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Catalyst: iodide and peroxodisulphate ions

S2O82- + 2I- I2 + 2SO42-

Hello! I am currently looking at this reaction. I know it is catalysed by Fe(II) ions, does anyone know in what form / reagent these are supplied to the reaction in? Thank you
Reply 1
The Fe(II) ions act as a catalyst. Typically, Fe(II) ions are supplied to the reaction in the form of a compound called ferrous sulfate (FeSO₄).
When ferrous sulfate is dissolved in water, it dissociates into Fe(II) ions (Fe²⁺) and sulfate ions (SO₄²⁻). These Fe(II) ions then serve as the catalyst for the reaction between S₂O₈²⁻ (persulfate ion) and I⁻ (iodide ion) to produce I₂ (diiodine) and SO₄²⁻ (sulfate ions).
the balanced equation for the reaction, including the Fe(II) ions as the catalyst:

S₂O₈²⁻ + 2I⁻ + 2Fe²⁺ I₂ + 2SO₄²⁻ + 2Fe³⁺

So, in the context of this reaction, ferrous sulfate (FeSO₄) is often used to supply Fe(II) ions as the catalyst.

Original post by chipperpotato
S2O82- + 2I- I2 + 2SO42-

Hello! I am currently looking at this reaction. I know it is catalysed by Fe(II) ions, does anyone know in what form / reagent these are supplied to the reaction in? Thank you
Original post by chipperpotato
S2O82- + 2I- I2 + 2SO42-

Hello! I am currently looking at this reaction. I know it is catalysed by Fe(II) ions, does anyone know in what form / reagent these are supplied to the reaction in? Thank you

Water-soluble salts of iron (in either the +2 or +3 oxidation state) work, but generally you want to avoid using any salts that contain anions that also would react with the peroxodisulphate or iodide ions.

As above, iron (II) sulphate works very well, since the sulphate ion doesn’t interfere with the reaction.
(edited 9 months ago)
Original post by swixxson
The Fe(II) ions act as a catalyst. Typically, Fe(II) ions are supplied to the reaction in the form of a compound called ferrous sulfate (FeSO₄).
When ferrous sulfate is dissolved in water, it dissociates into Fe(II) ions (Fe²⁺) and sulfate ions (SO₄²⁻). These Fe(II) ions then serve as the catalyst for the reaction between S₂O₈²⁻ (persulfate ion) and I⁻ (iodide ion) to produce I₂ (diiodine) and SO₄²⁻ (sulfate ions).
the balanced equation for the reaction, including the Fe(II) ions as the catalyst:

S₂O₈²⁻ + 2I⁻ + 2Fe²⁺ I₂ + 2SO₄²⁻ + 2Fe³⁺

So, in the context of this reaction, ferrous sulfate (FeSO₄) is often used to supply Fe(II) ions as the catalyst.


The equation you’ve included should show Fe^2+ on both sides, because in it’s current state it implies that Fe^2+ ions are consumed and not regenerated.
Original post by swixxson
The Fe(II) ions act as a catalyst. Typically, Fe(II) ions are supplied to the reaction in the form of a compound called ferrous sulfate (FeSO₄).
When ferrous sulfate is dissolved in water, it dissociates into Fe(II) ions (Fe²⁺) and sulfate ions (SO₄²⁻). These Fe(II) ions then serve as the catalyst for the reaction between S₂O₈²⁻ (persulfate ion) and I⁻ (iodide ion) to produce I₂ (diiodine) and SO₄²⁻ (sulfate ions).
the balanced equation for the reaction, including the Fe(II) ions as the catalyst:

S₂O₈²⁻ + 2I⁻ + 2Fe²⁺ I₂ + 2SO₄²⁻ + 2Fe³⁺

So, in the context of this reaction, ferrous sulfate (FeSO₄) is often used to supply Fe(II) ions as the catalyst.


Original post by TypicalNerd
Water-soluble salts of iron (in either the +2 or +3 oxidation state) work, but generally you want to avoid using any salts that contain anions that also would react with the peroxodisulphate or iodide ions.

As above, iron (II) sulphate works very well, since the sulphate ion doesn’t interfere with the reaction.


Amazing thank you!!
Reply 5
Original post by swixxson
The Fe(II) ions act as a catalyst. Typically, Fe(II) ions are supplied to the reaction in the form of a compound called ferrous sulfate (FeSO₄).
When ferrous sulfate is dissolved in water, it dissociates into Fe(II) ions (Fe²⁺) and sulfate ions (SO₄²⁻). These Fe(II) ions then serve as the catalyst for the reaction between S₂O₈²⁻ (persulfate ion) and I⁻ (iodide ion) to produce I₂ (diiodine) and SO₄²⁻ (sulfate ions).
the balanced equation for the reaction, including the Fe(II) ions as the catalyst:

S₂O₈²⁻ + 2I⁻ + 2Fe²⁺ I₂ + 2SO₄²⁻ + 2Fe³⁺

So, in the context of this reaction, ferrous sulfate (FeSO₄) is often used to supply Fe(II) ions as the catalyst.


Just a quick add on/correction for the equations for the catalyst:

The first step is Fe2+ reacting with the persulfate. It's redox so you need to generate 2 half equations and combine;
[2e- + S2O8(2-) -> 2 SO4(2-)]
[Fe2+ -> Fe3+ + e-]
Overall (step one):
S2O8(2-) + 2Fe2+ -> 2 SO4(2-) + 2Fe3+

Second step is oxidation of the Idodide (this also regenerates the Fe2+);
[2I- -> I2 + 2e-]
[Fe3+ + e- -> Fe2+]
Overall: step 2:
2I- + 2Fe3+ -> I2 + 2Fe2+

This is how you'll need to give the equations in the exam

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