Basic Idea: When a change is imposed on a system at equilibrium, the system shifts to undo the change and relieve stress.
Analogy: If equilibrium is represented as a straight line, any attempt to raise or lower it prompts a counter-movement to restore balance.
Equilibrium in Chemical Reactions
Dynamic Equilibrium: The rate of the forward reaction equals the rate of the reverse reaction, leading to no net change in concentrations of reactants and products.
Example Reaction:
Reactants: A and B (B in gaseous form)
Products: C (gas) and D (liquid)
Equilibrium is affected by changes in concentration or pressure.
Effect of Changes on Equilibrium
Increasing Concentration of Reactant (e.g., B):
Reaction shifts to right (towards products) to decrease the increased reactant.
Decreasing Concentration of Reactant (e.g., B):
Reaction shifts to left (towards reactants) to increase the decreased reactant.
Increasing Concentration of Product (e.g., C):
Reaction shifts to left to decrease the increased product.
Decreasing Concentration of Product (e.g., C):
Reaction shifts to right to increase the decreased product.
Effect of Adding Solid or Liquid
Example: Adding more of solid A or liquid D does not affect equilibrium as they are not included in the equilibrium constant k.
Practice Problems on Le Chatelier's Principle
Shifting Reaction Directions:
Adding or removing reactants/products shifts equilibrium to restore balance.
Adding Catalyst: Speeds up reaction but does not affect equilibrium position.
Effect of Gaseous Changes
Volume and Pressure:
Increasing volume decreases pressure; reaction shifts to side with more moles of gas.
Decreasing volume increases pressure; reaction shifts to side with fewer moles.
Key Points to Remember
Direction of Shift:
Towards increased reactants/products to decrease them.
Away from decreased reactants/products to increase them.
Catalysts: Affect rate but not the position of equilibrium.
Solid/Liquid Changes: Generally do not affect equilibrium unless in gaseous or aqueous states.
Additional Considerations
Partial Pressure vs. Concentration: Both play roles in determining shifts but are context-dependent (gas vs. solution).
Net Shifts: Determined by comparing shifts toward or away from reactants/products.