# How would I find the half-reactions?

1. Feb 19, 2012

### Bipolarity

$$2MoS_{2} + 7O_{2} \rightarrow 2MoO_{3} + 4SO_{2}$$

So here I have a redox reaction, it's already balanced but just for kicks I'm trying to find the half-reactions so that I know what is happening. I'm having some trouble though as I don't know what's being oxidized and what's being reduced. Any advice?

Thanks!

BiP

2. Feb 20, 2012

### Staff: Mentor

Two things are oxidized and one is being reduced.

The one that is being reduced should be obvious.

3. Feb 20, 2012

### Bipolarity

Sorry I don't catch it still. Which one is being oxidized and how do you know?

BiP

4. Feb 20, 2012

### moouers

Did you try assigning oxidation numbers?

If you're not sure about how to do that, this should help:

5. Feb 21, 2012

### Bipolarity

The problem is both Molybdenum and Sulphur have variable oxidation numbers so I don't know what they are.

BiP

6. Feb 21, 2012

### Staff: Mentor

Polysulfides are a little bit tricky. In general they have a form Sn2-. For S22- you can safely assume both sulfur atoms are identical and each has an oxidation number of -1. This is analogous to hydrogen peroxide where both oxygen atoms are assigned ON of -1.