Question

If enthalpy is a function of temperature only then why enthalpy is change in condensation

Answer 1

Enthalpy is specific for a reaction at a particular temperature. In condensation vapour is converted into liquid. This is a reaction in which physical state of reactant and product is different. The temperature although remains constant but there is a change in heat involved (latent heat). The heat is required to change the 'physical state' of the substance. That is why we call it latent heat because this heat does not change temperature but only the state.

So there is enthalpy change involved because of the reaction. It's not true that if change in temperature is zero then the change in enthalpy would also be zero.

Answer 2

Enthalpy of any substance increases with temperature, which means both the products and the reactants' enthalpies increase. The overall enthalpy of the reaction will change if the increase in the enthalpy of products and reactants is different.

The enthalpy of condensation is equal to the enthalpy of vaporization with the opposite sign enthalpy changes of vaporization are always positive (heat is absorbed by the substance), whereas enthalpy changes of condensation are always negative (heat is released by the substance)