OCR GCSE Chemistry A: Energetics (J248) – 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.

Every question below is on sub-topic C3.2 Energetics of the OCR GCSE (9–1) Chemistry A (Gateway Science) J248 specification (Version 4.0, August 2026), between them testing learning outcomes C3.2a–C3.2d, mathematical learning outcomes CM3.2i and CM3.2ii, and the PAG C8 temperature-change practical. Questions on C3.2a–C3.2c suit both tiers. Questions that use bond energies (C3.2d) are labelled Higher Tier only.

Before you start, try the Energetics study guide or the Energetics revision notes. The OCR GCSE Chemistry hub and the J248 checklist show where this sub-topic fits.

Questions

1. A student mixes four pairs of reagents, W to Z, in a polystyrene cup and records the temperature before and after mixing.

Pair	Reagents	Start (°C)	After (°C)
W	calcium oxide + water	21.5	30.3
X	sodium hydrogencarbonate + citric acid solution	20.5	15.3
Y	sodium chloride solution + potassium nitrate solution	22.0	22.0
Z	magnesium powder + dilute hydrochloric acid	19.5	41.0

(a) Calculate the temperature change for each pair. [2] (b) Identify the endothermic reaction. Explain your answer in terms of energy transfer. [2] (c) Suggest what the result for pair Y shows. [1]

2. (a) State what is meant by activation energy. [1] (b) Burning natural gas is exothermic, but a gas hob needs a spark before the gas will burn. Explain why. [2]

3. Sketch and label a reaction profile for an endothermic reaction. Label the reactants, the products, the activation energy and the energy change. [4]

4. A reaction profile shows the reactants at 120 kJ/mol, the top of the curve at 300 kJ/mol and the products at 40 kJ/mol.

(a) Calculate the activation energy. [1] (b) Calculate the energy change, including its sign. [2] (c) State whether the reaction is exothermic or endothermic, and give a reason from the profile. [1]

5. (Higher Tier only) A student writes: “In an exothermic reaction, energy is released when the bonds in the reactants break.” Explain what is wrong with this statement, and explain in terms of bonds why a reaction is exothermic. [3]

6. A student measures the temperature rise when a metal oxide reacts with dilute acid. Four repeats give rises of 7.2 °C, 7.8 °C, 9.9 °C and 7.5 °C.

(a) Which repeat does not fit the pattern of the others? [1] (b) Leaving that value out, calculate the mean rise. [2] (c) Estimate the uncertainty in the mean, using the range of the results you kept. [1] (d) The student used a glass beaker with no lid. Explain how this affects the measured temperature rise, and suggest one improvement. [2]

Use these bond energies (kJ/mol) for questions 7 to 11:

Bond	Energy	Bond	Energy	Bond	Energy
C–H	413	C=O	805	N–N	158
C–C	348	O–H	464	N≡N	945
C=C	612	O=O	498	N–H	391
C–O	358	H–H	436	Cl–Cl	243
H–Cl	432

7. (Higher Tier only) Ethene reacts with hydrogen to make ethane: CH₂=CH₂ + H–H → CH₃–CH₃. Calculate the energy change for this reaction. [3]

8. (Higher Tier only) Hydrazine, H₂N–NH₂, burns in oxygen: N₂H₄ + O₂ → N₂ + 2H₂O. Calculate the energy change for this reaction and state whether it is exothermic or endothermic. [4]

9. (Higher Tier only) Methanol, CH₃OH, burns: 2CH₃OH + 3O₂ → 2CO₂ + 4H₂O.

(a) Calculate the energy change for the equation as written. [4] (b) Calculate the energy change per mole of methanol burned. [1]

10. (Higher Tier only) Methane reacts with chlorine: CH₄ + Cl₂ → CH₃Cl + HCl. The energy change is −114 kJ/mol. Calculate the bond energy of the C–Cl bond. [3]

11. (Higher Tier only) A company is comparing ethanol and methanol as fuels. Ethanol, CH₃CH₂OH, burns: CH₃CH₂OH + 3O₂ → 2CO₂ + 3H₂O.

