# Why are oxidation reduction reactions coupled?

Jul 31, 2018

Oxidation is the loss of electrons, while reduction is the gain of electrons. During a reaction, if a certain reactant gained electrons (get reduced), this would mean that another reactant lost those electrons (get oxidised).

For instance:

$\boldsymbol{2} M g \left(s\right) + {O}_{2} \left(g\right) \to \boldsymbol{2} M g O \left(s\right)$

It's clear that Mg got oxidised (lost electrons) to become two $M {g}^{2 +}$ ions. But where would those electrons go?

Look at those half-ionic equations:

$\boldsymbol{2} \left(M g \left(s\right) \to M {g}^{2 +} \left(a q\right) + 2 {e}^{-}\right)$

${O}_{2} \left(g\right) + 2 {e}^{-} \to {O}^{2 -} \left(a q\right)$

Here, it is clear that the electrons cancel out each other to give you the balanced equation:

$\boldsymbol{2} M g \left(s\right) + {O}_{2} \left(g\right) \to \boldsymbol{2} M g O \left(s\right)$

It is also clear that $M g$ is oxidised, while ${O}_{2}$ is reduced.