Energy Changes Exam Practice | AQA GCSE Chemistry Topic 5

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.

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.

(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.

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.

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]

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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.

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]

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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.

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]

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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.

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]

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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)

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]

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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.

Topic 5: Energy Changes Exam Practice

📋 Structured Questions

Question 1: Exothermic Reactions

Question 2: Temperature Changes in Reactions

Question 3: Fuel Cells vs Rechargeable Cells

Question 4: Reaction Profiles

Question 5: Overall Energy Change Calculation

Question 6: Calculating an Unknown Bond Energy

Question 7: Planning an Energy Change Investigation Extended Response

Question 8: Required Practical. Temperature Changes

Question 9: Required Practical. Measuring Energy Changes Extended Response