Page 52 - Basic physical chemistry for the atmospheric sciences

38 Basic physical chemistry

rate very slightly, the relative humidity just above its surface would
increase above 1 1 0% [see Eq. (2.56) ] . This would cause water vapor
to diffuse away from just above the surface of the droplet to the
(
drier 1 10% relative humidity) air in the enclosure. The droplet would
therefore continue to evaporate in order to maintain the relative hu­
midity just above its surface. A similar argument will show that if the
droplet should grow very slightly, it would continue to grow. Hence, a
droplet with radius . 0 1 0 µ,m is in unstable equilibrium in an environ­
0
ment with a relative humidity 1 1 0%. This is an example of a physical
equilibrium that is controlled by chemical potentials.

Exercises
2. 10. Answer, interpret, or explain the following in light of the
principles discussed in this chapter.
(a) An equilibrium chemical state represents a compromise
between the drive for molecules to assume a state of
minimum energy and the drive toward a state of maxi­
mum entropy (i. e . , maximum molecular chaos). Con­
sider, for example, the reaction H2(g) � 2 H (g).
(b) The development of life (a relatively ordered state) on
Earth does not contradict the second law of thermody­
namics.
(c) The work done by a system in a reversible process is
greater than the work done by the system in an irrevers­
ible proce s between the same two states.
s
(d) The second law of thermodynamics is sometimes ex­
pressed in the form: "The entropy of the universe is
continua l y increasin . "
g
l
(e) Water can freeze spontaneously below 0°C even though
this produces a decrease in the entropy of the system.
(f) Some exothermic chemical reactions do not occur spon­
taneously.
(g) List some spontaneous processes that are not exo­
c
thermi .
(h) A l l chemical compounds will dissociate into the ele­
ments at sufficiently high temperatures. (Hint: Consider
n
the Gibbs-Helmholtz equatio . )
t
(i) Neither the heat absorbed (or released) nor h e work