Advanced level chemistry kinetics: Catalytic oxidation of iodide by peroxodisulfate ion

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Doc Brown's Advanced A Level Chemistry Advanced A Level Chemistry - Kinetics-Rates revision notes Part 7

7.2 The iron ion catalyzed oxidation of iodide ions by the peroxodisulfate ion

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Case study 7.2: Iron(II)/iron(III) ions catalyse the oxidation of iodide ions by the peroxodisulfate(VI) ion

Explanation and derivation of orders of reactants and how to write the rate expression

  • Another examples of a transition metal ion effecting a catalysis.

  • The uncatalysed reaction is overall is ...

  • (a) S2O8(aq) + 2I– (aq) ==> 2SO4(aq) + I2(aq) 

  • However, this 'direct' uncatalysed reaction involves the collision of two highly repelling negative ions and so has a very high activation energy (Ea3 in the diagram below). 

  • doc b

  • BUT, the collision of an Fe3+ ion and an I– ion involves a positive ion–negative ion attraction, reducing repulsion, so this interaction which has a much lower activation energy.

  • Initially, the 1st step overall for the catalysed reaction is ... (Ea1 in diagram above)

    • (b) 2Fe3+(aq) + 2I–(aq) ==> 2Fe2+(aq) + I2(aq) 

  • Fe2+ is the 'intermediate', and in the 2nd step overall, it is oxidised to Fe3+ and the peroxodisulfate ion is reduced to sulfate ion ... (Ea2 in diagram above)

    • (c) 2Fe2+(aq) + S2O8(aq) ==> 2SO4(aq) + 2Fe3+(aq) 

  • So, the iron(III) ion is regenerated in the cycle, showing the iron(II/III) ions act in a genuine catalytic cycle but remember it cannot be simply two steps, the above must represent the summations of at least four steps.

  • Note 1: It doesn't matter whether you start with the iron(II) or iron(III) ion, catalysis will occur because the peroxodisulfate would oxidise some Fe2+ to Fe3+ (reaction b) and the Fe3+ then oxidises the iodide!

  • Note 2: If you added up the two equations (b + c) of the cycle you get equation (a) showing the overall reaction change.

  • Note 3: The full mechanism must be quite complex e.g. at least 4 steps because the chances of three particles colliding in the right way (a termolecular collision) and with sufficient frequency is unlikely. Most mechanisms proceed by bimolecular collisions, whatever the overall order of the reaction!

  • The rate expression (explained in section 6.3)for the uncatalysed reaction is:

    •   rate = k[S2O8(aq)][I–(aq)], so what will it be for the catalysed?

    • maybe rate = k[S2O8(aq)][I–(aq)][Fe2+(aq)]  ?  

    • or [rate = k[Fe2+(aq)][I–(aq)]  ?  

    • or rate = k[S2O8(aq)][Fe2+(aq)?

  • depending on the activation energies of the steps, but I don't know on this one???


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