Final answer:
The change in internal energy of a liquid during vaporization is not equal to the heat added because the energy is used for the phase change, specifically as the enthalpy of vaporization, to overcome intermolecular forces rather than change temperature.
Step-by-step explanation:
When a liquid is vaporized, its change in internal energy is not equal to the heat added because of the energy required for the phase change. During vaporization, the energy is used to overcome the intermolecular forces in the liquid and convert it to gas, which is a process that involves changes in enthalpy.
Specifically, this energy is referred to as the enthalpy of vaporization. This is an endothermic process where the added heat is utilized for breaking the bonds between the molecules without increasing the temperature until the phase change is complete.
Vaporization and condensation involve latent heat. For vaporization, latent heat is absorbed from the surroundings, which is why the internal energy change of the liquid is not solely dependent on the heat added.
Instead, the internal energy change includes both the heat absorbed and the work done against the intermolecular forces. Correspondingly for condensation, this same amount of latent heat is released, increasing the temperature of the surroundings.