Edexcel A-Level Chemistry AS Paper 2, June 2019: Question 8

9 marks · Medium difficulty · Practical Techniques and Data Analysis

Plot concentration-time data for a catalytic reaction, describe how to find initial and maximum rates from tangents, explain why initial and maximum rates differ due to autocatalysis, and state why the initial rate measurement is less accurate.

Practise this question

Question

An exam question showing a chemical equation for compound X reacting with water to form Y, Z minus, and H plus. A table provides experimental data of concentration of Y against time, with some points already plotted on a grid showing concentration of Y versus time from 0 to 400 seconds. Subsequent parts ask to complete the graph, describe how to find initial and maximum rates from it, explain why initial and maximum rates are different, and evaluate the accuracy of the initial rate measurement.
Question text

8 Compound X reacts slowly with water according to the following equation.

X(s) + H O(l) → Y(aq) + Z–(aq) + H+(aq)

The reaction is catalysed by hydrogen ions and eventually goes to completion.

Compound X was added to water and the concentration of compound Y determined at

various times at a constant temperature.

The results of the experiment are shown.

Concentration of Y

Time / s –3

/ mol dm

0 0.000

25 0.002

40 0.005

50 0.010

65 0.020

75 0.030

85 0.040

100 0.050

115 0.060

140 0.070

200 0.080

260 0.085

380 0.090

(a) (i) Complete the graph of concentration against time by adding the six missing points.

Draw a line to pass through all the points.

(2)

Graph of concentration of Y against time

0.10 –

0.09 –

0.08 –

0.07 –

*P55611A02228*

0.06 –

0.05 –

0.04 –

0.03 –

0.02 –

0.01 –

0.00 –

0 50 100 150 200 250 300 350 400

Time /s

(ii) Describe how you would find a numerical value for the initial rate of reaction

and for the maximum rate of reaction in this experiment from the graph.

No actual calculations are required.

(4)

… *P55611A02328*

(b) For many reactions, the values of the initial rate and the maximum rate are the same.

Explain why the values of the two reaction rates obtained in this experiment are

different from each other.

(2)

(c) Give a reason why the measurement of the initial rate of reaction is likely to be

less accurate than the measurement of the maximum rate.

(1)

(Total for Question 8 = 9 marks)

Mark scheme

Show the mark scheme The mark scheme for the question showing the completed graph with all points correctly plotted and joined by a smooth curve. Detailed marking points for part (a)(ii) require taking a tangent at time equals zero for initial rate, finding the steepest part for maximum rate, and calculating the gradient. Part (b) awards marks for stating that hydrogen ions catalyse the reaction and their concentration starts low and increases. Part (c) accepts that it is difficult to judge the tangent accurately at the start compared to other points.

How to answer it

Kinetics and Autocatalytic Reactions

📌 What this question tests

This question assesses your ability to plot experimental kinetic data, construct and interpret concentration-time graphs, determine initial and maximum rates using tangents and gradients, and explain the mechanistic concept of autocatalysis where a product acts as a catalyst.

Question 8 (a) (i) — Graph Plotting and Curve Fitting

Completing the concentration-time graph

✅ Correct Answer

  • Accurately plot the remaining 5 missing data points from the table onto the grid.
  • Draw a single smooth curve passing through all plotted points (allowing for minor anomalies if specified, though here points follow a clean autocatalytic S-shape).

💡 Key Knowledge

  • An autocatalytic reaction profile is typically sigmoidal (S-shaped).
  • Initially, the rate is very slow (incubation period), then it accelerates steeply as the catalyst forms, before plateauing as reactants are depleted.

🧠 Exam Technique

  • Use a sharp pencil and a transparent ruler/curve guide.
  • Ensure crosses or dots are centred precisely on the coordinate intersections.

❌ Common Errors

  • Connecting points with straight line segments ("connect-the-dots") instead of drawing a smooth curve.
  • Misplotting data points due to misreading graph scale increments.
Mark Breakdown (2 marks): 1 mark for correct plotting of points; 1 mark for a smooth curve passing through all points.

Question 8 (a) (ii) — Initial and Maximum Rates

Describing graphical rate determination methods

✅ Correct Answer

  • Initial rate: Take a tangent to the curve at time = 0 (or at the very start of the reaction).
  • Maximum rate: Take a tangent to the steepest part of the curve (where the slope is greatest / closest to vertical).
  • Calculation method: Find the gradient of the respective tangent by measuring change in concentration divided by change in time (delta y / delta x).

🧠 Exam Technique

  • Use the word "tangent" explicitly—examiners penalise vague phrases like "draw a line at the start".
  • When calculating gradients on graph paper, always draw a large triangle to minimise percentage error.
Mark Breakdown (4 marks): 1 mark for mentioning tangent at time = 0; 1 mark for tangent at the steepest/maximum gradient point; 1 mark for stating gradient calculation (delta y / delta x). Note: Read the prompt carefully—no actual numerical calculations were required for this specific sub-part.

Question 8 (b) — Explaining Rate Differences

Why initial rate and maximum rate differ in this experiment

✅ Correct Answer

  • The reaction is catalysed by hydrogen ions ( H⁺(aq) ), which are a product of the reaction.
  • Initially, the concentration of hydrogen ions is zero (or very low), so the initial rate is slow.
  • As the reaction proceeds, H⁺ ions are produced, increasing the concentration of the catalyst, which causes the rate to speed up to a maximum. Therefore, the reaction is autocatalytic.

❌ Common Errors

  • Stating that reactant concentration is highest at the start (true for standard reactions, but misses the key catalytic role of H⁺ here).
  • Forgetting to link the accumulation of products directly to the increase in reaction rate.
Mark Breakdown (2 marks): 1 mark for noting that H⁺ ions act as a catalyst and their initial concentration is very low/zero; 1 mark for explaining that H⁺ ions are formed as the reaction progresses, increasing the catalyst concentration and rate.

Question 8 (c) — Experimental Accuracy

Evaluating measurement accuracy for initial vs. maximum rate

✅ Correct Answer

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  • It is much more difficult to accurately judge and draw a tangent at time = 0 compared to the steepest part of the curve.
  • The curve is very flat at the origin, making the exact placement of the tangent subjective and prone to drawing error.

💡 Key Knowledge

  • Steep, well-defined slopes (like the point of inflection / maximum rate) allow constructible right-angled triangles with clear coordinate intercepts, reducing percentage uncertainty.
Mark Breakdown (1 mark): 1 mark for identifying that drawing/judging a tangent at the start (time = 0) is more difficult than at the steepest part of the curve.

Topics

Physical Chemistry · Core Practicals · Core Practical 13b: Use a clock reaction to determine a rate equation · Topic 9: Kinetics I

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 2, June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.