Describe how the rate of oxidation of iodide ions with persulfate ions depends on the initial concentration of the reactants.

1. Determine which of the following statements are true by comparing the rate (1/t) in

Experiment 1 with the rates in Experiments 2 and 3 where [I–(aq)] and [S2O28–(aq)] are

halved. Explain your choices. Rate  [I–(aq)]2

Rate –1

[I (aq)]

Rate  [I–(aq)] Rate  [I–(aq)]

Rate  [S O2–(aq)]

28 28

Rate 

[S O2–(aq)] 28

Rate[S O2–(aq)] 28         I will provide you with a screenshot for this question***

 

  1. Use your answers to the previous question to write an experimentally determined rate law for the oxidation of iodide ions with persulfate ions.
  2. i) Describe how the rate of oxidation of iodide ions with persulfate ions depends on the initial concentration of the reactants.

    ii) Would you expect the rate of all reactions to be related to reactant concentrations in the exact same way as you saw here?

Consider the following proposed mechanisms for the reaction of iodide ions with persulfate ions:

 

Rate  [S O2–(aq)]2 1

 

 

Proposed Mechanism 1

– – slow * 2–

I (aq) + I (aq)  [I….I] (aq)

*2–2–fast 2– [I….I] (aq) + S2O8 (aq)  I2(aq) + 2SO4 (aq)

rate law : rate = k[I–]2

Proposed Mechanism 2

– 2– fast * 3–

I (aq) + S2O8 (aq) Û equilibrium

* 3– – slow [I…..S2O8] (aq) + I (aq) 

rate = rate law : rate =

[I….S2O8] (aq)

2– I2(aq) + 2SO4 (aq)

k[[I…..S2O8]3–][I–] therefore, K .k[I–]2[S O2–]

e28

The rate law is derived by writing an expression for the equilibrium constant, Ke, for

the first step. It is not necessary for you to know how to derive it but your demonstrator will help you if you are interested.

Proposed Mechanism 3

Proposed Mechanism 4

4. i) Place a tick √ next to the rate determining step (RDS) in each of the mechanisms above.

ii) Which mechanism(s) support/s your experimental results? State your reasoning clearly, using the RDS to support your answer.

iii) Which mechanism(s) do/does NOT support/s your experimental results? State your reasoning clearly, using the RDS to support your answer.

Consider your results for Experiments 1, 4, 5 and 6.

  1. Did the reaction rate increase or decrease with increasing temperature? Explain

    briefly why this should be so.

  2. Showing your calculations, use your data from Experiments 4 and 5 to justify or

    reject the following general statement:

    ‘The rates of most simple reactions approximately double

    for each 10K rise in temperature.’

  3. Why is it not as accurate to use your data for Experiments 1 and 4 to answer the above question?

Compare your results for Experiments 1 and 7.

8. Describe the effect of copper ions on your reaction rate.

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