How do you determine the oxidizing or reducing agent in a redox equation?
If the oxidation number increases upon reaction, the species is a reducing agent. And if the oxidation number decreases, the species is an oxidizing agent.
Redox transfer is formalized on the basis of loss or gain or electrons. Recall the old mnemonic:
And thus if something has been oxidized, it is A SOURCE of electrons; likewise, if something has been reduced it is an electron sink, and has ACCEPTED electron from somewhere.
We can go to a simple redox reaction; the oxidation of elemental carbon:
Zerovalent carbon has been oxidized to