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Topic 5: Energy Changes — Exam Practice

AQA GCSE Chemistry (8462) • Paper 1 • Structured Questions

📝 Exam Practice

Topic 5: Energy Changes Exam Practice

Test your knowledge with exam-style questions from past AQA GCSE Chemistry papers. Complete the multiple choice questions, then attempt the structured questions and check your answers against the mark scheme.

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📋 Structured Questions

These questions test key concepts from Topic 5. Attempt each question on paper, then click "Show Mark Scheme" to check your answer.

Question 1: Exothermic Reactions

2 marks

The reaction between hydrogen and chlorine is exothermic.

(a) Explain why this reaction releases energy to the surroundings. [2]

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  • Energy is needed to break bonds and energy is released when bonds form [1]
  • (and) the energy released (making bonds) is greater than the energy needed (breaking bonds) [1]
Examiner tip: A common mistake is implying that energy is required for both steps, or only mentioning bond making while omitting bond breaking. Never say "gases are lost and take energy with them" — that's a misconception.

Question 2: Temperature Changes in Reactions

2 marks

A student investigated the temperature change when zinc reacts with copper sulfate solution.

(a) Explain why the student used a polystyrene cup rather than a glass beaker for the reaction. [2]

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  • Polystyrene is a better (thermal) insulator [1]
  • (so) there is less energy transfer to the surroundings / less heat lost [1]
Examiner tip: Always use comparatives — write "better insulator" and "less heat lost" rather than just "it is an insulator". This is a practical evaluation question so link to accuracy of results.

Question 3: Fuel Cells vs Rechargeable Cells

2 marks

(a) Give two advantages of using hydrogen fuel cells instead of rechargeable cells to power cars. [2]

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Any two from:

  • No toxic chemicals to dispose of at end of cell's life [1]
  • Take less time to refuel (than to recharge) [1]
  • Travel further before refuelling / greater range [1]
  • No loss of efficiency over time [1]
Examiner tip: Avoid vague answers like "better for the environment" without specifics. Both fuel cells and rechargeable cells produce no emissions during operation — focus on disposal, range, and refuelling speed.

Question 4: Reaction Profiles

3 marks

A student drew part of a reaction profile for the exothermic reaction between hydrogen and chlorine. The reactants (H₂ + Cl₂) are drawn on a horizontal line.

(a) Complete the reaction profile. You should: complete the profile line showing the products, label the activation energy, and label the overall energy change. [3]

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  • Profile completed with the product energy level drawn below the reactant energy level [1]
  • Activation energy labelled with an arrow from the reactant energy line to the top of the curve [1]
  • Overall energy change labelled with an arrow from the reactant energy line to the product energy line [1]
Examiner tip: Use a ruler for arrows. A very common error is labelling the top of the curve as the activation energy rather than showing the gap between reactants and the peak.

Question 5: Overall Energy Change Calculation

3 marks

4NH₃ + 3O₂ → 2N₂ + 6H₂O. Bond energies (kJ/mol): N–H = 391, O=O = 498, N≡N = 945, O–H = 464.

(a) Calculate the overall energy change for the reaction. [3]

Show Mark Scheme
  • Energy for bonds broken = (12 × 391) + (3 × 498) = 6186 [1]
  • Energy for bonds made = (2 × 945) + (12 × 464) = 7458 [1]
  • Overall energy change = 6186 − 7458 = −1272 kJ [1]
Examiner tip: The most common mistake is forgetting the stoichiometry — 4NH₃ has 4 × 3 = 12 N–H bonds, not just 3. Always split your working into "bonds broken" and "bonds made" steps for maximum clarity.

Question 6: Calculating an Unknown Bond Energy

4 marks

CH₄ + Br₂ → CH₃Br + HBr. The overall energy change = −51 kJ/mol. Bond energies (kJ/mol): C–H = 412, Br–Br = 193, H–Br = 366.

