Stretch and Challenge – Student Advice

What is Stretch and Challenge?

· What it is:

· the application of knowledge and/or problem solving rather than just recalling data

· demonstrating a breadth of knowledge (including facts learned during the AS course) – not the narrow testing of one specification topic

· being examined by more open-ended questions which may have no single correct answer

· applying scientific knowledge to new contexts the student hasn’t met before

· very similar, in many ways, to the synoptic assessment seen in previous specifications

· part of the normal A2 examination paper.

· What it isn’t:

· additional content to learn

· lots of extra essay questions – although some Stretch and Challenge questions may be tested by extended prose questions

· being examined by questions that are flagged as Stretch and Challenge

· only for those students aiming for an A* grade – whatever grade the student is aiming for, they need to answer all the questions to the best of their ability.

Teaching Stretch and Challenge

It is not expected that you will teach Stretch and Challenge explicitly within your lessons. However, the guidance given here is intended to offer you suggestions on how you can start to get your students thinking in ways that will help them prepare for meeting Stretch and Challenge questions in the exam. There is no requirement for you to teach any additional content to those students whom you want to do well in Stretch and Challenge. Probably the best way to teach Stretch and Challenge is simply to integrate it into your general teaching rather than treating it as a separate issue for only the brightest students.

The OCR A2 Chemistry Student Book that accompanies this Teacher Support CD contains Stretch and Challenge boxes which you may find useful in highlighting opportunities for trying out the skills needed to tackle such questions. The Exam Café CD at the back of the book also contains advice for students as well as some sample questions and answers for them to try.

Teaching strategies which develop students' thinking and problem-solving skills or which provide opportunities for them to apply existing knowledge in unfamiliar contexts will all provide good preparation – some examples are given below. This is not intended to be an exhaustive list of how you could teach Stretch and Challenge or where it might be examined – virtually any topic on the specification could be examined in this way.

Stretch and Challenge in chemistry can be applied to almost any area of the specification. The questions will be designed to take the students to a level of understanding above that which is initially seen in the specification. Stretch and Challenge should not be thought of as another technique of asking synoptic questions, although it may well be tested in this way. The Stretch and Challenge questions will be at a standard that involves A level students applying their skills, knowledge and understanding to novel situations. The questions will be targeting grade C to grade A* candidates, so are not likely to involve simple definitions.

Some ideas for teaching strategies or opportunities for Stretch and Challenge are given in the following table. However, please realise that since the very nature of Stretch and Challenge is aimed at combining elements of work from different modules, it is often difficult to fit these strategies in a fixed place in the specification. Consequently, you may feel that some of the suggestions would fit better in a different module; the choice is yours!

Table 1 Ideas for Stretch and Challenge

Specification topic

Teaching strategy

Student book link

Arenes

4.1.1 a,b. Structure of benzene

4.1.1 f–g Phenols

4.1.1 h Uses of phenols

· Explain the resonance energy of benzene based upon the resonant hybrids of the two Kekulé structures. You could do some bond energy calculation work here.

· Describe the use and effect of polychlorinated biphenyls (PCBs) upon the environment – there is some scope for a How Science Works (HSW) discussion here.

· Consider the historical advantages and disadvantages of using phenol as an antiseptic.

1.1.3 The delocalised model of benzene

1.1.6 The reactivity of alkenes and benzene

1.1.7 Phenols

Carbonyl compounds

4.1.2 a,b Oxidation of carbonyls

4.1.2 c Reduction of carbonyls

· Describe the use of oxidising agents upon aldehydes. This can be extended to look at the chemical processes taking place as the oxidising reagent is itself reduced.

· Using NaBH4 as a reducing agent that targets carbonyl groups – this can lead onto the selectivity of a reagent depending upon its strength as a reducing/oxidising agent: e.g. ‘Why is Tollens’ reagent used as the test for aldehydes?’

1.1.10 Oxidation of alcohols and aldehydes

(Links to 5.2.3 Redox and 5.3.1 Redox calculations as well as from AS 2.2.1 Alcohols)

1.1.11 Reactions of aldehydes and ketones

Carboxylic acids and esters

4.1.3 i Use of fatty acid esters as biodiesel

· The development of biodiesel from fatty acids. Take a closer look at the claims of carbon neutrality and the effect on the economies of poorer countries (ties in with HSW statement 7c).

1.1.16 Triglycerides, diet and health

Amines

4.1.4 d,e Synthesis of azo dyes

· Describe the manufacture of azo dyes. There is scope here for taking the basic process of azo dye manufacture beyond the simple version quoted in the specification, as well as introducing different phenolic compounds for benzene diazonium ions to couple with.

