Solubility Product Constant, Ksp Chapter 12 – Chemical Equilibrium

Solubility The solubility of a substance, at a given temperature, is the maximum amount that dissolves in a fixed quantity of solvent. Different solutes may have different solubilities. A dilute solution contains less dissolved solvent than a concentrated one. A major factor that will determine whether or not a solute dissolves in a given solvent, making a solution, is the relative strength of the intermolecular forces within and between solute and solvent.

Solubility Equilibrium Solubility Equilibrium: the excess undissolved solute is in equilibrium with the dissolved solute. Solute (undissolved)  Solute (dissolved) A saturated solution contains the maximum amount of dissolved solute at a given temperature in the presence of undissolved solute. An unsaturated solution contains less solute than the maximum amount; if you add more solute, it will dissolve (until the solution becomes saturated).

Factors that can affect the solubility of a substance Temperature: Solubility increases with an increase in temperature if the dissolution is endothermic. Solubility decreases with an increase in temperature if the dissolution process is exothermic. Gas solubility decreases with rising temperature because gases have weak intermolecular forces; with an increase in temperature gas particles have increased kinetic energy which allows them to overcome the weak intermolecular interactions and renter the gas phase.

Factors that can affect the solubility of a substance Pressure: Little effect on liquids and solids due to their incompressibility Major effect on gas solubility When the pressure on a gas increases, the gas volume decreases resulting in more collisions between the gas and the surface of the solvent. This results in more gas in solution when equilibrium is reestablished. The solubility of a gas is directly proportional to the partial pressure of the gas above the solution (Henry’s Law).

How about the equilibria of slightly soluble ionic compounds?

Solubility rules for ionic compounds in water: Compounds that are insoluble will form precipitates in water.

Slightly Soluble Compounds When a soluble compound dissolves in water, it dissociates completely into ions. In the case of slightly soluble compounds, we will make the assumption that the small amount of solute that dissolves dissociates into ions completely (this is not always the case, but this is complex).

Equilibrium exists between solid solute and dissolved ions. Let’s look at the following dissolution equation: XnYm (s)  nX+(aq) + mY-(aq) We can represent the equilibrium constant for this dissolution as: Keq = [X+]n[Y-]m The equilibrium constant for solubility is referred to as the solubility product constant, Ksp. The expression is now written as: Ksp = [X+]n[Y-]m

Ksp The value of Ksp depends on temperature and not on the individual ion concentrations; when changes in concentration occur, the Ksp is maintained (as with other equilibrium constants. The value of Ksp is a measure of how far to the right the dissolution proceeds at equilibrium, saturation.

We can: determine the Ksp from solubility and determine the solubility from the Ksp.

Determining Ksp from solubility Lead(II) sulfate is used is used as a white pigment in paints and as a key component in common lead-acid car batteries. Its solubility in water at 25C is 4.25 x 10-3 g/100 mL solution. What is the Ksp of lead(II) sulfate? When lead(II) fluoride is shaken with pure water at 25C, the solubility is found to be 0.64 g/L. Calculate the Ksp of lead(II) fluoride.

Determining solubility from the Ksp Calcium hydroxide is a major component of mortar, plaster, and cement. Solutions of calcium hydroxide are used in industry as a cheap, strong base. Calculate the solubility of calcium hydroxide if the Ksp is 6.5 x 10-6.

Solubility Ksp values can be used to compare the solubilities of compounds with the same total number of ions the higher the Ksp, the greater the solubility.

Common-ion Effect The presence of a common-ion decreases the solubility of a slightly soluble compound. The Ksp of a compound depends only on the ion concentrations and if either goes up or goes down, then the other will go up or down in order to maintain a constant Ksp value. This is important to consider if we are looking at the solubility of a solute in a solvent other than water. This is not something that you need to know how to calculate in this course.

The effect of pH on solubility If the compound contains the anion of a weak acid, the addition of H3O+ (from a strong acid) will increase the solubility of the compound. If the compound contains the anion of a strong acid, then there is no effect on the equilibrium if H3O+ is added.