AQA A-Level Chemistry Paper 3, June 2025: Question 2

15 marks · Medium difficulty · Practical Techniques & Data Analysis

Investigate the rate of reaction between hydrogen peroxide, iodide ions, and acid using an iodine clock method to determine orders of reaction, rate equation, initial rate, and evaluate experimental uncertainties.

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Question text

02 The effect of reactant concentrations on the rate of a reaction is investigated.

The equation for the reaction is shown.

H O (aq) + 2 I–(aq) + 2 H+(aq) → I (aq) + 2 H O(l)

22 2 2

The experiment is done five times in the presence of thiosulfate ions and starch.

The reactant concentrations are varied by changing the volume of each reactant in the

mixture.

The thiosulfate ions react with the iodine as it is produced, until all the thiosulfate has

reacted.

I (aq) + 2 S O 2–(aq) → 2 I–(aq) + S O 2–(aq)

22 3 4 6

After all the thiosulfate has reacted, the iodine produced in the mixture then reacts

with the starch and the solution turns blue-black.

The time taken for the solution to turn blue-black is recorded.

If the amount of thiosulfate is the same in each experiment, the time recorded

represents the time to produce the same amount of iodine in each experiment.

Table 3 shows the results of the investigation.

Table 3

[H O ] [I–] [H+] Time for reaction mixture

Experiment –3 –3 –3

/ mol dm / mol dm / mol dm to turn blue-black / s

1 0.010 0.010 0.10 60

2 0.020 0.010 0.10 30

3 0.030 0.010 0.10 20

4 0.030 0.020 0.10 10

5 0.030 0.020 7 0.20 10

02.1 The amount of thiosulfate is the same in each experiment. To allow the order of

reaction with respect to each reactant to be calculated, several other variables need to

be controlled.

Identify three other control variables.

[3 marks]

Control variable 1

Control variable 2

Control variable 3

02.2 Deduce the order of reaction with respect to each reactant.

*06Explain how you deduced the order of reaction with respect to iodide ions.*

[3 marks]

Order with respect to H2O2

Order with respect to I–

Order with respect to H+

Explanation

02.3 Give the rate equation for the reaction.

Determine the units of the rate constant.

[2 marks]

Rate equation

Units of rate constant

02.4 In each experiment, 1.00 cm3 of 1.00 × 10–3 mol dm–3 sodium thiosulfate solution is

used to react with the iodine initially produced.

I (aq) + 2 S O 2–(aq) → 2 I–(aq) + S O 2–(aq)

22 3 4 6

The total volume of the reaction mixture in experiment 3 is 40.0 cm3

Calculate the initial rate of production of iodine, in mol dm–3 s–1, for experiment 3.

[3 marks]

Initial rate mol dm–3 s–1

02.5 The total uncertainty in the times recorded for each experiment is ±0.5 s but the

stopwatch used has a measurement uncertainty of ±0.01 s

Suggest why, in each experiment, the uncertainty in the times recorded is greater than

the measurement uncertainty of the stopwatch.

[1 mark]

02.6 Calculate the percentage uncertainty in the time recorded in experiment 3.

[1 mark]

Percentage uncertainty

02.7 The five experiments are repeated with the same stopwatch and the same

concentrations and volumes of H O , I– and H+

Suggest and explain how the method can be changed to decrease the

*08percentage uncertainty in the times recorded.*

[2 marks]

Mark scheme

Show the mark scheme

Question Answers Additional comments/Guidelines Mark

Any 3

• temperature

• amount/volume of starch ALLOW concentration/mass of starch

• total volume / volume of reaction mixture

02.1 • (constant) stirring/agitation

(3 x AO3)

• same person observing colour change / same shade of

colour when timing stopped IGNORE same stopwatch

IGNORE concs/vols/amounts of thiosulfate &

other reagents

IGNORE references to (same) apparatus

IGNORE same person unqualified

M1 1st / one for H O AND 1st / one for I– AND zero / 0 for H+

M2 Doubling [I–] halves the time M2 is independent of M1

M2 ALLOW doubling [I–] doubles the rate

02.2 + –

M3 (Use experiments 3 and 4) because [H ] and [H2O2] stay the ALLOW M2 AND M3 for when [I ] doubles and (3 x AO3)

same OR because only [I–] changes [H+] and [H O ] stay the same the time halves /

rate doubles

ALLOW any correct explanation based on two

experiments and orders wrt H O and/or H+

– A-LEVEL CHEMISTRY22 – 7405/3 –

M1 rate = k [H O ] [I–] M1 ALLOW rate = k [H O ]1 [I–]1 [H+]0

22 2 2

ALLOW M1 independent of 02.2 if correct

but ECF from 02.2 for other answers

NOT () 2

02.3

(2 x AO2)

