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Electrode potential and reduction

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Roxo:
An exam question asked for a suitable reagent to reduce Fe3+ to Fe2+ and no further. Their answer was Zn metal in acid solution. This bothers me because looking at electrode potentials Zn appears capable of reducing Fe3+ all the way to iron metal. My thought was that KI would be a better selection. This is a national level exam board so I’m clearly missing something as to why Zn is suitable but I can’t figure out what it is. Help would be appreciated. Thanks

Hunter2:
The redoxpotential Fe3+ + e-=> Fe2+ is +0,77 V its very noble to - 0,76 V other reaction Zn2+ +2e- => Zn

Explanation from other forum


--- Quote ---First of all, consider whether the product is Fe²⁺ ions or Fe for the reaction between Zn and Fe³⁺ ions.
[1]: Zn(s) + 2Fe³⁺(aq) → Zn²⁺(aq) + 2Fe²⁺(aq) …… E°(cell) = +1.53 V
[2]: 3Zn(s) + 2Fe³⁺(aq) → 3Zn²⁺(aq) + 2Fe(s) …… E°(cell) = +0.72 V
Due to the higher cell potential of reaction [1], reaction [1] is preferred than reaction [2]. In other words, Zn will reduce Fe³⁺ ions to Fe²⁺ ions, but not Fe metal.

However, when Zn in the excess, the excess Zn will further reduce the Fe²⁺ ions formed to Fe metal.
Zn(s) + Fe²⁺(aq) → Zn²⁺(aq) + 2Fe(s) …… E°(cell) = +0.32 V
The reaction is feasible for E°(cell) > 0 V.

Overall speaking, the final product is Fe²⁺ ions when Zn is the limiting reactant, but the final product is Fe metal when Zn is in excess.
--- End quote ---

Roxo:
Thanks very much for that. It helps a lot and sort of explains why iron gets deposited on zinc metal placed in a solution of iron ions.

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