OCR A-Level Chemistry AS Breadth in chemistry (01), June 2018: Question 14

1 mark · Easy difficulty · Multiple Choice

Identify the correct expression for the equilibrium constant Kc for the reversible reaction between nitrogen and hydrogen to form ammonia.

Practise this question

Question

Multiple choice question 14 presenting the equilibrium reaction N2(g) + 3H2(g) = 2NH3(g) and asking for the expression for Kc, with four fractional options A, B, C, and D, an answer box, and a mark allocation of [1].
Question text

14 The reversible reaction below is at equilibrium.

N2(g) + 3H2(g) 2NH3(g)

What is the expression for Kc?

[N (g)] [H (g)]3

A 2

[NH3(g)]

[NH (g)]2

B 3

[N2(g)] [H2(g)]

[N2(g)] + 3[H2(g)]

C

2[NH3(g)]

2[NH3(g)]

D

[N2(g)] + 3[H2(g)]

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme showing question number 14 with the correct answer option B and 1 mark.

14 B 1

How to answer it

Writing Equilibrium Expressions (Kc)

📌 What this question tests

This question assesses your ability to recall and construct an equilibrium constant expression ( Kc ) from a balanced reversible equation, correctly applying the rules of putting products on the numerator, reactants on the denominator, and converting stoichiometric balancing numbers into powers (indices).

Question 14

Determining the Expression for Kc

✅ Correct Answer: B

The correct expression is [NH₃(g)]² / ([N₂(g)][H₂(g)]³) .

💡 Key Knowledge

  • Products over Reactants: The concentrations of products go on top (numerator), and reactants go on the bottom (denominator).
  • Powers from Stoichiometry: The balancing numbers in the chemical equation become the powers (indices) of the concentration terms.
  • Square brackets [ ] denote equilibrium concentrations in mol dm⁻³ .

🧠 Exam Technique

  • Always double-check your products and reactants before writing the fraction.
  • Look out for common distractors where powers are written as coefficients (multipliers) instead of indices, or where the fraction is inverted.

❌ Common Errors

  • Inverting the fraction (Distractor A): Putting reactants on top and products on the bottom.
  • Multiplying instead of raising to powers (Distractor C & D): Writing terms like 3[H₂(g)] instead of [H₂(g)]³ .
Examiner Note: This is a straightforward recall and application question worth [1] mark. Candidates who understand the fundamental rules of equilibrium constants secure this mark instantly.

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 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.