WJEC A-Level Chemistry AS Unit 2, June 2025: Question 11

14 marks · Medium difficulty · Structured Questions

Analyse experimental data for the hydrolysis of halogenoalkanes using colorimetry and describe an iodine clock method to determine the effect of concentration on reaction rate.

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Question

Question 11 features two parts on reaction kinetics. Part (a) illustrates an experimental setup with a laptop connected to a light sensor inside a cardboard tube pointing at a test tube containing a reaction mixture illuminated by a lamp. Below it is a line graph of light intensity in lux versus time in seconds (0 to 180 s), showing three sigmoidal curves labeled A, B, and C decreasing from around 850–1000 lux down to around 50–100 lux. Sub-questions (i) to (iv) ask to explain why this method works, identify which curve corresponds to 1-iodobutane with reasoning, calculate the mean rate of reaction for curve B with units, and suggest a reason why results may not be valid. Part (b) gives two chemical equations for the persulfate-iodide clock reaction and asks for a 6-mark QER description of how to determine the effect of changing persulfate concentration on reaction rate.
Question text

11. (a) A student was investigating the rate of hydrolysis of three different halogenoalkanes:

1-chlorobutane, 1-bromobutane and 1-iodobutane

She added 2 cm3 of the halogenoalkane to a mixture of 6 cm3 of sodium hydroxide

dissolved in ethanol and aqueous silver nitrate solution. She then used a light sensor to

measure the amount of light transmitted through the reaction mixture over time.

light sensor

lamp

cardboard reaction

tube mixture

Her results are shown in the graphs below.

1000

A B C

0 30 60 90 120 150 180

Time/s

(i) Explain why this method can be used to monitor the rate of these hydrolysis

reactions. [2]

20 © WJEC CBAC Ltd. (2410U20-1)

(ii) Identify which of graphs A-C shows the rate of hydrolysis for 1-iodobutane.

Explain your reasoning. [3]

(iii) Calculate the mean rate of the reaction shown by graph B.

Give the appropriate unit. [2]

Rate =

Unit

(iv) Using the graphs, suggest why the results of this experiment could be considered

to be not valid. [1]

(b) Another student investigated the effect of concentration on rate using an iodine clock

reaction. This method uses a solution containing persulfate ions to oxidise iodide ions to

generate iodine in a slow reaction.

_ _ _

2l + S O 2 l + 2SO 2

28 2 4

Iodine is then reduced back to iodide by thiosulfate ions in a fast reaction.

_ _ _

I + 2S O 2 2I + S4O 2

22 3 6

When the thiosulfate has been consumed, excess iodine is formed which can then be

identified using an appropriate indicator.

21 Describe how the student could determine the effect of changing the concentration of©WJEC CBAC Ltd.(2410U20-1)

persulfate ions on the rate of the reaction.

You should

• give a brief method (specific quantities are not required)

• state how persulfate solutions of different concentrations are prepared from a

stock solution 23

• state how the data collected is used to determine the rate of the reaction

[6 QER]

Mark scheme

Show the mark scheme Mark scheme for Question 11 showing marking points: (a)(i) halide ions react with silver ions to form insoluble precipitate, so transmitted light decreases over time (2 marks); (a)(ii) curve A, fastest rate/finishes first, because C–I bond has lowest bond enthalpy (3 marks); (a)(iii) 850/100 = 8.5 with unit lux s⁻¹ (2 marks); (a)(iv) initial light intensity differs or precipitates have different obscuring abilities (1 mark). Part (b) is a 6-mark banded response providing indicative content for mixing solutions, using starch indicator, varying persulfate concentration while keeping total volume constant, timing to color change, and calculating rate as 1/time.

Marks available

Question Marking details

AO1 AO2 AO3 Total Maths Prac

11 (a) (i) award (1) each for any two of following

• halide ions react with Ag+ ions / silver halide forms

• precipitate forms / silver halides are insoluble 1 1 2 2

• so the amount of light passing through the mixture

decreases over time

(ii) A (1)

this reaction has the highest rate / finishes first (1) 2

C—I is the weakest bond / has the lowest bond enthalpy

(of the C—Hal bonds) (1) 1

(iii) 850

100 = 8.5 (1) accept any sensible values from graph

–1 1 1 2 1

lux s (1) accept lux / s

(iv) award (1) for either of following

light intensity does not start at the same level for all reactions 1 1 1

each precipitate has different ability to block light

Marks available

AO1 AO2 AO3 Total Maths Prac

(b) Indicative content

• mix appropriate volumes of iodide and thiosulfate

• add starch solution (as an indicator)

• add persulfate solution of appropriate concentration and

begin timing

• stop the clock when the solution turns blue/black

• repeat the method with different concentrations of persulfate

solution

• volume and concentration of all other solutions kept

constant 3 3 6 6

• 40 cm3 of stock persulfate solution is diluted with 10 cm3 of

deionised water for the second run, 30 cm3 diluted with 20

cm3 of water for the third and so on

• total volume of reaction mixture kept constant

• temperature kept constant

• calculate the (mean) rate using rate =

time

Marks available

AO1 AO2 AO3 Total Maths Prac

5-6 marks

Good description of the method; reference to total volume being constant; indication of how rate is determined

There is a sustained line of reasoning which is coherent, relevant, substantiated and logically structured. The information

included in the response is relevant to the argument.

