Latent heat Thermodynamics

Latent heat means the heat which remains latent with no visual effect during phase change.

Let us expand this statement. Latent heat is the heat absorbed or released by a substance during a phase change without a temperature change. It is called latent heat because the heat remains "hidden" or "latent" as it is not reflected in a temperature change, even though the molecules within the substance are gaining or losing energy.

When a substance undergoes a phase change, such as melting or vaporization, the added or removed heat energy is used to break or form intermolecular bonds, rather than increasing the average kinetic energy of the molecules, which would result in a temperature change.

For example, when water is heated, it eventually reaches its boiling point and begins to vaporize. During this phase change, the added heat energy is used to break the hydrogen bonds holding the water molecules together, allowing them to escape into the gaseous phase. This added heat energy is the latent heat of vaporization.

Why and when the phase change occurs?

 Every substance wants to stay in a state with minimum free energy and in a maximum stable condition. A phase change occurs when the substance is thermodynamically unstable and far from its internal thermodynamic equilibrium or minimum free energy. When the conditions in which the substance exists (e.g. temperature, pressure) result in the current phase being less stable than a different phase, a phase change will occur to move the substance towards a lower energy state or equilibrium. This can lead to transitions from solid to liquid, liquid to gas, or other phase changes depending on the specific conditions and properties of the substance.

Gibbs free energy change in phase transition

Gibbs free energy change, often denoted as ΔG, is a thermodynamic quantity that represents the maximum amount of reversible work that can be performed by a thermodynamic system at constant temperature and pressure.

 When there is no visible effect of heat, it means that the temperature of the system is not changing, and it is likely that the Gibbs free energy change is zero. In a system where the Gibbs free energy change is near zero, it indicates that the system is at thermodynamic equilibrium. This means that the system is in a state where the potential to do work is balanced, and the system is stable with no net change occurring. At thermodynamic equilibrium, the Gibbs free energy change is near zero, and there is no spontaneous change in the system. Why phase change occurs at a constant temperature at a given pressure?

Gibbs phase rule

You can predict what will stay constant and when by the Gibbs phase rule.

F=C−P+2 

which relates the degrees of freedom F (meaning intensive thermodynamic variables you can change) to the number of components C and number of phases P. When there is a single component (C=1) phase change (meaning P=2), there is only one degree of freedom. If you fix the pressure that completely sets the state and you have no freedom to change the temperature.

Pressure– Enthalpy [P-H] diagram is a powerful tool

A P-H diagram is a graphical representation of the thermodynamic properties of a substance. It plots pressure on the y-axis and enthalpy on the x-axis and can be used to visualize phase changes and other thermodynamic processes. When it comes to latent heat, the P-H diagram is particularly useful because it shows the heat transfer that occurs during a phase change. For example, when a substance undergoes a phase change from a liquid to a vapor, the latent heat of vaporization is the amount of heat that must be added to the substance at constant pressure for the phase change to occur. By looking at a P-H diagram, one can easily see the change in enthalpy that occurs during this phase change, as well as how the pressure and temperature of the substance change. This makes it a powerful tool for understanding latent heat and other phase-change properties of substances.

P-H diagram of water

Image credit: Google

Explanation

On the left of the dome, it is a water region. On the outline on the left side, it is the saturated water region or line. It has only sensible heat. At 1 bar / 100 degc it is 419 KJ/kg

Let us consider 1 kg water at 0 degc The sensible heat in saturated water with dryness fraction = 0

Q at 1 bar = m x Cp x dt m = 1 kg Cp = 4.187 KJ/kg / degc Q = 1 X 4.19 X 100 KJ/kg = 419 KJ/kg.

This is the enthalpy of saturated water at 100 degc when the dryness fraction of water is =0

You will find the same value on the P-H diagram. Inside the dome, it is the saturation region. Water at 0 dryness fraction converts to steam at constant temperature and pressure by breaking intermolecular bonds.

On the right outline, it is the saturated steam region. Here the dryness fraction has increased to 1. The steam is a dry saturated steam on this line. The width of the dome at 1 bar / 100 degc is the heat of vaporization, 2257 KJ/kg. This is the latent heat. The effect of heat is not visible here. Please refer to the P-H diagram. On the right-side outline which is the saturated steam line, the total energy is 419+ 2257 = 2676 KJ/kg. This is the total enthalpy of saturated steam at 1 bar. On the x-axis, you can find all enthalpy values