Mistake in solutions' manual for 19.34(b)

[Zellmer, Robert]

19.34(b) asks you how the enropy changes for the processes in
19.12.  The mistake is for 19.12(e).

19.12(e) is HCl(g) dissolving to give HCl(aq).  The solutions' manual
states the entropy for this process increases due to more motional
freedom (the HCl ionizes and you get ions).  This is incorrect.

It is true the HCl ionizes to give ions.  That process itself would have
a positive entropy change.  However, the HCl(g) dissolving in water
has a negative entropy change.  I've pointed out several times how
the entropy change for a gas dissolving in a liquid is negative. This
is a pretty large negative entropy change.  Negative enough to
outweigh the positive entropy change of the HCl ionizing to form
ions.  You can calculate the entropy change using the standard
entropy values in the back of the book (Appendix C, at 25.00 C,
298.15 K).

            HCl (g)  ---->     H+ (aq)   +   Cl- (aq)

S           186.69             0                    56.5

del(S) = [ 1 (0)  +  1 (56.5)] - 1 (186.69) = -130.2 J/K

This reaction is very spontaneous (at 25 C) even with this rather large
negative entropy change because it is very exothermic, del(H) = -74.9 kJ
and del(G) = -35.9 kJ (both calculated using the values in the table
in Appendix C).

Dr. Zellmer
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