Principle of Braun and Le Chatelier

In chap. VI., p. 171, we showed that the heat of reaction Qp at constant pressure is equal to the change during the reaction in the heat content H of the system. This follows from the equation 

If the change in volume at constant pressure is slight, so that may be neglected, as is usually the case for reactions between solids and liquids, we have 

The difference between Qp and is most marked in the case of gas reactions involving a change in the number of molecules, since the change in volume of the reacting system is considerable under these circumstances. 

Equation (1), however, is not restricted in application to chemical reactions, but may be applied to any change in a system involving absorption or emission of heat at constant pressure. The quantity Qp is named differently according to the nature of the change. Thus it is called  

We shall use the term latent heat L as a generic name for these different heats. 

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