How would I find the half-reactions?

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In summary, the conversation discusses a redox reaction with known coefficients but unknown half-reactions. The conversation provides tips on how to determine the half-reactions by assigning oxidation numbers and provides helpful resources for understanding redox reactions and oxidation states. The specific problem of variable oxidation numbers for Molybdenum and Sulphur is also addressed.
  • #1
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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  • #2
Two things are oxidized and one is being reduced.

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

BiP
 
  • #4
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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  • #5
The problem is both Molybdenum and Sulphur have variable oxidation numbers so I don't know what they are.

BiP
 
  • #6
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.
 

1. How do I determine the oxidation state of an element in a chemical reaction?

The oxidation state of an element can be found by using a set of rules. The general rule is that the oxidation state of an element in its elemental form is 0. For atoms in a molecule or ion, the sum of the oxidation states of all atoms must equal the overall charge of the molecule or ion. You can also use the electronegativity values of the elements to determine their oxidation states.

2. What is the purpose of balancing a chemical equation?

The purpose of balancing a chemical equation is to ensure that the number of atoms of each element on the reactant side is equal to the number of atoms of that element on the product side. This is based on the law of conservation of mass, which states that matter cannot be created or destroyed in a chemical reaction.

3. How do I identify the oxidizing and reducing agents in a redox reaction?

The oxidizing agent is the substance that causes another substance to lose electrons and undergo oxidation. It is typically the element or compound that is reduced in the reaction. The reducing agent is the substance that causes another substance to gain electrons and undergo reduction. It is usually the element or compound that is oxidized in the reaction.

4. What is the difference between an oxidation reaction and a reduction reaction?

An oxidation reaction is one in which an atom or molecule loses electrons, resulting in an increase in its oxidation state. A reduction reaction is one in which an atom or molecule gains electrons, resulting in a decrease in its oxidation state. These two types of reactions are always coupled together and are known as redox reactions.

5. How can I use the standard reduction potentials to determine the spontaneity of a redox reaction?

The standard reduction potentials (E°) of a redox reaction can be used to determine the spontaneity of the reaction. A more positive E° value indicates a stronger tendency for reduction, while a more negative E° value indicates a stronger tendency for oxidation. If the E° values for the two half-reactions are positive and negative, respectively, the reaction is spontaneous. If both values are negative, the reaction is non-spontaneous.

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