Solubility Product ConstantFactors Affecting SolubilitySelective Precipitation

• Objectives▫Today I will be able to:

Calculate the solubility product constant for a compound

Explain the factors that affect solubility Determine the compounds that selectively

precipitate from a solution when placed under stresses

• Informal assessment – monitoring student interactions and questions as they complete the practice problems

• Formal assessment – analyzing student responses to the practice problems

Lesson Sequence

•Evaluate: Warm Up•Explain: Notes•Elaborate: Practice Problems•Evaluate: Exit Ticket

Warm Up

Objectives

•Today I will be able to:▫Calculate the solubility product constant

for a compound▫Explain the factors that affect solubility▫Determine the compounds that selectively

precipitate from a solution when placed under stresses

Homework

•Continue working practice problems•Ch. 17•49, 51,53, 55, 57,59•69,70, 75,76

Agenda

•Warm Up•Notes•Practice Problems•Exit Ticket

Solubility Product ConstantFactors Affecting SolubilitySelective PrecipitationNotes

Solubility Equilibria

Solubility Product Constant

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

BaSO4(s) Ba2+(aq) + SO42−(aq)

Solubility Product Constant ExpressionThe equilibrium constant expression for this equilibrium is

Ksp = [Ba2+] [SO42−]

where the equilibrium constant, Ksp, is called the solubility product.

Ksp is not the same as solubility

What is solubility? What is Ksp?

• 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).

• Equilibrium constant for equilibrium between an ionic sold and its saturated solution

• Unitless• Larger magnitude means

more dissolving occurred• Only one value per

temperature• Does not change with pH

or other ions present in solution like solubility

Calculation•Solid silver chromate is added to pure water

at 25oC and some solid remains undissolved. The mixture is stirred for several days to ensure that equilibrium is achieved between the undissolved Ag2CrO4 and the solution. Analysis of the equilibrated solution shows that its silver ion concentration is 1.3 x 10-4

M. Assuming that the Ag2CrO4 solution is saturated , calculate Ksp for the compound.

Calculation

•The Ksp of CaF2 is 3.9 x 1o-11 at 25oC. Assuming equilibrium is established between solid and dissolved CaF2, calculate the solubility of CaF2 in grams/liter

Factors impacting solubility

1. 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)

2. pH▫ If a substance has a basic anion, it will be

more soluble in an acidic solution.▫ Substances with acidic cations are more

soluble in basic solutions.

3. Complex Ions

•Complex Ions▫ Metal ions can act as Lewis acids and form

complex ions with Lewis bases in the solvent.

4. Amphoterism▫ Amphoteric metal

oxides and hydroxides are soluble in strong acid or base, because they can act either as acids or bases.

▫ Examples of such cations are Al3+, Zn2+, and Sn2+

Complex Ions Cont.• The formation of these complex ions increases

the solubility of these salts.

Precipitation and Separation of Ions

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.

Sample Calculation•The Ksp for BaSO4 is 1.1 x 10¯10 and that for

BaSeO4 is 2.8 x 10¯11. A 1.0 M solution of BaCl2 is added slowly to a solution that is 1.0 x 10¯4 M in sodium sulfate, Na2SO4 and 1.0 x 10¯4 M in sodium selenate, Na2SeO4. What is the approximate percentage of one anion has precipitated at the point which the second anion just begins to precipitate? (Assume the addition of the BaCl2 solution does not change the overall solution volume to any significant degree.)

Solution

Selective Precipitation of Ions

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

Selective Precipitation Practice Problems•Visit the website and walk through the

sample calculations

•Try to solve the problem on your own and then walk through the worked out solution

•http://www.chemteam.info/Equilibrium/SelectivePrecip.html

Practice ProblemsCh. 1749, 51,53, 55, 57,59

69,70, 75,76

Closure

•Which problem did you find most challenging today?