International A Level (IAL)ChemistryEdexcelExam Questions4: Rates, Equilibria & Further Organic ChemistryKinetics

Kinetics (Edexcel International A Level (IAL) Chemistry): Exam Questions

Exam code: YCH11

1 hour15 questions

4 marks

A student investigated the kinetics of the reaction between bromate(V) ions and bromide ions in acidic conditions.

${BrO}_{3}^{-}$ (aq) + 5Br^– (aq) + 6H⁺ (aq) → 3Br₂(aq) + 3H₂O(l)

In the first experiment, the student measured the initial rate of the reaction at five different concentrations of bromate(V) ions, ${BrO}_{3}^{-}$ . In each case, the initial concentrations of bromide ions and hydrogen ions were constant and in large excess. The results obtained are shown.

Initial concentration of bromate(V) ions / mol dm^–3	Initial rate of reaction / mol dm^–3 s^–1
0.030	4.17 × 10^–7
0.060	8.34 × 10^–7
0.090	1.25 × 10^–6
0.120	1.67 × 10^–6
0.150	2.09 × 10^–6

i) Use the results to plot a suitable graph that can be used to show that the reaction is first order with respect to bromate(V) ions.

(3)

q16a-i-paper-4-2021-june-edexcel-ial-chemistry

ii) State how your graph shows that the reaction is first order with respect to bromate(V) ions.

(1)

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6 marks

In the second experiment, the student determined the initial rates of the same reaction starting with different concentrations of the reactants.

Run	[ ${BrO}_{3}^{-}$ ] / mol dm^–3	[Br^–] / mol dm^–3	[H⁺] / mol dm^–3	Initial rate of reaction / mol dm^–3 s^–1
1	0.062	0.21	0.40	1.52 × 10^–5
2	0.31	0.21	0.20	1.90 × 10^–5
3	0.062	0.63	0.40	4.56 × 10^–5

i) Use these results and your answer to (a) to deduce the orders with respect to Br^–ions and H⁺ions.

(2)

Br^- ions.................................................................................................................
H⁺ions.................................................................................................................

ii) Write the rate equation for the reaction.

(1)

iii) Use the results for Run 1 and your rate equation from (b)(ii) to calculate the value for the rate constant, k. Include units in your answer.

(3)

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3 marks

The presence of bromate(V) ions in drinking water is harmful to humans. Bromate(V) ions can be converted to less harmful bromide ions by passing the water through palladium with a reducing agent.

Describe how a heterogeneous catalyst, such as palladium, increases the rate of a reaction.

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5 marks

This question is about some compounds of bromine.

Potassium bromate(V) decomposes to form potassium bromide and oxygen.

$2 {KBrO}_{3} (s) \to 2 KBr (s) + 3 O_{2} (g) ∆_{r} H^{⦵} = - 67.2 kJ {mol}^{- 1}$

The standard molar entropies of these substances are given in the table.

Substance	KBrO₃(s)	KBr (s)	O₂(g)
$S^{⦵} / J K^{- 1} {mol}^{- 1}$	149.2	95.9	205.0

Calculate the total entropy change, ΔS_total, for this reaction at 298 K.

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8 marks

Bromide ions react with bromate(V) ions in acidic solution.

$5 {Br}^{-} (aq) + {BrO}_{3}^{-} (aq) + 6 H^{+} (aq) \to 3 {Br}_{2} (aq) + 3 H_{2} O (l)$

Two experiments are carried out.

i) Experiment 1

The concentration of Br⁻ions is determined at different times.

The concentrations of ${BrO}_{3}^{-}$ ions and H⁺ ions are in large excess and effectively constant.

The graph of concentration of Br⁻ions against time is shown.

q20b-paper-4-jan-2021-edexcel-ial-chemistry

Determine the order of the reaction with respect to bromide ions.

Show your working on the graph.

(3)

ii) Experiment 2

The initial concentrations of ${BrO}_{3}^{-}$ ions and H⁺ ions are changed and the initial rate of reaction is determined.

The initial concentration of Br⁻ions is constant and in large excess.

Run	[Br $O_{3}^{-}$ (aq)] / mol dm⁻³	[H⁺(aq)] / mol dm⁻³	Initial rate / mol dm⁻³ s⁻¹
1	0.1	0.1	3.6 × 10⁻³
2	0.2	0.1	7.2 × 10⁻³
3	0.3	0.2	4.3 × 10⁻²

Determine the order of reaction with respect to ${BrO}_{3}^{-}$ ions and to H⁺ions.

