AQA GCSE Chemistry 8462: Energy changes – 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 4.5 Energy changes of the AQA GCSE Chemistry (8462) specification, for teaching from September 2016 onwards and exams in 2018 onwards (version 1.1): spec points 4.5.1.1 to 4.5.2.2 and required practical activity 4. The topic is examined on Paper 1 at Foundation and Higher Tier. Questions 6, 7, 8 and 11(b) 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. Show every line of working in calculations – a correct method can earn marks even if the final number slips. For “Evaluate” questions, give points on both sides and finish with a judgement. Where bond energies are needed, they are supplied, as they are in the exam.

Questions

1. A student mixes two pairs of solutions and records the temperatures.

Reaction	Start (°C)	End (°C)
X	18.5	26.0
Y	22.0	17.5

(a) Calculate the temperature change for each reaction. [2] (b) State which reaction is endothermic. Give a reason for your answer. [2]

2. State whether each reaction is exothermic or endothermic.

(a) Methane burning in air. [1] (b) The thermal decomposition of calcium carbonate. [1]

3. A walker compares two hand warmers.

	Warmer P	Warmer Q
Highest temperature	55 °C	50 °C
Time it stays warm	8 hours	40 minutes
Can it be reused?	No – used once	Yes – reset in boiling water

Evaluate which warmer is better for a full day’s walk in cold weather. [4]

4. A student investigates the temperature change when hydrochloric acid neutralises sodium hydroxide solution. She repeats the experiment four times. The temperature rises are 4.5 °C, 4.9 °C, 6.8 °C and 4.7 °C.

(a) Identify the anomalous result. [1] (b) Calculate the mean temperature rise. [2] (c) The measured rise is lower than the true value. Suggest why, and give one improvement. [2] (d) Name the dependent variable. [1]

5. On a reaction profile, the reactants are at 60 kJ, the top of the curve is at 260 kJ and the products are at 180 kJ.

(a) Calculate the activation energy. [1] (b) Calculate the overall energy change and state whether the reaction is exothermic or endothermic. [2] (c) Explain what is meant by activation energy. [1]

6. (Higher tier only) Methane burns: CH₄ + 2O₂ → CO₂ + 2H₂O. Bond energies in kJ/mol: C–H 413, O=O 498, C=O 805, O–H 464. Calculate the energy change for the reaction and explain whether it is exothermic or endothermic. [4]

7. (Higher tier only) For H₂ + F₂ → 2HF the energy change is −540 kJ/mol. Bond energies in kJ/mol: H–H 436, F–F 158. Calculate the H–F bond energy. [3]

8. (Higher tier only) Hydrogen bromide decomposes: 2HBr → H₂ + Br₂. Bond energies in kJ/mol: H–Br 366, H–H 436, Br–Br 193.

(a) Calculate the energy change for the reaction. [3] (b) State whether the products are higher or lower than the reactants on the reaction profile. Give a reason. [2]

9. A student makes cells using different metals paired with copper in the same electrolyte.

Metal paired with copper	Voltage (V)
Magnesium	2.6
Aluminium	1.9
Zinc	1.0
Tin	0.4
Copper	0.0

(a) Put magnesium, aluminium, zinc and tin in order of reactivity, most reactive first. [1] (b) Predict the voltage of a cell made from magnesium and tin. [2] (c) Explain why two copper electrodes give 0 V. [1] (d) Two magnesium–copper cells are joined in series to make a battery. State its voltage. [1]

10.

(a) Explain why an alkaline battery eventually stops working. [1] (b) Explain why a rechargeable battery can be used again. [2]

11. A city council plans to replace diesel buses with either hydrogen fuel cell buses or buses powered by rechargeable batteries.

(a) Evaluate the use of hydrogen fuel cells compared with rechargeable batteries for the buses. [6] (b) (Higher tier only) Write the half equations for the reactions at the two electrodes of a hydrogen fuel cell with an acidic electrolyte. [2]

12. A student adds different masses of zinc powder to 25 cm³ of copper sulfate solution. The starting temperature is 20.0 °C each time.

Mass of zinc (g)	0.5	1.0	1.5	2.0
Highest temperature (°C)	24.2	28.3	32.5	32.6

(a) Calculate the temperature rise for each mass. [2] (b) Describe the pattern in the temperature rises. [2] (c) Suggest why the temperature rise stops increasing above 1.5 g. [2] (d) Is the reaction exothermic or endothermic? Explain in terms of the energy of the reactants and products. [2]

Answers

1. (a) X: 26.0 − 18.5 = +7.5 °C [1]; Y: 17.5 − 22.0 = −4.5 °C [1] (b) Y [1]; the temperature of the surroundings decreased [1]. Examiner insight: The reason must refer to the temperature falling; “it took in energy” alone does not use the data you were given.

2. (a) Exothermic [1] (b) Endothermic [1] Examiner insight: One-word answers are fine here, but “exo” or “endo” abbreviations are risky – write the full word.

