# Are oxidation reactions always exothermic?

May 9, 2018

Well, they should be endothermic if you're oxidizing an element... Combustion is exothermic but is NOT JUST oxidation.

An oxidation half-reaction of an element in general is

${\text{M"(s) -> "M}}^{z +} \left(a q\right) + z {e}^{-}$

where $z$ is the magnitude of the metal cation charge and $\text{M}$ is a metal.

or

$\text{A"^(z-)(aq) -> "A} \left(s / l / g\right) + z {e}^{-}$

where the appropriate phase applies to the neutral $\text{A}$ and $\text{A}$ is a nonmetal.

This is really just the process of ionization.

In general then, you supply energy to eject an electron, and thus it is an endothermic process, regardless of the identity of $\text{M}$ or $\text{A}$.

Hence, all ionization energies are positive.

An oxidation of a hydrocarbon in combustion is ALWAYS accompanied by the reduction of oxygen...

$\text{CH"_4(g) + 2"O"_2(g) -> "CO"_2(g) + 2"H"_2"O} \left(g\right)$

Here, oxygen atom is reduced from an oxidation state of $0$ in ${\text{O}}_{2}$ to $- 2$ in ${\text{CO}}_{2}$ or $\text{H"_2"O}$, and carbon is oxidized from an oxidation state of $- 4$ in ${\text{CH}}_{4}$ to $+ 4$ in ${\text{CO}}_{2}$ or $\text{H"_2"O}$.

Such processes are TYPICALLY exothermic, but always involve reduction AND oxidation.