OCR A-Level Chemistry AS Depth in chemistry (02), November 2021: Question 4

6 marks · Medium difficulty · Calculations

Calculate the enthalpy change for the decomposition of magnesium carbonate using experimental data from calorimetry and an enthalpy cycle.

Practise this question

Question

An exam question about determining the enthalpy change for the decomposition of magnesium carbonate. It provides three reactions, an enthalpy cycle diagram in Fig. 4.1, experimental steps for Reaction 2, a results table showing masses and temperatures, and instructions to use Hess's law to find Delta H 1.
Question text

4 A student carries out an investigation to find the enthalpy change for the decomposition of

magnesium carbonate, ∆H1 (Reaction 1).

Reaction 1 MgCO3(s) MgO(s) + CO2(g) ∆H1

This enthalpy change cannot be found directly. It can be determined indirectly from the enthalpy

changes for the reactions below, which can be found by experiment.

Reaction 2 MgCO3(s) + 2HCl(aq) MgCl2(aq) + H2O(l) + CO2(g) ∆H2

Reaction 3 MgO(s) + 2HCl(aq) MgCl2(aq) + H2O(l) ∆H3

The enthalpy cycle is shown in Fig. 4.1.

Reaction 1

MgCO3(s) MgO(s) + CO2(g)

ΔH1

Reaction 2 ΔH2 ΔH3 Reaction 3

MgCl2(aq) + H2O(l) + CO2(g)

Fig. 4.1

Determination of ∆H2 for Reaction 2

Student’s method

• Weigh a 250 cm3 polystyrene cup.

• Add about 100 cm3 of 2.00 mol dm–3 hydrochloric acid (an excess) to the polystyrene cup and

record the initial temperature of the HCl(aq).

• Add 4.215 g MgCO3, stir the mixture, and record the final temperature.

• Weigh the polystyrene cup containing the final solution.

Results

Mass of polystyrene cup / g 21.415

Mass of polystyrene cup + final solution / g 124.425

Initial temperature of HCl(aq) / °C 20.40

Final temperature of solution / °C 25.40

Determination of ∆H3 for Reaction 3

The student uses the same method as for Reaction 2 but with MgO in place of MgCO3.

The student calculates ∆H for Reaction 3 as –136.1 kJ mol–1.

(a)* Use the student’s results to calculate ∆H2 for Reaction 2 and determine the enthalpy change

∆H , in kJ mol–1, for the decomposition of magnesium carbonate (Reaction 1), using the

energy cycle in Fig. 4.1.

Assume the specific heat capacity, c, of the reaction mixture is the same as for water.

Additional answer space if required.

… [6]

Mark scheme

Show the mark scheme The mark scheme for question 4(a) showing a Level of Response marking grid from Level 1 to Level 3 (total 6 marks). It details calculations for energy change using q=mc delta T, moles of MgCO3, Delta H 2, and the final enthalpy change for Reaction 1 using the energy cycle.

How to answer it

Enthalpy Change of Decomposition of Magnesium Carbonate

What this question tests

This 6-mark extended response question assesses your ability to combine calorimetry data processing with Hess's Law cycles. You must accurately calculate heat energy changes using q = mcΔT , determine moles and molar enthalpy changes with correct signs, and apply an enthalpy cycle to find an unmeasurable enthalpy change indirectly.

Question Part (a)*

Calculating Enthalpy Changes using Calorimetry and Hess's Law

✅ Correct Final Answers

  • Reaction 2 (ΔH₂): -43.1 kJ mol⁻¹ (or -43.06 kJ mol⁻¹)
  • Reaction 1 (ΔH₁): +93.0 kJ mol⁻¹ (or +93.04 kJ mol⁻¹)

💡 Key Knowledge

  • Hess's Law: Total enthalpy change is independent of the route taken. Looking at Fig. 4.1: ΔH₁ = ΔH₂ - ΔH₃ .
  • Calorimetry formula: q = mcΔT where m is the total mass of the solution (or solution + solid).
  • Sign convention: Temperature increase means an exothermic reaction, requiring a negative sign ( - ) for ΔH.

🧠 Exam Technique

  • This is a Level of Response question (6 marks max). Structure your working clearly with numbered steps so the examiner can follow your chemical reasoning.
  • Always state final values to 3 significant figures and include the correct sign ( + or - ) and units ( kJ mol⁻¹ ).

❌ Common Errors

  • Mass trap: Forgetting to add the mass of the added MgCO₃ (4.215 g) to the mass of the liquid ( 124.425 - 21.415 = 103.0 g total mass). Using just 100 g drops you into Level 2/1.
  • Sign inversion: Omitting the negative sign for an exothermic reaction or messing up the algebraic rearrangement of the Hess cycle ( ΔH₂ - ΔH₃ ).

📐 Step-by-Step Calculation Guide

  1. Calculate mass (m) and temperature change (ΔT):
    Mass of solution = 124.425 - 21.415 = 103.0 g . Total mass including solid = 103.0 + 4.215 = 103.01 g (Accept either 103.0 g or 100 g for partial credit, but 103.01 g is rigorous).
    ΔT = 25.40 - 20.40 = 5.0 °C .
  2. Calculate heat energy change (q):
    q = m × c × ΔT = 103.01 × 4.18 × 5.0 = 2152.9 J = 2.153 kJ .
  3. Calculate moles of MgCO₃:
    Molar mass of MgCO₃ = 24.3 + 12.0 + (16.0 × 3) = 84.3 g mol⁻¹
    Moles ( n ) = 4.215 / 84.3 = 0.0500 mol .
  4. Calculate ΔH₂ for Reaction 2:
    Enthalpy change per mole = -2.153 kJ / 0.0500 mol = -43.06 kJ mol⁻¹ (Exothermic, hence negative).
  5. Apply Hess's Law for Reaction 1 (ΔH₁):
    From Fig. 4.1: ΔH₁ = ΔH₂ - ΔH₃
    ΔH₁ = -43.06 - (-136.1) = +93.04 kJ mol⁻¹ -> +93.0 kJ mol⁻¹ (3 SF).
Mark Scheme Allocation: Level 3 (5–6 marks): Correct enthalpy change calculations with correct signs for both ΔH₂ and ΔH₁. Well-developed line of reasoning. | Level 2 (3–4 marks): Calculates a value for ΔH₂ using energy change and moles. | Level 1 (1–2 marks): Processes partial experimental data (e.g., just q = mcΔT or moles).

Topics

Module 3: Periodic table and energy · Module 1: Development of practical skills in chemistry · Practical Activity Groups · 3.2 Physical chemistry · 1.1 Practical skills assessed in a written examination · PAG 3: Enthalpy determination

Question and mark scheme from the OCR A-Level Chemistry examination, AS Depth in chemistry (02), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.