3. P stays warm for 8 hours compared with 40 minutes, so it covers the whole walk [1]; P reaches a higher temperature [1]; Q can be reused, so it is cheaper over time and makes less waste [1]; judgement: P is better for a full day because Q would need resetting in boiling water, which is not available on a walk [1]. Examiner insight: An evaluation needs a conclusion that follows from the data; listing points without a judgement caps the marks.

4. (a) 6.8 °C [1] (b) (4.5 + 4.9 + 4.7) ÷ 3 [1] = 4.7 °C [1] (c) Energy is transferred to the surroundings (air, cup) [1]; use a lid or insulate the cup [1]. (d) The temperature change [1] Examiner insight: Including the anomalous value in the mean loses the method mark, even if the arithmetic is correct.

5. (a) 260 − 60 = 200 kJ [1] (b) 180 − 60 = +120 kJ [1]; endothermic, because the products are higher than the reactants [1] (c) The minimum energy particles must have to react [1]. Examiner insight: Both arrows start at the reactant level; measuring activation energy from the products is a common source of a wrong value.

6. Bonds broken: (4 × 413) + (2 × 498) = 2648 kJ/mol [1] Bonds formed: (2 × 805) + (4 × 464) = 3466 kJ/mol [1] Energy change: 2648 − 3466 = −818 kJ/mol [1] Exothermic: more energy is released forming bonds than is needed to break bonds [1]. Examiner insight: Error carried forward is usually allowed – if one sum is wrong but you subtract correctly, you can still earn the later marks.

7. 436 + 158 − 2x = −540 [1] 2x = 594 + 540 = 1134 [1] H–F = 567 kJ/mol [1] Examiner insight: Forgetting that two H–F bonds form gives 1134, which earns only the setting-up mark.

8. (a) Broken: 2 × 366 = 732 kJ/mol [1]; formed: 436 + 193 = 629 kJ/mol [1]; change = 732 − 629 = +103 kJ/mol [1] (b) Higher [1]; the reaction is endothermic because more energy is needed to break bonds than is released forming them [1]. Examiner insight: A positive sign is part of the answer; write “+103” or state “endothermic” to show you know the direction.

9. (a) Magnesium, aluminium, zinc, tin [1] (b) 2.6 − 0.4 [1] = 2.2 V [1] (c) There is no difference in reactivity because both electrodes are the same metal [1]. (d) 5.2 V [1] Examiner insight: A prediction from data should show the subtraction; a bare “2.2” with no working risks losing the method mark if it is misread.

10. (a) One of the reactants has been used up [1]. (b) The chemical reactions are reversed [1] when an external electrical current is supplied [1]. Examiner insight: Two marks need two ideas: the reversal and the external current.

11. (a) Any six creditworthy points, including a conclusion: Fuel cells produce only water, so no pollution at the bus [1]; fuel cell buses do not need long recharging, only refuelling [1]; batteries can only be recharged a limited number of times [1]; batteries contain chemicals that are harder to dispose of [1]; hydrogen is a flammable gas that is hard to store and needs pressurised tanks [1]; conclusion: fuel cells are the better choice for long bus routes, but only if the hydrogen is made without fossil fuels, because hydrogen is often made using fossil fuels [1]. (b) 2H₂ → 4H⁺ + 4e⁻ [1]; O₂ + 4H⁺ + 4e⁻ → 2H₂O [1] Examiner insight: In a six-mark evaluation, points must be comparisons with batteries, not just facts about fuel cells, and a supported conclusion is needed for full marks.

12. (a) 0.5 g: 4.2 °C and 1.0 g: 8.3 °C [1]; 1.5 g: 12.5 °C and 2.0 g: 12.6 °C [1] (b) The rise increases roughly in proportion to the mass up to 1.5 g [1]; then it levels off [1]. (c) All the copper sulfate has reacted, so zinc is in excess [1]; no more reaction happens, so no more energy is transferred [1]. (d) Exothermic, because the temperature of the surroundings increases [1]; the products have less energy than the reactants [1]. Examiner insight: “Describe the pattern” needs both parts of the trend – the increase and the levelling off – quoted with a value where the change happens.

Where marks are usually lost

• Calling a reaction endothermic because “it needs heat” instead of using the temperature fall of the surroundings.
• Including anomalous results in a mean.
• Measuring activation energy from the products instead of the reactants.
• Missing the multiplier for identical bonds (four C–H in CH₄, two O–H in each H₂O).
• Subtracting formed − broken and ending with the wrong sign.
• Giving an energy change with no unit.
• Evaluations with no comparison and no conclusion.
• Saying cells in a battery are connected “in parallel” to increase voltage.
• Half equations where electrons do not balance, or electrons shown on the wrong side.

Next steps

• Energy changes revision notes
• Energy changes study guide
• The rate and extent of chemical change 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 4.5 Energy changes.