[rka]

I have to balance:
SnCl2 + HgCl2 → SnCl4 + HgCl

It's a redox reaction where the oxidation states change as follows
Sn: 2 -> 4
Hg: 2 -> 1

The half reactions are:
Oxidation: SnCl2 --> SnCl4
Reduction: HgCl2 --> HgCl

My steps say to balance the half reactions ignoring H and O at first. However, it is not possible to balance these for mass. What am I doing wrong? I know I balance H and O using H+ and H2O. I dont know how to balance the chlorines though.

[Borek]

Assume Cl^- to be product. You won't be able to balance tin half reaction without assuming Cl^- on the LHS as well. In the end they should cancel out.

It shouldn't be HgCl, but Hg₂Cl₂.

[rka]

Like wise, what about the following reaction: CdS + I2 + HCl → CdCl2 + HI + S

Can this be balanced using the half-reactions method? If so how?

CdS --> S
I2 --> HI

How do you balance the CdS --> S reaction?

Or is there a different method to use that works. When is it appropriate to use half reactions and when is it not?

[Nobby]

Cadmium doesn't change the oxidation number. I would recommend write down only the Ions who change.

Also : S^2- => S + 2 e^-

I₂ + 2 e^- => 2 I^-

Sum: S^2- + I₂ => S + 2 I^-

If you like you can complete the Reaction wit Cd²⁺ and HCl

[Borek]

Cadmium doesn't change the oxidation number. I would recommend write down only the Ions who change.

Also : S^2- => S + 2 e^-

While you are correct about cadmium being +2 before and after, treating S^2- as ions is incorrect, as CdS is insoluble (pK_sp around 27).

How do you balance the CdS --> S reaction?

I would try CdS -> Cd²⁺ + S

Or is there a different method to use that works. When is it appropriate to use half reactions and when is it not?

Each method has it better and weaker points. Half reactions are quite good, I guess the problem here is you are given a set of tricky reactions. That's good. Once you will get efficient at doing tricky ones, you will be able to do the easy ones in memory, while picking your nose  

[Nobby]

While you are correct about cadmium being +2 before and after, treating S2- as ions is incorrect, as CdS is insoluble (pKsp around 27).

Good point. So the reaction takes place only on the surface of the CdS crystalls.