Redox and redox equilibria
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What is the overall equation when (acidified) iron (III) is reduced with granulated zinc?
Ulex replies
This seems like a nice easy question, so I’ll give you a nice easy answer:
2Fe3+(aq) + Zn(s) -> 2Fe2+(aq) + Zn2+(aq)
If you’re happy with that, fine! I suspect, however, that there’s something bothering you – it bothers me, too!
If the solution is neutral, it doesn’t work, so what’s the role of the acid?
Is it. perhaps, the hydrogen evolved from the zinc/acid mixture which does the reducing? Well, no, it can’t be that because if you bubble hydrogen from a cylinder through the solution, no reduction takes place.
There was a theory once which suggested that the reduction was done by hydrogen atoms before they had time to combine into hydrogen molecules – ‘nascent’ hydrogen (at the moment of birth, literally). Personally, I was never comfortable with that theory.
I think the acid keeps the surface of the zinc clean, free from zinc oxide, so that the atoms on the surface can get on with reducing the iron(III) ions.
If this is true, the simple answer is the correct one after all!
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updated: 01 December 2006