1.1.18 Amines and their reactions

Amino acids and chirality

4.2.1 h Identification of chiral centres

· Investigate the number of optical isomers which exist for molecules with more than one chiral carbon atom.

1.2.3 Optical isomerism

Polyesters and polyamides

4.2.2 f–h Hydrolysis and degradable polymers

· Discuss the advantages of the increased use of degradable polymers and the development of new polymer materials based upon vegetable sources for everyday use (ties in with HSW statement 6b).

1.2.7 Breaking down condensation polymers

Synthesis

4.2.3 b Devising multi-stage synthetic routes

· As well as developing more and more complex reaction schemes, there is also scope to introduce retro-synthesis – i.e. planning backwards from the product (target) molecule.

1.2.8–1.2.9 Organic synthesis of aliphatic and aromatic compounds

Chromatography

4.3.1 d,e Interpretation of chromatograms

4.3.1 g,h Interpretation of GC-MS results

· Chromatography lends itself well to stretching the students. Get them to interpret the chromatograms of more and more complex mixtures, including GC-MS results.

1.3.1–1.3.4 Separation; Thin-layer chromatography; Gas chromatography; Mass spectrometry

Spectroscopy

4.3.2 b Analysis of C-13 NMR

4.3.2 c Analysis of proton NMR

4.3.2 i Analysis of NMR, infrared and mass spectra

· All types of spectroscopy can be used to stretch students. The Royal Society of Chemistry (RSC) provides examples of spectra which can readily be accessed. Look online for other spectra examples – this resource provides spectra of most of the common organic molecules; however, it is easy to get carried away with this investigative procedure to the detriment of other parts of the specification!

1.3.13 Combined techniques

(Links to AS topics 2.2.3 Infrared spectra and 2.2.3 Mass spectra)

How fast?

5.1.1 b Half-lives of first order reactions

5.1.1 d Orders from rate–concentration graphs

5.1.1 e Initial rate graphs

· This section of the specification presents many opportunities for Stretch and Challenge; however, the caveat is that the students will need the necessary mathematical skills – or they will need to be taught these skills as part of their chemistry course. The specification asks that students recognise concentration–time graphs and rate–concentration graphs – beyond this, there exists the mathematical approach.

· For a reaction which is first order with respect to X, a plot of ln[X] against time should reveal a straight line and the gradient will equal –k, where k is the rate constant.

· For a reaction which is second order with respect to X, a plot of rate against [X]2 will be a straight line and a plot of 1/[X] against time will reveal a straight line and the gradient will equal k, where k is the rate constant.

· The relationship between the rate constant k and temperature T involves the Arrhenius equation and a plot of ln[X] against 1/T reveals a straight line where the gradient is equal to –Ea/R, where Ea is the activation energy and R is the universal gas constant.

2.1.4 Half-lives

2.1.5 Orders from rate–concentration graphs

2.1.6 Initial rates and the rate constant

How far?

5.1.2 d Effect of temperature upon Kc

5.1.1 g Rate constants

· Once again, there is some scope for stretching the more mathematically able. There is no requirement for the use of quadratic equations, so this one obvious development may not serve a purpose.

· However, the relationship between the equilibrium constant Kc and temperature T can be investigated graphically by plotting ln K against 1/T – the gradient will give –∆H/R.

· On a slightly simpler level, students might like to investigate the relationship Kc = kf/kr (where kf is the rate constant of the forward reaction and kr is the rate constant of the reverse reaction).

2.1.10 The equilibrium position and Kc

2.1.11The equilibrium constant, Kc, and the rate constant, k

Acids, bases and buffers

5.1.3 a-c Brønsted–Lowry acids and bases

5.1.3 h The ionic product of water

5.1.3 g pH calculations

5.1.3 n Calculation of pH of buffers

· The development of the understanding of acids can be taken from Arrhenius through to Brønsted–Lowry, and continued through to Lewis acids.

· Ask students to consider that water can be both acidic and basic by using:

H2O + H2O ⇌H3O+ + OH–

as indeed can ammonia

NH3 + NH3 ⇌ NH4+ + NH2–

· Develop the idea that for water pH 7 is neutrality

(i.e. [H+] = [OH–]) at 25 °C, yet, because Kw is temperature dependent at higher temperatures, neutrality occurs at a pH lower than 7.

· Investigate the relationship between ratios of concentrations of weak acids to their sodium salts and the pH of the buffer solution produced.

