AP Chemistry: Solubility
pH Effect on Solubility
Sometimes, dissolved ions can undergo further reactions, like with WATER/H3O+
If the anion is the conjugate base of a weak acid, it will react with H3O+, affecting solubility.
AgCN (s) Ag+1 (aq) + CN­ (aq)
Conjugate Base of HCN CN­ (aq) + H3O+1 HCN (aq) + OH­ (aq)
Silver cyanide is more soluble in acidic solutions
LeChatelier: CN­ removed, reaction shifts right
pH Effect on Solubility
AgCl (s) Ag+1 (aq) + Cl­ (aq)
Conjugate Base of HCl Cl­ will not react with additional H3O+
solubility not affected by the pH of the solution
Which salt would you expect to dissolve more readily in acidic solution, BaSO4 or BaF2?
BaSO4 (s) Ba+2 (aq) + SO4­2 (aq)
BaF2 (s) Ba+2 (aq) + 2 F­1 (aq)
17.4­17.6
AP Chemistry: Solubility
Complex Ions
ions formed from a metal cation with a Lewis Base attached to it by a co­ordinate covalent bond
many metal cations, especially transition metals like Fe, Cu, Zn, Ag as well as Al do this
ions attach to molecules/anions having lone pairs
(H2O, N)
Ligand: the Lewis Base that bonds to the metal cation
Cu(NH3)4+2 Complex Ions
Cu+2 (aq) + 4 NH3 (aq) Cu(NH3)4+2 (aq)
Kf = formation (stability) constant
Kf =
[Cu(NH3)4+2]
[Cu+2] [NH3]4
= 4.8 x 1012
very stable ion!
Kd = dissociation constant (1/Kf)
2.1 x 10­13
17.4­17.6
AP Chemistry: Solubility
Complex Ions
A couple times this year we created Cu(NH3)4+2 in the lab. We started by adding concentrated ammonium hydroxide to copper (II) sulfate, creating a precipitate of copper (II) hydroxide:
Cu+2 (aq) + 2 OH­ (aq) Cu(OH)2 (s)
As more base was added, the precipitate dissolved and created the very colorful complex ion:
Cu+2 (aq) + 4 NH3 (aq) Cu(NH3)4+2 (aq)
More technically, this is what is happening...
Cu(H2O)6+2 (aq) + 4 NH3 (aq) Cu(NH3)4(H2O)2+2 (aq) + 4 H2O (l) Complex Ions
Ultimately, a precipitate that may normally form may not in a solution where a complex ion could form...
If we added NaCl solution of CuCl2 solution, we would expect to see a Cu(OH)2 precipitate under appropriate solution conditions.
Cu+2 (aq) + 2 OH­ (aq) Cu(OH)2 (s)
However, if ammonia was added to the mixture, the formation of the complex ion would be favored over the precipitate formation.
Kf > Ksp
4.8 x 1012
17.4­17.6
2.6 x 10­19
AP Chemistry: Solubility
Complex Ions
LAST THING!!! Complex ions also explain why salts made with metal ions from weak bases like Al(OH)3 or Fe(OH)3 or Zn(OH)2 create acidic solutions:
Why is Al(NO3)3 an acidic salt...
The Al+3 ion creates a complex ion with water, which then reacts with water to create H3O+
Al(H2O)6+3 (aq) + H2O (l) Al(H2O)5(OH)+2 (aq) + H3O+ (aq) 17.4­17.6