(a) Calculate the bond energy X for the C–Br bond. [4]

Show Mark Scheme
  • Energy of bonds broken = (4 × 412) + 193 = 1841 [1]
  • Energy of bonds formed = (3 × 412) + 366 + X = 1602 + X [1]
  • −51 = 1841 − (1602 + X) [1]
  • X = 290 kJ/mol [1]
Examiner tip: Show your working clearly so partial credit (ECF) can be awarded. Many students forget brackets around "bonds made" values, leading to maths errors. Remember: Energy Change = Bonds Broken − Bonds Made.

Question 7: Planning an Energy Change Investigation ⭐ Extended Response

6 marks

A student wishes to investigate which of three metals (zinc, magnesium, or copper) will produce the largest exothermic reaction when added to hydrochloric acid.

(a) Describe a method the student could use to carry out this experiment safely, ensuring it is a fair test. [6]

Show Mark Scheme

Level 3 (5–6 marks): Coherent, logically sequenced method that would produce valid results. Explicitly details measuring temperature change and clearly identifies control variables for a fair test.

Level 2 (3–4 marks): Clear method describing temperature measurement, with at least one control variable identified. May lack minor procedural details.

Level 1 (1–2 marks): Basic steps given (e.g. "add metal to acid and take temperature"), but lacks control variables and logical structure.

Indicative content:

  • Measure a fixed volume of HCl using a measuring cylinder; pour into a polystyrene cup (insulation)
  • Control variables: same concentration and volume of acid; same mass of each metal; same state of division (e.g. all powder or all ribbons)
  • Measure initial temperature of acid with a thermometer
  • Add the first metal, stir, and record the maximum temperature
  • Calculate temperature change (max − initial); repeat for each metal; compare
Examiner tip: Don't just list equipment — explain what you do with it and why. Clearly state your independent variable (type of metal), dependent variable (temperature change), and control variables to access top marks.

Question 8: Required Practical — Temperature Changes 🔬

4 marks

A student investigates the temperature change when dilute hydrochloric acid is added to sodium hydroxide solution in a polystyrene cup.

(a) The temperature rose from 21°C to 35°C. Explain what this tells us about the reaction and describe one step the student should take to improve the accuracy of their results. [4]

Show Mark Scheme
  • The temperature increased, so the reaction is exothermic [1]
  • Energy is transferred to the surroundings / released [1]
  • Use a polystyrene cup with a lid to reduce heat loss to the surroundings [1]
  • Take the initial temperature of both solutions before mixing; repeat the experiment and calculate a mean [1]
Examiner tip: Always state the direction of energy transfer — "to the surroundings" for exothermic, "from the surroundings" for endothermic. Just saying "energy is released" without mentioning the surroundings often loses a mark.
Links to: Topic 6 — Factors Affecting Rate (temperature also affects reaction rate)

Question 9: Required Practical — Measuring Energy Changes ⭐🔬 Extended Response

6 marks

A student wants to compare the temperature changes produced when different metals react with dilute sulfuric acid.

(a) Describe a method the student could use to measure and compare the temperature changes. Explain how they should ensure the experiment is a fair test and how they should improve the accuracy of their results. [6]

Show Mark Scheme

Level 3 (5–6 marks): Coherent, detailed method with explicit control variables, clear measurement procedure, and accuracy improvements (insulation, repeats, mean).

Level 2 (3–4 marks): Method described with some control variables or accuracy improvements, but may lack detail in one area.

Level 1 (1–2 marks): Basic procedure mentioned (e.g. "mix metal and acid, check temperature") without control variables or accuracy measures.

Indicative content:

  • Measure a fixed volume of acid using a measuring cylinder into a polystyrene cup
  • Record the initial temperature of the acid
  • Add a fixed mass of the first metal; stir; record the maximum temperature
  • Control variables: same volume and concentration of acid, same mass of metal, same surface area of metal
  • Accuracy: use a lid on the polystyrene cup to reduce heat loss; repeat and calculate a mean
  • Calculate temperature change (ΔT = max − initial) for each metal and compare
Examiner tip: State all three variable types: independent (type of metal), dependent (temperature change), and control (volume, concentration, mass). The polystyrene cup acts as insulation — always explain why it's used.
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