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:
http://chemistry.about.com/od/generalchemistry/a/oxidationno.htm

Also, khanacademy.org has some excellent videos (I think three in total) about redox reactions and oxidation states. Start with this one: http://www.khanacademy.org/video/introduction-to-oxidation-states?topic=chemistry and work your way through the next few.

I also recently asked a question about balancing by half-reaction method here, and got some good help which may in turn help you: https://www.physicsforums.com/showthread.php?t=578120

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.

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