Entropy of vaporisation

Most non-polar liquids have an entropy of vaporisation approximating to 85 J K 1 mol 1. 

Entropy of Vaporisation (DSvap) The vaporisation of a substance occurs at a definite temp, (i.e., its b.p) and is accompanied by absorption of heat (enthalpy of vaporisation). The change in entropy known as entropy of vaporisation is given by... 

TABLE 18.2 Molar enthalpy and entropy of vaporisation and boiling temperature of some simple liquids... 

Trouton s rule states that the molar entropy of vaporisation has for many kinds of liquids the same value of about 87 J/(K mol) ... 

ASvapjn is equal to ratio between the molar enthalpy of vaporisation and the boiling temperature, which leads to Eq. (18.10). The molar enthalpy and entropy of vaporisation and the boiling temperature of some simple liquids are presented in Table 18.2 Some liquids deviate sharply from the rule. This is often because these liquids have structure and so a greater amount of disorder is introduced when they evaporate. Examples are water and methanol due to hydrogen bonds between the molecules. (Atkins and De Paula, 2006)... 

CIF3 forms no corresponding compounds. Both BrFg and CIF3 have high entropies of vaporisation, suggesting there is association in the liquid. ICI3 is much less reactive than the other two AXg compounds. 

The problem that must be faced is how to evaluate 5gtr. On first consideration one might expect the deviation from Trouton s rule to be proportional to but unfortunately this describes the structure of the various liquids only at their respective boiling points, which may be markedly different from their structures at 25°C. Another approach would entail the use of the entropy of self-association of the various solvents in some inert solvent . This raises the question as to what is meant by an inert solvent , and in any event data are not presently available to make such calculations. A third method would be to calculate the difference in the experimentally measured entropy of vaporisation at 25°C and a statistical mechanical calculation in which hydrogen bonding and dipole-dipole interactions have been purposely neglected. To the knowledge of the authors no such calculations have been performed for these systems. 

Hence the entropy of vaporisation is the same for A and B at the same reduced temperature. It is then sufficient to determine the temperatures Ta and Tb which correspond to an arbitrarily chosen value of s. The ratio TbITa will be equal to (1 + 6). We may take different values for s if the theorem of corresponding states is satisfied the ratio TbJTa should remain constant. 

Because of the increase in entropy accompanying fusion and vaporisation, the standard molal entropies of gases are usually higher than those of liquids of similar molecular complexity, which are in turn higher than those of solids. As the figures show, the generalisation is approximate nevertheless it is very useful. 

The saturated vapour pressure of a-CdSe was measured in the temperature range 1016 to 1170 K using a gas flow method. Values for the entropy and the enthalpy of formation of a-CdSe at 298.15 were derived using the second law and an estimated heat capacity of a-CdSe. The experimental results were therefore re-evaluated by the review using both the second and the third law, the selected thermodynamic functions of selenium, the data for Cd in [89COX/WAG], the selected heat capacity of a-CdSe, and the selected entropy of a-CdSe in the case of the third law. The vaporisation was assumed to occur according to the reaction a-CdSe Cd(g) + The results were... 

Table 3.2 presents a selection of the most used thermodynamic options for phase equilibrium with suitable enthalpy and entropy methods. The accuracy of both phase equilibrium and enthalpy/entropy computation must be examined when using EOS models. For example, often a cubic EOS underestimates the enthalpy of vaporisation. In this case other methods are more accurate, as those based on three-parameters corresponding states law (Lee-Kesler, Curl-Pitzer, etc.). Mixtures rich in components with particular behaviour, as or CH, need special methods for accurate simulation. When binary interaction parameters for liquid activity models are absent, the UNI FAC predictive method may be employed. It is worth to note that UNIFAC is suitable only for exploratory purposes, but not for final design. When high non-ideal mixtures are involved at higher pressure then the combination of EOS with liquid activity models is recommended (see Chapter 6). 

The improvement in AT-values does not automatically ensure accuracy of other properties. In this example the estimation of liquid volume is poor for SRK, but acceptable for PR. Without interaction coefficients the prediction of the liquid volume is even better Note that when the volumetric properties are important, as in reservoir engineering, special equation of state or mixing rules should be applied, as Teja-Sandler EOS given in Chapter 5. The same observation holds for the enthalpy of vaporisation, which could be in serious error. Another method for enthalpy/entropy computation should be used, as for example based on the principle of corresponding states. 

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