You must explain your working.

(3)

iii) Give the overall rate equation for this reaction.
Include the units for the rate constant.

(2)

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7 marks

The rate constant for the reaction between bromoalkane and cyanide ions is determined at five different temperatures.

The results are given in the table.

Temperature (T ) / K	1/Temperature (1/T ) / K⁻¹	Rate constant (k) / s⁻¹	ln k
300	3.33 × 10⁻³	3.72 × 10⁻⁵	−10.20
310	3.23 × 10⁻³	1.34 × 10⁻⁴	−8.92
320	3.13 × 10⁻³	5.48 × 10⁻⁴	−7.51
330	3.03 × 10⁻³	2.01 × 10⁻³	−6.21
340	2.93 × 10⁻³	7.23 × 10⁻³	−4.93

Plot a graph of ln k against 1/T and use it to determine the activation energy, E_a.

Include the sign and units of the gradient and the activation energy.

The Arrhenius equation can be expressed as

$\ln k = - \frac{E_{a}}{R} \times \frac{1}{T} + constant$

$[R = 8.31 J {mol}^{- 1} K^{- 1}]$

q20c-paper-4-jan-2021-edexcel-ial-chemistry

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3 marks

The reaction between bromate(V) ions and bromide ions in acidic solution is given by the equation:

BrO₃^- (aq) + 5Br^- (aq) + 6H⁺ (aq) → 3Br₂ (aq) + 3H₂O (l)

A student investigated the rate of this reaction. The results of four experiments carried out at 298 K are shown below.

Experiment	[BrO₃⁻] / mol dm⁻³	[Br⁻] / mol dm⁻³	[H⁺] / mol dm⁻³	Initial rate / mol dm⁻³ s⁻¹
1	0.10	0.10	0.10	1.20 × 10⁻³
2	0.20	0.10	0.10	2.40 × 10⁻³
3	0.20	0.20	0.10	4.80 × 10⁻³
4	0.20	0.20	0.20	1.92 × 10⁻²

Use the data in the table to deduce the order of reaction with respect to BrO₃⁻, Br⁻ and H⁺. Show your reasoning.

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3 marks

Write the overall rate equation for this reaction. Use the data from experiment 1 to calculate a value for the rate constant, k, and state its units.

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2 marks

A student proposes two possible mechanisms for this reaction.

Mechanism 1

Step 1 (slow): BrO₃⁻ + 5Br⁻ + 6H⁺ → 3Br₂ + 3H₂O

Mechanism 2

Step 1 (slow): BrO₃⁻ + Br⁻ + 2H⁺ → HBrO + HBrO₂

Step 2 (fast): HBrO + Br⁻ + H⁺ → Br₂ + H₂O

Step 3 (fast): HBrO₂ + 3Br⁻ + 3H⁺ → 2Br₂ + 2H₂O

Deduce which mechanism is consistent with the rate equation from part (b). Explain your answer.

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5 marks

In a separate investigation, the student measures the rate constant k for this reaction at five different temperatures. The data collected is shown below.

T / °C	T / K	1/T / 10⁻³ K⁻¹	k / dm⁹ mol⁻³ s⁻¹	ln k
15			3.43
25			12.0
35			39.8
45			119
55			338

The Arrhenius equation is:

$\ln k = - \frac{E_{a}}{R} (\frac{1}{T}) + \ln A$

On the grid, plot a graph of ln k against 1/T .

Graph with ln k on the y-axis, ranging from 0 to 9, and (1/T) x10⁻³ K⁻¹ on the x-axis, ranging from 3.05 to 3.50, with empty gridlines.

Use the graph to determine the activation energy, E_a, for this reaction. Give your answer in kJ mol⁻¹.

(R = 8.31 J K⁻¹ mol⁻¹)

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Time / s	[C₆H₅N₂Cl] / mol dm⁻³
0.0	0.500
40.0	0.410
100	0.285
200	0.165
280	0.100
350	0.070
400	0.050

Experiment number	[Br $O_{3}^{-}$ (aq)] /mol dm^–3	[Br^–(aq)] /mol dm^–3	[H⁺ (aq)] /mol dm^–3	Initial rate of reaction / mol dm^–3 s⁻¹
1	0.10	0.25	0.30	3.36×10^–5
2	0.10	0.25	0.60	1.34×10^–4
3	0.15	0.50	0.30	1.01×10^–4
4	0.15	0.25	0.60	2.01×10^–4