Q.Enthalpy of sublimation of a substance is equal to

The key idea is Hess's Law of Constant Heat Summation, which states that the total enthalpy change for a chemical or physical process is the same whether the process occurs in one step or in several steps.

Sublimation is the direct conversion of a substance from its solid state to its gaseous state. This process can be conceptually broken down into two sequential phase changes:

1. Fusion: The conversion of solid to liquid, associated with the enthalpy of fusion ($\Delta H_{fusion}$).
2. Vaporisation: The conversion of liquid to gas, associated with the enthalpy of vapourisation ($\Delta H_{vapourisation}$). …

The enthalpy of sublimation is the energy required to transform a substance directly from its solid state to its gaseous state without passing through the liquid phase. This process can be understood by considering the energy changes involved in breaking intermolecular forces.

Imagine a substance undergoing sublimation. The solid particles absorb energy to overcome the forces holding them in a rigid lattice and then absorb more energy to completely separate into a gas. This direct transition can be conceptually broken down into two steps, even if the liquid phase is not observed:

1. Melting (Fusion): The solid absorbs energy to convert into a liquid. This involves overcoming some of the intermolecular forces to allow particles to move past each other.
2. Boiling (Vaporisation): The liquid then absorbs additional energy to convert into a gas. This involves overcoming the remaining intermolecular forces to allow particles to move freely and independently.

According to Hess's Law, the total enthalpy change for a process is independent of the path taken, as long as the initial and final states are the same. Since enthalpy is a state function, the energy required to go from solid to gas directly must be the same as the energy required to go from solid to liquid and then from liquid to gas.

Let's define the relevant enthalpy changes:

1. Enthalpy of Sublimation ($\Delta H_{sub}$): This is the enthalpy change when one mole of a solid substance converts directly into its gaseous state at a constant temperature and pressure.

2. Enthalpy of Fusion ($\Delta H_{fus}$): This is the enthalpy change when one mole of a solid substance converts into its liquid state at its melting point.

3. Enthalpy of Vaporisation ($\Delta H_{vap}$): This is the enthalpy change when one mole of a liquid substance converts into its gaseous state at its boiling point. …