Ksp The Solubility Product Constant Learning Goals I
Ksp: The Solubility Product Constant Learning Goals: I will understand the solubility product constant, Ksp, and use it to compare to Q, the trial ion product, to determine if a precipitate will form I will understand how the common ion effect affects solubility
Ksp: The Solubility Product Constant. An equilibrium can exist between a partially soluble substance and its solution:
For example: Ba. SO 4 (s) Ba 2+ (aq) + SO 42 - (aq) – When writing the equilibrium constant expression for the dissolution of Ba. SO 4, we remember that the concentration of a solid is constant. The equilibrium expression is therefore: K = [Ba 2+][SO 42 -] K = Ksp, the solubility-product constant. Ksp = [Ba 2+][SO 42 -]
The Solubility Expression Aa. Bb(s) a. Ab+ (aq) + b. Ba- (aq) Ksp = [Ab+]a [Ba-]b Example: Pb. I 2 (s) Pb 2+ + 2 IKsp = [Pb 2+] [I-]2 v. The greater the Ksp the more soluble the solid is in H 2 O.
Solubility and Ksp Three important definitions: 1) Solubility: quantity of a substance that dissolves to form a saturated solution 2) Molar solubility: the number of moles of the solute that dissolves to form a liter of saturated solution 3) Ksp (solubility product): the equilibrium constant for the equilibrium between an ionic solid and its saturated solution
Comparing Q and Ksp to Determine Precipitate Formation -The Reaction Quotient, Q, is used to determine if a system is at equilibrium (but is called trial ion product when comparing to Ksp) -If Q=Ksp, then the solution is perfectly saturated and a precipitate will not form -If Q<Ksp, more solid can still dissolve, therefore, no precipitate forms -If Q>Ksp, a precipitate forms
Example #1: Calculating Molar Solubility when given Ksp Calculate the molar solubility of Ag 2 SO 4 in one liter of water. Ksp = 1. 4 x 10 -5
Example #1: Continued If the Ksp =1. 4 x 10 -5 for Ag 2 SO 4 and the concentrations of Ag+ and SO 42 - are measured to be 1. 5 M and 0. 75 M respectively, will a precipitate form?
Predicting if Precipitation Occurs Step 1: Write the balanced equilibrium: Step 2: Write the Ksp equation: Step 3: Use the concentration of ions to Calculate Q (trial ion product) Step 4: Compare Q to Ksp
You Try #1 • The concentration of nickel (II) ion in a solution is 1. 5 x 10 -6 mol/L. If enough Na 2 CO 3 is added to make the solution 6. 0 x 10 -4 mol/L in the carbonate ion, CO 3 -2, will precipitation of nickel (II) carbonate occur? The Ksp of Ni. CO 3 is 6. 6 x 10 -9.
Example #2 • Find the Ksp of Copper(II)phosphate if the molar solubility is 5. 9 x 10 -5 mol/L.
You Try #2 • What is the Ksp of a saturated solution of barium sulphate if the molar solubility is 2. 5 x 10 -2 mol/L?
Factors Affecting Solubility • Some of the main factors that affect solubility of a solute are: (1) Temperature (2) Nature of solute or solvent (3) Pressure (4) common ion effect
Temperature • Generally solubility increases with the rise in temperature and decreases with the fall of temperature but not in all cases. • In endothermic process solubility increases with the increase in temperature and vice versa. • In exothermic process solubility decrease with the increase in temperature. • Gases are more soluble in cold solvent than in hot solvent.
Nature of Solute and Solvent • A polar solute dissolved in polar solvent. • A polar solute has low solubility or is insoluble in a non-polar solvent. • In general like dissolves like
Pressure • The effect of pressure is observed only in the case of gases. • An increase in pressure increases of solubility of a gas in a liquid. • For example carbon dioxide is filled in cold drink bottles (such as coca cola, Pepsi 7 up etc. ) under pressure.
Common Ion Effect • The presence of other solutes can also influence solubility – although they do not effect Ksp • Ca. F 2(s) Ca+2(aq) + 2 F • addition of calcium or fluoride ion will shift the equilibrium left favoring solid formation and decreasing solubility of the compound
Discussion: Common Ion Effect Pb. I 2(s) Pb 2+(aq) + 2 I–(aq) How will adding KI(aq) to the above system affect precipitation?
How Did We Do? Learning Goals: I will understand the solubility product constant, Ksp, and use it to compare to Q, the trial ion product, to determine if a precipitate will form I will understand how the common ion effect affects solubility
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