Ch. 16 Equilibrium: Le Chatelier's Principle

Le Chatelier’s Principle

  • Definition: When a reaction that is at equilibrium is disturbed, the reaction will move in such a way as to try to undo the disturbance.

    • Shifts occur to reestablish equilibrium positions.

    • Consider the reaction quotient, Q.

    • The effect of catalysts:

    • Catalysts have NO effect on the equilibrium constant K.

    • They establish equilibrium faster.

Factors Affecting Equilibrium

  1. Temperature

    • Exothermic Reactions: Heat is a product.

    • Endothermic Reactions: Heat is a reactant.

    • Increasing Temperature:

      • The reaction wants to use up the extra temperature.

      • Shifts towards the endothermic reaction (the side that absorbs heat).

    • Decreasing Temperature:

      • The reaction wants to increase the extra temperature.

      • Shifts towards the exothermic reaction (the side that releases heat).

  2. Adding or Removing a Reagent

    • The shift tries to reestablish equilibrium according to Le Chatelier’s principle.

    • Adding Reactant:

      • The reaction tries to use up the added reactant.

      • Shifts right, towards the products.

    • Adding Product:

      • The reaction tries to use up the added product.

      • Shifts left, towards the reactants.

  3. Change in Pressure

    • Changes in pressure primarily affect systems involving gases.

    • Important Note: If the moles of gas are the same on both sides, then changes in volume have NO effect on the equilibrium position.

    • Increasing Pressure:

      • Results in a decrease in volume, leading to more collisions between molecules.

      • The system moves towards the side of the reaction with fewer gas moles (favors the side with the smaller number of moles of gas).

    • Decreasing Pressure:

      • Results in an increase in volume, lowering the number of collisions.

      • The system moves towards the side of the reaction that has more gas moles (favors the side with the greater number of moles of gas).

    • Change in Volume: Affects the pressure
      —considerations similar to changes in pressure apply.

    • Adding an Inert Gas:

      • Increasing pressure by adding an inert gas at constant volume has NO effect on the equilibrium because what matters for equilibrium are the partial pressures of the reactants and products.

      • Adding an inert component to a system at constant volume will change the total pressure but not the partial pressures of the compounds involved.

  4. Pneumonics to Remember This

    • Le Chatelier is a Stubborn Child:

      • Imagine Le Chatelier as a child who always resists what you give or take.

      • If you give him more of something (reactant/product, heat), he tries to push it away or use it up.

      • If you take something away from him, he tries to make more of it.

    • Temperature is a Party Guest:

      • Think of heat as an unwanted guest at an equilibrium party.

      • If you turn up the heat (increase temperature), the reaction shifts to the side that "uses up" the heat (endothermic side), trying to kick the guest out.

      • If the room gets cold (decrease temperature), the reaction shifts to the side that "produces" heat (exothermic side), trying to warm up.

    • Pressure is a Shrinking Room:

      • For gases, imagine the reaction taking place in a room.

      • If you increase the pressure, the room is shrinking! The molecules want to make more space for themselves, so the equilibrium shifts to the side with fewer gas moles.

      • If you decrease the pressure, the room is expanding! Now there's plenty of space, so the equilibrium shifts to the side with more gas moles.