OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2018: Question 13

1 mark · Medium difficulty · Multiple Choice

Identify which of the given enthalpy changes (bond enthalpy of C-H, second electron affinity of oxygen, and standard enthalpy change of formation of magnesium) are endothermic.

Practise this question

Question

Multiple-choice question 13 asking which enthalpy changes are endothermic among: 1. The bond enthalpy of the C-H bond, 2. The second electron affinity of oxygen, 3. The standard enthalpy change of formation of magnesium. Four options A (1, 2 and 3), B (Only 1 and 2), C (Only 2 and 3), and D (Only 1) are provided along with a box for the final answer.
Question text

13 Which enthalpy change(s) is/are endothermic?

1 The bond enthalpy of the C–H bond

2 The second electron affinity of oxygen

3 The standard enthalpy change of formation of magnesium

A 1, 2 and 3

B Only 1 and 2

C Only 2 and 3

D Only 1

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme indicates that the correct answer for question 13 is option B, worth 1 mark.

13 B 1 AO1.1

How to answer it

Analysing Endothermic Enthalpy Changes

What this question tests

This question assesses your core knowledge of energetics and enthalpy definitions (AO1.1). You must know whether specific types of enthalpy changes—namely bond enthalpies, successive electron affinities, and standard enthalpies of formation—release or absorb energy.

Question 13

Exam Breakdown & Analysis

✅ Correct Answer: B (Only 1 and 2)

Statements 1 and 2 are endothermic (energy absorbed), while statement 3 describes an element in its standard state, which is not an enthalpy change definition and equals 0 kJ mol⁻¹.

💡 Key Knowledge

  • Bond Enthalpy (1): Breaking bonds requires energy input, making it always endothermic (+).
  • Second Electron Affinity (2): Adding an electron to a negative ion (e.g., O⁻(g) + e⁻ → O²⁻(g)) forces two like charges together, requiring energy input to overcome repulsion (endothermic, +).
  • Standard Enthalpy of Formation of an Element (3): By definition, the standard enthalpy of formation of an element in its standard state (like Mg(s)) is zero, not an endothermic process.

🧠 Exam Technique

Tackle multiple-choice statements one by one and eliminate options. Statement 3 is a classic distractor: remember that elements in their standard states have a formation enthalpy of exactly zero, ruling out options containing statement 3 (A and C).

❌ Common Errors

Students frequently confuse first electron affinities (which are generally exothermic as the nucleus attracts the incoming electron) with second electron affinities (which are always endothermic due to inter-electronic repulsion).

Mark Allocation: [1] mark total for selecting option B.

Topics

Module 3: Periodic table and energy · Module 5: Physical chemistry and transition elements · 3.2 Physical chemistry · 5.2 Energy

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.