Question

Why are oxidation reduction reactions coupled?

Answer 1

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.