Question

Butane has a …………………. boiling point to that of propanol
(a) Higher
(b) Similar
(c) Lower
(d) None of these

Answer 1

Hint: Molecules with more intermolecular forces, which hold them together, are strongly bonded. This resists them to move easily or rapidly. Less movement leads to low kinetic energy and molecules could not escape the liquid state to boil, and hence, have higher boiling point.

Complete step by step answer:
Butane has structural formulas as $C{H_3} - C{H_2} - C{H_2} - C{H_3}$. So, it has only a weak Van der Waals force of attraction to hold each atom in the molecule. A substance starts boiling when its vapour pressure becomes equal to the atmospheric pressure. Vapour pressure is the measure of kinetic energy of the molecules and this kinetic energy is determined by the mass, velocity, and the temperature. At a temperature where substances start to boil, the average kinetic energy helps to overcome the force of attraction between the molecules to move them to gaseous state from the liquid state.
On the other hand, propanol has a structural formula of \[C{H_3} - C{H_2} - C{H_2} - O - H\], apart from the Van der Waals interaction, propanol can also form intermolecular hydrogen bonding because it has hydrogen atom connected to electronegative oxygen atom. Hydrogen bonds are much stronger than weak Van Der Waals force of attraction. So, propanol will need more kinetic energy to break the force of attraction between its molecules, and hence, have higher boiling point than butane.
Hence, butane has a lower boiling point than that of propanol,

So, the correct answer is Option C .

Note:
Molecules with higher molar masses have higher boiling point because more energy is needed to break the heavier molecules. So, apart from the forces of interaction, the boiling point is also directly proportional to the molecular mass of the molecules.