GCE AS CHEMISTRY Teachers' Guide 1
Contents
GCE AS Level Chemistry
Teachers’ Guide
Page
1.
Introduction
2
1.1 - Overview of New Specification
1.2 - Rationale
1.3 - Changes for teaching from September 2008
3
4
5
2.
Internal Assessment of practical skills, CH3
11
3.
Contributors to the Teachers’ Guide
16
GCE AS CHEMISTRY Teachers' Guide 3
1.
INTRODUCTION
The WJEC AS CHEMISTRY specification has been modified and updated for delivery from
September 2008. 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 subject criteria for GCE CHEMISTRY require 3 units for AS, 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
matthew.roberts@wjec.co.uk
(Subject Support Officer)
Subject page
www.wjec.co.uk
GCE AS CHEMISTRY Teachers' Guide 4
1.1 Overview of the AS Specification
UNIT CH1 – Controlling and Using Chemical Changes (in order to make things, produce
energy and solve environmental problems)
TOPIC 1
1.1
Basic ideas about atoms
1.2
Chemical calculations
TOPIC 2
2.1
Chemical equilibrium and acid-base reactions
2.2
Energetics
2.3
Kinetics
TOPIC 3
Application of the principles studied in Unit 1 to problems encountered in the production of
chemicals and of energy.
UNIT CH2 – Properties, Structure and Bonding
TOPIC 4
4.1
Chemical Bonding
4.2
Forces between molecules
4.3
Shapes of Molecules
4.4
Solubility of compounds in water
TOPIC 5
Solid Structures
TOPIC 6
6.1
The Periodic Table
6.2
Trends in properties of the elements of the s-block and Group 7 (17)
TOPIC 7
7.1
Organic compounds and their reactions
7.2
Hydrocarbons
7.3
Halogenoalkanes
7.4
Alcohols
TOPIC 8
Analytical techniques
UNIT CH3 – Assessment of Practical Skills
Based on the topic areas of 1.2, 2.1, 2.2 and 2.3
GCE AS CHEMISTRY Teachers' Guide 5
1.2 Rationale
The content at AS has been significantly reorganised in order to achieve the following:
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ease of transition from GCSE
immediate development of practical skills
retention of interest
shortening of the unit (without its underweighting)
Thus, the CH1 unit now contains the topics that naturally give rise to the opportunity for
assessment of practical skills in unit CH3. This will give students the opportunity to begin
developing their practical skills very early on in their AS study and ought greatly to assist in
the retention of candidates’ interest. These topics feature prominently in the GCSE course
and the AS treatment builds on this foundation, easing transition from GCSE.
This reorganisation may enable the unit to be taught in a shorter time by delaying the
assessment of practical skills until its completion. However, the unit is still weighted equally
with the CH2 unit, so that the student’s performance at AS is not determined mainly by
performance in the CH2 unit.
The CH2 unit now contains ideas involving bonding, structure and properties, which students
traditionally find more challenging. The intention is that they now meet these later in the
course when maturation has occurred.
CH1 and CH2 papers were 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 AS units will be available for summer entry only.
GCE AS CHEMISTRY Teachers' Guide 6
1.3 Changes to the specification for delivery in September 2008
Changes in Assessment Structure for AS
(a)
Changes to Assessment Objectives
The assessment objectives are listed on page 9 of the specification. A simplified
interpretation of these is as follows:
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AO1 – k + u
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AO2 – applying k + u
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AO3 – prac skills (How Science Works)
(b)
Changes to Assessment units
There are 3 units for AS. Each assessment unit must now test all three AOs
UNIT
%of AS
UM
Raw mark
AO1
AO2
AO3
CH1
40
120
80
35
35
10
CH2
40
120
80
35
35
10
CH3
20
60
60
6
6
48
There are no split units allowed, so that the CH3a paper (the so-called Prac/Theory
interface paper) of the old specification will cease to exist in the new specification but
the style of question, which did much to cement the theoretical understanding of the
students' practical work, will be transferred to the CH1 paper.
Changes in Content
These have arisen in the following two ways:
(a)
60% is specified by QCA criteria and these have resulted in
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(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 AS CHEMISTRY Teachers' Guide 7
Details of changes in content
The following indicates new learning outcomes and some deletions.
