nexus

Le Chateliers Principle Notes

Le Chateliers Principle states that if a change of condition is applied to a system in equilibrium, the system will shift in a direction that relieves the stress. This applies to changes in concentration, pressure, volume, and temperature.

Types of Stress and System Response:

• Change in Concentration:

  • Increasing the concentration of a reactant shifts the equilibrium to the right (towards products).
  • Increasing the concentration of a product shifts the equilibrium to the left (towards reactants).
  • Decreasing the concentration of a reactant shifts the equilibrium to the left.
  • Decreasing the concentration of a product shifts the equilibrium to the right.

• Change in Pressure/Volume (for gaseous systems):

  • Increasing pressure (or decreasing volume) favors the side with fewer moles of gas.
  • Decreasing pressure (or increasing volume) favors the side with more moles of gas.
  • If the number of moles of gas is the same on both sides, a change in pressure/volume has no effect.

• Change in Temperature:

  • This is more complex and depends on whether the reaction is endothermic or exothermic.
    • Exothermic Reaction ( $ \Delta H < 0 $ ): Increasing temperature shifts the equilibrium to the left (towards reactants); decreasing temperature shifts it to the right (towards products). Heat is treated as a product.
    • Endothermic Reaction ( $ \Delta H > 0 $ ): Increasing temperature shifts the equilibrium to the right; decreasing temperature shifts it to the left. Heat is treated as a reactant.

Mathematical Representation (using $ K_{eq} $ ):

The equilibrium constant, $ K_{eq} $ , remains constant at a given temperature. Changes in concentration, pressure, or volume will cause a shift in equilibrium, but $ K_{eq} $ itself will not change unless the temperature changes.

Examples:

• Consider the reaction: $ N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g) $ (This is an exothermic reaction)
  • Increasing the concentration of $ N_2 $ will shift the equilibrium to the right.
  • Increasing the pressure will shift the equilibrium to the right (fewer moles of gas on the product side).
  • Increasing the temperature will shift the equilibrium to the left (favoring reactants as heat is a product).

Equilibrium (For a detailed explanation of equilibrium constants, $ K_{eq} $ , $ K_p $ , $ K_c $ , etc.)

Acids and Bases (For how Le Chatelier’s Principle]] applies to buffer systems)

Endothermic and Exothermic Reactions (A deeper look at enthalpy and its effect on equilibrium)