Chapter 16 Solubility and Complex Ion Equilibria Solubility

Chapter 16 Solubility and Complex Ion Equilibria

Solubility - It’s Kind of A Big Deal

• When salt is placed in water, the number of ions in solution gradually increase. • As the ions’ concentrations increase, they become more likely to collide with one another and reform the solid • Eventually, a dynamic equilibrium is reached (a double arrow is used in the reaction) where no more solid will dissolve • The equilibrium expression for solubility product ([products]/[reactants]) can be written for the balanced reaction (Ksp)

Ca. F 2(s) <-> Ca 2+(aq) + 2 F-(aq) • Ksp = [Ca 2+][F-]2 • Ksp = solubility product constant or solubility product • Pure liquids and solids are not included in the equilibrium expression (normally reactant) • **There is a difference between solubility (equilibrium position) and solubility product (equilibrium constant with ONE value for given solid/temp. ) • Usually the units for Ksp are omitted

Solubility Expressions • Write products and equilibrium expressions for the following dissolution reactions: – Ba(OH)2(s) – Pb. CO 3(s) – Ca 3(PO 4)2(s)

Calculations • Copper (I) bromide has a measured solubility of 2. 0 X 10 -4 mol/L at 25°C. Calculate its Ksp value. • Answer: 4. 0 X 10 -8 • Calculate the Ksp value for bismuth sulfide (Bi 2 S 3), which has a solubility of 1. 0 X 10 -15 mol/L at 25°C. • Answer: 1. 1 X 10 -73 • The solubility of silver sulfide (Ag 2 S) at 25°C is 3. 4 X 10 -17 M. Calculate Ksp for Ag 2 S. • Answer: 1. 6 X 10 -49

The Other Way… • The solubility of an ionic solid can be calculated knowing its Ksp value • Example: The Ksp value for copper(II) iodate, Cu(IO 3)2, is 1. 4 X 10 -7 at 25°C. Calculate its solubility at 25°C. – Answer: 3. 3 X 10 -3 mol/L • Calculate the solubility in mol/L and g/L of Ni. CO 3 (Ksp=1. 4 X 10 -7) • For Ba 3(PO 4)2 (Ksp = 6 X 10 -39)

How Does p. H Relate • When a reaction is shifted right/left with a change in p. H (change in [H+] and [OH-]), it affects solubility • Depending on whether the p. H is decreased (add [H+]/decrease [OH-]) or increased (add [OH-]/decrease [H+]), the solubility will change • General rule: if the anion X- is an effective base (if HX is a weak acid), the salt will show increased solubility in acidic solution (OH-, S 2 -, CO 32 -, C 2 O 42 -, Cr. O 42 -) <= more soluble in acidic solution than in pure water

Will A PPT Form? ? • Ion Product (Q): (calculated just like Ksp), but initial concentrations are used rather than equilibrium concentrations (just like Q from equilibrium expressions in ch. 13) • If Q > Ksp, precipitation occurs and will continue until concentrations are reduced enough so Q is not greater than Ksp • If Q < Ksp, no precipitation occurs

Example • A solution is prepared by adding 750. 0 m. L of 4. 00 X 10 -3 M Ce(NO 3)3 to 300. 0 m. L of 2. 00 X 10 -2 M KIO 3. Will Ce(IO 3) 3 (Ksp = 1. 9 X 10 -10) precipitate from this solution?