Question

(i) Why do the members of a homologous series show similar chemical properties?
(ii) What is the difference in number of carbon and hydrogen atoms between two consecutive members of a homologous series?
Give one example each of (i) Alkanes, (ii) Alcohols, (iii) Aldehydes.

Hint:

Homologous series members differ by a constant –CH$_2$ group and show similar chemical behavior due to the same functional group.

Answer 1

(i) Reason for similar chemical properties:
Members of a homologous series have:
The same functional group (e.g., –OH in alcohols, –CHO in aldehydes).
A common general molecular formula.
Similar structural framework.
Because of the same functional group, their chemical reactivity remains similar, even though physical properties (melting/boiling point, mass) vary.
(ii) Difference in atomic composition:
Each successive member of a homologous series differs by: \[ \boxed{\text{–CH}_2 \text{ group} \Rightarrow \text{1 carbon and 2 hydrogen atoms}} \] Examples:
Alkanes: CH$_4$ (methane), C$_2$H$_6$ (ethane)
Alcohols: CH$_3$OH (methanol), C$_2$H$_5$OH (ethanol)
Aldehydes: HCHO (formaldehyde), CH$_3$CHO (acetaldehyde)
Each next compound has one additional carbon and two hydrogen atoms.