A quantitative measures of
DS and the effect
of T on DS
At constant pressure (standard conditions),
Trouton's rule states that most liquids have approximately the same value for the entropy of vaporisation as illustrated in the table below (about 85 J K-1 mol-1). Exceptions may be due to:
(i) Liquids where the molecules have an 'extra' amount of order in them, due for example, hydrogen bonding (e.g. water). In such cases, the DS is larger than the typical 85 J K-1 mol-1 for systems having the 'normal' amount of disorder in the liquid phase.
(ii) Very light molecules (e.g. methane) are difficult to excite to rotation. This means that the gas phase has 'less' disorder then in 'normal' cases. In such cases, the DS is lower than the typical 85 J K-1 mol-1 for systems having the 'normal' amount of disorder in the gas phase.
(2) Isothermal expansion of an ideal gas
For an siothermal process, DU = 0, i.e.:
(3) Changes in entropy during the heating of an ideal gas - effects of temperature on entropy.
When we heat an ideal gas we have:
(a) Assuming that over the range (To,Tf) cp is constant, we have: