Calculate Latent Heat: L = (S_after - S_before)T

[KFC]

We could use the change of entropy before and after eh phase changed to calculate the latent heat

$$L = (S_{after} - S_{before})T$$

and in some case, if we know the relation of pressure and temperature, we can also apply Clausius-Clapeyron relation to calculate the latent heat.

I wonder if there is any usual way to calculate the latent heat for phase change. If the pressure is unchanged we see that then change of enthalpy is same the

$$\Delta H= T\Delta S$$

So can I say the latent heat of phase change b/w any two phase for any system is equal to the change of enthalpy of this system during the phase change?

 

[olgranpappy]

Yes, it's okay since the transition occurs at constant temperature and pressure.

$$L\equiv\Delta Q = T\Delta S=\Delta H - V\Delta P=\Delta H\;.$$

 

[astrokreng]

I would first commend you on your understanding of the relationship between entropy, temperature, and latent heat. Your approach of using the change in entropy before and after the phase change is a valid method for calculating latent heat. Additionally, the Clausius-Clapeyron relation is another useful tool for calculating latent heat, especially when the pressure is known.

To answer your question, the change in enthalpy during a phase change is indeed equal to the latent heat of that phase change, as long as the pressure remains constant. This is because the enthalpy is a measure of the total energy of a system, including both the internal energy and the work done by or on the system. During a phase change, there is no change in temperature, so the change in internal energy is zero. Therefore, the change in enthalpy is solely due to the work done by or on the system, which is equal to the latent heat.

However, it is important to note that this relationship is only true when the pressure remains constant. If the pressure changes during the phase change, then the change in enthalpy will not be equal to the latent heat. In this case, the change in enthalpy will also include the work done by or on the system due to the change in pressure.

In conclusion, your understanding of calculating latent heat using the change in entropy is correct, and the relationship between the change in enthalpy and the latent heat is valid as long as the pressure remains constant.