Download 15.8 Complex Ion Equilibria Complex Ion Solubility:

15.8 Complex Ion Equilibria
Demo:
Cu2+(aq) + 2OH-(aq) 
 Cu(OH)2(ppt.) white
Cu(OH)2 + 4 NH3(aq) 
 Cu(NH3)42+ + 2OHCharged species consisting of a metal ion surrounded by ligands (a Lewis Base). Have lone pair of electrons to donate.
Examples: H2O, NH3, Cl-, CNNumber of ligands is refered to as the coordination number: Ex. Co(H 2O)62+, Coordination number of 6
Equilibria:
Kf formation constant (stability constant)
Ex.
Ag+ + NH3  Ag(NH3)+
K1= 2.1x103
Ag(NH3)+ + NH3  Ag(NH3)2+
K2= 8.2x103
In Equilibria:
Concentrations of [Ag(NH3)2+] >> [Ag(NH3)+], NH3, Ag+
Sample Problem: 100.0 ml of 2.0 M NH3 is added to 100.0ml of 1.0x10-3M AgNO3:
What species are present? Ag+, NO3-, NH3, and H2O
What chemistry occurs? reaction of Ag+ + 2 NH3 
 Ag(NH3)2+
Stoichiometry Reaction:
Ag+
+
[5.0x10-4]
-5.0x10-4
[0]
2 NH3 

[1.0]
-2(5.0x10-4)
[1.0]
Ag(NH3)2+
+5.0x10-4
[5.0x10-4]
Equilibrium Calculations:
K2 
K1
[ Ag ( NH 3 ) 2  ]
[ Ag ( NH 3 )  ][ NH 3 ]

[ Ag ( NH 3 )  ]


[5.0 x10 4 ]
, x= Ag(NH3)+ =6.1x10-8 mol/L
[ x][1.0]
[6.1x10 8 ]
, x= Ag+ = 2.9x10-11 mol/L
[ x][1.0]
[ Ag  ][ NH 3 ]
Major species is Ag(NH3)2+
Complex Ion Solubility:
Group I cations; Ag+, Hg22+, Pb2+
Chloride precipitates all as AgCl, Hg2Cl2, PbCl2
Pb is moderately soluble in hot water
NH3 selectively separates Ag+ as Ag(NH3)+
AgCl  Ag+ + Cl- , Ksp = 1.6x10-10, NH3 removes Ag to drive equilibrium to the right and
increase solubility of AgCl.
Ag+ + NH3  Ag(NH3)+
K1= 2.1x103
Ag(NH3)+ + NH3  Ag(NH3)2
K2= 8.2x103
Overall reaction:
AgCl + 2NH3  Ag(NH3)2+ + ClK= Ksp x K1 x K2 = 2.8x10-3 = 
[ Ag ( NH 3 ) 2  ][Cl  ]
[ NH 3 ] 2
Determine Solubility of AgCl in water:
AgCl  Ag+ + Cl-, Ksp = 1.6x10-10, x= solubility = [Ag+] = 1.3x10-5 mol/L
Calculate Solubility of AgCl in 10.0 M NH3:
AgCl + 2NH3  Ag(NH3)2+ + ClK= Ksp x K1 x K2 = 2.8x10-3 = 
AgCl
K
+
[ Ag ( NH 3 ) 2  ][Cl  ]
[ NH 3 ]
2
2NH3
10.0
-2x
[10-2x]


[ NH 3 ] 2
Ag(NH3)2+
+
+x
[x]
[ x][ x]
[10  2 x]
[ Ag ( NH 3 ) 2  ][Cl  ]
2
Cl+x
[x]
Perfect square solution: K  2.8 x10 3 
x
, x  0.48
(10  2 x)
Summary of Solubility:
o
o
o
Solubility of salts with conjugate base increases with H+, lower pH.
Solubility of metal ions that form complex ions increases in combination with ligand.
Solubility increases with temperature.
Selected Problems: P. 744 #105, 106, 107, 108, 109, 111, 112, 113