Question

Which of the statements is correct?
i.Melting point of alkane increases with increase of C atom and with increase in branching.
ii. Boiling point of alkane increases with increase of C atoms but with decrease in branching.
iii. Cycloalkanes have lower boiling points than normal alkane with the same number of carbon atoms.
iv. Alkenes have lower boiling points than the same number of carbon atoms in alkanes.
A. (I), (II)
B. (I), (II), (III)
C. (III), (IV)
D. (IV)

Answer 1

The hydrocarbons have certain carbon atoms arranged in a straight chain, or in a branched manner. These branched and a straight chain carbon atom has an effect on the surface area of the molecule, which in turn affects the boiling and melting points of these molecules.

Complete answer:
We have been given certain statements regarding the melting and boiling points of hydrocarbons. We have to tell the correct statements.
- Statement (i) states that the melting point increases with increase in branching and increase in the carbons. This is true, as the melting point of any hydrocarbon, depends on the packing of a molecule. With branching, the surface area of the molecule decreases, which means that the atoms come closer to each other and are packed closely, which increases their intermolecular forces, and thus an increase in melting point.
- Statement (ii) suggests that there is an increase in boiling point when branching decreases but carbon atoms increases. This is also true, as boiling points of hydrocarbons depend on the Van der Waal forces between the atoms. When the branching decreases, the Van der Waals forces also decrease due to less surface area so the boiling point increases. But when the carbon chain increases, the boiling point decreases due to Van der Waal forces having maximum activity.
- Statement (iii) suggests that cycloalkanes have lower boiling point than alkanes. This is incorrect as due to the physical structure of cycloalkanes, the London forces come into action, which increases the boiling point of cyclic structures.
- Statement (iv) suggests that alkenes have a lower boiling point than alkanes with the same carbon atoms, this is also incorrect. As in case of alkenes the double bonds tend to attract the atoms of the molecule more closely resulting in higher boiling point, while in alkanes the surface area is more and weak dispersion forces are present which decreases the boiling point.

Hence statements (i) and (ii) are correct. So option A is correct.

Note:
Strong Van der waal forces are responsible for an increased boiling point, as the energy required to break the molecule will be more due to strong Van der Waal forces. As Van der Waal forces result in polarization and are dependent on the distance between atoms, hence surface area affects these forces.