Le Chatelier’s Principle
Le Chatelier’s Principle
• When a chemical system at equilibrium is
disturbed by a stress, the system adjusts
(shifts) to oppose the change
• Stresses include:
• Change in concentration
• Change in pressure (or volume)
• Change in temperature
Change in Concentration
A(g) + 3B(g)  2C(g) + heat
• Increasing the concentration of the reactants OR
• Decreasing the concentration of the products
• Will favour the forward reaction, causing the equilibrium to
shift to the RIGHT
• Decreasing the concentration of the reactants OR
• Increasing the concentration of the products
• Will favour the reverse reaction, causing the equilibrium to
shift to the LEFT
• RECALL: Addition or removal of solid or liquids does not change the
concentration. Therefore does not cause a shift. I.e. only applies to gases
and aqueous solutions.
Change in Concentration
N2(g) + 3H2(g)  2NH3
Change in Pressure
A(g) + 3B(g)  2C(g) + heat
 volume  pressure
 volume  pressure
• Increasing the volume of the container OR Decreasing the
pressure
• Will cause a shift to the side with MORE gas molecules
• In our example, it will shift left (4 molreactants > 2 molproducts)
• Decreasing the volume of the container OR Increasing the
pressure
• Will cause a shift to the side with LESS gas molecules
• In our example, it will shift right (4 molreactants > 2 molproducts)
Change in Temperature
In an exothermic reaction:
• Increasing the
temperature will cause a
shift to the LEFT
• Decreasing the
temperature will cause a
shift to the RIGHT
Change in Temperature
In an endothermic reaction:
• Increasing the temperature
will cause a shift to the
RIGHT
• Decreasing the
temperature will cause a
shift to the LEFT
Change in Temperature
Recall: Keq is temperature dependent.
Therefore, changes in temperature will also
affect Keq
Shift right =  products,  Keq
Shift left =  reactants, Keq
DEMONSTRATION
Variables that do NOT Affect Equilibrium
• Catalysts
• Increases reaction rate by lowering activation energy (of
BOTH the forward and the reverse reactions equally)
• Decreases the time required to reach equilibrium but does
not affect the final position of equilibrium
• Inert Gases
• Increases the pressure, which will increase reaction rate
• Increases the probability of successful collisions for BOTH
products and reactants equally
• Decreases the time required to reach equilibrium but does
not affect the final position of equilibrium
Practice