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7.9 Key Thermodynamic Properties 151

TABLE 7.2 Spectroscopic Entropy
Minus Calorimetric
Entropy

Entropy Excess,
Substances J Ki mot i
CO 4.64
N 20 4.77
NO 3.14
H2O 3.39

about each of the N oxygens, only (6/16) of the possible configurations are likely. For the
crystal as a whole, this means that only (6/16)N of the 22N configurations are to be con-
sidered in calculating W:

W = 22N(6/16t =(3/2t. [7.33)
Putting this W into formula (5.55) yields the residual entropy
I
So =kln(3/2t =.Nkln(3/2) = Rln(3/2) = 3.37 JK- mol-I, [7.34}
which checks the observed So in table 7.2.

Z9 Key Thermodynamic Properties
Data from many experiments on common substances are summarized in tables 7.3
and 7.4.
The entropies for the gases have been corrected to the ideal gas state. Also, allowances
have been made for any frozen-in disorder. However, the contributions from mixing dif-
ferent isotopes of an element and the contributions from degeneracies arising from
nonzero nuclear spins have been omitted since they do not contribute to Scalor and they
cancel out in common chemical reactions.
The enthalpy of formation tllflf is the heat of reaction for the formation of 1 mole of
the compound in its standard state from the elements in their standard states. From the
tabulated entropies and enthalpies, standard Gibbs energy changes can be calculated
with the formula
[7.35}
which follows from definition (5.79). They can also be calculated from the listed Gibbs
energies of formation tlGo f.
The entropy change in a reaction equals the entropies of the products minus the
entropies of the reactants. Similarly, the enthalpy change equals the enthalpies of for-
mation of the products minus the enthalpies of formation of the reactants. And the Gibbs
energy change equals the Gibbs energy of formation of the products minus the Gibbs
energy of formation of the reactants.

ExampleZ6
Using tables 7.3 and 7.4, calculate ~, tllfl, and !J.Go for the reaction

NO (g) +.! O 2 (g) ~ N0 2 (g).
2