(a) “Keerthi thinks that Substitution reaction occurs in saturated Hydrocarbons, on the contrary Krishi thinks, it occurs in unsaturated Hydrocarbons.” Justify with valid reasoning whose thinking is correct.

(b) “Methane and Propane and their Isomers are used as fuels” Comment. Draw the electron dot structure of the immediate lower homologue of Propane. Give any two characteristics of homologues of a given homologous series.

(c) A mixture of oxygen and ethyne is burnt for welding. Can you predict why a mixture of ethyne and air is not used?

(a) Keerthi's thinking is correct as substitution reactions are characteristic of saturated hydrocarbons (alkanes) where all bonds are single (C–C, C–H). With no reactive double/triple bond, an incoming atom/group can replace a hydrogen atom.
By contrast, unsaturated hydrocarbons (alkenes/alkynes) contain reactive double and triple bonds. These double and triple bonds readily open and add atoms across it, so they usually undergo addition, not substitution.

(b) Methane and propane (and their isomers) are used as fuels because they are saturated hydrocarbons that burn easily in air, releasing a large amount of heat energy and producing mainly carbon dioxide and water without leaving any residue. This makes them clean fuels suitable for domestic and industrial purposes.

The immediate lower homologue of propane (C₃H₈) is ethane (C₂H₆) and ethane has the following electron dot structure :

Characteristics of homologous series :

• All members of a homologous series have the same general formula.
• Each member of the series differs from the preceding one by the addition of CH₂ group and by molecular mass of 14 a.m.u.

(c) A mixture of oxygen and ethyne is used for welding because this mixture produces a very high flame temperature of about 3000 °C, sufficient to melt metals. However, a mixture of ethyne and air is not used, because air contains nitrogen and less oxygen, which leads to incomplete combustion of ethyne and this incomplete combustion produces sooty flame with a lower temperature, making it unsuitable for welding purposes.