M2 mol–1 dm3 s–1 M2 dependent on a rate equation in M1

M2 ALLOW any order of mol dm s

M2 ALLOW if no answer to M1

1.00 × 10-3

M1 (amount of S O 2– =

23 1.00

1000

= 1.00 × 10–6 mol)

amount of I = ½ × amount of S O 2–

22 3

= 5.00 × 10–7 mol

M1 = 1.25 × 10–5 mol dm–3

M2 [I2] = 40 3

1000 For M2, M1 must be an attempt at calculating

02.4

amount of I2 (3 x AO2)

M2 = 6.25 × 10–7 (mol dm–3 s–1) (= 3 marks)

M3 rate = 20 –7

(at least 2sf so ALLOW 6.3 × 10 )

– A-LEVEL CHEMISTRY – 7405/3 –

M3 if no M2 ALLOW mol of iodine from M1/20

e.g. 5 × 10–7 / 20 = 2.5 × 10–8 (= 2 marks, M1 &

M3)

(observer) reaction time / time taken to stop timer (is more IGNORE ‘human error’ unqualified 1

02.5 significant) / hard to determine ‘end point’ / colour change not

(1 x AO3)

instantaneous

02.6 2.5 (%) NOT 2.55

(1 x AO3)

M1 increase the time (and so reduces percentage uncertainty)

M2 use a greater concentration/amount of thiosulfate ALLOW greater volume of thiosulfate

ALLOW lower temperature

02.7 IGNORE use of colorimeter

(2 x AO3)

IGNORE references to starch

IGNORE references to repeats / averages

NOT lower/greater total volume / adding water /

lower/greater concentrations of reactants

How to answer it

Iodine Clock Kinetics: Orders, Rate Equations & Uncertainty

WHAT THIS QUESTION TESTS

Core Concepts & Practical Skills:

Designing controlled kinetics investigations (identifying independent, dependent, and control variables).
Deducing reaction orders from continuous initial-rate clock experiment data (where rate ∝ 1/time).
Writing rate equations and determining correct derived units for the rate constant ( k ).
Multi-step stoichiometric calculations of initial reaction rate involving limiting reagents and reaction volume.
Distinguishing between procedural uncertainty and instrumental reading uncertainty, and evaluating methods to reduce percentage uncertainty.

PART 02.1 • 3 MARKS

Control Variables in the Clock Reaction

Identifying variables that must remain constant

✅ Acceptable Control Variables (Any Three)

Temperature (or use of a thermostatted water bath)
Total volume of the reaction mixture (maintained using deionised water)
Volume / mass / concentration of starch solution added
Constant stirring / agitation
Same colour end-point (same person observing the sudden colour change / same depth of blue-black)

❌ Common Mistakes & Rejected Answers

Mentioning thiosulfate: The question stem explicitly says "The amount of thiosulfate is the same in each experiment", so this cannot score.
"Same person" unqualified: Must specifically refer to the observer judging the end-point colour.
"Same stopwatch" or "same apparatus": Instrumental replication is standard lab practice, not a chemical control variable.

Mark Scheme: 3 × AO3 marks. Award 1 mark each for any 3 distinct valid variables.

PART 02.2 • 3 MARKS

Determining Orders of Reaction

Deducing orders from initial rate (1/t) data

✅ Correct Deductions

Order wrt H₂O₂: 1 (or 1st / first)
Order wrt I⁻: 1 (or 1st / first)
Order wrt H⁺: 0 (or zero)

✅ Required Explanation for I⁻

Compare Experiments 3 and 4: [H₂O₂] and [H⁺] remain constant. When [I⁻] is doubled (0.010 → 0.020 mol dm⁻³), the time is halved (20 s → 10 s), meaning the initial rate doubles. Therefore, the reaction is first order with respect to I⁻.

🧠 Exam Technique & Data Breakdown

Rate relationship: Since fixed moles of I₂ are formed before the blue-black colour appears, Rate ∝ 1/time . Halving time means rate doubles!
For H₂O₂: Compare Exp 1 & 2: [I⁻] and [H⁺] constant. [H₂O₂] doubles (0.010 → 0.020), time halves (60 s → 30 s) → Rate doubles → 1st order.
For H⁺: Compare Exp 4 & 5: [H₂O₂] and [I⁻] constant. [H⁺] doubles (0.10 → 0.20), time stays constant (10 s) → Zero order.

Mark Breakdown:
• M1: All 3 orders deduced correctly (1, 1, 0).
• M2: Stating that doubling [I⁻] halves the time (or doubles the rate).
• M3: Explaining that Experiments 3 & 4 are used because [H⁺] and [H₂O₂] are kept constant (or only [I⁻] changes).

PART 02.3 • 2 MARKS

Rate Equation and Units of k

Formulating the rate law and deriving rate constant units

✅ Correct Answer

Rate equation: rate = k[H₂O₂][I⁻]

Also allowed: rate = k[H₂O₂][I⁻][H⁺]⁰

Units of k: mol⁻¹ dm³ s⁻¹ (or dm³ mol⁻¹ s⁻¹ )

🧠 How to Derive the Units

Rearrange for k:
k = rate / ([H₂O₂][I⁻])

Substitute units:
k = (mol dm⁻³ s⁻¹) / ((mol dm⁻³) × (mol dm⁻³))

Cancel terms:
k = s⁻¹ / (mol dm⁻³) = mol⁻¹ dm³ s⁻¹

Mark Scheme: 1 mark for the correct rate equation (brackets required, no round brackets). 1 mark for correct units (ECF allowed from an incorrect rate equation in 02.2).

