Balancing Redox Reactions: CH4 + O2 --> CO2 + H2O

[tica86]

a. CH4(g) + 2O2(g) ----------> CO2(g) + 2H2O(g)

I need to determine which is reduced and which is oxidized, I already know that C in methane is Oxidized. O2 is reduced but how can you determine that?? I know it has to do with electron transfer but I just don't see it

 

[Borek]

Assign oxidation numbers to all atoms.