Edexcel A-Level Chemistry Paper 2, June 2018: Question 9

16 marks · Hard difficulty · Practical Techniques and Data Analysis

Investigate the reaction kinetics of iodine clock reactions including rate equations, rate constants, activation energy from Arrhenius plots, and experimental design.

Practise this question

Question

Question text

9 This question is about the reaction kinetics of an ‘iodine clock’ reaction.

One example of an ‘iodine clock’ reaction that involves the iodate(V) ions and

iodide ions in acidic solution is

IO−(aq) + 5I−(aq) + 6H+(aq) → 3I (aq) + 3H O(l)

32 2

(a) State why the order of reaction with respect to iodide ions cannot be five, even

though 5mol of iodide ions are shown in the equation.

(1)

(b) A series of experiments was carried out by a student to determine the order of reaction

with respect to iodate(V) ions. The concentrations of the iodide ions and the acid were

in large excess and the volume of the iodate(V) solution was varied.

The total volume of the reaction mixture was kept constant by the addition of suitable

volumes of deionised water.

The following results were obtained:

Experiment Number 1 2 3 4 5 6

Volume of iodate(V) solution / cm3 10.0 7.0 5.0 3.0 2.0 1.0

Time (t)/s 180 260 357 606 900 800

(1000/t) / s−1 5.56 1.11 1.25

(i) In experiment 6, the student forgot to add deionised water to keep the total

volume the same for each experiment.

State why the total volume should be kept the same.

(1)

(ii) Complete the table and use the results from experiments 1, 2, 3, 4 and 5 to

plot a graph of 1000/t against volume of iodate(V) ions.

(4)

*P52293A02024*

(iii) Deduce the order of reaction with respect to the iodate(V) ions. Justify your answer.

(2)

aqueous solutions of potassium iodide, sodium thiosulfate and starch.*P52293A02124*

The main reaction is

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

22 2 2

The reaction is first order with respect to hydrogen peroxide and iodide ions but

zero order with respect to hydrogen ions.

(i) In one experiment, the following data were obtained:

Reactants Initial concentration / mol dm−3

H O2(aq) 1.50 × 10−3

I−(aq) 2.10 × 10−3

H+(aq) 2.10 × 10−3

Initial rate = 1.24 × 10−3 mol dm−3 s−1

Write the rate equation and hence deduce the value of the rate constant, k,

from these data. Include units and give your answer to an appropriate

number of significant figures.

(2)

(ii) Explain the purpose of the starch present in the reaction mixture when starch

is neither in the rate equation, nor in the reaction equation.

(2)

(d) Another ‘iodine clock’ reaction produced data that enabled the following graph of*P52293A02224*

lnk against 1/T to be drawn.

1 –

1 / T / K–1

0 –

1.3 × 10–3 1.4 × 10–3 1.5 × 10–3 1.6 × 10–3 1.7 × 10–3 1.8 × 10–3

–1 –

–2 –

–3 –

ln k

–4 –

–5 –

–6 –

–7 –

–8 –

(i) The Arrhenius equation can be expressed as

Ea 1

ln k = − × + constant

R T

From the gradient of the graph, determine the activation energy, Ea, for this reaction.

Include a sign and units in your answer.

(3)

(ii) Give a reason for the point at lnk = −7 not being included in the line drawn

on the graph.

(1)

(Total for Question 9 = 16 marks)

Mark scheme

Show the mark scheme

Question

Acceptable Answer Additional Guidance Mark

Number

9(a) A statement that makes reference to the following: (1)

The chance of five or more ions colliding in the Allow ‘at the same time’ for the RDS

rate determining step is negligible

Question

Acceptable Answer Additional Guidance Mark

Number

9(b)(i) A statement that makes reference to the following: (1)

So that the volume of iodate(V) ions is Allow the volume of iodate(V) ions can

proportional to the concentration. be used instead of the concentration in

plotting the graph.

Ignore reference to “fair tests”.

Question

Acceptable Answer Additional Guidance Mark

Number

9(b)(ii) calculation of all three 1000/t values (1) (5.56) 3.85, 2.80, 1.65, (1.11) (4)

Do not award 2.8 or ≥3SF

axes: correct way round, labelled and including

units (1) Do not award use of T for t

suitable scale (1) Plotted points must cover at least ½ the

graph paper on each axis

all points plotted correctly, with best-fit straight

Allow ±½ square

line through the origin (1)

Ignore plotting of experiment 6

Exemplar graph

Question

Acceptable Answer Additional Guidance Mark

Number

9(b)(iii) An answer that makes reference to the following: Mark independently (2)

first order (with respect to iodate(V) ions) (1)

because straight line goes through the origin / rate Allow “volume” for “concentration”

is (directly) proportional to concentration (1) Do not award references of

proportionality to time

Ignore references to half life

Question

Acceptable Answer Additional Guidance Mark

Number

9(c)(i) Example of calculation: (2)

value of rate constant to 2 or 3 SF (1) (Rate=k[H O ][I−] so

k= rate ÷ ([H O ][I−])

=1.24x10−3÷(1.50x10−3x2.10x10−3)

=393.65…)

= 390/394

units of rate constant (1) dm3 mol−1 s−1

Accept units in any order

No TE on incorrect rate equation

Question

Acceptable Answer Additional Guidance Mark

Number

9(c)(ii) An explanation that makes reference to (2)

starch is an indicator (to react with the iodine) Do not award references to iodide/I−

(1) Allow

Reacts with iodine/produces a blue-

black colour when the reaction is

complete.