(a) Calculate the energy change when one mole of ethanol burns. [4] (b) Use your answer to (a) and your answer to question 9(b) to calculate the energy released per gram of each fuel. Relative atomic masses: H = 1, C = 12, O = 16. [3] (c) State which fuel releases more energy per gram. [1] (d) Suggest why the values from bond energies are only estimates. [1]

12. A student adds sodium hydrogencarbonate powder to citric acid solution in a polystyrene cup. The temperature falls from 20.4 °C to 15.1 °C.

(a) Calculate the temperature change. [1] (b) Explain, in terms of energy transfer, why the temperature falls. [2] (c) Describe how the reaction profile for this reaction differs from the profile for an exothermic reaction. [2] (d) The reaction fizzes and some solution splashes out of the cup. Suggest and explain one change to the method that would make the result more accurate. [2]

Answers

1. (a) W: 30.3 − 21.5 = +8.8 °C; X: 15.3 − 20.5 = −5.2 °C [1]. Y: 0.0 °C; Z: 41.0 − 19.5 = +21.5 °C [1] (b) X is endothermic [1]. The temperature fell, so energy was transferred from the surroundings to the reacting chemicals [1]. (c) No measurable energy transfer, so probably no reaction took place [1]. Examiner insight: For classification, the direction of the change carries the credit, so a sign on every value makes the reasoning in (b) easy to follow.

2. (a) The energy needed for a reaction to occur (the minimum energy particles need when they collide) [1]. (b) The spark supplies the activation energy [1]. Once the gas is burning, the energy released by the reaction keeps it going, and overall more energy is released than taken in [1]. Examiner insight: Answers that say the spark makes the reaction endothermic lose credit; tie the spark to activation energy, not to the overall energy change.

3. Axes labelled energy (vertical) and progress of reaction (horizontal) [1]. Products line drawn higher than the reactants line, labelled [1]. Activation energy arrow from the reactants line up to the peak [1]. Energy change arrow from reactants up to products [1]. Examiner insight: Arrows must start and end at the right lines; an activation energy arrow from the axis or from the products does not show the correct quantity.

4. (a) 300 − 120 = 180 kJ/mol [1] (b) 40 − 120 [1] = −80 kJ/mol [1] (c) Exothermic: the products are lower in energy than the reactants [1]. Examiner insight: In (b) the negative sign is part of the answer; a bare 80 does not show which way the energy moved.

5. Bond breaking takes in energy; it does not release it [1]. Energy is released when new bonds form in the products [1]. A reaction is exothermic when the energy released making bonds is greater than the energy taken in breaking bonds [1]. Examiner insight: The comparison in the final point needs both sides named; “making bonds releases energy” alone does not explain why the overall change is exothermic.

6. (a) 9.9 °C [1] (b) (7.2 + 7.8 + 7.5) ÷ 3 [1] = 22.5 ÷ 3 = 7.5 °C [1] (c) Range = 7.8 − 7.2 = 0.6 °C, so uncertainty = ±0.3 °C [1] (d) Energy is lost to the surroundings through the glass and the open top, so the measured rise is smaller than the true value [1]. Use a polystyrene cup with a lid [1]. Examiner insight: “Heat loss” earns more when you say which way it pushes the result (too small) and that it affects every repeat, which makes it a systematic error.

7. Bonds that change. Broken: C=C + H–H = 612 + 436 = 1048 kJ [1]. Made: C–C + 2 × C–H = 348 + 826 = 1174 kJ [1]. Energy change = 1048 − 1174 = −126 kJ/mol [1] Examiner insight: Counting every bond on both sides (2700 − 2826) gives the same answer; mixing the two approaches on one side does not.

