Question: 70. Given below are two statements :
Statement I: The boiling point of three isomeric pentanes follows the order n-pentane > isopentane > neopentane
Statement II : When branching increases, the molecule attains a shape of sphere. This results in smaller surface area for contact, due to which the intermolecular forces between the spherical molecules are weak, thereby lowering the boiling point.
In the light of the above statements, choose the most appropriate answer from the options given below:
(1) Both statement I and Statement II are correct
(2) Both Statement I and Statement II are incorrect
(3) Statement I is correct but Statement II is incorrect
(4) Statement I is incorrect but Statement II is correct
Answer: Option (1)
Explanation:
The boiling point of alkanes depends on the strength of intermolecular forces,
mainly London dispersion forces.
n-pentane has a straight-chain structure, which provides a larger surface area for intermolecular contact,
resulting in stronger dispersion forces and a higher boiling point.
Isopentane is a branched-chain alkane, so its surface area is smaller than that of n-pentane,
leading to weaker intermolecular forces and a lower boiling point.
Neopentane is highly branched and nearly spherical in shape,
giving it the smallest surface area among the three,
resulting in the weakest intermolecular forces and the lowest boiling point.
Thus, the boiling point order is n-pentane > isopentane > neopentane, making Statement I correct.
Statement II correctly explains that increased branching makes molecules more spherical, reduces surface area for contact, weakens intermolecular forces, and lowers boiling point.
Therefore, both Statement I and Statement II are correct.