How would I find the half-reactions?

[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

 

[Borek]

Two things are oxidized and one is being reduced.

The one that is being reduced should be obvious.

 

[Bipolarity]

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

BiP

 

[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

 

[Bipolarity]

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

BiP

 

[Borek]

Polysulfides are a little bit tricky. In general they have a form S_n^2-. For S₂^2- 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.