Enthalpies of Physical Changes

The standard enthalpy of transition is the amount of heat required to change a substance from one phase to another under standard conditions.

At constant pressure, the heat absorbed or released equals the enthalpy change of the phase transition.

For example, the standard enthalpy of vaporization is the heat required to convert 1 mole of liquid into vapor at 1 bar.

Enthalpy is a state function, so its change does not depend on the path. The overall change stays the same whether a solid evaporates directly or melts first and then evaporates.

The reverse process experiences an equal enthalpy change but with an opposite sign.

The heat involved in a phase transition is proportional to the amount of material, and the proportionality constant is the enthalpy change of the transition, which characterizes the type of phase transition.

For the vaporization of water, this constant is the heat of vaporization, expressed in Joules per gram or Joules per mole.

Its high value reflects the strong hydrogen-bond network in liquid water, which requires substantial energy to break into individual molecules.