1. Characteristics of Equilibrium
Notes 17.2

2. The Equilibrium Constant
aA + bB  cC + dD
A, B, C, D represent chemical species (compounds or molecules) and
a, b, c, d are their coefficients
Equilibrium constant= K
[ ] represent concentrations
K= [C]c [D]d
[A]a [B]b

3. The Equilibrium Constant
The equilibrium expression is a special ratio of the concentrations of the products to the concentrations of the reactants
The product concentrations are the numerator and the reactant concentrations are the denominator

4. Example Write the equilibrium expression for the following reaction:
4NH3 (g) O2 (g)  4NO2 (g) H2O (g)
[NO2]4 [H2O]6
[NH3]4 [O2]7

5. Example 2SO2 (g) + O2 (g)  2SO3 (g) [SO2] = 1.50 M [SO2] = 0.590M
[O2] = 1.25 M [O2] = M
[SO3]= 3.50 M [SO3] = M
Solve for the equilibrium constant K for both
( answer = 4.36 and 4.32)

6. answer K= [SO3]2 [SO2]2 [O2] K= (3.50)2 (1.50)2 (1.25) K= (0.260)2
(0.590)2 (0.045)
K= K= 4.32

7. The Equilibrium Constant
For a given reaction at a given temperature, the special ratio of the concentrations of the products to reactants defined by the equilibrium expression will always be equal to the same number--- K
There is only one equilibrium constant for a particular system at a particular temperature, but there is an infinite number of equilibrium positions. (page 607)

8. Heterogeneous Equilibria
The position of a heterogeneous equilibrium does not depend on the amounts of pure solids or liquids present
The concentrations of pure solids or pure liquids in a chemical reaction are not included in the equilibrium expression for the reaction
It does not apply to solutions or gases because their concentrations can vary