Le Chatelier's Principle describes how a chemical system at equilibrium reacts when it experiences a change in conditions. Think of it as a balancing act: if you push a system out of balance, it will shift its weight to counteract that push and find a new state of stability.
The formal definition is: If a system at equilibrium is subjected to a change in concentration, temperature, or pressure, the system will shift its equilibrium position to counteract the effect of the disturbance.
The Three Main Stressors
There are three primary ways to stress a system at equilibrium:
Concentration
Add more reactant: The system shifts right (toward products) to use up the extra reactant.
Remove a product: The system shifts right to replace what was lost.
Add more product: The system shifts left (toward reactants) to use up the extra product.
Temperature
To predict temperature shifts, you must treat heat as either a reactant or a product:
Exothermic Reactions (negative Delta H): Heat is a product. Increasing temp shifts the reaction left.
Endothermic Reactions (positive Delta H): Heat is a reactant. Increasing temp shifts the reaction right.
Pressure (Gases Only)
Pressure only affects systems with different numbers of gas molecules on each side:
Increase Pressure: The system shifts toward the side with fewer moles of gas to reduce the crowding.
Decrease Pressure: The system shifts toward the side with more moles of gas.
Practice Problems
Use the following reversible reaction for the questions below:
N2(g) + 3H2(g) <=> 2NH3(g) + heat
Concentration Change
If you inject more N2 gas into the container, which way will the equilibrium shift?
A) Toward the products (Right)
B) Toward the reactants (Left)
C) No change
Temperature Change
If the reaction vessel is placed in an ice bath (decreasing the temperature), which way will the equilibrium shift?
A) Left, favoring N2 and H2
B) Right, favoring NH3
Pressure Change
If the volume of the container is decreased (increasing the pressure), how will the system respond?
A) Shift left (4 moles of gas to 2 moles of gas)
B) Shift right (4 moles of gas to 2 moles of gas)
C) No shift because both sides are gases.
The Catalyst Question
If a catalyst is added to this reaction at equilibrium, what happens to the equilibrium position?
A) Shifts Right
B) Shifts Left
C) No change in position, but it reaches equilibrium faster.
Answer Key
A (Right): The system tries to consume the added N2.
B (Right): Since it is exothermic, removing heat pulls the reaction toward the heat-producing side.
B (Right): The left side has 4 moles (1+3), and the right side has 2 moles. Higher pressure favors the side with fewer moles.
C (No change): Catalysts speed up both the forward and reverse reactions equally!
