1. Equilibrium
March 28, 2007

2. Chemical Reactions Reactants  Products A + 3B  2C
But the reverse can also happen
2C  3B + A
So we can describe the reaction as equilibrium
A + 3B 2C
Forward Reaction
Reverse Reaction

3. rate of forward reaction = rate of reverse reaction
Equilibrium
rate of forward reaction = rate of reverse reaction

4. Equilibrium - Concentration
At equilibrium, the concentration of product and reactants stays constant

5. Equilibrium Visualization

6. Law of Mass Action
Once a reaction has reached equilibrium, the relative concentration of products remains constant
We call this constant K, the equilibrium constant
Example: N2 + 3H2  2NH3

7. Equilibrium Expression
Things that appear in the equilibrium expression
Concentration of solutions
Pressure of gases
Reaction coefficient
Things that do NOT appear
Pure liquids
Pure solids
Units

8. Greater number of reactants – K < 1
Learning Check
Does this graph represent a K > 1, K< 1, or K =1 ?
Greater number of reactants – K < 1

9. Learning Check
Determine the Equilibrium Expression (K) for each of the following reactions
CaCO3 (s) CaO (s) + CO2(g)
2NO2(g) N2O4(g)
H2CO3(aq) CO2(g) + H2O(l)

10. Calculating the Equilibrium Constant
Calculate the equilbrium constant if the equilibrium concentrations of NO2 and N2O4 are 2.0 mol/L.
2NO2(g) N2O4(g)

11. Equilibrium Position
At constant temperature….reaction can only have one equilibrium constant but many equilibrium positions
N2 + 3H2 2NH3 K = 640 (25 °C)
Equilibrium concentration of each product can be…
675 45 5
800 40 10
900 30
NH3 (mol/L)
H2 (mol/L)
N2 (mol/L)

12. Value of Equilibrium Constant
A + 2B 2C + D
If K >>>>1, forward reaction is favored
Large concentration of products
If K <<<<1, reverse reaction predominates
Large concentration of reactants
If K = 1, reverse reaction and forward reaction equal
Equal concentration of reactants

13. Changing the Equilibrium Constant
Change the temperature
Change the reaction coefficients
N2 + 3H NH3
2N2 + 6H NH3
Relationship between K1 and K2
K2 = K12 K2 = (640)2 = x 105

14. Changing the Equilibrium Constant
Change the temperature
Change the reaction coefficients
N2 + 3H NH3
2NH3 N2 + 3H2
Relationship between K1 and K3
K3 = 1/K1 K3 =1/ 640 =

15. Learning Check
Determine the value of the equilibrium constant for the following reaction
2NO2 N2O4
½ N2O NO2

16. Disturbing the Equilibrium

17. Heat as a Reactant/ Product
UO2(s) + 4HF(g)  UF4(g) + 2H2O(g

18. FeSCN2+ Equilibrium KSCN + Fe(NO3)3  FeSCN2+ + KNO3
SCN- + Fe3+  FeSCN2+
Fe3+ + HPO42-  FeHPO4+
Ag+ + SCN-  AgSCN
Ag+ + Cl-  AgCl
Spectator ions….ignore