3-4 marks

Some key steps of the method; attempt at dilution

There is a line of reasoning which is partially coherent, largely relevant, supported by some evidence and with some

structure. Mainly relevant information is included in the response but there may be some minor errors or the inclusion of

some information not relevant to the argument. 15

1-2 marks

Some indication of measuring the time taken for colour change to be seen

There is a basic line of reasoning which is not coherent, supported by limited evidence and with very little structure.

There may be significant errors or the inclusion of information not relevant to the argument.

0 marks

No attempt made or no response worthy of credit.

Question 11 total 6 3 5 14 1 11

How to answer it

Rates of Hydrolysis & Iodine Clock Kinetics

What this question tests

This question assesses core laboratory techniques and physical-organic principles from WJEC AS Chemistry Unit 2:

  • Halogenoalkane Reactivity: Explaining nucleophilic substitution rates using carbon–halogen bond enthalpies rather than bond dipoles.
  • Continuous Monitoring Methods: Measuring light transmission via a light sensor and datalogger as precipitates form.
  • Data Analysis & Rate Calculation: Calculating mean reaction rate (change in quantity ÷ time) and identifying correct units.
  • Experimental Evaluation: Identifying systematic errors and validity issues in turbidity/precipitation experiments.
  • QER Experimental Design (Iodine Clock): Formulating a full practical method including dilution series, control variables, indicator chemistry, and mathematical analysis ( Rate ∝ 1/time ).
Part (a)(i) — 2 Marks

Monitoring Hydrolysis with a Light Sensor

Explaining why turbidity monitors reaction progress

✅ Correct Answer (Any 2 of the following)

  • Halide ions (produced during hydrolysis) react with silver ions (Ag⁺) to form an insoluble silver halide precipitate. [1 mark]
  • A precipitate forms / silver halides are insoluble in water. [1 mark]
  • As the precipitate forms, the reaction mixture turns opaque/cloudy, so the amount of light transmitted through the mixture decreases over time. [1 mark]

🧠 Exam Technique & Insight

Always link the chemistry to the physics of detection:

  1. State the chemical product: Ag⁺(aq) + X⁻(aq) → AgX(s) .
  2. Explain the physical change: cloudiness/turbidity increases due to the solid suspension.
  3. Relate to the instrument: light transmission drops as light is scattered/absorbed.
Mark Allocation: 2 marks maximum (AO1 + AO3). Do not simply state "it changes colour" — you must mention the formation of a precipitate or reduced light transmission.
Part (a)(ii) — 3 Marks

Identifying 1-Iodobutane & Bond Enthalpy Trend

Relating rate of hydrolysis to carbon–halogen bond strength

✅ Correct Answer

  • Identification: Curve A [1 mark]
  • Graph deduction: Curve A has the steepest slope / highest rate / finishes first / light intensity reaches a minimum quickest. [1 mark]
  • Chemical reason: The C—I bond is the weakest / has the lowest bond enthalpy of the carbon–halogen bonds, so it breaks most readily. [1 mark]

❌ The #1 Student Pitfall: Electronegativity Trap

Common Error: Claiming 1-chlorobutane reacts fastest because chlorine is the most electronegative, making the C—Cl bond most polar.

The Truth: Rate of hydrolysis is governed by bond enthalpy, not bond polarity. Down Group 7, atomic radius increases, orbital overlap decreases, and C—X bond enthalpy drops dramatically (C—Cl: 338 kJ mol⁻¹ vs C—I: 238 kJ mol⁻¹). C—I breaks easiest!

Mark Breakdown: 1 mark for Graph A; 1 mark for stating curve A has the highest rate / completes first; 1 mark for lowest C—I bond enthalpy.
Part (a)(iii) — 2 Marks

Mean Rate Calculation for Curve B

Determining average rate of change from experimental data

📐 Step-by-Step Calculation

  1. Initial & final light intensities for Curve B:
    Initial reading at t = 0 s ≈ 950 lux (or from curve shelf ≈ 950 lux).
    Final plateau reading at t ≈ 100 s ≈ 100 lux.
    Change in intensity = 950 − 100 = 850 lux.
  2. Time taken:
    Reaction reaches plateau at 100 s.
  3. Mean Rate:
    Mean rate = Δintensity / Δtime = 850 / 100 = 8.5 [1 mark]
  4. Determine units:
    Intensity unit (lux) divided by time (s) = lux s⁻¹ (or lux / s) [1 mark]

🧠 Top Tip on Units

Always inspect the axis labels! The y-axis is in lux and the x-axis is in s. Mean rate is simply Δy / Δx.

