Why are oxidation reduction reactions coupled?

1 Answer

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:

bb2Mg(s) + O_2(g) -> bb2MgO(s)

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

Look at those half-ionic equations:

bb2(Mg(s) -> Mg^(2+)(aq) + 2e^(-))

O_2(g) + 2e^(-) -> O^(2-)(aq)

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

bb2Mg(s) + O_2(g) -> bb2MgO(s)

It is also clear that Mg is oxidised, while O_2 is reduced.