Edexcel A-Level Chemistry Paper 3, June 2024: Question 10

14 marks · Hard difficulty · Practical Techniques and Data Analysis

Analyze the acid-catalysed hydrolysis of ethyl ethanoate using experimental data, graphical methods, and a multi-step reaction mechanism.

Practise this question

Question

Question text

10 The progress of the reaction between ethyl ethanoate and water with a

hydrochloric acid catalyst can be followed in an experiment using a titrimetric

method. An outline procedure is shown.

Step 1 Add 100 cm3 of 0.500 mol dm–3 HCl(aq) to a conical flask.

Step 2 Using a pipette, add 5.0 cm3 of ethyl ethanoate to the conical flask

while mixing.

Step 3 Immediately remove a 5.0 cm3 sample of the reaction mixture from Step 2 and

add it to a separate conical flask containing crushed ice.

Step 4 Titrate the removed sample using 0.200 mol dm–3 NaOH(aq).

Step 5 Repeat Steps 3 and 4, removing 5.0 cm3 samples every 10 minutes.

Step 6 After 60 minutes, heat the remaining reaction mixture under reflux for

15 minutes.

Step 7 Remove a 5 cm3 sample of the reaction mixture after completing Step 6,

titrating this sample with 0.200 mol dm–3 NaOH(aq).

(a) (i) Explain the purpose of the crushed ice in Step 3.

(2)

(ii) Explain why Step 6 is carried out before the final sample is removed and

titrated in Step 7.

(2)

(b) Some data from the experiment are shown. Vfinal is the titre volume from Step 7,

Vt is the titre volume at time t.

Time reaction sample is added to 27

*P74455A02732*0102030 40 50 60

crushed ice / min

V – V of sodium hydroxide / cm3 11.60 6.50 3.80 1.80 0.90 0.80 0.80

final t

(i) Plot a graph of the Vfinal – Vt of sodium hydroxide against the time the reaction

sample is added to the crushed ice.

(3)

(ii) Explain why Vfinal – Vt is plotted rather than the concentration of

sodium hydroxide solution for each titre.

(2)

… 28

… *P74455A02832*

(iii) Determine two half-lives for the reaction, showing your working on the graph.

(2)

(iv) Deduce the order of the reaction, using your answer from (b)(iii) to justify

your deduction.

(1)

A student researching the mechanism for the reaction found this example.

O–

+ OH–

Step 1

O O

slow

O– O

Step 2

O OH + –O

fast

– Step 3 –

H2O + O HO + OH

fast 29

Deduce a rate equation that is consistent with this reaction mechanism.*P74455A02932*

Justify your answer.

(2)

(Total for Question 10 = 14 marks)

Mark scheme

Show the mark scheme

Question

Answer Additional Guidance Mark

Number

10(a)(i) An explanation that makes reference to the following points: (2)

• to stop / freeze / quench the reaction (1) Allow to prevent further hydrolysis/negligible reaction

Ignore reference to cooling the reaction

Ignore prevention of evaporation

Ignore just slow the reaction

• to allow time for the titration to be carried out (1) Accept to prevent change/negligible change in

concentration of reactants / products

Allow so the concentration can be found at that time

Question

Answer Additional Guidance Mark

Number

10(a)(ii) An explanation that makes reference to the following points: (2)

• to ensure the hydrolysis / reaction reaches Allow hydrolysis reaction goes to completion

equilibrium (1) Do not award reference to oxidation

• so the maximum amount of carboxylic acid (formed) Allow reference to final equilibrium/at the end for

can be determined (1) maximum

Question

Answer Additional Guidance Mark

Number

10(b)(i) (3)

• both axes labelled with units with time on horizontal axis (1)

• suitable scale where points cover at least half the available

space horizontally and vertically (1)

• all points plotted correctly and line of best fit shown (1) Line of best fit to be with +/− 2 small square of plotted

points

Do not award a line that goes up by 1 small square after

60 minutes

Exemplar graph

Question

Answer Additional Guidance Mark

Number

10(b)(ii) An explanation that makes reference to any two of the following points: (2)

• Vfinal is proportional to concentration of / amount of ester initially

plus the acid catalyst (1)

• Vfinal − Vt is proportional to concentration of / amount of ester at

each time / at t (1)

• the concentration (of NaOH) does not change

(so the graph would be a straight line) (1)

Question

Answer Additional Guidance Mark

Number

10(b)(iii) An answer that makes reference to the following points Check the graph that 6 and 3 are not used (2)

