Overview
This lecture explains Le Chatelier’s principle and how changes in concentration, temperature, and pressure affect chemical equilibrium in reactions.
Le Chatelier’s Principle
- Le Chatelier's principle states that a system at equilibrium will adjust to counteract any applied stress and restore balance.
- Stresses that can affect equilibrium include changes in concentration, temperature, and pressure/volume.
Effects of Concentration Changes
- Adding reactants shifts equilibrium toward the products (to the right) as the system tries to use up added reactants.
- Adding products shifts equilibrium toward the reactants (to the left) to reduce the excess products.
- Removing reactants causes the system to produce more reactants (shift left), while removing products prompts the system to produce more products (shift right).
Effects of Temperature Changes
- If a reaction is exothermic (ΔH negative), heat is a product; increasing temperature shifts equilibrium left, decreasing temperature shifts it right.
- If a reaction is endothermic (ΔH positive), heat is a reactant; increasing temperature shifts equilibrium right, decreasing it shifts left.
- Heat can be treated as an additional reactant or product based on the reaction’s enthalpy change.
Effects of Volume and Pressure Changes (for Gaseous Reactions)
- Decreasing volume increases pressure, causing equilibrium to shift toward the side with fewer gas particles.
- Increasing volume decreases pressure, causing equilibrium to shift toward the side with more gas particles.
- The direction of the shift depends on the number of gas molecule moles on each side of the reaction.
Key Terms & Definitions
- Equilibrium — A state in a chemical reaction where the forward and reverse reactions occur at the same rate.
- Le Chatelier’s Principle — The concept that a system at equilibrium reacts to oppose applied stress.
- Exothermic Reaction — A reaction that releases heat (ΔH negative).
- Endothermic Reaction — A reaction that absorbs heat (ΔH positive).
- ΔH (Delta H) — Change in enthalpy (heat content) of a reaction.
Action Items / Next Steps
- Review example reactions and practice predicting equilibrium shifts given changes in concentration, temperature, or pressure.
- Prepare questions for the next session if concepts remain unclear.