Allow

Changes colour when all the thiosulfate

is used up.

the time taken for the formation of the blue-black Allow indication of ‘time taken’

complex (can be used to calculate the reaction

rate) (1)

How to answer it

Reaction Kinetics: The Iodine Clock

What this question tests

This comprehensive kinetics question assesses your understanding of collision theory, multi-step reaction mechanisms, graphical analysis of rate data, rate equations, rate constants with units, the role of indicators in clock reactions, and the Arrhenius equation. You will need to apply proportionality logic, plot data accurately, calculate activation energy from gradients, and handle significant figures.

Question 9 (a)

Collision Probability & Reaction Mechanisms

✅ Correct Answer

The chance of five or more ions colliding simultaneously in the rate-determining step is negligible.

💡 Key Knowledge

Overall stoichiometric equations show overall mass balance, not molecularity. Elementary steps rarely involve more than two (at most three) species colliding because the probability of simultaneous multi-particle collisions is vanishingly small.

🧠 Exam Technique

Keep your answer concise. Examiners accept "at the same time" for the rate-determining step (RDS), but explicitly referencing collision probability secures the mark.

Mark: 1 mark

Question 9 (b)(i)

Maintaining Constant Volume

✅ Correct Answer

So that the volume of iodate(V) ions is proportional to its concentration.

❌ Common Errors

Students often lose this mark by mumbling about "making it a fair test" or referring loosely to "keeping conditions the same" without explaining that total volume controls concentration scaling when volume is varied.

Mark: 1 mark

Question 9 (b)(ii) & (iii)

Data Processing & Graphical Determination of Order

📐 Calculations & Data Processing (9b.ii)

Calculate 1000/t for experiments 2, 3, and 4:
- Exp 2 (t = 260): 1000 / 260 = 3.85
- Exp 3 (t = 357): 1000 / 357 = 2.80
- Exp 4 (t = 606): 1000 / 606 = 1.65
Plot 1000/t (y-axis) against volume of iodate(V) (x-axis). Ensure proper scaling covering over half the grid.
Draw a best-fit straight line forcing through the origin (0,0) . Ignore anomalous point 6.

✅ Correct Answer (9b.iii)

First order with respect to iodate(V) ions because the straight line goes through the origin, showing that rate ( 1000/t ) is directly proportional to concentration (volume).

Marks: 4 marks (graph) + 2 marks (justification)

Question 9 (c)

Rate Equations, Rate Constants, and Starch Indicators

📐 Calculation: Rate Constant k (9c.i)

Deduce rate equation: Rate = k [H₂O₂][I⁻]
Rearrange for k : k = Rate / ([H₂O₂][I⁻])
Substitute values:
1.24 × 10⁻³ / ((1.50 × 10⁻³) × (2.10 × 10⁻³))
Evaluate: 390 or 394 dm³ mol⁻¹ s⁻¹ (2 or 3 SF).

✅ Starch Purpose (9c.ii)

Starch acts as an indicator that reacts with iodine, producing a striking blue-black color when the reaction reaches a specific point, allowing accurate timing for the clock reaction.

❌ Common Errors

Writing incorrect units for second-order overall rate constants (e.g. omitting negative indices or mixing up order). Ensure units match your rearranged algebraic expression.

Marks: 2 marks (rate constant) + 2 marks (starch)

Question 9 (d)

Arrhenius Equation & Activation Energy

📐 Step-by-Step Activation Energy

Identify Arrhenius linear form: ln k = (-Ea / R) × (1/T) + constant
Gradient equals -Ea / R . Calculate gradient from the provided graph using a large triangle.
Multiply by gas constant R = 8.31 J mol⁻¹ K⁻¹ and negate to find Ea .
Convert joules to kilojoules ( ÷ 1000 ) and provide with a + sign and units ( kJ mol⁻¹ ).

💡 Anomalous Point (9d.ii)

The point at ln k = -7 is excluded from the line of best fit because it is an anomaly (outlier) caused by experimental error during measurement.

Marks: 3 marks (Ea calculation) + 1 mark (anomaly)

Topics

Physical Chemistry · Core Practicals · Core Practical 13b: Use a clock reaction to determine a rate equation · Topic 9: Kinetics I · Topic 16: Kinetics II

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 2, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.