work or instruction sheets should be included with the work.
Teacher Result Sheets are a vitally important part of most of the experiments since
many of the Implementation marks are allocated by comparing the candidate’s
results with those of the teacher. All relevant sections of these must be completed
without ambiguity and it must be very clear if different candidates or sets of
candidates have different solutions or materials and which candidate has which.
All work should be in ink and any temporary pencil work written over in ink.
GENERAL POINTS
1.
Theory. For each experiment the section headed Theory (presented on a separate
page) gives full details of how any required answers may be deduced. So long as
candidates obtain these answers somehow, they are at liberty to use either the
approach outlined or to follow any alternative method which they may prefer. The
Theory Sections are not intended to be prescriptive but are meant simply to assist
candidates in understanding the experiment in question.
2.
Consequential marking. Any error on one section will not affect the mark given in a
subsequent section. In particular, a candidate's plan should be checked by the
teacher, deficiencies noted on the work, and the candidate given a correct plan to
work with.
3.
Equipment variations across centres have to be taken into account in mark schemes
and teachers should report any features such as thermometer precision, availability
of data logging and interfacing equipment and, especially, limitation in the type of
balance available. Note that experiments 6.I1, 6.I3 and 6.I4 need at least a threeplace balance.
4.
When experiments lead to results which differ significantly from those expected, this
should be reported, together with any appropriate comment. In such circumstances it
would be reasonable for the candidates to be marked on the basis of the teacher's
results, so long as this is to their benefit. It should be appreciated, that if the teacher's
result is not reported, it is not possible for the Board to take any remedial action
should the candidate score badly by virtue of poor agreement with literature values.
GCE A2 CHEMISTRY Teachers' Guide 16
5.
Choice of experiments. Centres are required each year to choose from the given list
of experiments one inorganic and one organic experiment. Not more than one
planning type of experiment (i.e. 6.I2 and 6.O2) is allowed. This is to ensure an
appropriate spread of experimental skills in the assessment.
Centres are
encouraged to vary their choices from year to year so that the exercises do not
become repetitively stereotyped to the students.
6.
Specimen pro formas are included to be used by the candidates if the centre wishes.
If additional space is needed for any section then additional sheets should be
attached to the pro formas in the appropriate position. Pro formas may be modified to
suit individual centre needs.
7.
Format of the Experiments
The mark allocation described above (following the QCA/DECWL rules) has been
designed to test all the skills previously examined for the A2 Work. The nature of the
exercises, requiring the integration of skills within AO3 as well as the AO1 and AO2
question components, means that the work is intrinsically synoptic in character and
thus needs no additional element of synoptic questions as in the previous
specification. 10% of the marks are awarded for AO1 and for AO2 questions and
these questions are very largely directed towards reinforcing the linkage between
practical work and theory and helping to ensure that students carry their theoretical
knowledge into the laboratory and their practical knowledge and experience into the
examination room.
It is, however, recognised that, for most teachers, laboratory time is both limited and
precious. It is therefore advised that only the actual execution of the experimental
work has necessarily to be carried out during laboratory periods. The great bulk of
the listed questions can be answered either before or after the experimental work is
undertaken but in Experiment 6.I2 and 6.O2 the required plans must obviously be
formulated first. Teachers are therefore free to arrange the timing of the candidates'
responses to the questions posed as they think most appropriate so long as they
ensure that this is done under controlled conditions.
GCE A2 CHEMISTRY Teachers' Guide 17
Inorganic Experiments
Two types of experiment are offered here. Thus experiment 6.I2 is an identification exercise
and the marks awarded will mostly relate to correct planning, identities and explanations and
rationalisations thereof. By contrast, experiments 6.I1, 6.I3 and 6.I4 contain sections which
involve accurate analysis by volumetric techniques.
