AQA A-Level Chemistry: Energetics (7405) – 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 3.1.4 Energetics of the AQA AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards: 3.1.4.1 Enthalpy change, 3.1.4.2 Calorimetry (with Required practical 2), 3.1.4.3 Applications of Hess’s law and 3.1.4.4 Bond enthalpies. None of this content is “A-level only”, so every question suits both AS and A-level. All data you need is given in the questions.

Links: Energetics study guide | Energetics revision notes | AQA A-level Chemistry hub | printable checklist

Take c = 4.18 J g⁻¹ K⁻¹ for water and all solutions, and assume 1.00 cm³ of solution has a mass of 1.00 g.

Questions

1. Define the term standard enthalpy of combustion. [3]

2. This question is about standard conditions.

(a) State the pressure used for standard enthalpy changes. [1] (b) Explain why ΔfH⦵ of Br₂(l) is zero but ΔfH⦵ of Br₂(g) is not. [2]

3. Write equations, with state symbols, for:

(a) the standard enthalpy of formation of propan-1-ol, CH₃CH₂CH₂OH(l) [2] (b) the standard enthalpy of combustion of ethane, C₂H₆(g). [2]

4. Excess zinc powder is added to 25.0 cm³ of 0.400 mol dm⁻³ copper(II) sulfate solution in a polystyrene cup. The extrapolated temperature rise is 20.5 K. Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s) Calculate the enthalpy change for this reaction in kJ mol⁻¹. [4]

5. A spirit burner containing propan-1-ol (Mr = 60.0) is used to heat 100 g of water in a copper can. The burner’s mass falls from 64.37 g to 63.89 g. The water temperature rises from 19.5 °C to 42.0 °C.

(a) Calculate the enthalpy of combustion of propan-1-ol. Give your answer to an appropriate number of significant figures. [4] (b) Suggest two reasons why your answer is less exothermic than a data-book value. [2]

6. Use the data to calculate the standard enthalpy of formation of butane, C₄H₁₀(g). ΔcH⦵ (kJ mol⁻¹): C(s) −394, H₂(g) −286, C₄H₁₀(g) −2877. [3]

7. Calculate ΔH for 4NH₃(g) + 5O₂(g) → 4NO(g) + 6H₂O(g). ΔfH⦵ (kJ mol⁻¹): NH₃(g) −46, NO(g) +90, H₂O(g) −242. [3]

8. Chloroethane forms in the reaction C₂H₆(g) + Cl₂(g) → C₂H₅Cl(g) + HCl(g). Mean bond enthalpies (kJ mol⁻¹): C–H 412, Cl–Cl 243, C–Cl 338, H–Cl 432, C–C 348.

(a) Use the mean bond enthalpies to calculate ΔH for the reaction. [3] (b) All the species are gases. Explain why your answer may still differ from a value calculated from enthalpies of formation. [2]

9. For H₂(g) + F₂(g) → 2HF(g), ΔH = −542 kJ mol⁻¹. The mean bond enthalpies of H–H and F–F are 436 and 158 kJ mol⁻¹. Calculate the H–F bond enthalpy. [3]

10. A student finds the enthalpy change for MgSO₄(s) + 7H₂O(l) → MgSO₄·7H₂O(s) using two experiments. Ar: Mg 24.3, S 32.1, O 16.0, H 1.0.

• Experiment 1: 3.01 g of anhydrous MgSO₄ dissolves in 50.0 g of water. The temperature rises by 10.9 K.
• Experiment 2: 6.16 g of MgSO₄·7H₂O dissolves in 50.0 g of water. The temperature falls by 1.9 K.

Use 50.0 g as the mass that changes temperature in both experiments.

