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International A LevelChemistryEdexcelExam Questions4: Rates, Equilibria & Further Organic ChemistryKinetics

Kinetics (Edexcel International A Level Chemistry): Exam Questions

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Kinetics

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1a4 marks

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

Br O subscript 3 to the power of minus sign(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, Br O subscript 3 to the power of minus sign . 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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1b
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In the second experiment, the student determined the initial rates of the same reaction starting with different concentrations of the reactants.

Run

[bold BrO subscript bold 3 superscript bold minus] / 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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1c3 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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2a
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The decomposition of benzenediazonium chloride is a first order reaction.

C6H5N2Cl + H2O → C6H5OH + N2 + HCl

The activation energy of this reaction was determined by measuring the rate constant
at various temperatures.

In an experiment at 333 K, the concentration of a sample of benzenediazonium chloride was measured at various times during its decomposition. The results of this experiment are shown.

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

i) Plot a graph of concentration of benzenediazonium chloride against time.

(3)

q17a-i-paper-4-oct-2021-edexcel-ial-chemistry

ii) Determine a value for the half-life, t1⁄2 , of this reaction.

You must show your working on the graph.

(1)

iii) Calculate the rate constant, k, for the reaction at 333 K.
Include units in your answer.

Use the expression ln2 = kt1⁄2

(2)

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2b
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The experiment described in (a) was repeated for five temperatures and the data used to plot a graph for the Arrhenius equation in the form

lnk  =  −E subscript a over R italic cross times italic 1 over T +  constant

q17b-paper-4-oct-2021-edexcel-ial-chemistry

i) Use the rate constant that you have calculated in (a)(iii) to obtain data for a point on the graph for 333K.

(2)

ii) Plot your data from (b)(i) on the graph.

(1)

iii) Determine the gradient of the graph by drawing a best-fit line.
Include a sign and units in your answer.

(3)

iv) Use the gradient determined in (b)(iii) to calculate the activation energy for the decomposition of benzenediazonium chloride.
Include a sign and units with your answer.

(3)

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3
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The hydrolysis of halogenoalkanes by alkali is a nucleophilic substitution reaction.

RX + OH− → ROH + X−

The mechanism of this reaction for primary halogenoalkanes is different from the mechanism for tertiary halogenoalkanes.

Describe how knowledge of the rate equations for the hydrolysis of halogenoalkanes provides evidence for the mechanisms of these reactions.

Curly arrow mechanisms are not required.

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1a
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Bromate(V) ions, Brbegin mathsize 16px style O subscript 3 to the power of minus sign end style, oxidise bromide ions, Br–, in dilute acid.

Brbegin mathsize 16px style O subscript 3 to the power of minus sign end style(aq) + 5Br–(aq) + 6H+ (aq) → 3Br2 (aq) + 3H2O(l)

Experiments to determine initial reaction rates were carried out using different initial concentrations of the three reactants.

The results are shown in the table.

Experiment number

[Brbegin mathsize 16px style O subscript bold 3 to the power of bold minus sign end style(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

The reaction is first order with respect to bromate(V) ions.

i) Deduce the rate equation for the reaction. Justify your answer using the data.

(4)

ii) Use the data from Experiment 4 and your answer to (a)(i) to calculate the rate constant for the reaction. Include units in your answer.

(3)

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1b1 mark

Give one possible reason why the rate equation shows that the reaction cannot proceed in one step.

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2a
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This question is about some compounds of bromine.

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

2 KBrO subscript 3 space left parenthesis straight s right parenthesis space rightwards arrow space 2 KBr space left parenthesis straight s right parenthesis space plus space 3 straight O subscript 2 space left parenthesis straight g right parenthesis space space space space space space space space space space increment subscript straight r space H to the power of ⦵ equals space minus 67.2 space kJ space mol to the power of negative 1 end exponent


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

Substance

KBrO3 (s)

KBr (s)

O2 (g)

bold S to the power of bold ⦵ bold space bold divided by bold space bold J bold space bold K to the power of bold minus bold 1 end exponent bold space bold mol to the power of bold minus bold 1 end exponent

149.2

95.9

205.0

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

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2b
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Bromide ions react with bromate(V) ions in acidic solution.

5 Br to the power of minus space left parenthesis aq right parenthesis space plus space BrO subscript 3 superscript minus space left parenthesis aq right parenthesis space plus space 6 straight H to the power of plus space space left parenthesis aq right parenthesis space rightwards arrow space 3 Br subscript 2 left parenthesis aq right parenthesis space plus space 3 straight H subscript 2 straight O left parenthesis straight l right parenthesis

Two experiments are carried out.

i) Experiment 1

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

The concentrations of begin mathsize 16px style Br O subscript 3 to the power of minus sign end style 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 begin mathsize 16px style Br O subscript 3 to the power of minus sign end style 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

[Brbegin mathsize 16px style O subscript bold 3 to the power of bold minus sign end style(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 subscript 3 superscript minus 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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2c7 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, Ea.

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

The Arrhenius equation can be expressed as

ln space straight k space equals space minus E subscript a over R space cross times 1 over T plus space constant

left square bracket R space equals space 8.31 space straight J space mol to the power of negative 1 end exponent space straight K to the power of negative 1 end exponent right square bracket

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

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