12.4 - Solution Equilibrium and
Factors Affecting Solubility
Solution Equilibrium
Stages in the Equilibrium process of a Solution:
Initial - The solute begins to dissolve in the solvent
Dissolving - The Solution becomes more concentrated, and
the solute may recrystallize (Rate of dissolution> Rate of
recrystallization)
Dynamic Equilibrium - The Solution is highly concentrated
and the Rate of recrystallization = Rate of dissolution
Saturation
Solutions in Dynamic Equilibrium are Saturated, any more
added solute will not dissolve.
Solutions with less than the Equilibrium amount of solute are
Unsaturated, and added solute will continue to dissolve.
Solutions containing more that the Equilibrium amount of
solute are Supersaturated. These solutions are unstable
and will rapidly recrystallize if disturbed. Solutions like this
can only be attained by manipulating solubility.
Temperature Dependence of the
Solubility of Solids
In general, the solubility of solids in water increases with
temperature. Supersaturated solutions are made by heating
water and adding more solute than could be dissolved at
room temperature, then letting it cool.
This technique is used for purification
because the resulting crystal rejects
impurities.
Factors affecting the Solubility of Gases
in Water
The solubility of gases in liquids is affected by both
temperature and pressure.
As temperature increases solubility decreases.
This is why small air bubbles form when heating water. The
gases become less soluble and come out of solution.
Factors affecting the Solubility of Gases
in Water
The solubility of gases in liquids increases with pressure.
This is why soda fizzes when the container is opened. The
pressurized carbon dioxide escapes, lowering the solubility of
the carbon dioxide dissolved in the soda and letting it escape
solution.
Henry’s Law
The above image depicts the increase of pressure on a
system in equilibrium. The increase in pressure causes an
increased rate of dissolution of gas molecules, causing an
increase of gas molecules in solution until a new equilibrium
is formed.
Henry’s Law
We can use Henry’s Law to determine the solubility of gases
with an increase of pressure
Henry’s Law:
Sgas=kHPgas
Where: Sgas is the solubility of the gas (in M)
Pgas is the partial pressure of the gas (in atm)
kH
is the Henry’s Law constant of the gas
which depends on the solute, solvent, and temperature (in
M/atm)
Henry’s Law
Practice Problem!
What pressure of carbon dioxide is required to keep the
carbon dioxide concentration in a bottle of soda at 0.12 M at
25℃?
Answer = 3.5 atm
12.5 - Expressing Solution
Concentration
Concentration
Concentration is a measure of how much solute is present in
a solution relative to the amount of solvent.
A Dilute solution has a relatively small amount of solute
A Concentrated solution has a relatively large amount of
solute
Measurements include molarity, molality, parts by mass,
parts by volume, mole fraction, and mole percent
Measurements
Uses
The different measurements of concentration are useful for
different purposes.
Molarity for making and diluting solutions
Molality for comparing concentrations at different
temperatures
Parts by mass for comparing by weight
Parts by volume for comparing liquids
Mole fraction and Mole percent for largely varying ratios
Practice Problems
A solution is prepared by dissolving 17.2 g of ethylene glycol
(C2H6O2) in 0.500 kg of water. The final volume of the
solution is 515 ml. Calculate your answer in Molarity
Answer = 0.538 M
0.005 moles of salt (NaCl) are added to a 20 L barrel
of water (H2O). Assuming the density of water is
1g/mL, what is the concentration of the solution in ppm
by mass?
Answer = 14.61 ppm