OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2024: Question 10

1 mark · Easy difficulty · Multiple Choice

Identify the correct rate-concentration graph for a zero-order reaction with respect to a reactant.

Practise this question

Question

Multiple choice question 10 asking which rate-concentration graph for a zero-order reactant is correct, with four options A to D showing different graph shapes. Option A shows a downward sloping straight line, option B shows an upward sloping straight line passing through the origin, option C shows an exponential curve increasing upwards, and option D shows a horizontal straight line parallel to the concentration axis.
Question text

10 A reaction is zero order with respect to a reactant.

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

A

B

C

D

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer is D for question 10, worth 1 mark.

10 D 1

How to answer it

Identifying Zero-Order Rate-Concentration Graphs

What this question tests

This question assesses your understanding of chemical kinetics, specifically how the order of a reaction with respect to a reactant relates to the shape of its rate-concentration graph. You must be able to link rate equations (e.g., Rate = k[A]⁰ ) to graphical representations.

Question 10

Multiple Choice: Rate-Concentration Graphs for Zero-Order Reactions

✅ Correct Answer: D

Graph D shows a horizontal straight line parallel to the concentration axis. This indicates that the rate of the reaction remains constant regardless of changes in the concentration of the reactant.

💡 Key Knowledge

  • For a zero-order reaction, the rate equation is: Rate = k[A]⁰
  • Since anything to the power of zero equals 1, this simplifies to: Rate = k (where k is the rate constant).
  • Therefore, rate is independent of concentration.

🧠 Exam Technique

Always double-check the axis labels before answering. Students often confuse concentration-time graphs with rate-concentration graphs. A zero-order concentration-time graph slopes downwards linearly, but a zero-order rate-concentration graph is completely horizontal.

❌ Common Errors

  • Choosing B: Graph B represents a first-order rate-concentration graph (directly proportional, straight line through the origin).
  • Choosing A: This shape does not represent a standard order profile for these axes and often traps students who confuse gradient trends.
Mark Allocation: 1 mark awarded for selecting 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, Periodic table, elements and physical chemistry (01), June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.