Examiner Tips For OCR A Level Chemistry Paper 2

As an examiner for this paper, I have prepared the below tips for tackling the OCR A Level Chemistry Paper 2 - Synthesis and Analytical Techniques. These tips are based on my experience marking hundreds of scripts. 

Many students understand the chemistry, but lose lots of marks due to poor exam techniques. Therefore, I hope the below comments will help you avoid some common mistakes.

Synthesis involves the production of new chemical compounds by using an array of different chemical reactions. Analysis involves using a variety of techniques to identify a chemical compound, to assess the compound's purity and exact composition.

General Tips to Prepare You before Your Exam

In order to excel in this paper, you must make sure you have a sound understanding of the variety of laboratory techniques used in organic chemistry. 

You will familiarise yourself with these techniques during your core practicals in the laboratory. Make sure you understand why each of these techniques is used. 

So, for example, you should understand:

1. How and why Quickfit apparatus is used

2. The process of distillation and heating under reflux

3. How to use a separating funnel to remove an organic layer from an aqueous layer

4. Use of a drying agent

5. When to use filtration under reduced pressure

6. Recrystallisation

7. Measurement of melting points 

Make sure you are confident in writing and balancing chemical equations. This is an incredibly important skill throughout A-level Chemistry that should be able second nature by exam season. 

You should understand what a reaction mechanism is, and you must know the difference between free radical substitution, nucleophilic substitution and addition, and electrophilic substitution and addition. Many students mix up these mechanisms. 

Practice drawing out the mechanism and perfect where the curly arrow should be positioned. This is often something that goes wrong - forgetting to show the difference between single electrons and electron pairs is common (be clear with your handwritting!).

Make sure you learn by heart all of the different chemical reactions you have learned. For example, polymerisation, hydrolysis, esterification etc. Use mind maps to summarise all of these reactions, which will be useful for being able to devise two-stage synthetic routes. 

Have a sound understanding of all of the analytical techniques used in this course. Make sure you are using the datasheet correctly when interpreting data for the analysis of a compound.

Be confident in answering problem-solving questions. The only way you will be proficient at this is to practise as many examination questions as possible. Start with easier examples then move to more challenging questions once you feel more confident to tackle them. Save some of the latest papers until the end and practise whole papers under timed conditions. 

Note down your timings for each question and if a specific question has taken longer to answer then review that area later. 

Self-mark them and annotate the questions you got incorrect. You can look up the examiner's comments as well, which can give useful advice. You should then be able to look up the grade boundaries online and see how you performed. 

Note down any areas of weakness and re-address these by practising further questions aimed at those topics using online resources to hone your skills in that topic. 

Exam Technique Tips for Paper 2

Be careful with significant figures! 

OCR usually requires the final answer given to 3 significant figures (unless otherwise instructed in the question). 

For calculation questions worth 3 or more marks make sure that for each stage you write down the value to 3 or more significant figures (including a trailing zero). In fact, OCR will allow the whole calculator value. 

If you start to truncate values to 2 significant figures then you will lose marks. These marks can add up during a whole paper. 

Make sure you know the difference between molecular, structural, skeletal and displayed formulas. 

Usually, if not specified, any representation can be given or a hybrid structure. Some questions will be asked in bold for a specific representation which many students ignore which can lose them many marks.

Practice drawing the horseshoe intermediate in the electrophilic substitution reaction. 

Make sure that 𝝿-ring covers approximately 4 of the 6 sides of the benzene ring structure and the direction of this ring is correct. 

Make sure you know definitions word-for-word and learn reaction conditions by heart. 

When using spectroscopy to work out a chemical structure, annotate the spectrums. 

Also, draw the molecule and label which peaks in the NMR would be associated with each part of the molecule. This way it makes it clear to the examiner how you came to derive your chosen structure. Make sure you have commented on all of the data that was given. 

Tackling Analytical Problems

Stage 1

If given combustion analysis data first work out the empirical formula. Then work out the molecular formula from the empirical formula. From the mass spectrum state the M/Z of the molecular ion peak. The molecular ion mark is the peak with the greatest M/Z. Be careful not to quote the very small peak on the right of the molecular ion peak as it is due to the presence of carbon 13 isotopes in the molecule. 

Stage 2

Work out what is the functional group. Look at the infra-red spectrum and list the bonds and their wavenumbers both present and also which ones are absent. 

Stage 3

The question may give information on chemical tests, chemical reactions or smells. This will give you an idea of what the isomer may be. Make a note of your deductions from this information. 

Stage 4

If the carbon NMR is provided, note down how many carbon environments are present. Note down what potential carbon environments are present in the structure. 

Stage 5

If the proton NMR is provided then you could provide your information in a table format. You could label each proton Ha, Hb etc. Which could then be annotated onto the spectrum itself. 

The table should include the chemical shift, multiplicity, number of protons in each environment and what the proton environments are for each peak. As an examiner, I find this format is more clear than just writing the information in a paragraph. It also avoids missing any important information. 

Stage 6

If the mass spectrum is provided then note down any noticeable fragments. 

Stage 7

Draw out of the possible isomers that could be present using the information from the molecular formula, the functional groups from the infra-red spectrum, carbon and proton environments. Use the M/Z ratio of fragments that you would expect to find for the possible isomers. If they are not present then you can use this information to eliminate the isomer. 

Stage 8

Draw out and ring your final isomer. You could even then use this final drawing to annotate the proton environments present. 

I have included an example of an exemplary student gaining full marks on an analytical problem using the stages given above where necessary: 

Infrared Spectrum

Use the results to identify the unknown compound. Show all your reasoning.