OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2017: Question 10

1 mark · Easy difficulty · Multiple Choice

Identify the correct concentration-time graph shape for a reactant A in a zero order reaction.

Practise this question

Question

Multiple choice question asking which concentration-time graph shape corresponds to a zero-order reactant A. Four graphs are shown: A is a straight downward-sloping line, B is a curve decreasing exponentially, C is a horizontal straight line, and D is an upward-sloping curve.
Question text

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

Which concentration–time graph for reactant A is the correct shape?

A

B

C

D

Your answer [1]

Mark scheme

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

10 A 1

How to answer it

Concentration-Time Graphs for Zero Order Reactions

What this question tests

This question assesses your ability to recall and recognise the graphical representation of reaction kinetics, specifically linking a zero-order reaction with respect to a reactant to its corresponding concentration-time graph shape.

Question 10

Identifying the Correct Concentration-Time Graph

✅ Correct Answer: Graph A

Graph A shows a straight line sloping downwards from left to right with a constant negative gradient. This represents a constant rate of reaction as concentration decreases over time, which is the hallmark of a zero-order reactant.

💡 Key Knowledge

  • Rate equation: Rate = k[A]⁰ = k
  • Since rate is independent of concentration ([A]⁰ = 1), the rate remains constant throughout the reaction.
  • The gradient of a concentration-time graph represents the rate of reaction ( d[A]/dt ). A constant gradient equals a straight, downward-sloping line.

🧠 Exam Technique

When approaching multiple-choice kinetics questions, quickly write down the rate equation in your working space. Match the mathematical relationship to the straight-line ( y = mx + c ) or curve shape before looking closely at the options.

❌ Common Errors

  • Confusing graph shapes: Graph C (a horizontal straight line) is often incorrectly chosen by students who confuse concentration-time axes with rate-concentration axes. On a rate-concentration graph, zero order is horizontal.
  • Curved lines: Mistaking zero-order graphs for first-order (exponential decay curve, like Graph B) or second-order curves.
Mark Scheme Allocation: 1 mark awarded for selecting A. No alternative answers accepted.

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 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.