Redox...

A question from email:

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Hey Dr. Bodwin I had a question on balancing equations. I can't tell if this equation is a redox or not

2 C2H2(g) + 5 O2(g) 􀀧 4 CO2(g) + 2 H2O(g)

I was looking for some guidelines on how to tell if an equation is redox or not, and if it is I am also looking for help on how to write the half reactions.

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OK, I'll answer the easier part of this question first. If a reaction is redox, then the oxidation numbers of some (or all) of the elements have to change. Let's look at ox#'s here...

In C2H2(g), let's use the "rules"... if each of the H's is +1 and the molecule is neutral, then the two C's must total -2. (The sum of all the oxidation numbers has to equal the charge of the molecule or polyatomic ion.) That means each C must have ox# = -1. {I'm assuming that the two C's are identical.}

Since O2(g) is an uncombined element in it's natural/standard state, the ox# = 0.

For CO2(g) it's back to the rules... if each oxygen is -2, then the carbon must be +4.

For H2O(g), the rules pretty much do all the work for us, H is +1, O is -2.

So in this reaction, carbon is going from -1 to +4 and oxygen is going from 0 to -2, there are changes in ox#'s, therefore this is a redox reaction. {Which one is reduction and which one is oxidation? Hmm...}

Now for the harder part of this specific reaction... writing half reactions for a combustion reaction like this is a little more involved than writing half reactions for metal/metal redox reactions because the oxygen is incorporated into both products. That means that our stepwise procedure for balancing redox reactions doesn't work all that great here... fortunately, hydrocarbon combustion reaction can usually be balanced by inspection/trial-and-error. If you have to balance a redox reaction (and write out half reactions) on the final exam, it will not be a hydrocarbon combustion. You should still be able to assign oxidation numbers and identify which element is being oxidized or reduced, but these are probably easier to balance by inspection.

Other questions? Let me know...