Question

Assertion: Hexane has higher melting points than both pentane and heptane.
Reason: Melting point of a straight chain alkane with an odd number of carbon is always less than adjacent homologues with an even number of carbons.
A. Both the assertion and reason are correct and reason is the correct explanation for assertion.
B. Both the assertion and reason are correct and reason is not the correct explanation for assertion.
C. Assertion is incorrect but reason is incorrect.
D. Assertion is wrong and reason is correct.

Answer 1

We know that the melting point of the hydrocarbon increases with increase in carbon chain length. This is because the van der Waals interaction, which is London dispersion force, is greater for the longer chain hydrocarbon.

Complete step by step answer:
We must remember that the melting point of hexane is higher than pentane and heptane because the melting point of a straight chain alkane with an odd number of carbons is always fewer than adjacent homologues with an even number of carbons. Alkanes which have an even number of carbons in their parent chain have enhanced packing efficiency than those that have an odd number of carbons in their parent chain.
Thus, Assertion and Reason are correct but Reason does not have the correct explanation of the assertion.

Hence option B is correct.

Note: We also remember that the intermolecular forces between the atoms determine the boiling and melting point of the compounds stronger the intermolecular forces higher will be the boiling and melting point.
In both the hydrocarbons there is a weak London dispersion force but the strength the intermolecular forces increases with the number of atoms in the molecule.
Example:
Intermolecular forces in ethane:
Ethane has London forces like all covalent compounds.
Ethane has a polar bond, so it exhibits dipole-dipole interactions.
Ethane has hydrogen atom but there is no oxygen or fluorine or nitrogen atom to form intermolecular hydrogen bonding.
Intermolecular forces present in methanol:
Methanol has London forces like all covalent compounds.
Methanol has a polar bond, so it exhibits dipole-dipole interaction.
Methanol has Hydrogen, or an Oxygen, nitrogen or Fluorine so it has intermolecular hydrogen bonding.
We must know that the hydrogen bonds are relatively strong intermolecular forces and thus it takes a lot of energy to break the intermolecular forces thus the molecules with stronger hydrogen bonds have higher boiling and melting point. Thus, methanol has a high boiling point and melting point because it has a strong hydrogen bond.