Phase Transitions

A phase transition is a transformation from one phase to another at a specific transition temperature under given pressure conditions. It occurs spontaneously only when temperature and pressure conditions are met. During this process, both phases coexist, and the substance's temperature remains constant, making phase transitions isothermal processes.

During a phase transition, both the initial and final phases coexist in equilibrium. The chemical potentials of multiple phases of the same component are equal at equilibrium, meaning the system is at a stable state. All absorbed or released heat goes into breaking or forming intermolecular bonds rather than increasing molecular motion, so temperature remains constant.

Temperature and pressure changes alter the balance between intermolecular forces and molecular motion or spacing. These parameters directly govern whether a phase transition occurs. Higher temperatures increase molecular motion, favoring gas phases, while increased pressure favors denser phases like liquids or solids.

For an isothermal phase transition at equilibrium, the Gibbs free energy equals zero because chemical potentials of different phases are equal. By substituting this condition into the Gibbs free energy equation, the entropy change for the phase transition can be expressed as the ratio of the enthalpy of phase change to the transition temperature.

The amount of heat absorbed or released during a phase transition equals the product of the mass of the component and the enthalpy of phase change. This can also be expressed in terms of moles by replacing mass with the number of moles, allowing flexible calculation depending on available data.

Intermolecular forces determine the stability of each phase. Temperature and pressure changes alter the balance between these forces and molecular motion. When molecular motion overcomes intermolecular attractions, a substance transitions to a less ordered phase; when forces dominate, it transitions to a more ordered phase.

Entropy change quantifies the increase in disorder during a phase transition. By expressing entropy change as the ratio of enthalpy of phase change to transition temperature, we can predict spontaneity and understand the thermodynamic driving force behind phase transitions at specific conditions.