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AP Chemistry
3-4-17
Place your test corrections on your desktop for collection.
HW: Read 6.2-6.3 by Monday and do exercises 48, 50, 56,
60, 64, 66, 72, 74
Handouts: None
Warm-up/Objective and Notes
AP Chemistry
3-4-17
Objective: To master equilibrium of complex ions
Warm-up #3: When a complex ions forms, what is the
Lewis acid and what is the Lewis base?
Answer: The metal ion is the Lewis acid accepting a
pair of electrons from the ligand which is the Lewis
base
Qualitative Analysis
This is the process of precipitating out the ions
from each of the 5 groups sequentially and
identifying them.
This is a common type experiment in an
advanced inorganic chemistry lab in college.
We will come back to this at the end of today’s
talk. But first some equilibrium…
Equilibria Involving Complex Ions
A complex ion is a charged species consisting of
a metal ion (typically a transition metal)
surrounded by ligands.
A ligand is a Lewis base that donates a pair of
electrons to an empty orbital of the metal ion
(a Lewis acid) to form a dative covalent bond.
Common ligands are H2O, NH3, Cl-, and CN-
Coordination number
The number of ligands attached to the metal ion
is its coordination number.
The most common coordination numbers are:
6 with examples Co(H2O)62+ and Ni(NH3)63+
4 with examples CoCl42- and Cu(NH3)42+
2 with example Ag(NH3)21+
Example complex ions-all with
coordination number of 6
The ligand
donates a pair
of electrons to
a small, positively
charged metal
cation to form the
complex ion.
Equilibrium Involving Complex Ions
Metal ions add ligands one at a time with formation
constants, K1, K2, K3 etc.
Formation of Ag(NH3)2 occurs in two steps from Ag1+
and NH3:
Ag1+(aq) + NH3(aq) ⇌ Ag(NH3)1+(aq)
K1 = 2.1 x 103
Ag(NH3)1+(aq)+ NH3(aq) ⇌ Ag(NH3)21+(aq) K2 = 8.2 x 103
Equilibrium Involving Complex Ions
Consider a solution of 100.0 mL of 2.0 M NH3
with 100.0 mL 1.0 x 10-3 M AgNO3.
Before a reaction occurs, what are the major
species?
NH3, H2O, Ag1+ and NO31What reactions will occur in this solution?
Equilibrium Involving Complex Ions
What reactions will occur in this solution?
Well, NH3 will react with water:
NH3(aq) + H20(aq) ⇌ NH41+(aq) + OH1- Kb = 1.8 x 105
Will a significant amount of NH3 be removed
and converted to NH41+?
No, Kb is small so our 2.0 M NH3 conc. Won’t
change significantly.
Equilibrium Involving Complex Ions
What are our initial conc. of Ag1+ and NH3?
[Ag1+]0 = (0.100L)(1.0x10-3M)/0.200 L = 5.0 x 10-4 M
[NH3]0= (0.100L)(2.0 M)/0.200 L = 1.0 M
Assumptions will help us to figure out the
concentration of all species at equilibrium.
KEY assumption: both K1 and K2 are large (>103) so
almost ALL Ag1+ will end up Ag(NH3)21+
Equilibrium Involving Complex Ions
Setting up:
Ag1+
+
2NH3  Ag(NH3)21+
Before 5.0 x 10-4M
1.0 M
0
After
0
1.0-2(5.0x10-4)M 5.0x10-4M
Twice as much as
Ag1+ is required
Equilibrium Involving Complex Ions
At equilibrium, the conc of Ag(NH3)21+ will equal
[Ag1+]0 (5.0 x 10-4M) and the conc of NH3 will be
[NH3]0 (1.0M)
However, there will be small amounts of Ag(NH3)1+
and Ag1+, so those must be calculated
−4 ]
[𝐴𝑔 𝑁𝐻3 1+
]
[5.0×10
2
=
[𝐴𝑔(𝑁𝐻3 )][𝑁𝐻3 ] [𝐴𝑔(𝑁𝐻3 )1+ ][1.0𝑀]
𝐾2 = 8.2 × 103 =
Ag(NH3)1+=6.1 x 10-8M
Equilibrium Involving Complex Ions
Now determine [Ag1+]
𝐾1 = 2.1 × 103 =
[𝐴𝑔 𝑁𝐻3 1+ ]
[𝐴𝑔1+ ][𝑁𝐻3 ]
=
[6.1×10−8 ]
[𝐴𝑔1+ ][1.0𝑀]
Ag1+ = 2.9 x 10-11 M
[Ag(NH3)21+] >>[Ag(NH3)1+]>>[Ag1+]
Qualititative Analysis the Solubility of
Complex Ions
Suppose you want to identify WHICH ions
precipitate out of solution in the first 4 groups or
are left in solution (Group V).
We can actually identify which ions are present
based on the solubility of the complex ions.
I will show you how you can identify which ions
are present in the Group I precipitation (those
that precipitate in HCl).
Complex Ions and Solubility
Suppose we have a solution containing one or more Ag1+,
Pb2+ or Hg22+.
•First we treat the solution with HCl and form the
insoluble chlorides.
•Next, heat and filter to remove Pb2+ ions
•Then solubilized the AgCl and separate it from the
possible remaining precipitated chlorides.
How? Add concentrated NH3 to form the complex ions
Ag(NH3)21+ which will solubilize all Ag1+ ions.
Complex Ions and Solubility
Quantitation
First let’s write the reactions occurring in the solution for
Ag.
AgCl(s) ⇌ Ag+(aq) + Cl-(aq)
Ksp = 1.6 x 10-10
Ag+(aq) + NH3(aq) ⇌ Ag(NH3)+(aq)
K1 = 2.1 x 103
Ag(NH3)+(aq) + NH3(aq) ⇌ Ag(NH3)2+(aq) K2 = 8.2 x 103
------------------------------------------------------------------------AgCl(s) + 2NH3(aq) ⇌ Ag(NH3)2+(aq) +Cl- Koverall = 2.8 x 10-3
Complex Ions and Solubility
Koverall = 2.8 x 10-3 = [Ag(NH3)2+][Cl1-]/[NH3]2
In 10.0 M NH3, what is the solubility of AgCl(s)?
Let x = molarity of Ag(NH3)2+
2.8 x 10-3 = x2/(10.0-2x)2
2.8𝑥10−3
𝑥
=
10.0 − 2𝑥
x = 0.48 M of AgCl in 10.0 M NH3
Complex Ions and Solubility