AQA A-Level Chemistry Paper 3, June 2018: Question 13

1 mark · Easy difficulty · Multiple Choice

Identify which rate against concentration graph represents a reaction that is second order with respect to reactant X.

Practise this question

Question

Question 13 displays a graph plotting 'rate of reaction' on the y-axis against 'concentration of X' on the x-axis. Four labeled curves are shown: A is a downward sloping curve from high to zero; B is a downward sloping straight line; C is an upward sloping straight line through the origin; and D is an upward curving line with an increasing gradient starting at the origin. Candidates must select which letter corresponds to a second-order reaction with respect to X.
Question text

13 A series of experiments was carried out to find the order of reaction with respect to

reactant X. In these experiments, only the concentration of X was changed.

Which graph would show that the reaction is second-order with respect to X?

[1 mark]

A

B

C

D

Mark scheme

Show the mark scheme Mark scheme for question 13 showing the correct answer as D.

13 D

How to answer it

Rate-Concentration Graphs & Reaction Orders

📌 What this question tests

This question assesses your ability to relate mathematical rate equations to graphical representations in chemical kinetics, specifically:

  • Interpreting Rate vs Concentration graphs for reactants.
  • Distinguishing between zero-order, first-order, and second-order relationships graphically.
  • Preventing confusion between rate vs concentration graphs and concentration vs time graphs.

Question 13

Multiple Choice — Rate of Reaction vs Concentration of X

✅ Correct Answer

D

1 Mark awarded for correctly identifying curve D.

For a reaction that is second-order with respect to X, the rate equation is:

Rate = k[X]²

Plotting Rate (y-axis) against [X] (x-axis) yields a quadratic relationship ( y = kx² ), which is an upwardly curving parabola starting at the origin (0, 0).

💡 Key Knowledge: Rate vs [Reactant] Profiles

  • Zero Order ( Rate = k ): Horizontal straight line. Changing [X] does not affect the rate.
  • First Order ( Rate = k[X] ): Straight line with a constant positive gradient passing through the origin (Curve C).
  • Second Order ( Rate = k[X]² ): Upward curve with an increasing gradient starting at the origin (Curve D).
  • Negative Orders: As [X] increases, rate decreases (Curves A and B). These are not standard positive integer orders tested at A-Level.

📐 Mathematical Analysis

Consider what happens to the rate as concentration doubles from 1 to 2 units:

  1. First Order (Line C):
    Rate₁ = k(1) = k
    Rate₂ = k(2) = 2k
    Gradient is constant (straight line).
  2. Second Order (Curve D):
    Rate₁ = k(1)² = k
    Rate₂ = k(2)² = 4k
    Rate quadruples when concentration doubles; the gradient increases as [X] increases (exponential-style upward curve).

🧠 Exam Technique: 2-Step Graph Check

Step 1: Always check the axes first!

  • If y-axis = Rate and x-axis = [Concentration], rate goes up with concentration for positive orders.
  • If y-axis = [Concentration] and x-axis = Time, the curve goes down as reactant is used up!

Step 2: Match order to algebraic power:

  • Order 1 → Power 1 ( y = mx ) → Straight line
  • Order 2 → Power 2 ( y = mx² ) → Upward curve

❌ Common Examiner Traps & Mistakes

  • Selecting C instead of D: The most common error. Students remember that rate increases with concentration for both 1st and 2nd order, but fail to realise that a straight line (C) represents a directly proportional first-order relationship, not second-order.
  • Selecting A: Students frequently mix up this graph with a concentration-time graph. A downward curve like A is characteristic of a concentration-time graph for a first-order or second-order reaction, but on a rate-concentration axis, it would imply a negative order.
  • Selecting B: A downward straight line on a concentration-time graph represents a zero-order reaction. On this graph, it represents an inverse linear relationship.

Topics

Physical Chemistry · 3.1.9 Rate Equations

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.