NEED HELP + EXPLANATION IDENTIFYING THE SUBSTANCE OXIDIZED AND REDUCED.

Question

idek-ok · Answer

I am not looking for an answer, but rather an explanation.  My teacher is too busy right now, and the tutoring room is closed.  Okay, so, I have to identify what is being oxidized and reduced in CH4 + 2O2 yields CO2 + 2H2O.  I know that carbon goes from a charge of -4 to +4, so it's definitely being oxidized.  However, I get that hydrogen goes from a charge of +4 to +2, and oxygen goes from 0 to -2 in 2H2O and -4 in CO2.   So how can I tell when which is being reduced, when it seems both hydrogen and oxygen are being reduced?

aaronq · Answer

The formal charge of hydrogen doesn't change, its +1 in both $CH_4$ and $H_2O$.

Additionally, the formal charge of oxygen in both water and carbon dioxide is -2. Note that there are two O atoms in $CO_2$ and the -4 is divided by 2.

sweetburger · Answer

@idek-ok remember that elements in their "free" elemental state have an oxidization number of 0. This also applies to elements that are naturally diatomic.

sweetburger · Answer

Think of the oxidization number of the $$O_2$$ on the reactant side of the reaction.