OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2022: Question 9

1 mark · Medium difficulty · Multiple Choice

Identify the equation that represents the standard enthalpy change of atomisation of bromine.

Practise this question

Question

Multiple-choice question 9 asking which equation represents the standard enthalpy change of atomisation of bromine, with four options: A (1/2 Br2(l) -> Br(g)), B (Br2(l) -> 2Br(g)), C (1/2 Br2(g) -> Br(g)), and D (Br2(g) -> 2Br(g)), followed by a box for the answer.
Question text

9 Which equation represents the change that accompanies the standard enthalpy change of

atomisation of bromine?

A ½ Br2(l) Br(g)

B Br2(l) 2Br(g)

C ½ Br2(g) Br(g)

D Br2(g) 2Br(g)

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme table shows question number 9 has the correct answer A, worth 1 mark.

9 A 1 1.1

How to answer it

Standard Enthalpy Change of Atomisation

What this question tests:

This question assesses your precise definition-based knowledge of enthalpy changes, specifically the standard enthalpy change of atomisation. You must know both the physical state under standard conditions and the stoichiometric requirement to produce exactly one mole of gaseous atoms from the element in its standard state.

Question 9 Analysis

Identifying the Correct Equation for Bromine Atomisation

✅ Correct Answer: A

0.5 Br₂(l) ➔ Br(g)

Bromine is a liquid at standard room temperature and pressure (SATP). By definition, the enthalpy of atomisation forms one mole of gaseous atoms, requiring a stoichiometric coefficient of 0.5 for the diatomic Br₂ molecule.

💡 Key Knowledge

  • Definition: The enthalpy change that accompanies the formation of one mole of gaseous atoms from the element in its standard state under standard conditions.
  • Standard State of Bromine: Liquid ( l ) at 298 K and 100 kPa.
  • Atom Product: Must always yield single gaseous atoms ( Br(g) ), never diatomic molecules ( Br₂(g) ).

🧠 Exam Technique

  • Check two things systematically for any atomisation equation:
    1. Is the product exactly one mole of atoms? (Eliminates B and D).
    2. Is the reactant in its correct standard state at room conditions? Bromine is liquid, not gaseous! (Eliminates C).

❌ Common Errors

  • Choosing B ( Br₂(l) ➔ 2Br(g) ): Students often forget that enthalpy changes are defined per mole of product formed, leading them to form two moles of atoms instead of one.
  • Choosing C or D: Confusing the standard state of bromine. Bromine is a liquid ( l ), unlike chlorine and fluorine which are gases.
Mark Scheme Allocation: 1 mark available for selecting option A. No alternative pathways accepted.

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 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.