Physical Chemistry , 1st ed.

where the S(products) and S(reactants) represent the absolute entropy of the

chemical species involved in the process. If standard conditions apply, every

entropy term can have a degree symbol ° appended:

rxnS°

0

products

S°

0

reactants

S°

Changes in entropy for chemical processes can be considered using the above

Hess’s-law type of approach.

Example 3.7

Using the table in Appendix 2, determine the change in entropy for the fol-

lowing chemical reaction occurring at standard pressure and the stated tem-

perature:

2H 2 (g) + O 2 (g) →2H 2 O ()

Solution

From the table,S°[H 2 (g)] 130.7 J/molK,S°[O 2 (g)] 205.1 J/molK, and

S°[H 2 O, ()] 69.91 J/molK. Keeping in mind that the balanced chemical

reaction gives molar ratios of reactants and products, equation 3.27 yields

rxnS°[2 69.91] [2 130.7 205.1] J/K



0

products

S° 

0

reactants

S°

where the entropies of the products and reactants are labeled. The mol units

cancel because we are including the stoichiometry explicitly: 2 mol H 2 O as

products, and 2 mol H 2 and 1 mol O 2 as reactants. Evaluating:

rxnS°326.7 J/K

That is, during the course of the reaction, the entropy is decreasing by 326.7 J/K.

Does this make sense, in terms of entropy as a measure of the number of

accessible states? The balanced chemical reaction is showing 3 moles of gas

reacting to make 2 moles of liquid. It can be argued that a condensed phase

will have fewer accessible states than a gas will, and the actual number of

molecules is decreasing. Therefore, a decrease in entropy is understandable.

As with H, there are many times when Sneeds to be determined for a

process that occurs at different temperatures and pressures. Equation 3.18, or

82 CHAPTER 3 The Second and Third Laws of Thermodynamics

S

Total entropy of reactants

rxnS  Sproducts  Sreactants

Total entropy of products

Figure 3.8 Entropy can change for a reaction, just like enthalpy can change. In this case, the

entropy of the products is less than that of the reactants, so the rxnSis negative.

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