AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2024: Question 2

✅ Correct answers

• Measure a fixed volume of hydrochloric acid using a measuring cylinder.
• Measure the mass of calcium carbonate using a balance.
• Place the calcium carbonate in the conical flask.
• Add the hydrochloric acid to the flask, then quickly fit the stopper and delivery tube.
• Start the timer as soon as the reactants are mixed.
• Measure the volume of carbon dioxide at set time intervals, or time how long it takes to collect a fixed volume of gas.
• Repeat with different sized pieces of calcium carbonate.
• Keep other variables the same: mass of calcium carbonate, volume of acid, concentration of acid, and temperature.
• Repeat each test and compare results.

💡 Key knowledge

• The investigation is about particle size, so that is the only thing that should change.
• Smaller pieces have a larger surface area, so the reaction is faster.
• A valid result needs a fair test and repeat measurements.

🧠 Exam technique

• Write the method in the correct order, like a real experiment.
• Use precise practical verbs: measure, add, start timer, record, repeat.
• To reach Level 3, the method must clearly produce a valid outcome and be logically sequenced.
• Include at least one way of measuring rate, not just “it reacts faster”.

📐 Step-by-step model method

1. Use a balance to measure the same mass of calcium carbonate for each test.
2. Use a measuring cylinder to measure the same volume of hydrochloric acid each time.
3. Keep the acid concentration and temperature the same.
4. Put the calcium carbonate into the conical flask.
5. Add the acid, then immediately fit the stopper and delivery tube.
6. Start the timer as soon as the acid is added.
7. Record the volume of carbon dioxide at regular time intervals, or time how long it takes to collect a fixed volume.
8. Repeat the experiment using different sized pieces of calcium carbonate.
9. Repeat each test and calculate a mean if needed.

❌ Common errors

• Changing more than one variable, such as mass and particle size.
• Forgetting to say how the gas is measured.
• Not mentioning a control variable, especially temperature or acid volume.
• Writing “use the same equipment” instead of explaining what is kept constant.
• Not repeating results, which loses marks for validity.

✅ Full-mark answer

Increasing the temperature increases the rate of reaction because particles have more energy and move faster. This means the frequency of collisions increases, so more collisions happen in a given time.

💡 Key knowledge

• Higher temperature = faster-moving particles.
• More collisions per second = faster reaction.
• You can also say a greater proportion of collisions have enough energy to react.

🧠 Exam technique

• To get all 3 marks, link the ideas: temperature → energy/speed → collisions → rate.
• Use particle language, not just “it goes faster”.
• One mark is for the rate change, one for energy/more movement, one for collision frequency or successful collisions.

✅ Correct answer

A catalyst is a substance that increases the rate of a reaction and is not used up during the reaction.

💡 Key knowledge

• Catalysts speed up reactions.
• They are unchanged at the end of the reaction.
• They do not get used up, so they can be reused.

🧠 Exam technique

• This is usually 2 marks: one for speeding up the reaction, one for not being used up.
• If you write “it changes the rate” AQA may allow it, but “increases the rate” is safer.

✅ Correct answer

Enzymes

💡 Key knowledge

• Enzymes are biological catalysts.
• They speed up reactions in living organisms.

🧠 Exam technique

• One-word answer only: enzymes.
• Spelling should be close enough to be clearly recognisable.

Reaction-rate experiment

• Measure variables accurately.
• Change only particle size.
• Keep other variables constant.
• Measure carbon dioxide produced over time.
• Repeat for reliability.

Temperature explanation

• Particles gain more energy.
• They move faster.
• Collisions happen more often.
• Rate increases.

Catalyst facts

• Speeds up reaction.
• Not used up.
• Biological catalysts are enzymes.