The third law of thermodynamics absolute entropies

Although thermodynamics makes no assumptions about the structure of matter, it is sometimes instructive to interpret thermodynamic results in terms of microscopic properties. For example, entropy may be considered as a measure of the degree of disorder of the elements of a system the more disorder the greater the entropy. From a molecular viewpoint, the disorder is associated with the molecules of a system. 

As in the case of energy or enthalpy, we are usually interested in differences in entropy rather than absolute values. However, in the case of entropy, it is possible to assign absolute values. This is a consequence of the third law of thermodynamics, which states that the entropy of perfect crystals of all pure elements and compounds is zero at the absolute zero of temperature (OK), Consequently, the absolute entropy of a substance at any temperature T is given by the change in the entropy of the substance in moving from OK to T. The absolute entropies of many substances (generally at 25°C and 1 atm - indicated by 5 for the molar absolute entropy under standard conditions) are 

The entropy changes associated with the forward reaction of the general chemical reaction (1.5) is given by 

Exercise 2.5, Calculate the change in entropy A5 at 298K associated with the reaction 

Note that the reaction decreases the number of molecules and the entropy decreases. In general, if a chemical reaction decreases the number of gaseous molecules, it decreases the entropy of a system conversely, if a chemical reaction increases the number of gaseous molecules, it increases the entropy of the system. 

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