1. Introduction to the Equilibrium Constant
East Wing Mobile, Alexander Calder,
American, 1977

2. Equilibrium
Equilibrium is the state when the concentrations of all reactants and products remain constant over time.
This occurs when the rate of the forward = rate of the reverse reaction.
Equilibrium happens because reactants and products never stop colliding!

3. Equilibrium Graph N2O4 (g) ↔ 2 NO2 (g)
At what time has this system reached equilibrium?
60 s
What are the equilibrium concentrations of N2O4 and NO2?
0.21 M and 1.55 M
Why is the pink line steeper than the blue line?
N2O4 (g) ↔ 2 NO2 (g)

4. Equilibrium Expression
An equilibrium expression tells us how much of each species (reactants and products) is present at equilibrium.
Keq is constant at a given temperature and depends on the equilibrium concentrations of products and reactants.

5. Reminder
Concentration is given in units of molarity (moles of solute / liter of solution)
M or mol/L
[ ] means molarity

6. Equilibrium Expression
Here is a general reaction:
aA + bB ⇄ cC + dD
The lower case letters represent the stoichiometric coefficients and the upper case letters represent reactants and products.
The way that we quantify the concentrations of each substance at equilibrium is the equilibrium constant

7. Example 1 [NO2]4[O2] [N2O5]2 Keq =
What is the equilibrium expression (Keq) for the following decomposition reaction?
2N2O5(g) ⇄ 4NO2(g) O2(g)
[NO2]4[O2]
[N2O5]2
Keq =

8. Example 2 Dinitrogen tetroxide decomposes as follows:
N2O4 (g) ⇄ 2NO2(g)
What is the equilibrium expression
K =
[NO2]2
[N2O4]

9. Example 3: Determining the value of Keq - Quantitative Equilibium
Calculate the equilibrium constant for the following reaction given the experimentally determined equilibrium concentrations.
2NO(g) +O2(g) ⇄ 2NO2(g)
Experimental equilibrium concentrations:
[NO]= 4.065x 10-2 M
[O2] =9.525 x 10-2M
[NO2] = M
Answer: Keq = 858,600

10. Equilibrium Constants
When Keq > 1 then products are favored in the reaction
This means that there are more products than reactants at equilibrium
When Keq < 1 then the reactants are favored in the reaction
This means there are more reactants then products at equilibrium.

11. Example 4
Which of the following reactions will be product favored and which will be reactant favored?
a) K = 24.3
product favored
b) K = 3.4x10-4
reactant favored
c) K =
d) K = 8.25x102

12. Example 5
If the equilibrium constant for the following reaction is 3.8x104 and the equilibrium concentration of N2 is 8.5x10-1 M and H2 is 3.1x10-3 M, what is the equilibrium concentration of NH3?
N2(g) + 3 H2(g) ↔ 2 NH3(g)
Answer: M

13. Solids and Liquids
Only aqueous solutions and gases are included in equilibrium expressions because only they can change their concentrations. Solids and liquids are not included because their concentrations remain constant.

14. Example 6 Write the equilibrium expression for the following reaction.
2 Cu2O (aq) + C (s) ↔ 4 Cu (s) + CO2 (g)