(a) Explain why this enthalpy change cannot be measured directly by calorimetry. [1] (b) Calculate ΔH₁, the enthalpy change for dissolving anhydrous MgSO₄. [3] (c) Calculate ΔH₂, the enthalpy change for dissolving MgSO₄·7H₂O. [3] (d) Use Hess’s law to calculate the enthalpy change for the hydration reaction. [2]

11. This question compares two ways of finding ΔH for the combustion of methanol.

(a) Calculate ΔcH⦵ for CH₃OH(l) + 1½O₂(g) → CO₂(g) + 2H₂O(l). ΔfH⦵ (kJ mol⁻¹): CH₃OH(l) −239, CO₂(g) −394, H₂O(l) −286. [3] (b) Use mean bond enthalpies to calculate ΔH for CH₃OH(g) + 1½O₂(g) → CO₂(g) + 2H₂O(g). (kJ mol⁻¹): C–H 412, C–O 360, O–H 463, O=O 496, C=O 743. [3] (c) Explain why your answers to (a) and (b) are different. [2]

12. In Required practical 2, a student records the temperature every 30 seconds before and after mixing two solutions.

(a) Explain why the student plots temperature against time and extrapolates back to the time of mixing. [2] (b) The student repeats the experiment in a glass beaker instead of a polystyrene cup. State and explain the effect on the calculated value of ΔH. [2]

Answers

1. The enthalpy change when 1 mole of a substance [1] is completely burned in oxygen [1] under standard conditions, with all reactants and products in their standard states [1]. [3] Examiner insight: each idea is a separate mark, so “burned in air” or leaving out “1 mole” costs a mark even when the rest is perfect.

2. (a) 100 kPa [1] (b) Br₂(l) is the standard state of bromine, so forming it from the element involves no change [1]. Br₂(g) is not the standard state, so its value includes the enthalpy change of turning Br₂(l) into Br₂(g) [1]. Examiner insight: the mark needs the idea of a standard state; “because it is an element” alone does not explain why Br₂(g) is different.

3. (a) 3C(s) + 4H₂(g) + ½O₂(g) → CH₃CH₂CH₂OH(l): elements on the left, 1 mol of product [1]; balanced with correct state symbols [1]. (b) C₂H₆(g) + 3½O₂(g) → 2CO₂(g) + 3H₂O(l): 1 mol of ethane, balanced [1]; state symbols with H₂O as a liquid [1]. Examiner insight: doubling the equation to avoid fractions loses the mark, because the definitions require exactly 1 mole of the named substance.

4. q = 25.0 × 4.18 × 20.5 = 2142 J [1] n(CuSO₄) = 0.400 × 0.0250 = 0.0100 mol [1] ΔH = −2.142 ÷ 0.0100 [1] = −214 kJ mol⁻¹ [1] [4] Examiner insight: the final mark needs the negative sign and kJ mol⁻¹; an answer of −214 000 shows the J to kJ conversion was missed and loses the last mark.

5. (a) Mass burned = 0.48 g; n = 0.48 ÷ 60.0 = 0.0080 mol [1] q = 100 × 4.18 × 22.5 = 9405 J [1] ΔcH = −9.405 ÷ 0.0080 = −1176 kJ mol⁻¹ [1] = −1200 kJ mol⁻¹ to 2 s.f., because the mass change has only 2 significant figures [1] (b) Any two of: heat lost to the surroundings or to the can; incomplete combustion; evaporation of propan-1-ol from the wick. One mark each [1] [1] Examiner insight: using 0.48 g as m in q = mcΔT is a common error that loses the q mark, but later marks can still be awarded as error carried forward.

6. Correct cycle or expression: ΔfH = ΣΔcH(reactants) − ΔcH(butane) [1] 4(−394) + 5(−286) = −3006 [1] ΔfH⦵ = −3006 − (−2877) = −129 kJ mol⁻¹ [1] Examiner insight: +129 kJ mol⁻¹ shows the subtraction was reversed and usually scores only the first mark.

7. Products: 4(+90) + 6(−242) = −1092 [1] Reactants: 4(−46) + 5(0) = −184 [1] ΔH = −1092 − (−184) = −908 kJ mol⁻¹ [1] Examiner insight: forgetting to multiply by the coefficients gives a wrong sum; giving O₂ a non-zero value loses the reactants mark.

