HW #13d and 15 Balancing Half Reactions and Cell Diagrams

[Shannon Han 2B]

I'm confused on #13d and #15 on the chapter 13 textbook problems. The question asks to write the half-reactions, balanced equation, and cell diagram for each equation. On 13, why is the equation for the anode Au(s) ---> Au3+ + 3 e- and not Au+ ---> Au3+ + 2e-?

On 15, I'm in general confused on how the solutions manual came up with these half reactions for the anode. On 15a, where did the Ag(s) come from in the equation for the anode? I don't understand why the equation is Ag (s) + Br- (aq) ---> AgBr (s) if the original equation has Ag+ (aq).

On 15b, why is O2(g) in the cell diagram and not H2O(l)?

Also, how do you know when to add Pt(s) to the left or right or both sides of the cell diagram?
Any help would be appreciated on these problems. Thanks!

[Chem_Mod]

On 13d, there are only 2 possible half-reactions listed in the table which involve Au:

1) Au+ (aq) + e- —> Au (s)
2) Au3+ (aq) + 3e- —> Au (s)

The first equation is used for the cathode, so the second equation must be used for the anode.

Please see this post regarding 15a:
https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=140&t=3415&sid=fa7c89dc87538d34bbec1f2ed1ff9929

On 15b, you do not need to add water if there are aqueous species because it is already implied.

Please see this post regarding Pt(s):
https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=140&t=2116&sid=fa7c89dc87538d34bbec1f2ed1ff9929

[Justin Le 2I]

You would add Pt to one of the sides if you need a metal electrode that can conduct the electrons. For example, if you have aqueous, gas, or liquid, then you would use Pt.