Chemical Equilibrium

1
Chemical Equilibrium
Read Chapter 6-1 to 6-5 and Chapter 8-1 to 8-4.
We will begin with a review of equilibria and
then introduce activities (8-1 to 8-4).
aA bB cCdD
Consider the reaction
The equilibrium constant is
2
Last semester you used Ks for

• Strong acids
• Weak acids
• Buffers

This semester we extend this to include
solubilities, complex ion formation combined with
acid-base equilibria.
3
Equilibrium Constants
What determines the size of an equilibrium
constant?
Equilibrium constants come from thermodynamics.
What are the dimensions of the equilibrium
constant?
Each quantity is really a ratio of concentrations.
4
Equilibrium Constants
When we write the equilibrium constant
what we are really writing (for a liquid) is
See page 100 of text for more details.
5
Lets consider a few examples.
Hg2(IO3)2(s) Hg22 2IO3-
init. conc. Solid 0
0 final conc. Solid x
2x
Solve for x in Ksp Hg22IO3- 2x(2x)2
xKsp/41/3
Ksp 1.3x10-7M
6
Solubilities

• If you have a salt AB it will dissolve to some
  extent in water.
• Ksp values tell you to what extent the salt will
  dissolve.
• The equation KspAB only applies if there is
  solid present and it is in equilibrium with the A
  and B ions in solution.
• As a chemist, you will find that solubility rules
  are useful. These rules are now given.

7
Solubility Rules
Solubility Rules
1. Salts containing Group I elements are soluble
(Li, Na, K, Cs, Rb). Exceptions to this rule
are rare. Salts containing the ammonium ion
(NH4) are also soluble. 2. Salts containing
nitrate ion (NO3-) are generally soluble. 3.
Salts containing Cl -, Br -, I - are generally
soluble. Important exceptions to this rule are
halide salts of Ag, Pb2, and (Hg2)2. Thus,
AgCl, PbBr2, and Hg2Cl2 are all insoluble. 4.
Most silver salts are insoluble. AgNO3 and
Ag(C2H3O2) are common soluble salts of silver
virtually anything else is insoluble. 5. Most
sulfate salts are soluble. Important exceptions
to this rule include BaSO4, PbSO4, Ag2SO4, and
CaSO4.

8
6. Most hydroxide salts are only slightly
soluble. Hydroxide salts of Group I elements are
soluble. Hydroxide salts of Group II elements
(Ca, Sr, and Ba) are slightly soluble. Hydroxide
salts of transition metals and Al3 are
insoluble. Thus, Fe(OH)3, Al(OH)3, Co(OH)2 are
not soluble. 7. Most sulfides of transition
metals are highly insoluble. Thus, CdS, FeS, ZnS,
Ag2S are all insoluble. Arsenic, antimony,
bismuth, and lead sulfides are also insoluble.
8. Carbonates are frequently insoluble. Group
II carbonates (Ca, Sr, and Ba) are insoluble.
Some other insoluble carbonates include FeCO3,
PbCO3. Carbonates become soluble in acid
solution. 9. Chromates are frequently
insoluble. Examples PbCrO4, BaCrO4
9
10. Phosphates are frequently insoluble.
Examples Ca3(PO4)2, Ag2PO4 11. Fluorides are
frequently insoluble. Examples BaF2, MgF2,
PbF2.
Thanks to Professor Kenneth W. Busch from whose
Web page these data were extracted.
Memorize rules 1-4.
10
Lets consider a 2nd example.
Consider an aqueous solution made by adding
sufficient quantities of Hg2(IO3)2 and Hg2I2
salts that equilibrium is reached. Will the
mercurous ion concentration be greater than or
smaller than the previous problem? What is the
I- concentration at equilibrium?
For Hg2(IO3)2 Ksp 1.3x10-7
For Hg2I2 Ksp 1.1x10-28
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Common Ion Problem
Write down eq. expressions for Hg2(IO3)2 and
Hg2I2.
To find all the concentrations, first treat each
problem independently assuming the other salt is
not present.
For Hg2I2 Ksp 1.1x10-28
For Hg2(IO3)2 Ksp 1.3x10-7
Ksp Hg22I- 2x(2x)2
Hg221.3x10-4
Hg2 23.0x10-10
If we had a mixture of both, all the mercurous
ions would be due to the Hg2(IO3)2 . To solve
for I- assume Hg2 21.3x10-4 and solve the
following
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Common Ion Problem Continued
To solve for I- assume Hg221.3x10-4 and
set up the following table
Hg2I2(s) Hg22 2I-
init. conc. Solid 1.3x10-4
0 final conc. Solid
1.3x10-4x 2x
Solve for x in Ksp Hg22I- 2 (1.3x10-4x
)(2x)2
Ksp 1.1x10-28 1.3x10-4 (2x)2
x 4.6x10-13
Always test for self consistency.
13
Activities

• This is where we learn that life is more complex
  than we have previously thought.

14
Lets consider a 3rd example.
Consider an aqueous solution made by adding
sufficient Hg2(IO3)2 that equilibrium is reached.
Then add enough KNO3 so that the solution has a
.1M potassium concentration. What is the
concentration of the mercurous ion after
equilibrium is reached?
Based on our expression for K, adding potassium
nitrate should have no effect on the Hg22
concentration.
In fact we find that the concentration goes up.
Why?
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Here is another example of this effect.
Fe3 SCN- Fe(SCN)2
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An ionic atmosphere surrounds ions in solution.
The size of d determines the interaction between
the two spheres, and d is a function of the ionic
strength m.
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As the number of ions (of any flavor) in solution
increase (i.e. m increases) what happens to the
interactions between the two shaded regions. How
does this affect the concentration of the ions in
solution?
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Equilibrium Constants
This equilibrium constant expression must be
wrong.
Equilibrium constants come from thermodynamics.
The correct expression for K is
(8-5)
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The key equations
The extended Debeye-Huckel equation is
How do we solve for concentrations?
Where
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Sample problem
Calculate the Ca2 conc. obtained by dissolving
CaF2 in a 0.05M soln of NaClO4
CaF2(s) Ca2 2F-
init. conc. Solid 0
0 final conc. Solid x
2x
Solve for x in Ksp Ca2gCaF- 2 g2F
Ksp 3.9x10-11M
Ignore the contribution of Ca2 and F- to ionic
strength.
21
Calculate the ionic strength for a 0.05M soln of
NaClO4
22
we find gCa.485 and gF.81
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Calculate the Ca2 conc. obtained by dissolving
CaF2 in a 0.05M soln of NaClO4
CaF2(s) Ca2 2F-
init. conc. Solid 0
0 final conc. Solid x
2x
Solve for x in Ksp Ca2gCaF- 2 g2F
x gCa (2x)2 g2F
3.9x10-11 x(.485)(2x)2(.81)2
3.9x10-11/(.485)(.81)24x3
x3.1x10-4
check for consistency by recalculating m.
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Recalculate the ionic strength for the solution
Since the ionic strength has not changed we are
confident our answer is correct.
Practice calculating ionic strengths (prob. 8-5)
and activity coefficients (prob. 8-6), and
concenctrations (prob. 8-12). See also problem 5
of homework.