Edexcel A-Level Chemistry Paper 2, November 2021: Question 11

7 marks · Medium difficulty · Practical Techniques and Data Analysis

Plot experimental kinetics data for the reaction between a chloroalkane and potassium hydroxide, determine the order with respect to hydroxide ions, deduce the reaction mechanism, and classify the chloroalkane.

Practise this question

Question

Question text

11 A series of experiments was carried out to determine the kinetics of the reaction

between a chloroalkane, RCl, and potassium hydroxide in aqueous solution.

A large excess of the chloroalkane was used.

The data obtained are shown.

[OH−] / mol dm−3 Time / s

0.00100 39

0.00200 31

0.00300 23

0.00400 16

0.00500 8

(a) Plot a graph of the concentration of the hydroxide ions against time.

(2)

(b) State the order with respect to hydroxide ions.

Justify your answer by reference to your graph in (a).

(2)

… *P65464A02228*

Justify your answer.

(2)

(d) Give the classification of the chloroalkane in this reaction.

(1)

(Total for Question 11 = 7 marks)

Mark scheme

Show the mark scheme

Question Mark

Answer Additional Guidance

Number

11(a) Example of graph (2)

• M1 axes labelled with units on axes,

suitable uniform scale with points

covering at least half the available

space in both directions (1)

• M2 all points plotted correctly with

straight line of best fit (1)

Allow variables on either axis

Question Mark

Answer Additional Guidance

Number

11(b) An answer that makes reference to the following points (2)

• zero order with respect to hydroxide ions (1)

• The graph is a straight line so the rate of reaction is M2 dependent on M1

independent of the concentration of the hydroxide

ions (1)

Question Mark

Answer Additional Guidance

Number

11(c) An answer that makes reference to the following points Mark consequentially on order (2)

• SN1 (1) Allow TE from (b) e.g. if first order in (b) allow

SN2

• as there is only one reactant in the rate determining

step / as the hydroxide ions do not affect the rate

(1)

Question Mark

Answer Additional Guidance

Number

11(d) Allow TE from first order in (b) and/or SN2 in (c) (1)

• the chloroalkane is tertiary e.g. if SN2 in (c) allow primary

NOTE if first order wrt hydroxide ions in (b) but

SN1 given in (c) can score 1 mark in (d) for tertiary

(Total for Question 11 = 7 marks)

How to answer it

Kinetics, Orders of Reaction and Halogenoalkane Mechanisms

What this question tests

This multi-step question assesses your ability to process experimental kinetics data, plot graphs with correct scales and labels, deduce the mathematical order of a reaction from graphical trends, link rate equations to reaction mechanisms (SN1 vs SN2), and classify halogenoalkanes based on their kinetic behaviour.

Part (a): Plotting the Kinetics Graph

Question 11(a) — Plot a graph of the concentration of the hydroxide ions against time. (2 marks)

✅ Correct Answer

Axes: [OH⁻] / mol dm⁻³ on one axis and Time / s on the other, with correct units.
Scale & Plotting: Linear, uniform scale covering at least half of the grid in both directions, with all 5 points correctly plotted and joined by a straight line of best fit.

🧠 Exam Technique

Always check that your plotted points occupy >50% of the available grid space. Do not force the line through the origin unless the data specifically supports it; use a ruler to draw a single straight line of best fit balancing the points.

❌ Common Errors

Omitting units from axis labels (e.g., writing just "Time" instead of "Time / s").
Using non-linear or cramped scales that fail to use the grid effectively.

Mark breakdown: (1) M1 for properly labeled axes with units and sensible scale; (2) M2 for correct point plotting and straight line of best fit.

Part (b): Deducing Reaction Order

Question 11(b) — State the order with respect to hydroxide ions. Justify your answer by reference to your graph in (a). (2 marks)

✅ Correct Answer

Order: Zero order with respect to hydroxide ions.
Justification: The graph is a straight line, showing that the rate of reaction is independent of the concentration of hydroxide ions.

💡 Key Knowledge

Recognising graph shapes is vital: a straight line for concentration-time indicates zero order because the rate (gradient) remains constant as reactant concentration changes.

❌ Common Errors

Stating an incorrect order (such as first order) by confusing concentration-time graphs with rate-concentration graphs.
Failing to explicitly link the straight line feature to the rate being independent of concentration.

Mark breakdown: (1) M1 for stating zero order; (2) M2 for stating the graph is a straight line / rate is independent of [OH⁻] (M2 is dependent on M1).

Part (c): Mechanism Deduction

Question 11(c) — Deduce the type of mechanism occurring. Justify your answer. (2 marks)

✅ Error-Free Answer

Mechanism: SN1.
Justification: There is only one reactant in the rate-determining step (or hydroxide ions do not affect the rate / are zero order).

🧠 Exam Technique & Consequential Marking

This question allows Error Carried Forward (TE) from part (b). If you incorrectly stated first order in part (b), stating SN2 here will still earn credit as long as your justification matches your stated order!

❌ Common Errors

Forgetting to link the zero-order kinetics with respect to [OH⁻] directly to the slow step (rate-determining step) of the mechanism.

Mark breakdown: (1) M1 for SN1; (2) M2 for explaining that only one reactant is in the rate-determining step or [OH⁻] does not affect the rate.

Part (d): Halogenoalkane Classification

Question 11(d) — Give the classification of the chloroalkane in this reaction. (1 mark)

✅ Correct Answer

The chloroalkane is tertiary.

💡 Key Knowledge

Tertiary halogenoalkanes typically undergo nucleophilic substitution via an SN1 mechanism because the bulky alkyl groups stabilise intermediate carbocations, making a concerted SN2 backside attack sterically hindered.

❌ Common Errors

Guessing primary or secondary without considering how the SN1 mechanism correlates directly with tertiary structures. Note that examiners apply error-carried forward rules here if your previous parts were consistent.

Mark breakdown: (1) 1 mark for stating tertiary.

Topics

Physical Chemistry · Organic Chemistry · Topic 16: Kinetics II · Topic 6: Organic Chemistry I

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 2, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.