a) ‘Homologous series’ are prevalent for organic compounds
and not for inorganic compounds.
Homologous series are characteristic of
organic compounds because these compounds are based on carbon chains. In a homologous
series, each member differs from the next by a CH₂ unit. This regularity in
structure leads to a gradual variation in physical properties and similar
chemical properties among the members. Inorganic compounds, on the other hand,
do not form such series because their structures are more varied and less
amenable to the regular incremental changes seen in organic compounds.
b) Methane, ethane and propane have similar chemical
properties but differ in physical properties.
Methane (CH₄), ethane (C₂H₆), and propane
(C₃H₈) are all alkanes, which are saturated hydrocarbons. They have similar
chemical properties because they all undergo similar types of reactions, such
as combustion and substitution, due to the presence of only single bonds
between carbon atoms. However, they differ in physical properties such as
boiling and melting points. These differences arise because as the molecular
size increases, the strength of van der Waals forces between the molecules
increases, leading to higher boiling and melting points for larger molecules.
c) Hydrocarbons of the same homologous series have the
same general formula.
Hydrocarbons in the same homologous series
share a common general formula, which describes the ratio of carbon to hydrogen
atoms in the molecules. For example:
- Alkanes have the general formula (CnH{2n+2} -- Alkanes (paraffins)
- Alkenes have the general formula (CnH{2n} --- Alkenes (olefins)
- Alkynes have the general formula (CnH{2n-2}
This general formula is a result of the consistent structural pattern within
the series, where each successive compound differs by a CH₂ group.