1. Chemical equilibrium is a state in which the forward and reverse reactions balance each other because they take place at equal rates.
Rateforward reaction = Ratereverse reaction

2. Chemical Equilibrium: Basic Concepts
Topic 22
Reversible reactions
You can recognize that the ammonia-forming reaction reaches a state of chemical equilibrium because its chemical equation is written with a double arrow like this.
At equilibrium, the concentrations of reactants and products are constant.
However, that does not mean that the amounts or concentrations of reactants and products are equal.

3. Factors Affecting Chemical Equilibrium
Chemical Equilibrium: Basic Concepts
Topic 22
Factors Affecting Chemical Equilibrium
Le Châtelier’s principle states that if a stress is applied to a system at equilibrium, the system shifts in the direction that relieves the stress.
For example, consider the equilibrium system
If an additional amount of reactant (NO or Br2) is added to the system, the equilibrium will shift to the right, that is, more product (NOBr) will be formed.

4. Factors Affecting Chemical Equilibrium
Chemical Equilibrium: Basic Concepts
Topic 22
Factors Affecting Chemical Equilibrium
Conversely, adding more NOBr to the system will result in a shift to the left, forming more NO and Br2.
The removal of a reactant or product also results in a shift in the equilibrium.
Removing a reactant causes the equilibrium to shift to the left, forming more reactants.
Removing the product causes a shift to the right, forming more product.

5. Chemical Equilibrium: Basic Concepts
Topic 22
Changes in volume
Le Châtelier’s principle also applies to changes in the volume of the reaction vessel containing the equilibrium system.
Suppose the volume of the reaction vessel for the system is decreased, resulting in an increase in pressure.
The equilibrium will shift to relieve the stress of increased pressure.

6. Chemical Equilibrium: Basic Concepts
Topic 22
Changes in volume
In this case, the shift will be to the right because three moles of reactant gas combine to form only two moles of product gas.
Thus, a shift toward the product will reduce the pressure of the system.
If the volume of the reaction vessel was increased, the equilibrium would shift to the left, and more of the reactants would be formed.

7. Chemical Equilibrium: Basic Concepts
Topic 22
Changes in volume
Note that changing the volume of the reaction vessel causes no shift in the equilibrium when the number of moles of product gas equals the number of moles of reactant gas; an example is the equilibrium

8. Changes in temperature
Chemical Equilibrium: Basic Concepts
Topic 22
Changes in temperature
Consider the thermochemical equation for the reversible formation of hydrogen chloride gas from its elements.
The forward reaction releases heat, so you can consider heat as a product in the forward reaction and a reactant in the reverse reaction.

9. Changes in temperature
Chemical Equilibrium: Basic Concepts
Topic 22
Changes in temperature
Raising the temperature of this system requires the addition of heat, which shifts the equilibrium to the left and reduces the concentration of hydrogen chloride.
Lowering the temperature of the system means that heat is removed, so the equilibrium relieves the stress by shifting to the right, increasing both the concentration of hydrogen chloride.