Role of entropy in Liquid-Vapor phase change

A liquid heats up to the point at which its specific heat permits it to do so. The liquid loses its ability to absorb heat once it has no heat capacity to absorb heat further and the sensible temperature rise stops.

The liquid is now fully saturated with heat. At this point, water typically has a sensible heat of 419 KJ/kg. The saturated liquid starts to transition from dryness fraction X = 0 to X = 1 as heating continues, eventually turning into saturated vapor.

The following thermodynamic processes take place as it does so:

-The liquid starts to break down the intermolecular bonds producing vapor and generating potential energy. This is the heat at which the liquid vaporizes. It is called the heat of vaporization.

-Thermodynamic PV work causes the vapor to expand

-As vapor expands and with the onset of turbulence there is the generation of entropy

-thermal equilibrium between liquid and vapor phases establish with no further rise in temperature

In summary. the heating of saturated water at constant temperature and pressure until it becomes saturated dry vapor involves the following

-Generate heat of vaporization

-Generate entropy

-The system goes into thermodynamic equilibrium

Gibbs equation

dG = dH - TdS

According to the above equation, enthalpy gain (dH) decreases as entropy (dS) increases. As a result, dG, the Gibbs free energy change moves to near zero as the entropy increases. At dG = 0,?the system achieves a thermal equilibrium between the two phases.

The saturated liquid phase transitions to saturated vapor at X = 1 (where X is the dryness fraction), the point at which all intermolecular bonds break.

Recall that the heat required to change a saturated liquid into a saturated vapor goes to?(1) the vaporization process and (2) the PV thermodynamic work that causes the vapor to expand.

The third consumption of heat goes to generate entropy, Q/T.

Here, Q is the enthalpy and T is the temperature. In the steam table, the entropy losses are shown separately. For water, the heat content or the enthalpy at this point is about 2260 KJ/kg. This is called the latent heat of the liquid.