International A LevelChemistryEdexcelTopic Questions4: Rates, Equilibria and Further Organic Chemistry4.1 Kinetics

Kinetics (Edexcel International A Level Chemistry)

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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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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
Sme Calculator6 marks

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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