Q.iii. What is enthalpy of fusion?

A phase transition converts one physical form of a substance into another (solid to liquid, liquid to gas, or solid directly to gas) at constant temperature and pressure, without any change in chemical composition, and each such transition has its own characteristic molar enthalpy change. The enthalpy of FUSION (ΔfusH) is the heat absorbed when one mole of a solid melts into liquid at its melting point (e.g. ice to water at 0 degC absorbs +6.01 kJ/mol; the reverse process, freezing, releases the same amount, ΔfreezH = -6.01 kJ/mol). The enthalpy of VAPORIZATION (ΔvapH) is the heat absorbed when one mole of liquid turns to vapour at constant temperature (e.g. water at 100 degC absorbs about +40 kJ/mol, while at 25 degC -- since not all the liquid's molecules need as much extra energy to escape at the boiling point -- it is somewhat higher, about +44 kJ/mol); the reverse, condensation, releases heat. The enthalpy of SUBLIMATION (ΔsubH) is the heat absorbed when one mole of solid converts directly to vapour (e.g. ice to water vapour at 0 degC absorbs +51.08 kJ/mol). Because enthalpy is a state function, converting solid directly to vapour must release/absorb exactly the same total heat whether it happens in one sublimation step or via two steps (melting then vaporizing), giving the useful relation ΔsubH = ΔfusH + ΔvapH.