KINETICS 7.2 The iron(II/III) ion catalyzed oxidation of iodide ions by the peroxodisulfate ion

[Author ©  Dr Phil Brown PhD: Doc Brown's exam revision notes suitable for students of advanced pre-university/college level theoretical-physical chemistry courses: rates of reaction factors: KINETICS peroxodisulfate oxidation of iodide ions [kinetics page updated Mar 13th 2026 *]

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) S₂O₈^2–_(aq) + 2I^– _(aq) ==> 2SO₄^2–_(aq) + I_2(aq)

• However, this 'direct' uncatalysed reaction involves the collision of two highly repelling negative ions and so has a very high activation energy (E_a3 in the diagram below). 

• 

• 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 ... (E_a1 in diagram above)

  • (b) 2Fe³⁺_(aq) + 2I^–_(aq) ==> 2Fe²⁺_(aq) + I_2(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 ... (E_a2 in diagram above)

  • (c) 2Fe²⁺_(aq) + S₂O₈^2–_(aq) ==> 2SO₄^2–_(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[S₂O₈^2–_(aq)][I^–_(aq)], so what will it be for the catalysed?

  • maybe rate = k[S₂O₈^2–_(aq)][I^–_(aq)][Fe²⁺_(aq)]  ?  

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

  • or rate = k[S₂O₈^2–_(aq)][Fe²⁺_(aq)]  ?

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