How would I find the half-reactions?

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Discussion Overview

The discussion revolves around identifying half-reactions in a balanced redox reaction involving molybdenum and sulfur compounds. Participants explore the concepts of oxidation and reduction, as well as the assignment of oxidation numbers to clarify the redox process.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant presents a balanced redox reaction and seeks assistance in identifying which species are oxidized and reduced.
  • Another participant suggests that two species are oxidized while one is reduced, implying that the reduced species should be identifiable.
  • A participant expresses confusion about which species is oxidized and requests clarification on how to determine this.
  • It is proposed that assigning oxidation numbers could help in identifying the oxidized and reduced species.
  • A participant notes the complexity of determining oxidation numbers for molybdenum and sulfur due to their variable oxidation states.
  • One participant mentions that polysulfides can be tricky and provides a general form for their oxidation states, drawing an analogy to hydrogen peroxide.

Areas of Agreement / Disagreement

Participants do not reach a consensus on which specific species are oxidized or reduced, indicating ongoing uncertainty and differing interpretations regarding oxidation states.

Contextual Notes

Participants highlight the challenges posed by variable oxidation numbers for certain elements, which may complicate the assignment of oxidation states in the reaction.

Bipolarity
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[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?

Thanks!

BiP
 
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Two things are oxidized and one is being reduced.

The one that is being reduced should be obvious.
 
Sorry I don't catch it still. Which one is being oxidized and how do you know?

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

BiP
 
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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