# How do you determine the oxidizing or reducing agent in a redox equation?

Sep 16, 2016

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.

#### Explanation:

Redox transfer is formalized on the basis of loss or gain or electrons. Recall the old mnemonic: $\text{LEO SAYS GER}$, $\text{loss of electrons = oxidation; gain of electrons = reduction.}$

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:

$C + {O}_{2} \rightarrow C {O}_{2}$

Zerovalent carbon has been oxidized to ${C}^{I V +}$. But while carbon has been oxidized, its very oxidation has provided electrons to the dioxygen oxidant; i.e. carbon is here the reducing agent.