Chemistry Exam Technique: Maximise Your Potential

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A-level Chemistry By: Please log in to see tutor details
Subject: Chemistry » A-level Chemistry
Last updated: 06/03/2018
Tags: a-level chemistry, chemical, chemistry calculations, command words, exam technique

Overall Approach

- Scan the whole paper and look for questions you like the look of. Consider doing those first.
- Get the 'easy' marks in the bank quickly.
- Spend what time you have left working on harder questions

Approaching Individual Questions

- Read the whole question before you start.
- You may realise you have misread or misunderstood an earlier part when you read a later part of the question.
- You may find pointers to help with the answer of one part of the question in a later part.
- Look for the command words or verbs (see below) that tell you what type of answer the Examiner is looking for.
- If you are stuck on a particular part, don't spend to long – leave it and come back later. When you come back, read the question again as you may have misread or misunderstood the first time and it may now all become clear.
- Don't spend too long on calculations in multiple choice questions. A calculation that might be worth 2 or even 3 marks elsewhere will only get you 1 mark in the multiple choice section. If you have time, come back later.
- Look for short-cuts in calculations in multiple choice questions. These are often built in to the question to reward the brighest candidates; the less able candidates may still get the mark, but they will have spent a lot longer so risk missing out elsewhere.
- Look at the information given and think about how you can use it. This is particularly important in calculations (see below), but also applies to other sorts of questions.
- Look at the number of marks for each section and use it as a guide to how much you need to write (the space allowed is also a guide). There is usually one mark for every (correct) point you make, but don't think you will get credit for saying the same thing twice in slightly different ways.
- Don't give more information than you have to. If you have already made enough points and are not sure about something else then it is better left out. You can lose one or all of the marks you have gained if the examiner sees something that is just plain wrong. It's called the 'list principle' and is intended to penalise candidates who just regurgitate a whole load of information without thinking if it is right or relevant.

Command Verbs (Key Words)

These tell you what type of answer to give. Examiners always say that candidates don't read the question. Often, what they mean is that the candidates didn't pay attention to these key words.

Describe…
This may refer to a graph, a reaction / process or a set of data. It means "say what you see" without going into too much (if any) background information.

State…
"Say what it is" – it may be a definition or a relationship. Again, don't give too much unnecessary information – as long as you do actually state what was asked for.

Explain…
Again, this could be a graph, relationship, equation, process, etc. You have to both describe what is happening and then say why. In other words, what are the chemical / biological principles involved.

Evaluate…
Your answer normally needs to address both sides of the argument – for and against.

Economic or Industrial
When asked about the economic or industrial importance / relevance / significance of a process, reaction, etc. your answer MUST include some reference to value of or demand for the products. If the question asks about why one process, reaction, etc. would be favoured over another then you need to refer to issues such as safety (e.g. greater cost of high pressure containment equipment) or avoidance of toxic / hazardous / harmful substances (e.g. acids, halogens, etc.).

Edexcel publish a full list of command verbs in their specifications. You can also find a comprehensive list of command words in the OCR Practical Skills Handbooks (chemistry or biology). Take a look at either of these – they are available from the Edexcel and OCR websites - even if you are studying an AQA specification.

Chemical Equations

You are quite likely to be asked to "write an equation for the reaction between X and Y". This may well be an unfamiliar equation (or one you have forgotten). DON'T PANIC. Start with what you are given and (using your chemical knowledge!) work out what might happen. For example:

4 (b) Catalytic converters are used to remove the toxic gases NO and CO that are produced when alkane fuels are burned in petrol engines.

4 (b) (i) Write an equation for a reaction between these two toxic gases that occurs in a catalytic converter when these gases are removed.

The reaction is between NO and CO, so the starting point has to be: NO + CO -->

Next, we know that these gases are toxic and the purpose of the catalytic converter is to remove them, so the products must be (relatively) harmless. The obvious products are either CO2 and N2 or NO2 and C. Carbon (solid) would block the catalytic converter and NO2 isn't exactly harmless, so the products must be CO2 and N2. Now, all you have to do is balance the equation.

