Another example is the cooling effect when a salt such as ammonium
chloride is dissolved in water (right). The reaction:
absorbs enough heat to chill its surroundings, yet we do not expect
an ammonium chloride solution to separate spontaneously into salt
crystals and pure water just because in this direction the reaction
gives off heat.
An even simpler example is the vaporization of water. The heat
of vaporization at room temperature is DH0=
+10.5 kcal mole-1. Heats of vaporization
for all liquids are positive because energy is required to break
the attractive forces between molecules in the liquid and create
a gas. Yet evaporation frequently is spontaneous. If only heat-yielding
processes were spontaneous, then all gases in the universe would
condense to liquids, all liquids would freeze to crystalline solids,
and the world would be nothing but rock and ice. This obviously
is not so, and energy obviously cannot be the only factor in making
chemical reactions spontaneous.
Chloride crytals dropped into water absorb heat from the surroundings
as they dissolve. The tumbler feels cold to the touch.