OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2023: Question 15

1 mark · Hard difficulty · Multiple Choice

Assess statements regarding the redox, temperature dependence, and entropy change for the thermal decomposition equilibrium of chlorine trifluoride.

Practise this question

Question

Multiple choice question 15 presenting a chemical equation: 2 ClF3(g) reversible arrow Cl2(g) + 3 F2(g) with delta H = +318 kJ mol-1. Below are three numbered statements: 1. The decomposition is a redox reaction. 2. When the equilibrium mixture is cooled, the colour fades. 3. The decomposition has a negative entropy change. Options A, B, C, D follow, with options for 1, 2 and 3; Only 1 and 2; Only 2 and 3; Only 1.
Question text

15 Chlorine trifluoride can be decomposed into its elements forming the equilibrium mixture below.

2Cl F (g) Cl (g) + 3F (g) ∆H = +318 kJ mol–1

32 2

colourless green yellow

gas gas gas

Which statement(s) is/are correct?

1 The decomposition is a redox reaction.

2 When the equilibrium mixture is cooled, the colour fades.

3 The decomposition has a negative entropy change.

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 Mark scheme table showing the correct answer for question 15 is B, worth 1 mark under assessment objective AO2.1.

15 B 1 AO2.1

Total 15

How to answer it

Decomposition of Chlorine Trifluoride Equilibrium

OCR A-Level Chemistry • Multiple Choice Question

What this question tests

This question assesses your ability to integrate multiple core physical and inorganic chemistry concepts simultaneously: assigning oxidation numbers to determine redox reactions, applying Le Chatelier's principle alongside enthalpy changes to predict equilibrium shifts and visual changes, and understanding the qualitative link between the physical states of reactants/products and entropy change (ΔS).

Question Analysis & Statements Evaluation

Question 15 (Multiple Choice)

✅ Correct Answer: Option B (Only 1 and 2)

Statements 1 and 2 are correct, while statement 3 is incorrect, making B the correct choice.

🧠 Exam Technique

Use the elimination method. Test each numbered statement individually. Often, proving or disproving just one statement (like statement 3 regarding entropy) immediately cuts your options down to 50%.

Statement 1: Redox Reaction Check

Status: TRUE

  • In ClF₃ , chlorine has an oxidation state of +3 and fluorine is -1.
  • In the elements Cl₂ and F₂ , both atoms have an oxidation state of 0.
  • Since oxidation states change, chlorine is reduced (+3 to 0) and fluorine is oxidised (-1 to 0). This is definitely a redox reaction.

Statement 2: Cooling & Colour Fading

Status: TRUE

  • Forward reaction is endothermic ( ΔH = +318 kJ mol⁻¹ ). Therefore, the backward reaction is exothermic.
  • When you cool the mixture, Le Chatelier's principle states the equilibrium shifts in the exothermic (backward) direction to counteract the decrease in temperature.
  • Shifting backward forms more colourless ClF₃(g) and removes coloured gases ( Cl₂ is green, F₂ is yellow), so the colour fades.

❌ Statement 3 Analysis & Common Errors (Why it's FALSE)

Statement: "The decomposition has a negative entropy change." — FALSE

  • Check the moles of gas: Look at the stoichiometric equation: 2ClF₃(g) ⇌ Cl₂(g) + 3F₂O(g) (or 3F₂ ). We have 2 moles of gaseous reactants going to 1 + 3 = 4 moles of gaseous products.
  • An increase in the total number of moles of gas means disorder increases significantly. Therefore, the entropy change ( ΔS ) is positive, not negative.
  • Common Pitfall: Students often confuse enthalpy and entropy rules, or forget to count balancing coefficients when assessing gaseous moles.
Mark Scheme Allocation: 1 mark for selecting option B. Tested under Assessment Objective AO2.1 (applying chemical concepts to familiar contexts).

Topics

Module 2: Foundations in chemistry · Module 3: Periodic table and energy · Module 5: Physical chemistry and transition elements · 2.1 Atoms and reactions · 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 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.