[yoyo]

a. write the cell notation for, and balance, the reaction:

Co3+(aq) + Zn(s) --> Co2+(aq) + Zn2+ (aq)

Okay, here I think that the reduction occured at the cathode:
Co3+(aq) + 1e- --> Co2+ (aq)

And the oxidation takes place at the anode:
Zn(s)--> Zn2+(aq) + 2e-

So should the cell notation be:

Zn(s) | Zn2+(aq) || Co3+(aq) Co2+(aq)

Is this correct? The reason I didn't place any single vertical line between the last two (Co3+ and Co2+) is because I was made to understand that it's only for phase changes, so it wouldn't be necessary since they're both still aqueous. Is that right?



b. write the balanced reaction from the cell notation:

Mo(s)/Mo3+(aq)//Pb4+(aq)/Pb2+(aq)

Okay, for this one I came up with:

Pb4+(aq) + Mo(s) --> Pb2+(aq) + Mo3+(aq)

But I have no idea how to balance. I mean, what does that even mean? Just the charges? Like, does there have to be an equal amount of charges, whereas now there is 4+ on the left side and 5+ on the right?

Thanks.

[yoyo]

My attempt at balancing:

2Co3+ + Zn -> 2Co2+ + Zn2+

2Mo + 3Pb4+ -> 2Mo3+ + 3Pb2+.

Also, a friend tells me that the right hand side of the first cell should be //Co3+(aq),Co2+(aq)/Pt and the second //Pb4+(aq),Pb2+(aq)/Pt . Don't know what that's all about... but oh well!

Anyway, please feel free to correct me where I'm wrong. Thanks.

[tamim83]

Everything looks good.  Your friend is right, when making a galvanic cell (or even a electrolytic cell), you need electrodes.  Typically, you would use the elemental form of the oxidized or reduced metal if it participates in the cell reaction.  If not, you use an inert metal like Pt or graphite (C_s).