Rate and Equilibrium: Exam Practice

This final lesson in the AQA GCSE Chemistry Rates and Equilibrium topic brings together everything you have learned. It provides key definitions, common question types, model answers, and a comprehensive revision strategy. Use this lesson to consolidate your knowledge and prepare for the exam.

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Key Definitions to Memorise

The following definitions are taken directly from or aligned with the AQA GCSE Chemistry specification. You must be able to write them accurately under exam conditions.

Term	Definition
Rate of reaction	The speed at which reactants are used up or products are formed
Collision theory	For a reaction to occur, particles must collide with sufficient energy (and correct orientation)
Activation energy (Ea)	The minimum amount of energy that colliding particles must have for a reaction to occur
Catalyst	A substance that increases the rate of a chemical reaction without being used up; provides an alternative pathway with lower activation energy
Reversible reaction	A reaction that can proceed in both the forward and reverse directions
Dynamic equilibrium [H]	When the rate of the forward reaction equals the rate of the reverse reaction in a closed system; concentrations of reactants and products are constant
Le Chatelier's principle [H]	If a system at equilibrium is subjected to a change, the position of equilibrium shifts to oppose the change
Closed system	A system where no matter can enter or leave

Exam Tip: Memorise these definitions word for word. In AQA GCSE Chemistry, many 1-mark and 2-mark questions simply ask you to define a term. If your definition is imprecise, you may lose the mark. Practise writing them from memory until they are automatic.

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Common Question Type 1: Calculating Rate

Typical question: "24 cm3 of gas was collected in 60 seconds. Calculate the mean rate of reaction. Give your answer to 2 significant figures."

Model answer:

mean rate = volume of gas / time

mean rate = 24 / 60

mean rate = 0.40 cm3/s

Common Mistakes	How to Avoid Them
Forgetting units	Always write the unit (e.g. cm3/s, g/s)
Wrong number of significant figures	Read the question carefully and round correctly
Dividing time by volume (instead of volume by time)	Rate = quantity / time, not time / quantity

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Common Question Type 2: Explaining Factors Using Collision Theory

Typical question: "Explain why increasing the temperature increases the rate of reaction." (3 marks)

Model answer:

1. Increasing the temperature gives particles more kinetic energy, so they move faster (1 mark)
2. Particles collide more frequently (1 mark)
3. A greater proportion of collisions have energy equal to or greater than the activation energy, so more collisions are successful (1 mark)

graph TD
    A[Increase Temperature] --> B[More kinetic energy]
    B --> C[Faster-moving particles]
    C --> D[More frequent collisions]
    C --> E[Greater proportion with energy >= Ea]
    D --> F[More successful collisions per second]
    E --> F
    F --> G[Rate increases]

Exam Tip: For temperature, you MUST mention the activation energy to get full marks. Simply saying "particles move faster and collide more often" will typically earn only 2 out of 3 marks. The third mark comes from stating that "a greater proportion of particles have energy equal to or greater than the activation energy."

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Common Question Type 3: Interpreting Rate Graphs

Typical question: "The graph shows two experiments. Experiment A used 2.0 mol/dm3 HCl and Experiment B used 1.0 mol/dm3 HCl. Describe and explain the differences between the two curves."

Model answer:

Feature	Experiment A (2.0 mol/dm3)	Experiment B (1.0 mol/dm3)
Initial gradient	Steeper	Less steep
Initial rate	Faster	Slower
Final volume of gas	Same	Same
Time to reach completion	Shorter	Longer

Explanation: Experiment A has a higher concentration of HCl, so there are more acid particles per unit volume. This means collisions between acid particles and marble chips are more frequent, leading to more successful collisions per second and a faster rate. The final volume of gas is the same because the same mass of marble chips (the limiting reactant) was used in both experiments.

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Common Question Type 4: Required Practical Questions

Typical question: "Describe an experiment to investigate the effect of concentration on the rate of reaction between hydrochloric acid and marble chips." (6 marks)

Model answer structure:

Mark Point	What to Write
1	Place 5 g of marble chips (same size) in a conical flask
2	Add 50 cm3 of HCl at a known concentration; start the stopwatch and attach the bung and gas syringe
3	Record the volume of CO2 gas every 10 seconds for 2 minutes
4	Repeat with different concentrations of HCl (e.g. 0.5, 1.0, 1.5, 2.0 mol/dm3)
5	Control variables: same mass and size of marble chips, same volume of acid, same temperature
6	Repeat each experiment at least 3 times and calculate a mean for reliability

Exam Tip: In 6-mark extended response questions, the examiner is looking for a well-structured, detailed answer. Use paragraphs or bullet points to organise your response. Include specific values (e.g. "5 g", "50 cm3", "10-second intervals") to show practical understanding. Always mention control variables and repeats.

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Common Question Type 5: Le Chatelier's Principle [H]

Typical question: "The equation for the Haber process is: N2(g) + 3H2(g) <=> 2NH3(g). The forward reaction is exothermic. Predict and explain the effect of increasing the pressure on the yield of ammonia." (3 marks)

Model answer:

1. Increasing the pressure shifts the equilibrium to the right (towards the products) (1 mark)
2. There are 4 moles of gas on the left and 2 moles on the right; by Le Chatelier's principle, the system opposes the increase in pressure by shifting towards the side with fewer moles of gas (1 mark)
3. Therefore, the yield of ammonia increases (1 mark)

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Common Question Type 6: Explaining Compromise Conditions [H]

Typical question: "Explain why a temperature of 450 degrees Celsius is used in the Haber process rather than a lower temperature." (3 marks)

Model answer:

1. A lower temperature would shift the equilibrium to the right (exothermic direction), giving a higher yield of ammonia (1 mark)
2. However, a lower temperature would give a slower rate of reaction because fewer particles would have energy equal to or above the activation energy (1 mark)
3. 450 degrees Celsius is a compromise — it gives a reasonable yield AND a reasonable rate of reaction, making the process economically viable (1 mark)

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Revision Checklist

Use this checklist to ensure you have covered every part of the topic: