AQA GCSE Chemistry 8462: The rate and extent of chemical change – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover section 4.6 The rate and extent of chemical change of the AQA GCSE Chemistry (8462) specification, for teaching from September 2016 onwards and exams in 2018 onwards (version 1.1): spec points 4.6.1.1 to 4.6.2.7 and required practical activity 5. The topic is examined on Paper 2 at Foundation and Higher Tier. Questions 2(b), 3(c), 10 and 11 use content the specification marks (HT only) and are labelled Higher tier only.

Learn the content first in the study guide and the revision notes. The course hub is AQA GCSE Chemistry and the printable checklist lists every topic.

How to use this set. Write a unit with every rate. In “Explain” answers, link each idea to the next with “so” or “because”: particle change, then collision frequency or energy, then rate. For equilibrium questions, name the direction of shift and what happens to the amount of the product asked about.

Questions

1. Magnesium reacts with dilute hydrochloric acid. 36 cm³ of hydrogen is collected in 45 s. Calculate the mean rate of reaction. [2]

2. Marble chips react with acid in an open flask on a balance. The mass falls by 1.32 g in 100 s as carbon dioxide escapes.

(a) Calculate the mean rate of reaction in g/s. [2] (b) (Higher tier only) Calculate the mean rate in mol/s. Mr of CO₂ = 44. [2]

3. A student collects the gas from a reaction.

Time (s)	0	10	20	30	40	50	60	70
Volume (cm³)	0	22	38	50	57	60	60	60

(a) Calculate the mean rate of reaction over the first 20 s. [2] (b) State the time at which the reaction stopped. [1] (c) (Higher tier only) A tangent drawn to the curve at 20 s passes through (0 s, 10 cm³) and (40 s, 66 cm³). Calculate the rate at 20 s. [2] (d) Explain why the rate decreases as the reaction goes on. [2]

4. Explain why increasing the temperature increases the rate of a reaction. [3]

5. A block of calcium carbonate measures 4 cm × 2 cm × 1 cm.

(a) Calculate its surface area to volume ratio. [3] (b) The block is crushed into eight 1 cm cubes. The ratio becomes 6 : 1. Explain how this affects the rate of reaction with acid. [3]

6. A student investigates how the concentration of sodium thiosulfate affects the time for a cross to disappear when hydrochloric acid is added.

Concentration (mol/dm³)	0.05	0.10	0.15	0.20
Time (s)	124	62	41	31

(a) Calculate 1/time for 0.10 mol/dm³ and 0.20 mol/dm³. Give answers to 3 significant figures. [2] (b) Describe the relationship between concentration and rate shown by the data. [1] (c) Explain this relationship using collision theory. [2] (d) Give two control variables. [2]

7. Hydrogen peroxide decomposes slowly to water and oxygen. A student adds 0.50 g of manganese(IV) oxide. In 60 s, 8 cm³ of oxygen forms without it and 46 cm³ forms with it. At the end, 0.50 g of manganese(IV) oxide is recovered.

(a) Give two pieces of evidence that manganese(IV) oxide is a catalyst. [2] (b) Explain how a catalyst increases the rate of reaction. [2] (c) Explain why manganese(IV) oxide is not included in the equation for the reaction. [1]

8. Hydrated copper sulfate (blue) ⇌ anhydrous copper sulfate (white) + water

(a) What does the symbol ⇌ show? [1] (b) Water is added to white anhydrous copper sulfate. Describe what you would see and what happens to the temperature. [2]

9. State the two conditions needed for a reversible reaction to be at equilibrium. [2]

10. (Higher tier only) Sulfur dioxide reacts with oxygen: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g). The table shows the percentage of SO₃ in the equilibrium mixture at a fixed pressure.

Temperature (°C)	400	500	600
SO₃ at equilibrium (%)	98	93	80

(a) Use the data to deduce whether the forward reaction is exothermic or endothermic. [2] (b) Predict the effect of increasing the pressure on the amount of SO₃. Explain your answer. [2] (c) Predict the effect of removing SO₃ from the mixture as it forms. [2]

11. (Higher tier only) A factory makes gas Z from gases X₂ and Y₂: X₂(g) + 3Y₂(g) ⇌ 2Z(g). The forward reaction is exothermic.

(a) A very low temperature would give the highest yield of Z. Explain why it is not used. [3] (b) Explain the effect of a high pressure on the rate of reaction and on the yield of Z. [4] (c) Explain how using a catalyst helps the factory. [2]

Answers

1. 36 ÷ 45 [1] = 0.8 cm³/s [1] Examiner insight: The unit is part of the accuracy mark; “0.8” alone is incomplete.

2. (a) 1.32 ÷ 100 [1] = 0.0132 g/s [1] (b) 1.32 ÷ 44 = 0.030 mol [1]; 0.030 ÷ 100 = 3.0 × 10⁻⁴ mol/s (0.0003 mol/s) [1] Examiner insight: If the moles are wrong but divided correctly by the time, error carried forward still earns the second mark.

3. (a) 38 ÷ 20 [1] = 1.9 cm³/s [1] (b) 50 s [1] (c) (66 − 10) ÷ (40 − 0) [1] = 1.4 cm³/s [1] (d) The concentration of the reactant decreases as it is used up [1], so collisions become less frequent [1]. Examiner insight: For (b), 70 s or 60 s is wrong – the reaction stops at the first time the volume stops changing.

4. Particles move faster, so collisions are more frequent [1]; collisions are more energetic [1]; so more collisions have at least the activation energy (more successful collisions) [1]. Examiner insight: The energy point is needed for full marks; frequency alone gains only one mark.

5. (a) Surface area = 2 × (8 + 4 + 2) = 28 cm² [1]; volume = 4 × 2 × 1 = 8 cm³ [1]; ratio = 3.5 : 1 [1] (b) The surface area to volume ratio increases (3.5 : 1 to 6 : 1) [1], so more particles are exposed to the acid [1], so collisions are more frequent and the rate increases [1]. Examiner insight: Show surface area and volume separately; a bare ratio with no working risks losing both method marks.

6. (a) 1 ÷ 62 = 0.0161 s⁻¹ [1]; 1 ÷ 31 = 0.0323 s⁻¹ [1] (b) The rate is (directly) proportional to concentration – doubling concentration doubles 1/time [1]. (c) Doubling concentration doubles the number of particles in the same volume [1], so collision frequency doubles [1]. (d) Any two: temperature, volume of thiosulfate, volume of acid, concentration of acid, same cross, same observer [1] [1]. Examiner insight: “Amount of solution” is too vague for a control variable; name volume or concentration.

7. (a) The rate increased (46 cm³ compared with 8 cm³ in 60 s) [1]; the mass of manganese(IV) oxide was the same at the end (0.50 g) [1]. (b) It provides a different pathway [1] with a lower activation energy [1]. (c) It is not used up in the reaction [1]. Examiner insight: Evidence answers should quote the data; “it made it faster” without the volumes may not score.

8. (a) The reaction is reversible [1]. (b) The white solid turns blue [1]; the temperature increases because this direction is exothermic [1]. Examiner insight: A colour change needs both colours or the final colour; “changes colour” alone does not score.

9. A closed system (reactants and products cannot escape) [1]; the forward and reverse reactions occur at exactly the same rate [1]. Examiner insight: “The reaction stops” contradicts the second point and can cancel the mark.

10. (a) Exothermic [1]; as temperature increases, the percentage of SO₃ decreases [1]. (b) The amount of SO₃ increases [1]; the equilibrium shifts to the right, which has fewer molecules (2 compared with 3) [1]. (c) More SO₃ forms, or more SO₂ and O₂ react [1], until equilibrium is reached again [1]. Examiner insight: In (a), the mark for the reason needs the trend from the data, not just a recalled rule.

11. (a) Lowering the temperature increases the yield because the forward reaction is exothermic [1]; but the rate would be too slow [1], because collisions are less frequent and fewer have the activation energy [1]. (b) Rate increases [1] because particles are closer, so collisions are more frequent [1]; yield increases [1] because the equilibrium shifts to the side with fewer molecules (2 compared with 4) [1]. (c) A catalyst lowers the activation energy [1], so a fast rate is possible at a lower temperature, giving a reasonable yield [1]. Examiner insight: Rate and yield are separate ideas; answers that merge them (“more product faster”) cannot earn the separate marks.

Where marks are usually lost

• Leaving units off rates, or writing cm³ for a mass-based rate.
• Reading the stopping time as the last time in the table rather than when the volume first stays constant.
• “More collisions” instead of “more frequent collisions”.
• Missing “more energetic collisions” for temperature.
• Giving a surface area to volume ratio with no working.
• Saying a catalyst “adds energy” instead of lowering the activation energy.
• Claiming reactions stop at equilibrium.
• Counting atoms instead of molecules for pressure questions.
• Mixing up rate (how fast) and yield (how much).

Next steps

• Rate and extent of chemical change revision notes
• Rate and extent of chemical change study guide
• Energy changes practice questions
• AQA GCSE Chemistry course hub
• Printable checklist
• All free 10-minute diagnostics
• Book a free trial class

Official syllabus

AQA GCSE Chemistry (8462) specification, for teaching from September 2016 onwards, exams in 2018 onwards, version 1.1 (4 October 2019), published by AQA – section 4.6 The rate and extent of chemical change.