Solubility equilibrium
is base on the assumption that solids dissolve in water to give the basic
particles from which they are formed.
· Molecular solids dissolve
to give individual aqueous molecules.
· Ionic solids dissociate to give their
respective positive and negative ions:
The ions in formed from
the dissociation of ionic solids can carry an electrical current. Salt
solutions, therefore, are good conductors of electricity. Molecular solids,
however, do not dissociate in water to give ions, so no electrical current can
be carried.
1.
The ratio of the maximum
amount of solute to the volume of solvent in which this solute can dissolve.
1.
Generally expressed in
two ways:
1.
grams of solute per 100
g of water
2.
moles of solute per
Liter of solution
|
Soluble: Dissolve - Do
NOT form a solid precipitate.
|
|
Insoluble: Do NOT
Dissolve - Do form a solid precipitate.
|
- A salt is considered soluble
if it dissolves in water to give a solution with a concentration of at
least 0.1 M at room temperature.
- A salt is considered
insoluble if the concentration of an aqueous solution is less than
0.0001 M at room temperature.
- Salts with solubilities
between 0.0001 M and 0.1 M are considered to be slightly soluble.
Salts that have extremely low solubilities
dissociate in water according to the principles of equilibrium. For example,
the reaction for the dissociation of the salt AgCl is:
The reverse reaction for the dissolving of the salt would be the
precipitation of the ions to form a solid:
The system has reached equilibrium when the rate at which AgCl
dissolves is equal to the rate at which AgCl precipitates.
- Saturated solution - Contains the maximum
concentration of ions that can exist in equilibrium with the solid salt
at a given temperature.
The equilibrium reaction for the dissociation of
AgCl is:
Solubility product equilibrium constant (Ksp) - The product of the equilibrium concentrations
of the ions in a saturated solution of a salt. Each concentration is raised to
the power of the respective coefficient of ion in the balanced equation.
- NOTE: There is no denominator in
the solubility product equilibrium constant. The key word to remember is
PRODUCT which can remind you that you should have a
multiplication (or product) of the concentrations of the ions. The
reason that the solid reactant is not written is because its
concentration effectively remains constant.
For example, the solubility product equilibrium
constant for the dissociation of AgCl is:
Let's try another example of a solubility product equilibrium
constant. Consider the reaction for the dissociation of CaF2 in
water:
The solubility product equilibrium constant for this reaction
would be the product of the concentration of Ca2+ ion and the
concentration of the F- ion raised to the second power (squared):
NOTE: Unlike Ka
and Kb for acids and bases, the relative values of Ksp
cannot be used to predict the relative solubilities of salts if the salts being
compared produce a different number of ions.
The solubility product is literally the product of the
solubilities of the ions in units of molarity (mol/L)
- Ksp can be
calculated from the solubility of a salt. Conversely, the solubility of
a salt can be calculated from Ksp.
Let's try an example calculation problem to
demonstrate the relationship between the solubility and the solubility product
of a salt.
1) Calculate the solubility of CaF2 in g/L (Ksp
= 4.0 x 10-8)
First, write the BALANCED REACTION:
Next, set up the SOLUBILITY PRODUCT EQUILIBRIUM EXPRESSION:
In the above equation, however, we have two unknowns, [Ca2+]
and [F-]2. So, we have to write one in terms of the other
using mole ratios. According to the balanced equation, for every one mole of Ca2+
formed, 2 moles of F- are formed. To simplify things a little, let's
assign the the variable X for the solubility of the Ca2+:
If we SUBSTITUTE these values into the equilibrium expression,
we now only have one variable to worry about, X:
We can now SOLVE for X:
We assigned X as the solubility of the Ca2+ which is
equal to the solubility of the salt, CaF2. However, our units right
now are in molarity (mol/L), so we have to convert to grams:
Now, let's try to do the opposite, i.e., calculate the Ksp
from the solubility of a salt.
First, write the BALANCED REACTION:
Next, set up the SOLUBILITY PRODUCT EQUILIBRIUM EXPRESSION:
It is given in the problem that the solubility of AgCl is 1.3 x 10-5.
Since the mole ratio of AgCl to both Ag+ and Cl- is 1:1,
the solubility of each of the ions is equal to the solubility of AgCl. SUBSTITUTE the solubility given into the equilibrium expression to get Ksp:
Solute and Solvent
Structure/Polarity
Solute molecules are
held together by certain intermolecular forces (dipole-dipole, induced
dipole-induced dipole, ion-ion, etc.), as are molecules of solvent. In order
for dissolution to occur, these cohesive forces of like molecules must be
broken and adhesive forces between solute and solvent must be formed.
The solubility of a drug
in a given solvent is largely a function of the polarity of the solvent.
