Chemical Equilibrium

1
Chemical Equilibrium

• Lesson 5
• The Solubility Product

2
Ksp

• The solubility product, Ksp, is another
  equilibrium constant term for dissolution.
• When a substance is placed in water, if it is
  soluble in the solvent the solid will dissolve
  into its component ions, and the system obtains
  an equilibrium.
• This is a heterogeneous equilibrium, and so the
  concentration of the initial solid before
  dissolution will not be in the equilibrium
  expression only the concentration of the
  dissolved ions.

3
Ksp Solubility

• Ksp values help us determine solubility. If the
  number is very small, this means that very little
  products are formed (quite insoluble). The
  greater the value of Ksp, the more soluble the
  solid.

4
Example 1

• Write the equilibrium expression for the
  following reaction
• H2O
• BaSO4 (s) Ba2 (aq) SO42- (aq)

5
Example 2

• A chemist has a closed vessel containing a chunk
  of solid zinc hydroxide, surrounded by a
  saturated solution at 25C. He determines the
  concentration of aqueous zinc ions and aqueous
  hydroxide ions to be 2.7x10-6 mol/L and 5.4x10-6
  mol/L, respectively. Calculate the solubility
  product constant, and write a statement about the
  solubility of the solid.

6
Example 3

• A technician places a quantity of solid copper
  (I) bromide ions into a beaker of distilled
  water. The measured molar solubility of copper
  (I) bromide is 2.0x10-4 mol/L at 25C. Calculate
  the Ksp of copper (I) bromide.

7
Example 4

• The solubility product constant of solid copper
  (II) iodate is 6.9x10-8 at 25C. Calculate the
  molar solubility of copper (II) iodate.

8
Predicting Precipitation

• When compounds containing ions are mixed,
  sometimes a product with low solubility is forms,
  which then precipitates from the solution.
• This is commonly seen in double displacement
  reactions, and this was predicted using your
  solubility rules in grade 11.
• We can determine the reaction quotient, Q, for a
  reaction and compare it to the Ksp value.

9
Predicting Precipitation

• If Q is greater than Ksp, the equilibrium lies to
  the right (saturated) so will shift to the left
  and precipitation occurs.
• If Q is less than Ksp, the equilibrium lies to
  the left (unsaturated) so will shift to the right
  and no precipitation occurs.
• If Q is equal to Ksp, the solution is at
  equilibrium and no precipitation occurs.

10
Example 5

• A student is planning to combine a 1.0x10-3 mol/L
  aqueous solution of silver nitrate with a
  5.0x10-3 mol/L aqueous solution of potassium
  bromide at 25C. Will a precipitate form when
  the solutions are mixed? If so, identify the
  precipitate.