PART 02.4 • 3 MARKS

Initial Rate Calculation for Experiment 3

Step-by-step quantitative determination of initial rate

📐 Step-by-Step Calculation

Step 1: Calculate moles of thiosulfate used

Amount of S₂O₃²⁻ = conc × vol = 1.00 × 10⁻³ mol dm⁻³ × (1.00 / 1000 dm³)
Amount of S₂O₃²⁻ = 1.00 × 10⁻⁶ mol

Step 2: Determine moles of I₂ produced (Reacting Ratio)

From equation: I₂(aq) + 2S₂O₃²⁻(aq) → 2I⁻(aq) + S₄O₆²⁻(aq)
Molar ratio of I₂ : S₂O₃²⁻ is 1 : 2.
Amount of I₂ produced = ½ × 1.00 × 10⁻⁶ mol = 5.00 × 10⁻⁷ mol

Step 3: Convert moles of I₂ to concentration in reaction mixture

Total reaction mixture volume = 40.0 cm³ = 40.0 / 1000 dm³ = 0.040 dm³
[I₂] = (5.00 × 10⁻⁷ mol) / 0.040 dm³ = 1.25 × 10⁻⁵ mol dm⁻³

Step 4: Calculate rate of production of I₂

Time for Experiment 3 = 20 s.
Rate = Δ[I₂] / Δt = 1.25 × 10⁻⁵ mol dm⁻³ / 20 s = 6.25 × 10⁻⁷ mol dm⁻³ s⁻¹
(Accept 6.3 × 10⁻⁷ mol dm⁻³ s⁻¹)

❌ Common Calculation Traps

Forgetting the 1:2 ratio: Using 1.00 × 10⁻⁶ mol for iodine loses M1.
Omitting total volume: Dividing moles of I₂ directly by 20 s without finding concentration first gives 2.5 × 10⁻⁸ mol s⁻¹ (loses M2).
Volume conversion: Forgetting to divide 40.0 cm³ by 1000 to convert to dm³.

💡 Why is this the "Initial Rate"?

The thiosulfate removes iodine as fast as it is generated. Because the amount of S₂O₃²⁻ is very small, it is consumed after only a tiny fraction of the reactants have reacted (<1%). Therefore, reactant concentrations are effectively constant during this period, giving an accurate initial rate.

Mark Breakdown:
• M1: Correct moles of I₂ (= 5.00 × 10⁻⁷ mol)
• M2: Correct concentration of I₂ (= 1.25 × 10⁻⁵ mol dm⁻³)
• M3: Correct rate (= 6.25 × 10⁻⁷ mol dm⁻³ s⁻¹)

PARTS 02.5, 02.6 & 02.7 • 4 MARKS

Experimental Uncertainties & Method Improvements

02.5: Stopwatch Uncertainty vs Total Uncertainty (1 Mark)

✅ Correct Answer

Any one of:

Human reaction time (time taken by observer to press stop).
Difficulty in judging the exact end-point / colour change is not completely instantaneous.

❌ Examiner Warning

Simply writing "human error" receives 0 marks. You must state reaction time or difficulty judging the sudden colour change.

Mark: 1 × AO3 mark.

02.6: Percentage Uncertainty Calculation (1 Mark)

📐 Calculation for Experiment 3 (time = 20 s, total uncertainty = ±0.5 s)

% Uncertainty = (Uncertainty / Measured Value) × 100

% Uncertainty = (0.5 s / 20 s) × 100 = 2.5%

Examiner Note: Do NOT write 2.55%. Use the given total uncertainty of ±0.5 s, not the ±0.01 s stopwatch precision.

Mark: 1 × AO3 mark.

02.7: Modifying Method to Decrease % Uncertainty (2 Marks)

✅ Suggested Modification & Explanation

Modification (M2): Use a greater concentration, volume, or amount of thiosulfate (OR conduct at a lower temperature).
Explanation (M1): This will increase the time taken for the colour change to appear, which decreases the percentage uncertainty in the recorded time.

❌ Disallowed Modifications

Changing H₂O₂, I⁻, or H⁺: The prompt explicitly forbids this: "repeated with the same concentrations and volumes of H₂O₂, I⁻ and H⁺".
"Change total volume": Changing total volume alters reactant concentrations.
"Take repeats": Repeating reduces anomalous errors, but does not modify the method to reduce the measurement percentage uncertainty.

Mark Breakdown:
• M1: Idea of increasing the measured time (to reduce % uncertainty).
• M2: Use a greater concentration/amount/volume of sodium thiosulfate (or lower temperature).

Topics

Physical Chemistry · Required Practicals · 3.1.9 Rate Equations · 3.1.2 Amount of Substance · Required Practical 7: Measuring the rate of reaction by an initial rate method

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.