Reactions with K>1, 13.31 anode vs cathode

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ShreyaPatel1K
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Joined: Fri Sep 26, 2014 2:02 pm

Reactions with K>1, 13.31 anode vs cathode

Postby ShreyaPatel1K » Wed Feb 04, 2015 6:03 am

Was wondering why in some cases, such as in K>1 circumstances listed in 13.31, the lower value in potential counts as an cathode instead of an anode.

Isn't the rule of thumb that the more negative/less positive potential is the anode?

Niharika Reddy 1D
Posts: 127
Joined: Fri Sep 26, 2014 2:02 pm

Re: Reactions with K>1, 13.31 anode vs cathode

Postby Niharika Reddy 1D » Wed Feb 04, 2015 11:50 am

If K>1, the forward reaction is favored and the reaction will be spontaneous as given. This is indicated by a positive E° value. The question asks to identify which of the reactions have K>1, so we look at the given reaction to determine which half reaction occurs at the cathode/anode, not which order will give us a spontaneous reaction (ie, smaller standard reduction potential associated with the anode).

For 31 b, looking up the standard reduction potentials of the half reactions shows the reduction of Ce4+ has the higher standard reduction potential, but in the given reaction, Ce3+ is oxidized, so we must assign this half reaction to the anode even though it has the larger potential, and make the reduction potential negative because we flip this reaction to make it an oxidation reaction (as it is given that way in the problem).

If the standard reduction potential of the anode is larger than that of the cathode, E° for the cell will be negative and K<1, thus the forward reaction as given will not be spontaneous.

If the question asked to design a galvanic cell, the smaller E° value would be associated with the anode since this must be the case to get a spontaneous reaction, and galvanic cells are spontaneous. So yes, that's the rule of thumb but only when you're looking to create a spontaneous cell.


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