Calculate the concentration ratio

Kqwert

Problem Statement
Hello,

If we have the half-reactions:

Fe3++e- -> Fe2+
and
2H+ + 2e- -> H2

Calculate the ratio [Fe3+] / [Fe2+] at pH 2.
Relevant Equations
.
Any hint? I was immediately thinking standard electrode potentials as we have two half reactions, but not sure exactly how to go on..

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Borek

Mentor
Not standard electrode potentials. Do you know any equation that says what the real potential is?

Kqwert

I think I can use Nernst's equation, but not sure how to calculate the real potential..

Borek

Mentor
In general question is incomplete, as it doesn't say anything about a pressure of gaseous hydrogen. I would go for a standard 1 atm.

Kqwert

How do I go on and solve it? I assume I can calculate the electrode potential of the hydrogen reaction, but I need the total electrode potential in order to find the Fe-ratio, right? Any hints?

Borek

Mentor
At equilibrium both cells have the same potential.

Kqwert

Ok, I thought Ecell was 0 at equilibrium??

Borek

Mentor
Yes, but it doesn't mean potentials of both half cells are zero. It means difference between them is zero, that is something entirely different.

Kqwert

Thank you. When saying "difference between them are zero". Is that when the cell potential is calculated on reduction form? I.e. both half reactions are given as reduction reactions. I know another way of looking at it is writing one reaction as an oxidation reaction and the other as a reduction rx.

Borek

Mentor
Doesn't matter whether the reaction is reduction of oxidation, it occurs at exactly the same potential.

Your problem stem from the fact you were probably taught to calculate the cell potential by adding half cells but reversing the sign of one of them - this is completely unnecessary. Just subtract them and math will do the trick of reversing the sign if necessary (-(-1) is 1 after all).

"Calculate the concentration ratio"

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