Question

Enthalpy of sublimation of a substance is equal to:
A) Enthalpy of fusion $+$ enthalpy of vaporization
B) Enthalpy of fusion
C) Enthalpy of vaporization
D) Twice the enthalpy of vaporization

Answer 1

We know that sublimation is a process in which a solid is directly converted into the gaseous state. So, the amount of heat energy required to convert a mole of solid directly into the gaseous state at constant pressure is known as the molar enthalpy of sublimation. The enthalpy of sublimation is usually expressed as $\Delta {H_{sub}}$ in joules per mole or kilogram of a substance.

Complete solution:
As we know that a solid cannot be directly converted into gaseous state without passing through an intermediate state i.e. liquid state.
So, this process of sublimation takes place in two steps.
In the first step:
$Solid \to liquid$
The process in which a solid is converted into the liquid state is known as melting or we can also say that the heat energy required to change one mole of a solid to liquid state without any change in temperature is known as the latent heat of fusion also known as the enthalpy of fusion.
In the second step:
$liquid \to gas$
So, the process in which a liquid is converted into gaseous state is known as vaporization or the heat energy required to change one mole of liquid at its boiling point to gaseous state is known as latent heat of vaporization also known as enthalpy of vaporization.

Therefore, from the above explanation we can say that the correct option is (A).

Note: here we have used the term latent heat because latent heat is the energy which can be absorbed or released by a substance, the pressure and temperature remaining constant during the process. Also, the heat of vaporization is always greater than the heat of fusion because there is a large separation of particles in gaseous state so more energy will be required to change a liquid into gas.