8. Broken: N–N + 4 × N–H + O=O = 158 + 1564 + 498 = 2220 kJ [1]. Made: N≡N + 4 × O–H = 945 + 1856 = 2801 kJ [1]. Energy change = 2220 − 2801 [1] = −581 kJ/mol, exothermic [1] Examiner insight: Hydrazine has a single N–N bond, while nitrogen gas has a triple bond; using the wrong one changes the answer by hundreds of kJ, so draw the displayed formulae first.

9. (a) Each CH₃OH has 3 C–H, 1 C–O and 1 O–H. Broken: 6 × C–H + 2 × C–O + 2 × O–H + 3 × O=O = 2478 + 716 + 928 + 1494 = 5616 kJ [1] [1]. Made: 4 × C=O + 8 × O–H = 3220 + 3712 = 6932 kJ [1]. Energy change = 5616 − 6932 = −1316 kJ/mol [1] (b) −1316 ÷ 2 = −658 kJ/mol of methanol [1] Examiner insight: The equation burns two moles of methanol, so a “per mole of fuel” answer must be halved; the working should show this division.

10. Broken: C–H + Cl–Cl = 413 + 243 = 656 kJ [1]. −114 = 656 − (C–Cl + 432) [1]. C–Cl = 656 + 114 − 432 = 338 kJ/mol [1] Examiner insight: Writing the full broken − made equation before rearranging shows the method clearly, even if an arithmetic slip follows.

11. (a) Ethanol has 1 C–C, 5 C–H, 1 C–O and 1 O–H. Broken: 348 + 2065 + 358 + 464 + (3 × 498) = 4729 kJ [1] [1]. Made: 4 × C=O + 6 × O–H = 3220 + 2784 = 6004 kJ [1]. Energy change = 4729 − 6004 = −1275 kJ/mol [1] (b) Mr: ethanol = 46, methanol = 32 [1]. Ethanol: 1275 ÷ 46 = 27.7 kJ/g [1]. Methanol: 658 ÷ 32 = 20.6 kJ/g [1] (c) Ethanol [1] (d) Bond energies are average values taken from many different compounds [1]. Examiner insight: In (b), state the relative formula masses before dividing; a correct method with a slip in Mr is easier to credit when every step is visible.

12. (a) 15.1 − 20.4 = −5.3 °C [1] (b) The reaction is endothermic [1]: energy is transferred from the solution (the surroundings) to the reacting chemicals, so the solution cools [1]. (c) The products are higher in energy than the reactants [1], so the energy change arrow points upwards [1]. (d) Use a larger cup or a lid with a small hole for the thermometer [1], so no solution (and the energy it carries) is lost, and the reading reflects all of the reacting mixture [1]. Examiner insight: Improvement questions earn the second mark only when the change is linked to why it helps; naming a piece of equipment is not enough on its own.

Where marks are usually lost

• Classifying a reaction by how big the temperature change is, rather than whether it rose or fell.
• Writing that the reaction “made cold” instead of describing energy moving from the surroundings to the chemicals.
• Drawing activation energy from the products or the axis instead of from the reactants.
• Missing the sign on an energy change, or giving no unit.
• Reversing the bond energy rule: breaking bonds takes energy in, making bonds gives energy out.
• Forgetting the equation’s numbers, such as 3O₂ (three O=O bonds) or 2CH₃OH.
• Confusing N–N in hydrazine with N≡N in nitrogen gas.
• Giving an answer for the whole equation when the question asks per mole of fuel.
• Including an anomalous result in a mean.

Next steps

• Recap with the Energetics revision notes.
• Re-read the Energetics study guide for any outcome you found difficult.
• Browse the OCR GCSE Chemistry hub and tick off C3.2 on the J248 checklist.
• Try one of the free 10-minute diagnostics.
• Book a free trial class.

Official syllabus

OCR GCSE (9–1) Chemistry A (Gateway Science) J248 specification, Version 4.0 (August 2026), for first assessment in 2018, published by OCR. Sub-topic C3.2 Energetics, learning outcomes C3.2a–C3.2d, mathematical learning outcomes CM3.2i–CM3.2ii and the linked practical suggestion (PAG C8).