Quick GCHEM question, help plz

[DDS4tehwin]

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Ok so maybe I am a little slow, because I'm having trouble understanding the Galvanic/Electrolytic stuff.

On Kaplan, they say that Anode of Electrolytic is +, and Cathode is -. BUT, TopScore states the exact opposite.

Also, doesn't this problem go against what Kaplan said? Help please?

 

[Decan]

I dont know where it says that in kaplan...but the anode is + in an electrolytic cell

 

[DDS4tehwin]

No no that is what Kaplan says, I agree. But TopScore's explanation on one of the problems say the opposite. Also (relating to the problem I posted), if Galvanic is a -, then why is the answer a positive?

 

[Hysteria24]

I am pretty sure you don’t have to worry about anodes or cathodes here.

The E will be the combination of the reduction potential and oxidation potential of the two species. The problem lists reduction potentials. Co3+ gets reduced so use the reduction potential listed (+1.82 V). The other species in the other reaction is oxidized, so we need the oxidation potential, which is the reverse of the reduction potential. This makes it -0.53 V.

Now just add them together (1.82 – 0.53 V = 1.29 V)

Kaplan does go into this.

 

[DDS4tehwin]

I am pretty sure you don’t have to worry about anodes or cathodes here.

The E will be the combination of the reduction potential and oxidation potential of the two species. The problem lists reduction potentials. Co3+ gets reduced so use the reduction potential listed (+1.82 V). The other species in the other reaction is oxidized, so we need the oxidation potential, which is the reverse of the reduction potential. This makes it -0.53 V.

Now just add them together (1.82 – 0.53 V = 1.29 V)

Kaplan does go into this.

How did you know that Co3+ gets reduced? The final Eo is positive then, but didn't they say that Galvanics are spontaneous (as in negative)?? Or am I mixing up the two? (G and Eo)? If so, can you please clear that up?

 

[fancymylotus]

How did you know that Co3+ gets reduced? The final Eo is positive then, but didn't they say that Galvanics are spontaneous (as in negative)?? Or am I mixing up the two? (G and Eo)? If so, can you please clear that up?

The one with the more positive E is the reduction half, so you switch the other one because obviously if the first one gets reduced, the second one must be oxidation. Since we are given REDUCTION potentials, you gotta change the sign on the one thats oxidized. Galvanics are spontaneous, electrolytics use an outside source of energy to do work. Both species are suspended in the same solution in electrolytic cells. Anodes in galvanics are negative, cathodes are positive, and the opposite is true for electrolytics.

The final E is positive, and if you use G=-nFE, you can see that in order to have a negative G, you need a positive E, and vice versa. Negative G=spontaneous.