Relations for ΔH and ΔS for Liquids | Additional Documents & Tests for Civil Engineering (CE) PDF Download

Relations for ΔH and ΔS for Liquids

One starts with the generic equations for dH and dS developed in section 5.4.

              .......(5.20)

               .......(5.22)

As discussed in section 2.3, for liquids it is often simpler to use volume expansivity and isothermal compressibility parameters for computing thermodynamic properties of interest.  Thus from eqn. 2.3: 

Using eqn. 2.3 in 5.20 and 5.22 the following relations obtain:

               .......(5.66, 5.67)

The above equations may also be used to compute the properties of a compressed liquid state. Since the volumetric properties of liquids are very weakly dependent of pressure one can often use the saturated liquid phase properties as reference points and integrate the eqns. 5.66 to 5.67 (at constant temperature) to obtain enthalpy and entropy respectively. The relevant equations are:

               .......(5.68, 5.69)

In the last two equations, the molar volume V i may be set equal to Vi sat (liq.) , and the volume expansivity approximated to that at the saturated liquid point at the given temperature.