OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2025: Question 5

1 mark · Medium difficulty · Multiple Choice

Determine the rate of reaction at 30 seconds from a graph of volume of hydrogen released against time.

Practise this question

Question

A line graph showing the volume of hydrogen gas released in cm cubed on the y-axis (from 0 to 100) against time in seconds on the x-axis (from 0 to 100). Plotted points show the curve starting at (0, 0), passing through (10, 28), (20, 49), (30, 64), (40, 75), (50, 82), (60, 87), (70, 90), (80, 92), (90, 93), and (100, 93). The question asks for the rate of reaction in cm cubed per second at 30 seconds, providing options A: 0.8, B: 1.2, C: 2.1, and D: 2.4.

Mark scheme

Show the mark scheme Mark scheme row for question 5 indicating the correct answer is B, awarded 1 mark.

How to answer it

Calculating Instantaneous Reaction Rate from a Gas Evolution Curve

📌 What This Question Tests

This question assesses your ability to determine the instantaneous rate of reaction at a specific time from continuous experimental data (a volume vs. time curve) and avoid classic distractor traps in multiple-choice questions.

  • Core Concept: Rate of reaction is equal to the gradient of the curve at that specific instant ( gradient of tangent = Δy / Δx ).
  • Distinction: Distinguishing clearly between instantaneous rate (tangent at time t ) and average rate ( total change / total time ).
  • Graphical Skills: Constructing a tangent to a curve accurately and reading coordinate intervals.
Question 5 (Multiple Choice) • 1 Mark

Rate of Reaction at t = 30 s

Identifying the correct value of the gradient at a given point on a curve

✅ Correct Answer

B — 1.2 cm³ s⁻¹

Drawing a tangent to the curve at exactly t = 30 s yields a gradient of approximately 1.2 cm³ s⁻¹.

💡 Key Knowledge

  • Rate definition: Rate is the change in concentration/amount/volume per unit time.
  • Instantaneous Rate: Because the rate decreases continuously as reactants are consumed, the rate at any exact instant t must be found by drawing a tangent line at that point and calculating its slope:
    Rate = gradient = ΔV / Δt
  • Units: cm³ s⁻¹ corresponds to y-axis change / x-axis change .

📐 Step-by-Step Gradient Calculation

  1. Locate the point on the curve: Find t = 30 s on the horizontal axis. Move up to the curve; at this point, the volume is 64 cm³ .
  2. Draw a tangent line: Place a ruler as a tangent against the curve at point (30 s, 64 cm³) so it just touches the curve without crossing it. Extend the tangent line across a wide range of the grid.
  3. Pick two easy-to-read points on the tangent line:
    For an accurate tangent at 30 s:
    • Point 1 (y-intercept or early time): (0 s, 28 cm³)
    • Point 2: (50 s, 88 cm³)
  4. Calculate gradient ( Δy / Δx ):
    Gradient = (88 - 28) cm³ / (50 - 0) s = 60 / 50 = 1.2 cm³ s⁻¹

❌ Deconstructing the Distractors

  • Trap C (2.1): The most common student error! Students calculated V / t = 64 / 30 = 2.13 cm³ s⁻¹ . This is the average rate over the first 30 seconds, not the instantaneous rate at 30 seconds.
  • Trap D (2.4): This represents the initial rate of the reaction (near t = 0 s ), where the curve is at its steepest ( ~28 / 10 = 2.8 or tangent at t = 0 ).
  • Trap A (0.8): This is the rate much later in the reaction (around t = 60 s ), where the curve begins to level off.

🧠 Exam Technique & Strategy

  • Always bring a transparent 30 cm ruler: A transparent ruler lets you see the curvature on both sides of your target point, ensuring equal daylight between ruler and curve.
  • Make your triangle large: When finding Δy / Δx , choose two points on the tangent line separated by at least half the length of the line to minimise reading error.
  • Quick mental check: Rate decreases as the reaction progresses:
    Initial rate (~2.4) > Rate at 30 s (1.2) > Rate at 60 s (~0.8).
    Since 64 / 30 = 2.1 includes the rapid early phase, the rate at 30 s must be substantially lower than 2.1!
Examiner Insight: In Multiple Choice questions, examiners specifically include y / x as one of the options because it is the most frequent blunder. Whenever you see a question asking for "rate at [time]", remember: Rate at a point = Tangent, never direct division!

Topics

Module 1: Development of practical skills in chemistry · Module 3: Periodic table and energy · Module 5: Physical chemistry and transition elements · Practical Activity Groups · 1.1 Practical skills assessed in a written examination · 3.2 Physical chemistry · 5.1 Rates, equilibrium and pH · PAG 9: Rates of reaction – continuous monitoring method

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.