OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2023: Question 7

1 mark · Easy difficulty · Multiple Choice

Determine the half-life of a first-order reaction with respect to reactant X when the initial concentration is doubled from 1.0 mol dm⁻³ to 2.0 mol dm⁻³.

Practise this question

Question

Question text

7 The reaction below is first order with respect to reactant X.

X(aq) products

When the initial concentration of X is 1.0 mol dm–3, the half-life is 16 minutes.

What is the half-life when the initial concentration of X is 2.0 mol dm–3?

A 2 minutes

B 4 minutes

C 8 minutes

D 16 minutes

Your answer [1]

Mark scheme

Show the mark scheme

7 D 1 AO1.2

How to answer it

Kinetics: First-Order Reactions and Half-Life

What this question tests

This question assesses your understanding of the defining characteristic of a first-order reaction: that its half-life ( t₁ₔ₂ ) is constant and completely independent of the initial concentration of the reactant.

Question 7 (Multiple Choice)

Exam Breakdown

✅ Correct Answer

D: 16 minutes

Because the reaction is first-order with respect to X, changing the initial concentration from 1.0 mol dm⁻³ to 2.0 mol dm⁻³ has zero effect on the half-life.

💡 Key Knowledge

First-order reactions: Have a constant half-life regardless of concentration changes.
Zero-order reactions: Half-life decreases as initial concentration decreases ( t₁ₔ₂ ∝ [A] ).
Second-order reactions: Half-life increases as initial concentration decreases ( t₁ₔ₂ ∝ 1/[A] ).

🧠 Exam Technique

Do not fall into the trap of scaling calculations when you see numbers change! Always identify the order of the reaction first before applying mathematical relationships.

❌ Common Errors

Students often incorrectly assume that if the concentration doubles, the half-life must also double or halve (applying proportional logic meant for rate constants or zero/second order systems).

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, Periodic table, elements and physical chemistry (01), June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.