Latent Heats of Pure Substances

Wlien a pure substance is liquefiedfrom the solid state or vaporized from tlie liquid at constant pressure, no change in temperature occurs however, tire process requires the transfer of a finite amount of heat to the substance. These heat effects are called the latent heat of fusion and tire latent heat of vaporization. Similarly, there are heats of transition accompanying die change of a substance from one solid state to another for example, the heat absorbed when rhombic crystalline sulfur changes to tire monoclinic structure at 368.15 K (95°C) and 1 bar is 360 J for each g atom. 

The characteristic feature of all these processes is the coexistence of two phases. According to the phase rule, a two-phase system consisting of a single species is univariant, and its intensive state is determined by the specification of just one intensive property. Tims the latent heat accompanying a phase change is a function of temperature only, and is related to other system properties by an exact thermodynamic equation  

A V = volume change accompanying the phase change p = vapor pressure 

The derivationof this equation, known as the Clapeyronequation, is given in Chap. 6. 

When Eq. (4.11) is applied to the vaporization of a pure hquid, dP jdT is the slope of the vapor pressure-vs.-temperature curve at the temperature of interest, A V is the difference between molar volumes of saturated vapor and saturated liquid, and AH is the latent heat of vaporization. Thus values of AH may be calculatedfrorn vapor-pressure and volumetric data. 

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