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Clarification on standard potentials with SHE!

Posted: Sun Feb 01, 2015 5:17 pm
by Regina Chi 2K
In section 13.6 of the chemistry textbook, at the bottom of page 530, Pt(s) l H2(g) l H+(aq) ll Cu2+(aq) l Cu(s) describes the copper electrode as the cathode and the hydrogen components the anode. I think I understand why that is so. However, further on (middle of) page 531 of the same section, they note that Pt(s) l H2(g) l H+(aq) ll Zn2+(aq) l Zn(s) has the zinc electrode as the anode and they hydrogen components as the cathode. I don't really understand why the exact same types of reactions are taking place (with the same electron transfer), but their roles of being the cathode and anode are switched. Wouldn't the zinc electrode in the second part still be the cathode and the hydrogen, the anode?

Re: Clarification on standard potentials with SHE!

Posted: Sun Feb 01, 2015 5:48 pm
by Chem_Mod
Copper's reduction potential (+0.34 V) is higher than hydrogen (0 V), so copper will be reduced spontaneously in this cell. For a galvanic cell, reduction occurs at the cathode, so copper is the cathode. Zinc's reduction potential (-0.76 V) is less than hydrogen (0 V), so it will be hydrogen that gets reduced instead, therefore hydrogen is the cathode. The spontaneous directions of reaction are opposite, for the two different metals.

The passage in the book is being rather sloppy with the shorthand notation, since the cathode should be written on the right and the anode on the left, as in:
Zn (s) | Zn2+ (aq) || H+ (aq) | H2 (g)
This ensures that reading each half-cell from left to right gives the spontaneous change.