Energy of Phase Changes

Phase changes involve an alteration in the physical state of matter, for example, from solid to liquid or gas. During phase changes, energy is either absorbed or released. This process is described as either endothermic or exothermic, respectively.

Phase Transitions and Energy

During a phase transition, the substance either absorbs (endothermic) or releases energy (exothermic).
For example, transforming from a solid to a liquid (melting), or a liquid to a gas (evaporation) are endothermic processes, as they involve the absorption of energy to break intermolecular bonds.
On the contrary, changes from a gas to a liquid (condensation), or from a liquid to a solid (freezing) are exothermic processes where energy is released upon the formation of intermolecular bonds.

Heat of Fusion and Heat of Vaporisation

The energy needed to change a substance from a solid to a liquid at its melting point is termed the heat of fusion. This is an endothermic process as energy is ingested.
The energy required to change a substance from a liquid to a gas at its boiling point is termed the heat of vaporisation. This, too, is endothermic as it demands energy absorption.
Conversely, when a gas condenses into a liquid or a liquid freezes into a solid, that energy is liberated or given off, thereby making these processes exothermic.

Enthalpy and Phase Changes

Enthalpy (H) is a measure of the total energy of a thermodynamic system.
The enthalpy change during phase changes (such as fusion, vaporisation, or sublimation) is equivalent to the energy absorbed from or released to the surroundings at constant pressure.
This change in enthalpy, typically measured for one mole of substance, is termed the molar heat of fusion or the molar heat of vaporisation.

Phase Diagrams

A phase diagram is a graphical representation of the phase behaviour of a substance.
The axes of these diagrams represent temperature and pressure. The diagram shows the phase (solid, liquid, or gas) in which a substance exists under different conditions of temperature and pressure.
Phase diagrams are crucial for understanding the conditions under which phase changes take place and the associated energy changes.