UNIT CH1 – Controlling and Using Chemical Changes (in order to make things, produce
energy and solve environmental problems)
TOPIC 1
1.1
Basic ideas about atoms
(d)
explain the formation of ions from atoms by the loss or gain of electrons;
This outcome was implied in the old spec but now is clearly stated.
(n)
explain the origin of emission and absorption spectra in terms of electron transitions
between atomic energy levels;
(o)
describe and interpret the visible atomic spectrum of the hydrogen atom (first 4 lines
in the Balmer Series only);
(p)
recall the direct proportionality between energy and frequency, as implied by E = hf,
and the inverse relationship between frequency and wavelength;
(No calculations will be set.)
(q)
show understanding of the relationship between the frequency of the convergence
limit of the Lyman Series and the ionisation energy of the hydrogen atom.
Outcomes (n) – (q) were moved from the old CH4 unit to CH1 where they were felt to
be more appropriately placed.
1.2
Chemical calculations
(h)
calculate the atom economy and percentage yield of a reaction using supplied data.
It is intended here that atom economy is calculated in the same way as at GCSE:
% atom economy 
mass of useful product
 100
total mass of reactants
N.B. The use of the skills listed in outcomes 1.2 will be expected in all units of the AS and
A2 specification.
The above statement has been corrected to read (c) to (h) – and not (d) to (h) as in the
original printed version of the specification.
Note that the gas laws are not now required.
GCE AS CHEMISTRY Teachers' Guide 8
TOPIC 2
2.1
Chemical equilibrium and acid-base reactions
(f)
recall that carbon dioxide is an acidic gas and its interaction with water including its
effect on the carbonate/hydrogen carbonate equilibrium in sea-water.
This is meant to allow students to discuss the acidification of sea water by absorption of CO2
Note that equilibrium constants have moved to A2 as required by the criteria.
2.2
Energetics
(e)
recall details of experimental procedures for determining enthalpy changes in
aqueous solution, and calculate such enthalpy changes from experimental data using
mcT
H  
n
where m and c are the mass and specific heat capacity of, for example, the water
used, T is the incremental change in temperature, and n is the number of moles;
Here, the minus sign has been introduced, and the scaling factor s replaced by 1/n.
2.3
Kinetics
(i)
appreciate the importance of finding new and better catalysts, including the use of
enzymes, in achieving some of the goals of green chemistry, e.g., allowing the
possibility of lower temperatures (less energy consumption), lower pressures, etc.;
Students are not expected to recall all the principles of green chemistry.
TOPIC 3
Application of the principles studied in unit 1 to problems encountered in the production of
chemicals and of energy.
No recall in addition to Topics 1 and 2 is expected here. Students will be expected to apply
knowledge gained in those topics and use given information in questions.
GCE AS CHEMISTRY Teachers' Guide 9
UNIT CH2 – Properties, Structure and Bonding
TOPIC 4
4.1
4.2
4.3
4.4
Chemical Bonding
Forces between molecules
Shapes of Molecules
Solubility of compounds in water
Note that the section on gases in the old spec is no longer required.
TOPIC 5
Solid Structures
(c)
recall and describe the structure of carbon nanotubes and appreciate the analogy
with the graphite structure;
(f)
understand that a so-called ‘smart’ material is able to exhibit a change in properties
with a change in conditions (temperature, pH, etc) and this is often caused by a
change in structure;
(g)
understand that nano-sized materials often exhibit different properties which can lead
to new uses.
These new statements imply no more knowledge or understanding than at GCSE level.
TOPIC 6
6.1
6.2
The Periodic Table
Trends in properties of the elements of the s-block and Group 7 (17)
TOPIC 7
7.1
Organic compounds and their reactions
(a)
write displayed, shortened and skeletal structural formulae of simple alkanes,
alkenes, halogenoalkanes, primary alcohols and carboxylic acids given their
systematic names, and vice versa;
The word “displayed” has replaced “graphic” and is intended to mean that a student needs to
show all the bonds. “Skeletal” has been introduced.
