Questions

Q1.
 
This question is about the kinetics of the reaction between bromoethane and aqueous hydroxide ions.
(a) The results of an experiment to find the initial rate of the reaction are shown in the table below.

The rate equation for the reaction is

(i) Calculate the value of k. Give your answer to three significant figures and
include units.
(3)

 
(ii) Calculate the initial rate if the concentrations of both reactants were changed to 0.020 mol dm−3.
(1)

 
(b) (i) State the order of the reaction.
(1)
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(2)

 

(c) The rate constant for the reaction between bromoethane and hydroxide ions was determined at five different temperatures. The results are shown in the table below.

(i) Complete the missing values in the table.
(2)
(ii) Plot a graph of ln k against 1/T. Calculate the gradient of your graph and use this to calculate the activation energy, EA. The Arrhenius equation can be expressed as
     
		[Gas constant, R = 8.31 J K−1 mol−1]
(5)

(Total for question = 14 marks)

Q2.
 
Persulfate ions, S2O82−, oxidize iodide ions in aqueous solution to form iodine and sulfate ions, SO42−.
(a)  Write the ionic equation for this reaction. State symbols are not required.
(1)

(b)  The effect of persulfate ion concentration on the rate of this reaction was measured.
A few drops of starch solution and a small measured volume of sodium thiosulfate solution were added to the potassium persulfate solution.
Potassium iodide solution was then added and the time taken for the mixture to change colour was measured.
The reaction was repeated using different concentrations of potassium persulfate, but the same volumes and concentrations of sodium thiosulfate solution and potassium iodide solution.
The rates of the reaction were compared using the reciprocal of the time (1/time) for the mixture to change colour as a measure of the initial rate.
(i)  What is the final colour of the reaction mixture?
(1)
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(ii)  What would happen if the reaction was carried out without the addition of sodium thiosulfate?
(1)
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(iii)  Explain why the concentration of iodide ions remains constant until the mixture changes colour.
(1)
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(c)  The results obtained from the experiment in part (b) were tabulated as follows.

(i)  Plot a graph of 1/time on the vertical axis against the concentration of the persulfate ions.
(2)

(ii)  1/time is a measure of the initial rate of the reaction.
Deduce the order of the reaction with respect to persulfate ions.
Justify your answer.
(2)
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 ............................................................................................................................................. 
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(iii)  The reaction is first order with respect to iodide ions. Write the overall rate equation for the reaction and deduce the units for the rate constant.
(2)
Rate =
Units for the rate constant .....................................

(d)  The reaction in part (b) is repeated at two different temperatures, keeping the initial volumes and concentrations of the solutions constant.

(i)  Calculate, without drawing a graph, the activation energy of the reaction. Remember to give a sign and units with your answer.
(3)

(ii)  Suggest how the reliability of the activation energy determination could be improved, without changing the apparatus, solutions or method.
(1)
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(Total for question = 14 marks)