a. The enthalpy change that accompanies the conversion of one mole of solid directly into its vapour at constant temperature and pressure is called its enthalpy of sublimation (ΔsubH).
b. Whether the conversion of solid to vapour takes place directly in one step or in two steps (i.e., first melting of solid into liquid and then its vaporization), the enthalpy change is the same because enthalpy is a state function.
Therefore,
ΔsubH = ΔfusH + ΔvapH
Where ΔfusH is enthalpy of fusion and ΔvapH is enthalpy of vaporization.
