Are oxidation reactions always exothermic?
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) -> "M"^(z+)(aq) + ze^(-)#
#z#is the magnitude of the metal cation charge and #"M"#is a metal.
#"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
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) -> "CO"_2(g) + 2"H"_2"O"(g)#
Here, oxygen atom is reduced from an oxidation state of
Such processes are TYPICALLY exothermic, but always involve reduction AND oxidation.