out (Section 14-2) that the increase in disorder (entropy increase) that accompanies
mixing often provides the driving force for solubility of one substance in another.
For example, when one mole of solid NaCl dissolves in water, NaCl(s) nNaCl(aq),
the entropy (Appendix K) increases from 72.4 J/molK to 115.5 J/molK, or
S^0 43.1 J/molK. The term “mixing” can be interpreted rather liberally. For
example, the reaction H 2 (g)Cl 2 (g) n2HCl(g) has S^0 0; in the reactants, each
atom is bonded to an identical atom, a less “mixed-up” situation than in the prod-
ucts, where unlike atoms are bonded together.
5.Increase in the number of particles,as in the dissociation of a diatomic gas such as
F 2 (g) n2F(g). Any process in which the number of particles increases results in an
increase in entropy, Ssys 0. Values of S^0 calculated for several reactions of this
type are given in Table 15-6. As you can see, the S^0 values for the dissociation
process X 2 n2X are all similar for XH, F, Cl, and N. Why is the value given
in Table 15-6 so much larger for XBr? This process starts with liquidBr 2. The
total process Br 2 () n2Br(g), for which S^0 197.5 J/molK, can be treated as
the result of twoprocesses. The first of these is vaporization,Br 2 () nBr 2 (g), for
which S^0 93.1 J/molK. The second step is the dissociation of gaseous bromine,
Br 2 (g) n2Br(g), for which S^0 104.4 J/molK; this entropy increase is about the
same as for the other processes that involve onlydissociation of a gaseous diatomic
species. Can you rationalize the even higher value given in the table for the process
I 2 (s) n2I(g)?When water, H 2 O, and propyl alcohol, CH 3 CH 2 CH 2 OH (left) are mixed to form a solution
(right), disorder increases. S 0 for the mixing of any two molecular substances.
15-14 Entropy, S 627TABLE 15-6 Entropy
Changes for
Some
Processes
X 2 n2XReaction S^0 ( J/molK)H 2 (g)88n2H(g) 98.0
N 2 (g)88n2N(g) 114.9
O 2 (g)88n2O(g) 117.0
F 2 (g)88n2F(g) 114.5
Cl 2 (g)88n2Cl(g) 107.2
Br 2 ()88n2Br(g) 197.5
I 2 (s)88n2I(g) 245.3