Rates of reaction - kinetics (A2)

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I am investigating the activation energy of the reaction between peroxydisulphate ions and iodide ions. I have carried out the reaction with two catalysts also which are iron(II) and copper(II). I wasn't sure which was meant to be a more effective catalyst and why? 260308

Ulex writes
 
The peroxodisulphate ions and the iodide ion are both negative which is presumed to be a significant factor in the reason for the slowness of the reaction between them. The redox potantials are very much in favour of reaction:
 
I₂/ 2I^- +0.54 V
S₂O₈^2-/ 2SO₄^2- +2.01 V
 
A potential catalyst would therefore have to satisfy two criteria:
• it should be positively charged
• it should have a redox potential between +0.54 and +2.01 V
 
The system Cu²⁺/ Cu⁺ satisfies the first criterion but, at +0.15 V, not the second.
 
The system Fe³⁺/ Fe²⁺ , at +0.77V, satisfies both criteria.
 
Potentially, therefore, iron(II) will reduce peroxodisulphate and the resulting iron(III) will oxidise iodide. It is likely that both of these reactions will be quick. You can’t be absolutely sure, from this bit of theory, that the reactions will be faster than the uncatalysed reaction but it is a reasonable expectation.

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updated: 28 March 2008