Examiners accept sensible variations read from the graph (e.g. 900–1000 lux initial, 70–120 lux final, time 95–105 s), but your calculation must match the readings taken from Graph B.

Mark Allocation: 1 mark for calculation with sensible graph values (e.g. 8.5); 1 mark for correct unit ( lux s⁻¹ ).
Part (a)(iv) — 1 Mark

Evaluating Experimental Validity

Flaws in comparing turbidity across different halides

✅ Correct Answer (Either of the following)

  • The initial light intensity does not start at the same value for all three reactions (Curve A starts at ~880 lux, B at ~950 lux, C at ~1000 lux). [1 mark]
  • OR: The precipitates have different colours/opacities (AgCl is white, AgBr is cream, AgI is yellow) and different particle sizes, so they have different abilities to block/scatter light. [1 mark]

💡 Validity vs Reliability

Validity means the experiment actually tests what it claims to test in a fair, controlled manner. Because AgCl, AgBr, and AgI precipitate with different crystal structures and absorption spectra, identical concentrations do not reduce lux by identical amounts. A direct comparison of transmission is therefore not strictly valid.

Part (b) — 6 Marks (QER)

Designing an Iodine Clock Experiment

Investigating the effect of persulfate concentration on reaction rate

💡 Reaction Mechanism Recap

Reaction 1 (Slow, rate-determining): 2I⁻ + S₂O₈²⁻ → I₂ + 2SO₄²⁻

Reaction 2 (Fast scavenger): I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻

A fixed, small quantity of thiosulfate (S₂O₃²⁻) reduces iodine back to iodide instantly. As soon as all thiosulfate is consumed, free iodine remains and complexes with starch to form an instantaneous deep blue-black colour.

✅ Model Answer: Sustained, Logically Structured Method (5–6 Marks)

1. Practical Procedure:

  • Using suitable measuring cylinders or pipettes, measure out and combine fixed volumes of potassium iodide solution and sodium thiosulfate solution into a conical flask or beaker.
  • Add a few drops of starch indicator to the mixture.
  • Add a known concentration/volume of persulfate solution (S₂O₈²⁻) and immediately start the stopclock and swirl.
  • Stop the clock the exact moment the solution changes from colourless to blue-black. Record the time taken ( t ).

2. Preparation of Different Persulfate Concentrations (Dilution Series):

  • Create at least 5 different concentrations of persulfate by diluting a stock persulfate solution with deionised water:
    • Run 1: 50 cm³ stock persulfate + 0 cm³ deionised water
    • Run 2: 40 cm³ stock persulfate + 10 cm³ deionised water
    • Run 3: 30 cm³ stock persulfate + 20 cm³ deionised water
    • Run 4: 20 cm³ stock persulfate + 30 cm³ deionised water
    • Run 5: 10 cm³ stock persulfate + 40 cm³ deionised water
  • Ensure the total volume of the reaction mixture remains constant across all runs so that the concentration of persulfate is directly proportional to its volume added.

3. Controlled Variables:

  • Keep the volume and concentration of iodide and thiosulfate constant.
  • Keep the volume of starch constant.
  • Maintain a constant temperature (using a water bath if necessary) as reaction rates are highly temperature-sensitive.

4. Data Processing to Determine Rate:

  • Since the amount of iodine produced before the colour change is fixed (dictated by the constant amount of thiosulfate), initial rate is inversely proportional to time:
    Rate ∝ 1 / time (or Rate = 1 / t)
  • Plot a graph of Rate (1/t) on the y-axis versus concentration/volume of persulfate on the x-axis to establish the relationship (order of reaction).

❌ Common QER Mistakes

  • Forgetting deionised water: Changing the volume of persulfate without adding water changes the total volume, meaning concentrations of iodide and thiosulfate are unintentionally diluted!
  • Vague dilution instructions: Saying "dilute it down" scores poorly. Give an explicit recipe (e.g. 40 cm³ persulfate + 10 cm³ water).
  • Forgetting 1/t: Stating that "time is the rate" will lose marks. Rate is proportional to 1/t.

🧠 Band 3 Descriptor (5–6 Marks) Checklist

  • ✔ Clear step-by-step practical method with timing to blue-black colour.
  • ✔ Explicit dilution method maintaining constant total volume.
  • ✔ Stating control variables (constant volumes, constant temp).
  • ✔ Mathematical determination of rate ( Rate = 1/time ).
  • ✔ Logical, coherent paragraph structure with standard chemical terminology.

Topics

Organic Chemistry · Physical Chemistry · Practical · 2.6 Halogenoalkanes · 2.2 Rates of reaction · AS Unit 2 practical work

Question and mark scheme from the WJEC A-Level Chemistry examination, AS Unit 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.