OCR A-Level Chemistry AS Breadth in chemistry (01), June 2024: Question 13

1 mark · Easy difficulty · Multiple Choice

Identify the correct expression for the equilibrium constant Kc for the reversible reaction 2SO2(g) + O2(g) = 2SO3(g).

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Question

Multiple choice question 13 showing a reversible reaction at equilibrium: 2SO2(g) + O2(g) reversible arrow 2SO3(g). Four options A, B, C, and D give different mathematical expressions for Kc. Option A is [SO2]^2[O2]/[SO3]^2, Option B is [SO3]^2/[SO2]^2[O2], Option C is 2[SO2] + [O2]/2[SO3], and Option D is 2[SO3]/2[SO2] + [O2]. An empty box labeled 'Your answer' is at the bottom left, with [1] mark indicated on the bottom right.
Question text

13 The reversible reaction below is at equilibrium.

2SO2(g) + O2(g) 2SO3(g)

What is the expression for Kc?

[SO (g)]2 [O (g)]

A 2 2

[SO (g)]2

[SO (g)]2

B 3

[SO (g)]2 [O (g)]

2[SO2(g)] + [O2(g)]

C

2[SO3(g)]

2[SO3(g)]

D

2[SO2(g)] + [O2(g)]

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme for question 13 indicating the correct answer is B.

13 B 1

How to answer it

Writing Equilibrium Constant Expressions (Kc)

What this question tests

This question assesses your ability to recall and construct the equilibrium constant expression ( Kc ) from a balanced chemical equation, correctly placing products on the numerator, reactants on the denominator, and raising each concentration term to the power of its stoichiometric coefficient.

Question 13 • Multiple Choice

Constructing the Kc Expression

Reaction: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g)

✅ Correct Answer: B

The correct expression is:

[SO₃(g)]² / ([SO₂(g)]² [O₂(g)])

Marks: 1 / 1
Examiner Note: Option B correctly puts products on top, reactants on the bottom, and applies the stoichiometric coefficients as powers.

💡 Key Knowledge

  • General Formula: Kc = [Products] / [Reactants]
  • Powers: The stoichiometric balancing numbers in front of chemical species become indices (powers) in the Kc expression.
  • Square Brackets: Always use square brackets [ ] to denote equilibrium concentrations in mol dm⁻³.

🧠 Exam Technique

  • Don't rush multiple-choice questions involving algebraic fractions; write out the general rule [Products] / [Reactants] on your rough paper first.
  • Double-check that coefficients are written as superscript powers, not multipliers (avoid options C and D which incorrectly multiply or add terms).

❌ Common Errors

  • Inverting the fraction: Placing reactants on the numerator and products on the denominator (as seen in Option A).
  • Treating coefficients incorrectly: Multiplying concentrations by the coefficient rather than raising them to a power (seen in Options C and D).

Topics

Module 5: Physical chemistry and transition elements · 5.1 Rates, equilibrium and pH

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.