Please explain how to assign oxidation numbers from equations?

[swampman]

I'm having trouble on how to assign oxidations from equations,

e.g. C + O2 = CO2

then the oxidation numbers of C is +4 and O2 is -2. Therefore, C is the reductant and O is the oxidant.

Could someone please explain why is this so, and how to work out other questions like this? Is there a formula?

Thanks!

[Stick]

In Year 11 Chemistry you should have studied the rules of oxidation numbers. If you still have that textbook, have a read over them because it's imperative you can understand and apply them correctly.

[polar]

The oxidation numbers of C and O in O2 are both 0 - free elements have an oxidation number of zero.
In general, oxygen has an oxidation number of -2, thus, carbon has an oxidation number of +4 in CO2.

carbon - 0 to +4 (increase) thus, reductant
oxygen - 0 to -2 (decrease) thus, oxidant

[watto_22]

http://chemistry.about.com/od/generalchemistry/a/oxidationno.htm
or
http://www.dummies.com/how-to/content/rules-for-assigning-oxidation-numbers-to-elements.html
should help if you can't find your textbook

[swampman]

okay what about when it comes to compounds?
could someone please provide a worked solution please

e.g. Fe203 (s) + 3CO (g) ----> 2Fe (s) + 3CO2 (g)

[REBORN]

Fe2O3

Rule -> in a neutral compound the oxidation numbers must sum to zero

Memorised fact -> O has an oxidation number of negative two (bar special cases)

Working out -> Let A = Fe. 2A -6 = 0      2A = 6     A = 3     therefore oxdiation number of Fe is +3