(c)
describe structural isomerism and be able to write down the structural isomers of
non-cyclic organic compounds (up to and including C6 homologues) including those
of different chemical class;
Compounds with up to 6 C atoms can now be used in questions.
GCE AS CHEMISTRY Teachers' Guide 10
(d)
describe E-Z isomerism in alkenes, give an example, and discuss such isomerism in
terms of restricted rotation about the C = C bond, and appreciate that E-Z isomers
may have different physical and chemical properties;
E is from the German entgegen (opposite) and replaces trans. Z is from the German
zusammen (together) and replaces cis.
7.2
Hydrocarbons
7.3
Halogenoalkanes
Much of this has moved to AS from A2 as required by the criteria.
7.4
Alcohols
This has moved to AS from A2 as required by the criteria.
TOPIC 8
Analytical techniques
This has moved to AS from A2 as required by the criteria. A simple treatment of these
statements is intended.
GCE AS CHEMISTRY Teachers' Guide 11
2.
INTERNAL ASSESSMENT OF PRACTICAL SKILLS CH3
INTRODUCTION
Practical work is an essential part of chemistry and many of us were first attracted to the
subject by the bangs, colours and smells achieved in chemical experiments. Chemistry plays
a vital role in modern life as in designing and developing new drugs, materials for the
electronics industry and producing the millions of tons of basic chemicals needed.
In addition to this, a major chemical activity lies in analysis. All our foods, medicines, water
supplies and raw materials have to be analysed for purity and suitability, fuels must be
tested for energy output and an increasingly important area lies in environmental analysis.
Industrial pollution, ozone depletion, acid rain and many other problems can only be studied
through chemical analysis. Measurements down to less than one part per billion of pollutant
require sophisticated machines costing many thousands of pounds. Clearly these are not
available in A-level laboratories but the essential principles of analysis are the same
whatever method is used.
The main aim of this module is to help candidates to develop the important practical skills of
planning, accurately performing the experiment, calculating and analysing the results and
evaluating their significance and reliability. A further aim is that actually doing the experiment
will help to consolidate the understanding of the associated theoretical background.
The AS coursework unit is based on three practical areas of which any two are to be
selected. The three areas chosen are:
 energetics and thermochemistry
 rates of reaction and kinetics
 volumetric analysis and stoichiometry
Any combination of areas may be used with of course only one experiment from any given
area. The experiments selected for this AS module are in the realm of physical chemistry,
with inorganic and organic work being done in the A2 year.
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 the
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 AS CHEMISTRY Teachers' Guide 12
STRUCTURE OF THE UNIT
The Practical Unit CH3 in AS makes up 20% of the AS mark and thus 10% of the total A
level mark. Each of the two pieces of coursework has a maximum of 30 raw marks, giving a
total of 60 raw 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 3 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 marks being approximately as shown below.
The following describes the general nature of these sections and full details for each
experiment are given when these are described.
Planning (5 marks)
This section requires a title giving the aim of the experiment and a short statement of how it
is to be carried out and also contains the questions on knowledge and understanding that
relate to the given experiment.
Implementing (15 marks)
This means actually doing the experiment and is of course different for each exercise. Note
that this is the most important single part of the work and carries one-half of the marks.
Analysing (5 marks)
This involves processing the results obtained in doing the experiment, such as by plotting
graphs and doing calculations of concentrations, enthalpy changes, etc., depending on the
particular experiment.
Evaluating (5 marks)
There are two parts here; the first is generally to estimate the likely errors in the experiment
and perhaps suggest improvements in the method, the second part is to answer questions
on applications related to the experiment.
The estimation of errors is an important part of all experimental work and is considered in
some detail here, with minor differences in the approach being discussed under each
experiment.
The fact is that no experiment gives perfect answers, with errors arising from the equipment
and materials used, the method chosen and from operator error. It is important to be able to
estimate what the size of this error or uncertainty is likely to be.
For example, in a trial on the efficiency of a new drug it might be found that 52% of patients
survived after 2 years as against 50% in the control group without the new drug. Is this an
improvement? Unless we know the error or uncertainty in the test we cannot answer the
question.
If the error is 5%, this difference could easily have arisen by chance, but if the error is 0.5%
the improvement is almost certainly real.
