Question

Assertion:The boiling point of hexane is greater than that of pentane.
Reason:n-Alkanes have higher boiling points than isoalkanes.
(A) If both assertion and reason are correct and reason is the correct explanation of the assertion.
(B) If both assertion and reason are correct but reason is not the correct explanation of the assertion.
(C) If the assertion is correct but reason is incorrect.
(D) If the assertion is incorrect but the reason is correct.

Answer 1

Hexane and pentane both are a member of the homologous series of alkane. Their chemical formula is ${C_6}{H_{14}}$ and ${C_5}{H_{12}}$ respectively. Alkanes are the simplest hydrocarbons that are composed of Carbon and hydrogen. They are also called paraffin.

Complete step by step answer:
In alkanes, the atoms are bonded together by single covalent bonds thus, they are referred to as saturated hydrocarbons. The general formula of alkanes is ${C_n}{H_{2n + 2}}$.
The intermolecular force that exists between the atoms of alkanes is van der waal force. It is a weak force as compared to hydrogen bonding or dipole-dipole interaction. Stronger van der waal force between molecules gives rise to a higher boiling point.
The boiling point is a property of surface area and the number of alkanes that surrounds the molecule. In the case of hexane, the carbon atom chain is of $6$ atoms while in the case of pentane it is of $5$ atoms. Hence, the surface area of hexane is larger than the pentane. So, it will have a higher boiling point than pentane.
While in the case of chain and branched molecules, a straight-chain will have a higher boiling point than branched-chain due to greater surface area. Thus, n-alkanes will have a higher boiling point than isoalkanes. Hexane and pentane both are straight-chain alkanes.
Therefore, we can say that both assertion and reason are correct but reason is not the correct explanation of the assertion.

Hence, option (B) is correct.

Note:
Generally, All alkanes are highly combustible in nature. Hence, they are valuable as clean fuels .The first four alkanes - methane, ethane, propane, and butane are gases at room temperature. These can be used directly as fuels.