CBSE Class 10 Science 2020 · Paper 31/2/1 · Question 26

(a) A homologous series is a series of carbon compounds that have the same functional group (and hence similar chemical properties) and the same general formula, in which successive members differ by a $-{\text{CH}}_2-$ unit (14 u in molecular mass); their physical properties change gradually along the series. Example: the alcohols ${\text{CH}}_3\text{OH}$ (methanol), ${\text{C}}_2{\text{H}}_5\text{OH}$ (ethanol), ${\text{C}}_3{\text{H}}_7\text{OH}$ (propanol) and ${\text{C}}_4{\text{H}}_9\text{OH}$ (butanol) all have the functional group $-\text{OH}$ and the general formula ${\text{C}}_n{\text{H}}_{2n+1}\text{OH}$, and each differs from the next by $-{\text{CH}}_2-$. (b) (i) Esterification: the reaction of a carboxylic acid with an alcohol in the presence of an acid catalyst (concentrated sulphuric acid) to form a sweet-smelling ester and water. Example: ${\text{CH}}_3\text{COOH}+{\text{C}}_2{\text{H}}_5\text{OH}\stackrel{{\text{conc. H}}_2{\text{SO}}_4}{\to }{\text{CH}}_3{\text{COOC}}_2{\text{H}}_5+{\text{H}}_2\text{O}$ (ethyl ethanoate and water). (ii) Addition reaction: a reaction in which atoms, such as hydrogen, add across a double or triple bond of an unsaturated compound, in the presence of a catalyst such as nickel or palladium. Example: ${\text{CH}}_2={\text{CH}}_2+{\text{H}}_2\stackrel{\text{Ni}}{\to }{\text{CH}}_3-{\text{CH}}_3$ (unsaturated ethene gives saturated ethane).