Heat and Thermodynamics

6.3 Internal and External Latent Heats

The heat that is supplied to a liquid to change it into vapour is spent in two ways: a portion is used up to change it to vapour or to change the internal energy of the liquid and the remainder is spent in doing external work against the superincumbent pressure in changing the volume. The former is called the true latent heat or internal latent heat L_i. and the latter is called the external latent heat. If v_vap and v_liq are the specific volume of the vapour and of the liquid, respectively, then the external latent heat is

 

L_e = p(v_vap– v_liq)

 

where p is the superincumbent pressure. The total latent heat is

 

L = L_e + L_i

 

so that

 

L_i = L – L_e = L –p(v_vap – v_liq)     (6.17)

 

Now, by Eq. 6.9