How can you recognize oxidation reduction reactions?

Feb 5, 2017

By a change in formal oxidation number of the participating reactants with respect to the products.

Explanation:

See this old answer for some tips and principles. Any reaction that involves a change in formal oxidation number is a redox reaction. Under this umbrella lies simple combustion reactions of hydrocarbons, e.g.:

$C {H}_{4} + 2 {O}_{2} \rightarrow C {O}_{2} + 2 {H}_{2} O$

We have balanced this directly with no pfaffing about with electron transfer. But we could assign oxidation numbers to the individual atoms, and use the method of half equations:

$\text{Oxidation (i):}$
${C}^{- I V} {H}_{4} + 2 {H}_{2} O \rightarrow {C}^{+ I V} {O}_{2} + 8 {H}^{+} + 8 {e}^{-}$

$\text{Reduction (ii):}$
${O}_{2} + 4 {H}^{+} + 4 {e}^{-} \rightarrow 2 {H}_{2} {O}^{- I I}$

And we add $\text{(i)" + "(ii)} \times 2 :$

${\text{^(-IV)CH_4 +2O_2^0 rarr }}^{- I V} C {O}_{2}^{- I I} + 2 {H}_{2}^{- I I} O$

To give the original equation as required.