## Entropy change of reaction the entropy ruleofthumb

The entropy change due of reaction (9.5) that converts pure, unmixed reactants to pure, unmixed products, all at the standard pressure of 1 atm, is given by:

ASo = So (products) - So(reactants) = (ds° + cs°) - (as°A + bs°) (9.10)

Just as for the molar enthalpy, the molar entropies in Eq (9.10) can be referenced to the values for the normal physical state of the substances at Tref. so can be obtained from Eqs (3.22) and (3.23) for solids and liquids (see Fig. 3.26), and for gases, from:

Equation (9.10) becomes:

Although Eqs (9.11) applies in principle to gases as well as to condensed phases (as long as CP is constant), the absolute entropy of an ideal gas can be calculated from statistical thermodynamics; the molar entropy of an ideal gas consists of contributions from translation (i.e., motion of the molecules as a whole) and from internal modes of motion, such as rotation and vibration. The translational contribution is much larger than the others, which means that all gas species have approximately the same entropy. The translational entropy of an ideal gas at the standard pressure of 1 atm is given by:

where M is the molecular weight of the gas.

For solids, the third law is s = 0 at T = 0 K, so the absolute entropy can be calculated from: 