8. (a) Bonds broken (after cancelling identical bonds): one C–H + one Cl–Cl = 412 + 243 = 655 [1] Bonds made: one C–Cl + one H–Cl = 338 + 432 = 770 [1] ΔH = 655 − 770 = −115 kJ mol⁻¹ [1] (b) Mean bond enthalpies are averages over a range of compounds [1], so the actual bond enthalpies in C₂H₆ and C₂H₅Cl differ from these values [1]. Examiner insight: the C–C value is not needed because that bond is unchanged; in (b) the “not gaseous” reason earns nothing here because the question says all species are gases.

9. −542 = (436 + 158) − 2x [1] 2x = 594 + 542 = 1136 [1] x = 568 kJ mol⁻¹ [1] Examiner insight: two H–F bonds form, so dividing by 2 is essential; 1136 scores only the first two marks.

10. (a) It is not possible to add exactly the right amount of water to the solid and measure its temperature change accurately [1]. (b) Mr(MgSO₄) = 120.4, so n = 3.01 ÷ 120.4 = 0.0250 mol [1] q = 50.0 × 4.18 × 10.9 = 2278 J [1] ΔH₁ = −2.278 ÷ 0.0250 = −91.1 kJ mol⁻¹ [1] (c) Mr(MgSO₄·7H₂O) = 246.4, so n = 6.16 ÷ 246.4 = 0.0250 mol [1] q = 50.0 × 4.18 × 1.9 = 397 J [1] ΔH₂ = +0.397 ÷ 0.0250 = +15.9 kJ mol⁻¹ (+16 to 2 s.f.) [1] (d) ΔH₁ = ΔH(hydration) + ΔH₂ [1] ΔH(hydration) = −91.1 − (+15.9) = −107 kJ mol⁻¹ [1] Examiner insight: the sign of ΔH₂ must be positive because the temperature fell; using −15.9 gives −75.2 kJ mol⁻¹, which can earn the cycle mark in (d) but not the final answer.

11. (a) Products: −394 + 2(−286) = −966 [1] Reactants: −239 (O₂ is zero) [1] ΔcH⦵ = −966 − (−239) = −727 kJ mol⁻¹ [1] (b) Broken: 3(412) + 360 + 463 + 1½(496) = 2803 [1] Made: 2(743) + 4(463) = 3338 [1] ΔH = 2803 − 3338 = −535 kJ mol⁻¹ [1] (c) Mean bond enthalpies are averages, not the values for the bonds in these particular molecules [1]. In (a) methanol and water are liquids, so the enthalpy changes of state are included there but not in (b) [1]. Examiner insight: the 1½ O=O bonds are easy to miss in (b); the bonds-made total must include both C=O bonds in CO₂ and all four O–H bonds in two water molecules.

12. (a) Heat is lost to the surroundings while the temperature is rising [1]. Extrapolating back gives the temperature at the moment of mixing as if no heat had been lost, so ΔT is more accurate [1]. (b) Glass is a better conductor, so more heat is lost and the measured ΔT is smaller [1]. The calculated ΔH is therefore smaller in magnitude (less exothermic or less endothermic) [1]. Examiner insight: “the result is less accurate” gains nothing; the marks need the direction of the change in ΔT and in ΔH.

Where marks are usually lost

• Using the mass of fuel or solid instead of the mass of water as m in q = mcΔT.
• Not converting J to kJ, giving answers 1000 times too large.
• Missing signs: temperature rise means negative ΔH, temperature fall means positive ΔH.
• Quoting too many significant figures when the data are only to 2 s.f.
• Reversing the subtraction in combustion-data Hess’s law calculations.
• Not multiplying ΔfH⦵, ΔcH⦵ or bond enthalpies by the coefficients in the equation.
• Leaving “1 mole” or “standard states” out of definitions.
• Giving the states reason for a bond enthalpy discrepancy when every species is already a gas.

Next steps

• Energetics revision notes
• Energetics study guide
• AQA A-level Chemistry hub
• Printable AQA A-level Chemistry checklist
• All free 10-minute diagnostics
• Book a free trial class

Official syllabus

AQA, AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards: section 3.1.4 Energetics.