Entropy Changes Accompanying Specific Processes

The entropy change during a phase transition, like freezing or boiling, is given by the ratio of the enthalpy of transition to the transition temperature.

In exothermic transitions, entropy decreases, while it increases in endothermic transitions.

At the transition temperature, the total entropy change is zero because energy is released from the system to the surroundings.

For a perfect gas expanding isothermally, the entropy increases logarithmically with volume ratio, rising quickly at small changes and gradually at larger ratios.

Entropy also increases during heating at constant pressure or volume, depending on the heat capacity and temperature.

It rises more rapidly at lower temperatures and more gradually at higher temperatures. Substances with higher heat capacities show steeper increases in entropy.

Also, mixing two or more ideal gases spontaneously within an isolated system at the same pressure and temperature results in an overall increase in entropy.

The total entropy of the mixture is measured by the number of moles and the mole fraction of components.