• first half-life correctly shown on graph and value determined (1) Allow values between 11 and 13 (mins)

• second half-life correctly shown on graph and value determined (1)

Exemplar graph

Penalise lack of working on graph once only

Question

Answer Additional Guidance Mark

Number

10(b)(iv) An answer that makes reference to the following point: (1)

• first order Standalone mark

and

(as) half-lives are constant No TE from half life values

Question

Answer Additional Guidance Mark

Number

10(c) An answer that makes reference to the following points: (2)

• Rate = k [CH CO CH CH ][OH−] (1) Allow name or any correct structure of ester in rate

32 2 3

equation

Allow C4H8O2 for ethyl ethanoate

Allow any suitable alkali such as NaOH

Allow r for rate

• as slow step involves 1 (mol) of ethyl ethanoate and Allow only 1 mol of each reactant in the rate

1 (mol) of OH– ions (1) determining step

Ignore just that step 1 is the rate determining step

Ignore just that they are both first order

No TE on incorrect rate equation

M2 dependent on M1 or near miss

(Total for Question 10 = 14 marks)

TOTAL FOR PAPER = 120 MARKS

How to answer it

Kinetics of Ester Hydrolysis and Reaction Mechanisms

What this question tests

This question evaluates your understanding of experimental kinetics, specifically using a titrimetric quenching method to monitor the acid-catalysed hydrolysis of an ester. Key skills assessed include explaining experimental design features (quenching and refluxing to completion), plotting and interpreting kinetic data (calculating half-lives to determine reaction order), and deducing rate equations from multi-step reaction mechanisms.

Question (a) - Experimental Design

Quenching and Forcing Equilibrium

✅ Part (i): Purpose of Crushed Ice

To stop, freeze, or quench the reaction instantly (1 mark), and to allow sufficient time for the titration to be carried out accurately without the reaction continuing (1 mark).

✅ Part (ii): Purpose of Step 6 (Reflux)

To ensure the hydrolysis reaction reaches completion / equilibrium (1 mark) so that the maximum amount of carboxylic acid formed can be determined as V_final (1 mark).

❌ Common Errors

Saying ice "cools the reaction down" vaguely, without stating it stops/quenches the reaction.
Confusing quenching with lowering the activation energy or preventing evaporation.
In (ii), incorrectly claiming reflux "speeds up the catalyst" or causes oxidation.

Question (b) - Graphical Analysis & Kinetics

Data Analysis, Half-Lives, and Reaction Order

✅ Part (i): Graph Requirements (3 Marks)

Axes: Labelled with units ( V_final - V_t of sodium hydroxide / cm³ on vertical, time / min on horizontal).
Scale: Must cover at least half of the available grid space in both directions.
Plotting & Line: All points plotted correctly within ±2 small squares, with a smooth line of best fit.

💡 Part (ii): Why Use ( V_final - V_t )?

V_final - V_t is directly proportional to the concentration (or amount) of the ester remaining at time t . Using this difference avoids needing complex concentration calculations while ensuring a direct proportional relationship.

📐 Part (iii): Determining Half-Lives

First half-life: Drop from value 12 down to 6 on the y-axis, read across to the curve, and drop down to the time axis. Value is approximately 11 to 13 minutes (e.g., 12.5 mins).
Second half-life: Drop from 6 down to 3 on the y-axis, read across, and find the corresponding time interval. Value is approximately 11 to 13 minutes (e.g., 11.5 mins).

✅ Part (iv): Reaction Order Deduction

First order (1 mark). Justification: The successive half-lives are constant (independent of concentration).

Examiner Tip: Always show clear construction lines on your graph when determining half-lives. Examiners penalise missing working out.

Question (c) - Reaction Mechanisms & Rate Equations

Alkaline Hydrolysis Mechanism

✅ Correct Answer

Rate = k [CH_3CO_2CH_2CH_3] [OH⁻] (or using structural formulas / names)

🧠 Examiner Marking Logic

Mark 1: Correct rate equation containing both reactants involved in the process.
Mark 2: Justification referencing the slow step (Step 1), which involves 1 molecule of ethyl ethanoate and 1 hydroxide ion.

❌ Common Errors

Students frequently try to include the catalyst or products in the rate equation, or fail to link the orders directly to the stoichiometry of the rate-determining (slow) step.

Topics

Physical Chemistry · Organic Chemistry · Core Practicals · Core Practical 13a: Follow the rate of the iodine-propanone reaction by a titrimetric method · Topic 16: Kinetics II · Topic 6: Organic Chemistry I

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.