(a) What is a homologous series? Explain with an example. (b) Define the following terms giving one example of each. (i) Esterification (ii) Addition reaction

(a) Homologous Series:

A homologous series is a family of organic compounds that share the same functional group and similar chemical properties, where successive members differ by a $-CH_2-$ unit in their molecular formula and $14\text{ u}$ in their molecular mass.

Example: The alkane series is a homologous series with the general formula $C_nH_{2n+2}$:

1. Methane ($CH_4$)
2. Ethane ($C_2H_6$)
3. Propane ($C_3H_8$)

Here, $C_2H_6 - CH_4 = CH_2$, and the difference in molecular mass is $14\text{ u}$ (since $C = 12\text{ u}$ and $H = 1\text{ u}$).

(b) Definitions and Examples:

(i) Esterification:

Definition: The reaction of a carboxylic acid with an alcohol in the presence of an acid catalyst to produce a sweet-smelling compound called an ester along with water is known as esterification.

Example: Ethanoic acid ($CH_3COOH$) reacts with ethanol ($CH_3CH_2OH$) in the presence of concentrated sulphuric acid to form ethyl ethanoate (an ester) and water.

Chemical Equation:

$$CH_3COOH + CH_3CH_2OH \xrightarrow{\text{Acid}} CH_3COOCH_2CH_3 + H_2O$$

(ii) Addition Reaction:

Definition: A reaction in which unsaturated hydrocarbons (alkenes or alkynes) add hydrogen or other atoms in the presence of catalysts like nickel ($Ni$) or palladium ($Pd$) to form saturated hydrocarbons is called an addition reaction.

Example: Hydrogenation of ethene ($C_2H_4$) in the presence of a nickel catalyst to form ethane ($C_2H_6$).

Chemical Equation:

$$CH_2=CH_2 + H_2 \xrightarrow{\text{Ni}} CH_3-CH_3$$