OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2024: Question 11

1 mark · Easy difficulty · Multiple Choice

Identify the point on a concentration-time graph where dynamic equilibrium is reached for the reaction between hydrogen and iodine to form hydrogen iodide.

Practise this question

Question

Multiple choice question 11 showing the reversible reaction H2(g) + I2(g)⇌ 2HI(g). A concentration-time graph plots concentration on the vertical axis against time on the horizontal axis, with curves showing reactants decreasing and product increasing. Four dashed vertical lines labeled A, B, C, and D mark different points on the graph, with an answer box provided below.
Question text

11 The reversible reaction between hydrogen and iodine to form hydrogen iodide is

H2(g) + I2(g) 2HI(g)

The graph shows how the concentrations of the reactants and product change as the reaction

reaches a dynamic equilibrium.

At which point on the graph is the equilibrium reached?

A B C D

concentration

product

reactants

time

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme table shows question number 11 with the correct answer as C and 1 mark allocated.

11 C 1

How to answer it

Identifying Dynamic Equilibrium on Concentration-Time Graphs

What this question tests

This question assesses your understanding of dynamic equilibrium in reversible reactions (H₂(g) + I₂(g) ⇌ 2HI(g)) and your ability to interpret concentration-time graphs. Specifically, it tests your knowledge of how to distinguish between the point where reactant and product lines cross versus the point where concentrations become constant and equilibrium is established.

Question 11 (Multiple Choice)

Interpreting Concentration-Time Graphs for Reversible Reactions

✅ Correct Answer

C

Marks: 1 / 1

💡 Key Knowledge

  • Dynamic Equilibrium: Reached when the rate of the forward reaction equals the rate of the reverse reaction.
  • Graphically: This is represented by the point where the concentrations of all reactants and products become constant (flat horizontal lines).
  • At point C and beyond, concentrations stop changing because reactants are turning into products at the exact same rate that products are turning back into reactants.

🧠 Exam Technique

  • Do not confuse equilibrium with equal concentrations! Point B simply marks the exact moment the reactant and product lines cross (where their concentrations happen to be equal).
  • Always scan for where the gradient of all curves becomes zero (flat lines) to locate the onset of equilibrium.

❌ Common Errors

  • Selecting B: The most frequent misconception is choosing point B because the lines intersect. Students often mistakenly assume that "equilibrium" means equal amounts of reactants and products.
  • Selecting D: While equilibrium *is* maintained at D, point C is the earliest point on the graph where equilibrium is first reached. Examiners look for the exact inflection point where the lines level off.

Topics

Module 3: Periodic table and energy · 3.2 Physical chemistry

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.