OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2018: Question 8

1 mark · Easy difficulty · Multiple Choice

Identify the correct rate-concentration graph for a reaction that is first order with respect to reactant X.

Practise this question

Question

Multiple choice question 8 asking to identify the correct rate-concentration graph for a reactant X that is first order. Four options A, B, C, and D show different graph shapes: A is a straight line through the origin with a positive gradient, B is a straight line with a negative gradient, C is a horizontal straight line, and D is an exponential curve increasing upwards.
Question text

8 A reaction is first order with respect to a reactant X.

Which rate–concentration graph for reactant X is the correct shape?

A

B

C

D

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme indicates the correct answer is A for question 8, worth 1 mark.

8 A 1 AO1.1

How to answer it

Rate-Concentration Graphs for First-Order Reactions

What this question tests

This question assesses your ability to link the order of a reaction with respect to a reactant to the correct graphical representation of rate versus concentration ( rate = k[X]^n ). Specifically, you must recognize how a first-order relationship translates into a linear graph passing through the origin.

Question 8

Exam Breakdown & Solution

✅ Correct Answer: A

Graph A is a straight line through the origin, representing a direct proportional relationship between rate and concentration.

Mark Awarded: 1 / 1 (AO1.1)

💡 Key Knowledge

  • For a first-order reactant, the rate equation is rate = k[X] .
  • This fits the algebraic linear equation format y = mx (where y = rate, x = [X], and gradient m = rate constant k ).
  • Because there is no intercept ( c = 0 ), the line must start directly at the origin (0, 0).

🧠 Exam Technique

Always check the axis labels carefully before answering. Ensure you distinguish between rate-concentration graphs and concentration-time graphs, as their shapes for the same reaction order look completely different!

❌ Common Errors

  • Graph C (horizontal line) represents a zero-order reaction ( rate = k[X]⁰ = k ).
  • Graph D (exponential curve pointing upwards) represents a second-order reaction ( rate = k[X]² ).
  • Students sometimes confuse rate-concentration axes with concentration-time decay curves (like Graph B).

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