After this lesson, you should be able to...
- Predict the effect of temperature on solubility of solids and gases and read a plot of solubility versus temperature.
- Predict the effect of pressure on solubility of solids and gases.
- Use Henry’s Law to calculate the solubility of gases.
We will revisit section 14.4 in the ebook relating to the solubility of gases.
The solubility of a gas in a solution is affected by the pressure. Pressure
does not, however, affect the solubility of liquids and solids in solutions.
The solubility of a dissolved gas is proportional to the partial pressure of
that gas (
P_g
) above the solution. This relationship is known as Henry’s Law, which is
C_g=kP_g
, where
C_g
is the concentration (i.e.,
solubility) of a gas (“g” is the identity of the gas),
P_g
is the partial pressure of the gas, and
k is the proportionality constant. The proportionality constant (k) varies
from one gas to the next, and its value is related to the strength of IMFs
that the gas can form with the solvent. Gases that form stronger IMFs with
water will have larger Henry’s law constants, thus larger amounts of these
gases will dissolve at a fixed partial pressure compared to weak IMF gases. In
other words gases with stronger IMFs will be more soluble in water. This is
shown by the graph on in Figure 1. The graph plots solubility versus partial
pressure of the gas, with each line representing a different gas. The slope of
the line, which equals the Henry’s law constant, is larger for gases that form
stronger IMFs.
Henry’s law explains why carbonated beverages release large amounts of gas
when they’re opened. These beverages are packaged with pressurized carbon
dioxide, with a typical can of soda having an airspace consisting of
CO_2
at a partial pressure of 1.5-2.0 atm.
Under this high partial pressure, the
CO_2
is quite soluble in the beverage. When
the soda is opened, however, the airspace in the can (which was previously
pressurized
CO_2
) is replaced with air at about 1 atm total pressure. Air only contains a
tiny amount of
CO_2
, so the partial pressure of
CO_2
goes from
1.5-2.0 atm to nearly zero. As such, the solubility of
CO_2
in the soda decreases dramatically upon
opening, resulting in a supersaturated solution. Since CO2 is supersaturated
in the opened soda we observe a net movement (in the form of bubbles and fizz)
of gas out of the beverage.