For the straightforward titrations, such as Part 1 of 6.I1 and Part 2 of 6.I3, an accuracy of
about ± 0.8-1.0% will be expected for full marks, tapering off to zero for deviations exceeding
about ± 2.5-4.0 %, depending on the difficulty of the titration involved. For titres of the order
of 25 cm3 these limits correspond approximately to deviations of ± 0.20-0.25 cm3 and
± 0.60-1.00 cm3 respectively. For the somewhat more demanding mass aliquot titrations in
I1, I3 and I4 the titre deviations will be more generously assessed. Note that accuracy will
here be assessed on the titres, the primary experimental quantity, rather than on derived
values such as percentage compositions in which apparently small deviations from
theoretical values may conceal significant experimental errors. Where several aliquots of a
solution are to be titrated the mutual consistency (or otherwise) of titres used in deriving an
average value will also be taken into account, since titres deviating significantly in opposite
directions from the correct result could otherwise lead to average values showing a spurious
closeness to the theoretical quantity. Note that here it is especially important that
supervising teachers should report fully their own results on the forms provided. Normally
volumetric results will be marked on the basis of the theoretical results since the compounds
prepared are stable and of well-defined composition but where this will clearly benefit the
candidates the teacher's values may be used. In exceptional circumstances the centre
average result may also be taken into account.
As detailed in the AS guide the sensible use of significant figures in the avoidance of
excessive truncation of calculator outputs of final results and the recording of measured data
to the full precision of the apparatus (burette readings to 0.05 cm3 including initial 0.00 cm3
and weighings – 0.200g and not 0.2 g - ) is important.
For the marking of yields (in experiment 6.I4) see under the same topic in the Organic
Experiments. For all the experiments, both Organic and Inorganic, there are various
questions, given on the pro formas, to be answered so as to explore the various areas listed
above and the students' understanding of them (see Structure of the UNIT section above).
GCE A2 CHEMISTRY Teachers' Guide 18
Organic Experiments
Again two types of experiment are offered: here experiment 6.O2 is essentially an
identification exercise and will be assessed in a fashion similar to that used for experiment
6.I2. The remaining experiments involve preparation and characterisation and, apart from
the questions posed on the pro formas, will be marked largely on the basis of yields obtained
and melting temperatures reported.
In general, even for preparations which in theory give yields close to quantitative, it is quite
easy for less experienced operators to lose product in the process of purification. See the
next section. For that reason the experiments involve solid products (rather than liquids
which are significantly more difficult to manipulate) and full marks are usually to be awarded
for yields of 70 % or more. (Note, however, that penalties will be applied for products which
are so wet that they lead to yields in excess of 100 %!) The marks given will taper off as the
yield decreases but will only fall to zero if no product is obtained. Similarly the melting
temperatures of the products obtained are also marked on a sliding scale. In most cases full
marks will be awarded for melting temperature values within ± 2 °C (usually –2 °C!) of the
literature value tapering off to zero where the deviation exceeds about ± 10°C. Again it is
most important that supervising teachers should report their own yield and melting
temperature value (together with any range over which melting occurs) on the forms
provided since, where appropriate, these will also be taken into account in the marking. As
for the Inorganic Experiments, the pro formas contain various AO1 and AO2-based
questions, relating to the experiment, to be answered, which are intended to strengthen the
theory-practical linkage.
Obtaining good yields
The main reason for poor yields is not so much a failure to carry out the reaction properly but
rather a failure to remove all the product from solution usually during recrystallisation. In
general the crude product contains the required product and impurities that are more and/or
less soluble than this in the solvent to be used. Normally low yields are caused by adding
excessive amounts of hot solvent in an attempt to dissolve the less soluble impurities under
the impression that these are required product. Consequently on cooling much of the
required product remains in solution. Clearly some judgement is needed but the best
procedure is to add the minimum of hot solvent needed to dissolve what appears to be
almost all of the product, filtering to remove insoluble impurities and cooling. Any solid then
crystallising in the filter can be removed with a minimum of hot solvent. The lower the
temperature to which the filtered solution is cooled the greater the yield of product since
many organic compounds have large temperature coefficients of solubility. The solubility of
benzenecarboxylic acid, for example, is about 0.2 g/100 cm3 of water at room temperature
and 5.8g at 90o. Reluctant products can sometimes be induced to crystallise by scratching
the inside of the vessel with a glass rod to provide nucleation centres.
Some solutions are prone to supersaturation, a non-equilibrium phenomenon in which,
although solid and solution should co-exist under the conditions prevailing, the rate of crystal
growth is less than that of re-solution so that no crystal forms. The effect of scratching above
or of the presence of dust particles, etc, is to provide a nucleus on which the rate of growth is
increased.
GCE A2 CHEMISTRY Teachers' Guide 19
Scale of experiments
The scale of organic preparations may be adjusted either upwards or downwards, as
teachers feel is necessary or expedient. It is, of course, their responsibility to ensure that
such preparations, so modified, do in fact proceed satisfactorily, and any such changes must
be made clear on the results sheets of both teachers and candidates.
