(a) Structures for ethanol and ethanoic acid are as follows: (i) $\underset{\text{Ethanol}}{ \mathrm{H}- \underset{ \mathrm{H} }{ \underset{ \vert }{ \overset{ \mathrm{H} }{ \overset{ \vert }{ \mathrm{C} } } } } - \underset{ \mathrm{H} }{ \underset{ \vert }{ \overset{ \mathrm{H} }{ \overset{ \vert }{ \mathrm{C} } } } } - \mathrm{O}-\mathrm{H} }$ (ii) $\underset{\text{Ethanoic acid}}{ \mathrm{H}- \underset{ \mathrm{H} }{ \underset{ \vert }{ \overset{ \mathrm{H} }{ \overset{ \vert }{ \mathrm{C} } } } } - \overset{ \mathrm{O} }{ \overset{ \parallel }{ \mathrm{C} } } }$ (b) Conversion of ethanol to ethanoic acid is considered as an oxidation reaction because oxygen is added to ethanol to convert it to ethanoic acid. In the above reaction alkaline $\mathrm{KMnO}_4$ or acidified $\mathrm{K}_2\mathrm{Cr}_2\mathrm{O}_7$ adds oxygen to ethanol hence they are the oxidising agent used in the reaction involved.