conditions. We will find some straightforward ideas for understanding how
pressure and temperature changes affect the value of the equilibrium constant
and the extent of the reaction at equilibrium.5.2 Equilibrium
The rock on the side of the mountain in Figure 5.1a is not at equilibrium be-
cause, according to the laws of physics, it should spontaneously roll down the
mountain. On the other hand, the rock in Figure 5.1b is at equilibrium because
we don’t expect any additional, spontaneous change. Rather, if we want to
change this system, we will have to put work into the system, but then the
change is not spontaneous.
Now consider a chemical system. Think about a 1-cm^3 cube of metallic
sodium in a beaker of 100 mL of water. Is the system at equilibrium? Of
course not! There ought to be a somewhat violent, spontaneous chemical re-
action if we try to put a cubic centimeter of sodium in water. The state of
the system as described originally is not at chemical equilibrium. However,
it’s not a question of gravitational potential energy now. It is a question of
chemical reactivity. We say that this Na-in-H 2 O system is not at chemical
equilibrium.
The sodium metal will react with the water (which is in excess) via the fol-
lowing reaction:
2Na (s) 2H 2 O () →2Na(aq) 2OH(aq) H 2 (g)
Once that reaction is over, there will be no further change in the chemical
identity of the system, and the system is now at chemical equilibrium. In a
sense, it is very much like the rock and mountain. The sodium in water rep-
resents a rock on the side of a mountain (Figure 5.1a), and the aqueous
sodium hydroxide solution (which is an accurate description of the products
of the above reaction) represents the rock at the bottom of the mountain
(Figure 5.1b).
Consider another chemical system, this one a sample of water, H 2 O, and
heavy water, D 2 O, in a sealed container. (Recall that deuterium, D, is the
isotope of hydrogen that has a neutron in its nucleus.) Is this a description
of a system at equilibrium? Interestingly, this system is notat equilibrium.
Over time, water molecules will interact and exchange hydrogen atoms, so
that eventually most of the molecules will have the formula HDO—a result
that can easily be verified experimentally using, say, a mass spectrometer.
(Such reactions, called isotope exchange reactions, are an important part of
some modern chemical research.) This process is illustrated in Figure 5.2.
Other processes like precipitation of an insoluble salt from aqueous solution
are also examples of equilibrium. There is a constant balance between ions5.2 Equilibrium 119(a) (b)
Figure 5.1 (a) A rock on the side of a mountain represents a simple physical system that is
not at equilibrium. (b) Now the rock is lying at the bottom of the mountain. The rock is at its
minimum gravitational potential energy. This system is at equilibrium.