standard reduction potential

[laczlacylaci]

I probably posted this about 3 times.
So the equation I have in mind is Ecat/red - Eanod/ox=E standard reduction.
I first look at the cathode (more positive E and switch the other one (ox))

In this problem.
Ag+ has the more positive value=cathode (no switch)
Zn2+ has the more negative value=anode (switch)
Giving me: (+0.337)+(+0.763)=+1.1V (which is not the correct answer)

From the reaction, I clearly know that Zn is the cathode and Ag is the anode.
So (-0.763)+(-0.337)=-1.1V <---- correct answer.

@theonlytycrane , I tried PMing you, but I'm not sure how to insert pictures through chat.

In what situations should I use your technique (way 1) and the other one that worked in this case? Maybe I didn't fully grasp your whole post from last time.

I think whenever there is an equation present (like this one) I use the equation to figure out who is the cat/ox. But when there is no equation, I use your way? Thanks.

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[theonlytycrane]

Just looking at the two reduction potentials given, for this reaction to spontaneously occur, Ag would get reduced and Zn would get oxidized with Ecell = 1.1V as you described.

The problem is that the question asks specifically about a reaction where Zn gets reduced and Ag gets oxidized. Following the given reaction, the Ecell = -1.1V.

Using deltaG = -n * F * Ecell, we can see that deltaG will be positive so the given reaction won't be spontaneous. For the reaction to occur, it would need to be battery powered. For the purposes of the question, just following the equation gives -1.1V. But we should know that the reaction won't happen that way spontaneously.

A better question would have been open-ended and specified whether this is a galvanic (spontaneous) or electrolytic (battery-powered cell) and asked for Ecell in one of those setups, only given the initial reduction potentials for each half reaction.

 

[laczlacylaci]

Just looking at the two reduction potentials given, for this reaction to spontaneously occur, Ag would get reduced and Zn would get oxidized with Ecell = 1.1V as you described.

The problem is that the question asks specifically about a reaction where Zn gets reduced and Ag gets oxidized. Following the given reaction, the Ecell = -1.1V.

Using deltaG = -n * F * Ecell, we can see that deltaG will be positive so the given reaction won't be spontaneous. For the reaction to occur, it would need to be battery powered. For the purposes of the question, just following the equation gives -1.1V. But we should know that the reaction won't happen that way spontaneously.

A better question would have been open-ended and specified whether this is a galvanic (spontaneous) or electrolytic (battery-powered cell) and asked for Ecell in one of those setups, only given the initial reduction potentials for each half reaction.

Ah-got it.