Simultaneous Equilibria Ksp And Kd

8/14/2013

Complex Ion Equilibria
• The dissociation of complex ions can be represented similarly to equilibria.

Simultaneous Equilibria:
Solubility Product Constants and
Complex Ion Equilibria

→ Ag + + 2 NH
Ag(NH3 ) +2 ←
3

[Ag ][NH ]
=
[Ag(NH ) ]

2

+

Kd

3
+
3 2

→ Cu 2+ + 4 NH
Cu(NH 3 ) 24+ ←
3

[Cu ][NH ]
=
[Cu(NH ) ]
4

2+

Kd

Complex Ion Equilibria

3
2+
3 4

3

Complex Ion Equilibria

• A metal ion coordinated to several neutral molecules or anions forms compounds called complex ions.
• Familiar examples of complex ions include:

•
•

Ag(NH3 )+2 Cu(NH3 )+4

Complex ion equilibrium constants are called dissociation constants.
Example: Calculate the concentration of silver ions in a solution that is 0.010 M in
[Ag(NH3)2]+.
– Kd = 6.3 x 10-8

Co(NH3 )2 Cl2 Pt(NH3 )+2
2

4

1

8/14/2013

Complex Ion Equilibria

[Ag ][NH ] = 6.3×10
=
[Ag(NH ) ]
2

+

Kd

Complex Ion Equilibria

−8

3
+
3 2

(x)(2x)2

Kd =

(0.010 − x)

• How many moles of ammonia must be added to 2.00
L of water so that it will just dissolve 0.010 mole of silver chloride, AgCl?

= 6.3 ×10−8

– What is the stoichiometric amount of NH3 needed to form Ag(NH3)2+ ?
• STOICHIOMETRIC amount

4x3 = 6.3×10−10

– What is the equilibrium NH3 concentration needed to maintain the silver-ammonia complex?

x3 = 1.6×10−10

[ ]

x = 5.4 ×10−4 M = Ag+

• EQUILIBRIUM amount
5

7

Complex Ion Equilibria and
Precipitate Dissolution
• Copper(II) hydroxide, which is light blue colored, dissolves in aqueous ammonia to form dark blue, [Cu(NH3)4]2+.

Cu(OH)2(s ) + 4 NH3(aq )
Cu(OH)2(s )


→
←


Cu(2aq+ ) + 4NH3


→
←


Cu(NH3 ) 24+(aq ) + 2 OH−

Cu(2aq+ ) + 2 OH(−aq )


→
←


Cu(NH3 ) 24+(aq )

2