Are oxidation reactions always exothermic?

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Are oxidation reactions always exothermic?

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Answer 1 · 2018-05-09T00:10:09

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

$M (s) \to M^{z +} (a q) + z e^{-}$ $"M"(s) -> "M"^(z+)(aq) + ze^(-)$

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

or

$A^{z -} (a q) \to A (s / l / g) + z e^{-}$ $"A"^(z-)(aq) -> "A"(s//l//g) + ze^(-)$

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

Hence, all ionization energies are positive.

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

${CH}_{4} (g) + 2 O_{2} (g) \to {CO}_{2} (g) + 2 H_{2} O (g)$ $"CH"_4(g) + 2"O"_2(g) -> "CO"_2(g) + 2"H"_2"O"(g)$

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

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

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Are oxidation reactions always exothermic?

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