The inorganic exercises (6.I1, 6.I3 and 6.I4) however present a rather more difficult situation
as regards the analyses which they contain. In these experiments the requirement to weigh
accurately samples of about either 0.1 or 0.2 g clearly necessitates access to balances
weighing to three decimal places. It is recognised that shortage of such equipment may
present difficulties for some centres, especially where large numbers of candidates are
involved, but here any change in the quantities stipulated would require corresponding
changes in the concentrations of the solutions used in the analyses. Consequently, for any
changes of scale in these cases, the onus must lie upon the centre concerned to ensure that
the modified procedure would then actually work satisfactorily and to advise the WJEC in
advance of their intentions, giving appropriate details.
GCE A2 CHEMISTRY Teachers' Guide 20
General Safety Considerations
The purpose of these general instructions is to acquaint students with many aspects of
safety practice which are common to much of the A2 laboratory work and need not therefore
be separately rehearsed in the Instruction Sheets for each Experiment. Specific dangers
and hazards will, however, be noted individually where they occur. As a general approach,
while students should not regard practical work with undue timidity, they must always treat
both apparatus and reagents with care and respect and take full note of any hazardous
properties which are indicated. Attention is drawn to the following:
Eye protection
This is of paramount importance in the laboratory. Centres are required to provide safety
goggles for all experiments for this purpose. These should be 'chemical' grade goggles
complying with the EN166 3 standard (formerly BS 2092 C). For students who normally
need to wear glasses suitably strengthened personal safety spectacles may be a satisfactory
alternative. Students should be warned of the possible hazards when goggles mist up.
Inorganic Experiments
Volumetric work
A safety pipette filler should always be used here: never pipette any solutions by mouth,
especially alkali since this is very caustic if it is swallowed. (Similarly solutions containing
copper(II) ions or ethanedioate ions, for example, are toxic and should on no account be
pipetted by mouth.)
Spillages
For solutions of dilute acids add excess water before mopping up. For solutions of dilute
sodium (or potassium) hydroxide stand clear of any spillage since these can be very slippery
as well as caustic: again dilute with water before mopping up. At the concentrations
involved, other reagent solutions present only minimal hazards (e.g. potassium iodide,
sodium thiosulfate, starch and iodine). No special precautions are necessary for the spillage
of solids but if large amounts are involved they may be mixed with sand before sweeping up.
GCE A2 CHEMISTRY Teachers' Guide 21
Contact with skin
Where appropriate plastic gloves should be worn to prevent adsorption of harmful reagents
by the skin. Remember though, that wet gloves are slippery and should not be used for
handling apparatus or reagent bottles.
If you splash any of the reagents (solutions or solids) onto your skin, notify your supervisor
and wash the affected area with a large amount of water.
Mercury thermometers
Should one be broken, try to retain the liquid mercury. The vapour is a cumulative poison. If
necessary droplets of mercury can be sucked up using a small aspirator pump. If mercury is
spilled into floor cracks the volatility can be reduced by brushing in sulphur or zinc dust.
Disposal of surplus material
Unless otherwise instructed, small quantities of the chemicals may be disposed of by adding
to running water in the sink and then discharging to waste.
Organic Experiments
The great bulk of organic compounds are to a greater or lesser extent flammable and should
not therefore be exposed to naked flames or sparks. (This is especially true, for example, of
fairly low boiling solvents such as propanone and ethanol.) In general, direct heating will
only be necessary where heating under reflux is specified and for all other purposes the
temperature attainable on a water bath (100 ºC) should be sufficient.
Other hazards
The procedures to be followed for spillages and contact with the skin are in general the
same as for the Inorganic Experiments. Special care should be taken when working with
concentrated acids. Note that water should never be added to concentrated sulfuric acid: if
this is to be disposed of it should be added to an excess of water (not vice-versa). Specific
hazards are indicated for individual experiments as appropriate.
Teachers should note that they are responsible for ensuring safe working practice and that
all practical work fulfils the requirements of the COSHH regulations and the Health and
Safety at Work Act.
GCE A2 CHEMISTRY Teachers' Guide 22
Contributors to the Teachers' Guide
CH4
Dr David Ballard
CH5
Dr Martin Anthoney
Internal Assessment of Practical Skills, CH6
GCE Chemistry (A2) - Teacher Guidance/ED
19 September 2013
Dr Peter Blake