Calculation of First Law Quantities I

A reversible phase change at constant temperature and pressure involves the transfer of energy between phases without any chemical reaction.

The heat exchanged during this process is the latent heat of transition, and the work done equals the pressure multiplied by the volume change.

Under constant pressure, the enthalpy change equals the heat exchanged during the phase change, while the internal energy change accounts for both heat and work.

In constant-pressure heating without a phase change, the system expands or contracts as temperature changes, so it performs pressure–volume work.

The heat supplied equals the enthalpy change, which depends on the heat capacity at constant pressure and the temperature interval. This holds even for irreversible processes if initial and final pressures are equal.

In constant-volume heating without a phase change, the volume is fixed, so no work is done. All the heat supplied changes the internal energy, which depends on the heat capacity at constant volume and the temperature change, and applies to both reversible and irreversible processes.

The corresponding enthalpy change follows from the internal energy change combined with the change in the pressure–volume term.