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How would I find the half-reactions?

  1. Feb 19, 2012 #1
    [tex]2MoS_{2} + 7O_{2} \rightarrow 2MoO_{3} + 4SO_{2} [/tex]

    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?


  2. jcsd
  3. Feb 20, 2012 #2


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    Staff: Mentor

    Two things are oxidized and one is being reduced.

    The one that is being reduced should be obvious.
  4. Feb 20, 2012 #3
    Sorry I don't catch it still. Which one is being oxidized and how do you know?

  5. Feb 20, 2012 #4
    Did you try assigning oxidation numbers?

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

    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 [Broken] 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
    Last edited by a moderator: May 5, 2017
  6. Feb 21, 2012 #5
    The problem is both Molybdenum and Sulphur have variable oxidation numbers so I don't know what they are.

  7. Feb 21, 2012 #6


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    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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