Adding Chemical Equations

This is just algebra! You may be given two half equations (e.g. in a redox reaction or ionic equation) and asked to write the equation for the reaction, or it might be two steps in a process and you are asked to write an equation for the overall process.

The principle is always the same. Add the equations together as if they were algebraic equations and cancel out anything that appears on both sides (watch out for numbers!) to simplify the equation.

With half equations for redox reactions, you will usually have two equilibrium reactions and one will have to be reversed, so make sure you include everything in your combined equation and that they are on the correct side.

Calculations

Without going into the different types of calculations you will encounter, the principles are always the same. Unless you can see immediately how to perform the calculation or solve the problem, look at what you are given and think about what you can calculate with it. For example, if you know the concentration and the volume of a solution, you can calculate the number of moles of reactant or product and then use the chemical equation to find the unknowns. Work from what you have and a route to the answer may become clear.

Conversions

You need to remember the following:
1 m3 = 103 dm3 = 106 cm3 so 1 cm3 = 10-3 dm3 = 10-6 m3
1 dm3 = 103 cm3
1 kg = 103 g = 106 mg = 109 µg
1 g = 103 mg = 106 µg so 1 µg = 10-3 mg = 10-6 g
1 m = 103 mm = 106 µm = 109 nm so 1 nm = 10-3 µm = 10-6 mm = 10-9 m

Notice that each step is a factor of 103 or so you multiply by 1,000 when going from smaller to larger units. Going from larger to smaller units the factor is 10-3, so you divide by 1,000. Some students like to write out a table of conversion factors at the start of the exam to help them remember. The centimeter (cm) is an exception, because there are 10mm in 1 cm and 100 cm in 1 m.

Units in Calculations

Watch out for calculations involving the ideal gas equation, PV = nRT . P is in Pa, but the data may be given in kPa or MPa, so don't forget to convert. Remember that k = 103 and M = 106.

Standard Enthalpy of Combustion / Formation / Ionisation, etc. always applies to 1 mole of substance under standard conditions (temperature, pressure, etc.).

And finally….

Rigour of Expression

Many marks are lost through being almost right. As the marking process becomes more rigorous and standardised, it is becoming harder for a generous examiner to give the benefit of the doubt to a candidate that seems to have an understanding of the correct answer, even though they may not have answered in accordance with the mark scheme. Now, if your answer doesn't correspond closely to the mark scheme, you won't get any credit. As you work through past papers, look carefully at the mark schemes to see what terms the examiner expects in answer to certain questions – the same type of thing comes up year after year. Look also at the additional guidance to see what alternative wording would be acceptable and what would not. Get into the habit of using the acceptable terms in your answers and avoiding the unacceptable ones.

The following terms can cause confusion in biology and chemistry practical and data interpretation questions:

Accuracy is an assessment of how close the obtained value is to the true value. Accuracy can be assessed by the calculation of (or comment on) the percentage error, or comment on the accuracy of pieces of apparatus. Accuracy can also be assessed by commenting on how the trend line compares with the theoretical trend line.

Reliability is a measure of the confidence that can be placed in a set of observations or measurements.

Precision is the number of decimal places to which any measurement can be recorded, as determined by the apparatus used.

Limitations are factors that have not been controlled or taken into account in the design of the procedure. These can be described as 'design faults' of the procedure, and will affect each run and replicate equally throughout the investigation.

Error is something that has occurred on one (or possibly more) occasion(s). This effects intermittent and random results, and may be due to a mistake by the investigator.

The validity of an experiment or investigation depends upon factors such as the range and reliability of the observations or measurements that underpin it, any assumptions made in developing hypotheses or planning the investigation, and the nature of the investigation itself.

Range bars plot the highest and lowest data (i.e. no mathematical skill is demonstrated), whereas error bars require that the standard deviation or standard error is calculated and then plotted above and below the mean.

There is more useful information on these aspects in the OCR Practical Skills Handbooks (Chemistry and Biology) that can be downloaded from the OCR website; they are useful even if you are studying a different specification.


S Hoare A-level Chemistry Tutor (Nottingham)

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