Estimation of error can be complicated but all that is needed at AS is a simple and direct
approach.
GCE AS CHEMISTRY Teachers' Guide 13
In experiments having a large number of repeat values the error (strictly the precision) is
found by finding the mean value and the (standard) deviation from the mean. This is of no
use here where only one or two measurements are made.
All that needs to be done is to decide where the biggest error in the experiment is and
express this as a percentage. Which is the part of the operation with the biggest uncertainty?
It is not going to be in any weighing with a three-place balance where one gram can be
weighed as 1.000 g with perhaps an error of 1 mg in the last place. The % error is thus 1 in
1000 or 0.1%.
Burette/pipette errors in titrations and temperature errors in thermochemistry will be larger.
Just focusing on the burette we can estimate an error of, say, ± 0.1 cm3 in 20 cm3, that is 1 in
200 or 0.5%. In thermochemistry, the temperature error could be estimated as 0.2 °C in 10 °C
or 2%. These estimates can be based on the smallest division in the equipment used. Thus
with the burette we could estimate an error of one-half of a division for both the initial and
final burette readings, giving a total estimated error of one division or 0.1 cm 3 as above. Note
that most measurements in science are based on differences, as with the burette volumes,
giving a maximum error as the sum of the errors in the initial and final burette readings as
above.
There is no need at this level to add up the errors from the various components; it turns out
that any error that is less than one-third of the main error is quite negligible.
Also remember that these estimates are essentially guesses and all that we really need to
know is whether the error is 0.1% or 1% or 10%.
Having estimated the error, the next important step is to express the result of the experiment
in a sensible way bearing this in mind. For example, suppose that we have estimated the
error in measuring a concentration to be 1% and the result on the calculator reads
0.1234567 mol dm-3. A 1% uncertainty means that we are not sure about the third place (3) which could be a 2 or a 4 - so that the 4567 is completely meaningless. There is no harm in
recording the 4 as one extra so our answer should be 0.123 or 0.123(4) ± 0.001 mol dm-3.
GCE AS CHEMISTRY Teachers' Guide 14
Significant figures and decimal places
This is an area that can easily cause confusion in handling the paragraph above. Taking a
number such as 0.0014203 as an example, the number of decimal places is clearly seven
and the number of significant figures is five (zeros in a run of numbers being significant).
Results should be given to the number of significant figures justified by the error estimate (or
one extra) such as three for a 1% error (0.00142) and two for a 10% error (0.014).
Decimal places confuse the issue and SHOULD NOT BE USED in this area. Quoting the
number above to three decimal places gives 0.001 thus losing over 30% of the information
provided by the experiment.
In the same way, hard-won information is lost by excessive truncation (shortening) of a
result, such as writing an answer of 0.092 as 0.09 or even 0.1, which is not uncommon in a
candidate’s pro forma.
To sum up this important matter:
1.
a result is calculated and displayed on the calculator,
2.
the % error (uncertainty) is estimated as above, and
3.
the result is recorded with the number of significant figures given by the % error, i.e.,
three for a 1% error.
NB The use of standard form may help to avoid the mistakes above, e.g., 0.00142 is written
as 1.42 x 10-3.
The estimation of errors in the kinetics experiments can be made as above or directly from
the scatter of points when graphs are plotted. A simple procedure here is to draw the line of
best fit to the points and then a steeper or flatter line that could also reasonably match the
points. The percentage difference in slope between these lines gives an estimate of the
percentage error. It may be noted that a temperature change of one degree would give a
change in rate of some five percent so that this would normally be the largest error in unthermostatted runs at room temperature.
GCE AS 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.
It must be very clear to the marker if the apparatus used differs from that in
the instructions, e.g., 20 cm3 pipettes rather than 25 cm3, or if the procedure
is different, e.g., acid rather than alkali in the burette in a titration.
All work should be in ink and any temporary pencil work written over in ink.
GCE AS CHEMISTRY Teachers' Guide 16
Contributors to the Teachers’ Guide
Internal Assessment of Practical Skills, CH3
GCE Chemistry – Teachers Guide (AS)/HJ
19 September 2013
Dr Peter Blake