1. Equilibrium
Chapter 19-2

2. Reversible Reactions
A reaction in which the conversion of reactants into products and the conversion of products into reactants occur simultaneously

3. Reversible Reactions
The reactions occur simultaneously in both directions.
2SO2 + O SO3
The double arrows in the equation show that the reaction is reversible.

4. Chemical equilibrium
It is a state in which the forward reaction and reverse reaction takes place at the same rate.

5. Le Châtelier’s Principle
When stress is applied to a system at equilibrium, the system changes to relieve the stress.
It is used to predict the changes that occur when a system at equilibrium is disturbed.

6. Le Châtelier’s Principle: Factors Affecting Equilibrium
Concentration
Changing the amount, or concentration, of any reactant or product in a system at equilibrium disturbs the equilibrium

7. Le Châtelier’s Principle: Factors Affecting Equilibrium
Example:
H2CO CO2 + H2O
Take away CO2
More CO2 will be produced

8. Le Châtelier’s Principle: Factors Affecting Equilibrium
2.- Temperature
Increasing the temperature causes the equilibrium position of a reaction to shift in the direction that absorbs heat.

9. Le Châtelier’s Principle: Factors Affecting Equilibrium
Example:
2SO2 + O SO3 + Heat
Heat is a product.
In this example, heating the reaction mixture at
equilibrium the product yield decreases.
Cooling, or removing heat, the product yield
increases

10. Le Châtelier’s Principle: Factors Affecting Equilibrium
3.- Pressure
A change in the pressure on a
system affects only an equilibrium
that has an unequal number of
moles of gaseous reactants and
products.

11. Le Châtelier’s Principle: Factors Affecting Equilibrium
Increasing pressure favors the formation of product.
There are less total moles in the product side.
N H NH3
P more products

12. Le Châtelier’s Principle: Factors Affecting Equilibrium
Increasing pressure favors the formation of reactant.
There are less total moles in the reactant side.
PCl PCl3 + Cl2

13. Equilibrium Constant Symbol for equilibrium constant = Keq
Is the ratio of product concentrations to reactant concentrations at equilibrium, with each concentration raised to the power equal to the number of moles of that substance in the balanced chemical reaction.

14. Equilibrium Constant
aA + bB cC + dD
[C]c x [D]d
Keq =
[A]a x [B]b

15. Equilibrium Constant
The [ ] square brackets indicate the concentrations of substances in moles per liter (mol/L)
mol/L = Molarity
mol/L = M

16. Equilibrium Constant
2SO2 + O SO3
[SO3]2
Keq =
[SO2]2 [O2]

17. Equilibrium Constant If Keq > 1 products favored at equilibrium.
If Keq < 1 reactants favored at equilibrium.

18. Sample Question
Calculate Keq for this reaction if the equilibrium concentrations are:
[SO2] = 0.42 M, [O2] = 0.21 M, [SO3] = M
2SO SO2 + O2