Solubility

Chapter 17

• No only do acids and bases dissolve in aqueous solutions but so do ionic compounds– Many ionic compounds tend to be strong

electrolytes.– When ionic compounds dissociate, entropy of

the system increases– Remember that breaking the ionic bonds

between the salt ions and the intermolecular forces of the solution is endothermic, where the creation of the solution is exothermic

Strong Electrolytes Are…• Strong acids• Strong bases• Soluble ionic salts

Always soluble…

• All alkali metals (Sodium, potassium, etc.), ammonium, acetate, and nitrate salts– Examples:

• NaCl• KCl

• NH4Cl

• KNO3

Solubility Products

Consider the equilibrium that exists in a saturated solution of BaSO4 in water:

BaSO4(s) Ba2+(aq) + SO42−(aq)

•Since barium sulfate is not a strong electrolyte, it does not completely dissociate.

•The solubility product describes the dissolution of solid and is an indication of how soluble the solid is in water

Solubility Products

The equilibrium constant expression for this equilibrium is

Ksp = [Ba2+] [SO42−]

where the equilibrium constant, Ksp, is called the solubility product.• Remember solids do not appear in equilibrium constant expressions for heterogeneous equilibria

Solubility Products

• Ksp is not the same as solubility.• Solubility is generally expressed as the mass of

solute dissolved in 1 L (g/L) or 100 mL (g/mL) of solution, or in mol/L (M).

• The solubility product of a compound, generally, equals the product of the concentration of the ions involved in the equilibrium, each raised to the power of its coefficient in the equilibrium equation.

Practice Problem

• What is the solubility product expression for each of the following ionic compounds?– CaF2

– BaCO3

– Ag2SO4

Practice Problem

The solubility product of calcium fluoride at 298 K is 3.9 x 10-11. Find its molar solubility at this temperature.

Factors Affecting Solubility

• The Common-Ion Effect– If one of the ions in a solution equilibrium is

already dissolved in the solution, the equilibrium will shift to the left and the solubility of the salt will decrease.

BaSO4(s) Ba2+(aq) + SO42−(aq)

• The solubility of a slightly soluble salt is decreased by the presence of a second solute that furnishes a common ion

• How would the solubility of CaF2 be impacted if NaF is added to the solution?– decreases

Factors Affecting Solubility• pH

– If a substance has a basic anion (anion of a weak acid), it will be more soluble in an acidic solution.• Example: Mg(OH)2 in acidic

solution becomes Mg2+ + H2O– The solubility of slightly

soluble salts containing basic anions increases as [H+] increases (as pH is lowered)• Bases that would create strong

acids would be less soluble in an acid solution

– Substances with acidic cations are more soluble in basic solutions.

Will a Precipitate Form?

• In a solution,– If Q = Ksp, the system is at equilibrium

and the solution is saturated.– If Q < Ksp, more solid will dissolve until Q

= Ksp.– If Q > Ksp, the salt will precipitate until Q

= Ksp.

– The greater the Ksp value, the greater the solubility of the ionic compound

Selective Precipitation of Ions

One can use differences in solubilities of salts to separate ions in a mixture.