More about equilibrium is discussed in Chapter 10.REMEMBER
Le Châtelier’s Principle. It will occur again in Chapter 10.In the discussion above, if the 20°C system is heated to 30°C, the number of
gas molecules will be increased while the number of liquid molecules will be
decreased:
Heat + H 2 O (l) H 2 O (g)The equation shifts to the right (any similar system that is endothermic shifts
to the right when temperature is increased) until equilibrium is reestablished at
the new temperature.
The molecules in the vapor that are in equilibrium with the liquid at a given
temperature exert a constant pressure. This is called the equilibrium vapor
pressure at that temperature.
BOILING POINT
The vapor pressure-temperature relation can be plotted on a graph for a closed
system. (See Figure 24.) When a liquid is heated in an open container, the liquid
and vapor are not in equilibrium, and the vapor pressure increases until it
becomes equal to the pressure above the liquid. At this point the average kinetic
energy of the molecules is such that they are rapidly converted from the liquid to
the vapor phase within the liquid as well as at the surface. The temperature at
which this occurs is known as the boiling point. Notice that in this graph, water’s
normal boiling point is at 760 mm Hg pressure and 100°C temperature.
TIP
Boiling point is defined as the temperature at which the liquid’s vapor pressure equals the
atmospheric pressure.