OCR A-Level Chemistry AS Breadth in chemistry (01), June 2019: Question 11

1 mark · Easy difficulty · Multiple Choice

Identify the correct statement about the exothermic reaction between hydrogen and chlorine gases from a multiple-choice list.

Practise this question

Question

Multiple choice question 11 showing the reaction equation H2(g) + Cl2(g) -> 2HCl(g) with delta H = -184.6 kJ mol-1. Four options A, B, C, and D provide different statements regarding bond energy, reverse enthalpy change, enthalpy of formation, and temperature change.
Question text

11 Hydrogen and chlorine react as shown below.

H (g) + Cl (g) → 2HCl (g) ∆H o = −184.6 kJ mol−1

Which statement about this reaction is correct?

A Less energy is released on bond making than is taken in during bond breaking.

B The enthalpy change for the reverse equation is +184.6 kJ mol−1.

C The enthalpy change of formation of HCl (g) is –184.6 kJ mol−1.

D The temperature decreases during the reaction.

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme for question 11 indicating the correct answer is B, worth 1 mark under AO1.1.

11 B 1 AO1.1

How to answer it

Analysing Enthalpy Changes in Chemical Reactions

What this question tests

This question assesses your fundamental understanding of thermochemistry concepts at AS Level, specifically linking the sign of enthalpy changes ( ΔH ) to bond breaking/making processes, the reversibility of chemical equations, enthalpy of formation definitions, and temperature changes in exothermic systems.

Question 11 (Multiple Choice)

Enthalpy Change Statement Analysis

✅ Correct Answer: B

The correct option is B. Reversing any reversible chemical equation reverses the sign of its enthalpy change ( ΔH ). Since the forward reaction has ΔH = -184.6 kJ mol⁻¹ , the reverse reaction has ΔH = +184.6 kJ mol⁻¹ .

💡 Key Knowledge

  • Exothermic reactions: Release energy to the surroundings ( ΔH is negative). More energy is released on bond making than is taken in during bond breaking.
  • Reversible equations: Reversing the equation multiplies ΔH by -1.
  • Enthalpy of formation: The enthalpy change when 1 mole of a compound is formed from its elements in their standard states. For this reaction, 2 moles of HCl(g) are formed, so ΔH for the equation equals 2 × ΔfH (making ΔfH = -92.3 kJ mol⁻¹ , not -184.6 kJ mol⁻¹ ).
  • Temperature: Exothermic reactions increase the temperature of the surroundings, not decrease it.

🧠 Exam Technique

  • Evaluate each statement systematically rather than guessing. Cross out options as you disprove them.
  • Look closely at stoichiometry: definitions like "enthalpy of formation" specify 1 mole of product, which often catches students out when equations produce 2 moles.

❌ Common Errors

  • Option A trap: A negative ΔH means energy *released* is greater than energy absorbed, so statement A is backwards.
  • Option C trap: Forgetting that standard enthalpy of formation refers strictly to 1 mole of product, leading students to confuse the reaction enthalpy with the molar enthalpy of formation.
  • Option D trap: Confusing the temperature of the system with the temperature of the surroundings (exothermic = surroundings get hotter).
Mark Scheme Allocation: 1 mark available for identifying option B (AO1.1 - Demonstrate knowledge and understanding of scientific ideas).

Topics

Module 3: Periodic table and energy · 3.2 Physical chemistry

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.