Question

Are oxidation reactions always exothermic?

Answer 1

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

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An oxidation half-reaction of an element in general is

`"M"(s) -> "M"^(z+)(aq) + ze^(-)`

where `z` is the magnitude of the metal cation charge and `"M"` is a metal.

or

`"A"^(z-)(aq) -> "A"(s//l//g) + ze^(-)`

where the appropriate phase applies to the neutral `"A"` and `"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 `"M"` or `"A"`.

Hence, all ionization energies are positive.

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

`"CH"_4(g) + 2"O"_2(g) -> "CO"_2(g) + 2"H"_2"O"(g)`

Here, oxygen atom is reduced from an oxidation state of `0` in `"O"_2` to `-2` in `"CO"_2` or `"H"_2"O"`, and carbon is oxidized from an oxidation state of `-4` in `"CH"_4` to `+4` in `"CO"_2` or `"H"_2"O"`.

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