Why are oxidation reduction reactions coupled?

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Why are oxidation reduction reactions coupled?

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Answer 1 · 2018-07-31T13:40:34

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:

$2 M g (s) + O_{2} (g) \to 2 M g O (s)$ $bb2Mg(s) + O_2(g) -> bb2MgO(s)$

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

Look at those half-ionic equations:

$2 (M g (s) \to M g^{2 +} (a q) + 2 e^{-})$ $bb2(Mg(s) -> Mg^(2+)(aq) + 2e^(-))$

$O_{2} (g) + 2 e^{-} \to O^{2 -} (a q)$ $O_2(g) + 2e^(-) -> O^(2-)(aq)$

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

$2 M g (s) + O_{2} (g) \to 2 M g O (s)$ $bb2Mg(s) + O_2(g) -> bb2MgO(s)$

It is also clear that $M g$ $Mg$ is oxidised, while $O_{2}$ $O_2$ is reduced.

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Why are oxidation reduction reactions coupled?

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