electrode potentials equation

Hi,

When working out the e.m.f value I believe you do the

cathode (more positive number) - anode (more negative number)

so for cr3+ +3e- --> cr ( -o.74)
and cd2+ + 2e- --> Cd (-0.40)

I'd do -0.40 - (-0.74)
and get 0.34 ( a positive value so the reaction is feasible)

This appears to be the right answer, but in this book I have there is a question...

Fe3+ Ag --> fe2+ Ag+

Fe3+ e- --> fe2+ is 0.77v
and
Ag+ --> Ag + e- is 0.88v

when I do cathode -anode I get 0.88 -(o.77) to get 0.11v so its feasible, but the answer is that it is not feasible.

Why? If I write then the other way round I get a negative answer, but I thought it was cathode-anode and the more negative being at the anode.
If its because of the direction of eq being left for the anode, how come it isn't in the question I did above and got right?


Thanks in advance! rep for you if you can help

To decide on feasibility you do E(red) - E(ox)

In the proposed reaction the iron(III) gets reduced and the silver gets oxidised, so the sum is 0.77 - 0.88 = -0.11V

Hi,

When working out the e.m.f value I believe you do the

cathode (more positive number) - anode (more negative number)

so for cr3+ +3e- --> cr ( -o.74)
and cd2+ + 2e- --> Cd (-0.40)

I'd do -0.40 - (-0.74)
and get 0.34 ( a positive value so the reaction is feasible)

This appears to be the right answer, but in this book I have there is a question...

Fe3+ Ag --> fe2+ Ag+

Fe3+ e- --> fe2+ is 0.77v
and
Ag+ --> Ag + e- is 0.88v

when I do cathode -anode I get 0.88 -(o.77) to get 0.11v so its feasible, but the answer is that it is not feasible.

Why? If I write then the other way round I get a negative answer, but I thought it was cathode-anode and the more negative being at the anode.
If its because of the direction of eq being left for the anode, how come it isn't in the question I did above and got right?


Thanks in advance! rep for you if you can help

If you look at the given equation Fe 3 + Ag-- > Fe 3+ Ag +
You can see the forward reaction (hence the cathode) is Fe 3 +e --->fe 2+ which is 0.77
So cathode- anode =0.77-0.88=-0.11

Hope that clears it up.

thank you! it makes sense, using OIL RIG and then the cathode'anode

I'm not sure why my teacher said the anode is always the negative value out of the too (so in this case 0.77)
if that was the case, all reactions would have a positive value and would be feasible.

Nevermind, atleast its clear to me.

She probably meant in a feasible electrochemical cell the anode is always the positive value (: (since anode is the positive electrode, and cathode is the negative electrode ).

In an electrochemical cell the anode is the NEGATIVE half cell. For example in a zinc/copper electrochemical cell the Zn²⁺|Zn half cell is the negative anode in which Zn --> Zn²⁺ + 2e

This is the opposite of an electrolytic cell. Oxidation always occurs at the anode and reduction at the cathode in both types of cell...