Solvents may be considered polar, semi-polar or non-polar. Polar solvents will
dissolve ionic and other polar solutes (i.e. those with an asymmetric charge
distribution [like dissolves like]), whereas, non-polar solvents will dissolve
non-polar molecules. Semi-polar solvents (eg. alcohols and ketones) may induce
a certain degree of polarity in non-polar molecules and may thus act to improve
the miscibility of polar and non-polar liquids. The relationship between
polarity and solubility may be used in practice to alter the solubility of a
drug in a pharmaceutical solution.
One approach is to alter
the polarity of the solute by shifting it between its molecular (undissociated)
and ionic (dissociated) states. A shift toward the ionic form improves
solubility of the solute in water and other polar solvents. A shift toward the
molecular species improves solute solubility in non-polar solvents. Such shifts
may be produced by altering the pH of the solution (or using the salt form of
the compound).
Another approach is to
mix solvents of different polarities to form a solvent system of optimum
polarity to dissolve the solute. Such solvents must, obviously, be miscible.
This method is referred to as solvent blending or cosolvency and uses the
dielectric constant as a guide to developing the cosolvent system. Since many
solvents may be toxic when ingested, most solvent blends are limited to
mixtures containing water, ethanol, glycerin, propylene glycol, polyethylene
glycol 400 or sorbitol solution. The list is somewhat expanded for solutions
for external application.
The dielectric constant
(δ) of a compound is an index of its polarity. A series of solvents of
increasing polarity will show a similar increase in dielectric constant.
Part 1 : "Like dissolves
Like" - To dissolve or not to dissolve
To dissolve or not to
dissolve, that is the question. ''Like dissolves like'' is an expression used
by chemists to remember how solvents work. It refers to ''polar'' and
''non-polar'' solvents and solutes. For instance, water is polar while oil is
non-polar. Therefore, water will not dissolve oil. In other words, they are
immiscible. Meanwhile, ionic salt such as sodium chloride is ionic (which is
considered to be extremely polar). Since like dissolves like, that means polar
dissolves polar, thus the ionic salt is soluble in water.
However, how do I know
whether a molecule is polar? There are two conditions that you need to consider
before you determine if the molecule is polar.
Condition 1: Is the bond polar? As long as the
bonding atoms are different, they will have different electronegativity, which
leads to a dipole moment in the bond i.e. the bond is polar. For example, H-Br
is a polar bond whereas Br-Br is not.
Condition 2: What is the molecular geometry?
Consider the molecule CO2, which is carbon
dioxide. The displayed structure looks like this i.e. O=C=O. The C=O bond is polar because the bonding
atoms are different and they have different electronegativity. However, the molecule is linear in shape. As
a result, the dipoles (treated as vectors) will cancel out each other. In the
linear molecules, these dipole moments have the same magnitude, but since they
are pointing in opposite directions, there is cancellation of dipoles that
renders the molecule non-polar.
Now, what about water? Isn't water a non-polar
molecule? The answer is NO! If the water molecule is linear too, then it will
not be polar. However if you were to draw the dot and cross diagram for water,
you will find that the central oxygen atom has 2 lone pairs of electrons.
As a result, the molecular geometry is
actually bent. As a result, there is a collective effect of the individual
polar O-H bond that give rise to a net dipole moment, and hence water is a
polar molecule.
Temperature and Pressure
Effects on Solubility
Effect of Temperature on
Solubility:
The solubility of
solutes is dependent on temperature. When a solid dissolves in a liquid, a
change in the physical state of the solid analogous to melting takes place.
Heat is required to break the bonds holding the molecules in the solid
together. At the same time, heat is given off during the formation of new
solute -- solvent bonds.
CASE I: Decrease in
solubility with temperature:
If the heat given off in
the dissolving process is greater than the heat required to break apart the solid,
the net dissolving reaction is exothermic (energy given off). The addition of
more heat (increases temperature) inhibits the dissolving reaction since excess
heat is already being produced by the reaction. This situation is not very
common where an increase in temperature produces a decrease in solubility.
CASE II: Increase in
solubility with temperature:
If the heat given off in
the dissolving reaction is less than the heat required to break apart the
solid, the net dissolving reaction is endothermic (energy required). The
addition of more heat facilitates the dissolving reaction by providing energy
to break bonds in the solid. This is the most common situation where an
increase in temperature produces an increase in solubility for solids.
The use of first-aid
instant cold packs is an application of this solubility principle. A salt such
as ammonium nitrate is dissolved in water after a sharp blow breaks the
containers for each. The dissolving reaction is endothermic - requires heat.
Therefore the heat is drawn from the surroundings, the pack feels cold.
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