Enthalpy of vaporization

In thermodynamics, the enthalpy of vaporization (symbol ∆Hvap), also known as the (latent) heat of vaporization or heat of evaporation, is the amount of energy (enthalpy) that must be added to a liquid substance to transform a quantity of that substance into a gas.

Above the critical temperature, the liquid and vapor phases are indistinguishable, and the substance is called a supercritical fluid.

As the liquid and gas are in equilibrium at the boiling point (Tb), ΔvG = 0, which leads to: As neither entropy nor enthalpy vary greatly with temperature, it is normal to use the tabulated standard values without any correction for the difference in temperature from 298 K. A correction must be made if the pressure is different from 100 kPa, as the entropy of an ideal gas is proportional to the logarithm of its pressure.

[1] These two definitions are equivalent: the boiling point is the temperature at which the increased entropy of the gas phase overcomes the intermolecular forces.

Enthalpies of vaporization of common substances, measured at their respective standard boiling points:

Temperature-dependency of the heats of vaporization for water , methanol , benzene , and acetone
Molar enthalpy of zinc above 298.15 K and at 1 atm pressure, showing discontinuities at the melting and boiling points. The enthalpy of melting (Δ H °m) of zinc is 7323 J/mol, and the enthalpy of vaporization (Δ H °v) is 115 330 J/mol .