2.1.12 The road to acids

2.1.18 The ionisation of water

2.1.18 The ionisation of water

2.1.21 pH values of buffer solutions

Lattice enthalpy

5.2.1 b Born–Haber cycles

· Lattice enthalpy tends to be a straightforward topic with many definitions and conventions. One possible extension here is to use Born–Haber cycles to calculate the enthalpy of formation of theoretical compounds such as MgCl3 (it is highly positive due to the high third ionisation energy of Mg).

2.2.2–2.2.4 Constructing Born–Haber cycles; Calculations; Further examples

Enthalpy and entropy

5.2.2 f ∆G calculations

· The variation of ∆G with temperature can lead on to the introduction of simple Ellingham diagrams. This provides a straightforward but not too mathematical extension of ∆G.

2.2.8 Free energy

Electrode potentials and fuel cells

5.2.3 g,h Feasibility of reactions

5.2.3 i–p Storage and fuel cells

· Explain the effect of increasing/decreasing concentrations of one (or more) of the solutions upon the e.m.f. of a cell.

· Discuss the development of fuel cell vehicles, particularly hydrogen-based ones – this is continually evolving so there will be much new material for students to read, often from scientific publications such as Education in Chemistry and New Scientist.

2.2.12 The feasibility of reactions

2.2.13–2.2.14 Storage and fuel cells; Hydrogen for the future

Transition elements

5.3.1 e–i Ligands and complex ions

5.3.1 c Catalytic behaviour

· Here, the scope for Stretch and Challenge is great. The specification keeps to a relatively small number of ligands and complex ions, but the ideas used can be applied to any complex. Consequently, complex ion shapes and ligand exchange reactions can be investigated involving transition elements outside the first row and involving any ligands – however, it should be stressed that the general concepts must stay within the specification.

· There are many reactions involving heterogeneous catalysis. Less frequent are those involving homogeneous catalysts. One example for students to investigate is the redox reaction:

S2O82–(aq) + 2I–(aq) → 2SO42–(aq) +I2(aq)

which is catalysed by Fe3+(aq) ions.

Reactions such as these can involve references to many parts of the specification.

2.3.4–2.3.7 Transition metals and complex ions; Stereoisomerism in complex ions; Bidentate and multidentate ligands; Ligand substitution

2.3.3 Catalysis and precipitation

(Links to A2 5.3.1 Redox reactions/titrations;

A2 5.2.3 Electrode potentials; and AS 2.3.2 Catalysis)

One valuable source of questions which are specifically designed to stretch students is the RSC Chemistry Olympiad, which takes place in late January/early February and is sat by Year 13 students. Although these questions are not specification specific, they are designed to get students thinking about chemistry as a whole. The questions are ramped up in terms of difficulty, so even the most able come away feeling taxed!

What type of skills will students need?

· In order to perform well on Stretch and Challenge questions, students will require a similar range of skills to those previously required on synoptic papers.

· They need to be able to make synoptic links across modules and potentially back to GCSE – although this is not directly required in the specification.

· Students must be able to think more widely or deeply about a subject. They must be able to form an opinion or consider possibilities they haven't encountered in the specification. However, this should be based on scientific principles not personal opinion.

· Although not all Stretch and Challenge questions will require extended writing skills, many will. The students will need to be able to:

· structure an argument

· provide evidence for any opinions offered

· provide a well-balanced summary if asked to weigh options.

· Stretch and Challenge questions could equally require the application of existing knowledge to new situations in different ways, such as:

· labelling an unfamiliar diagram

· performing calculations.

It is important that you give your students the opportunity to practise such skills.

How will Stretch and Challenge be examined?

· Approximately 5–10% of the marks available in each paper will be allocated to Stretch and Challenge questions. These questions may be stand-alone, or they may be the final part of a several-part question.

· There will not be any type of flag or icon for Stretch and Challenge in the question paper itself – the questions will look like any other. However, they are usually ramped in terms of difficulty and are likely to appear towards the end of the paper.

· The Stretch and Challenge questions could call for:

· data analysis

· extended writing

· annotating a diagram

· performing a calculation.

Consequently, students will benefit from experience gained in the application of existing knowledge to perform such tasks.

· The command words used should also give students a clue that a particular question is inviting them to demonstrate Stretch and Challenge skills. Common command words include:

· explain

· deduce

· predict

· discuss

· suggest

· calculate

· determine

· sketch.

· In addition to the teaching suggestions outlined above, you could use some of the synoptic questions from the previous specification to give students practice in tackling Stretch and Challenge questions. Past papers for this specification – as they become available from OCR – will also be useful.

· A Stretch and Challenge advice document containing example questions is also available to students via the Exam Café CD-ROM that accompanies the OCR A2 Chemistry Student Book in this series.

Stretch and Challenge