Le Chatelier's Principle: Summary

Le Chatelier's Principle

Equilibrium Analogy

  • Equilibrium is a state where a system appears static, like walking up a downward-moving escalator at the same rate it moves down.
  • An equilibrium system adjusts to remain in equilibrium when conditions change.

Le Châtelier’s Principle

  • If a system in equilibrium is subjected to an external stress, the equilibrium shifts to minimize the effects of that stress.
  • External stresses alter forward or reverse reaction rates, causing imbalance.
  • Factors affecting reaction rates:
    • Concentration
    • Pressure and Volume
    • Temperature
    • Catalyst

Changes in Concentration

  • Adding a reactant increases the forward reaction rate, shifting the equilibrium to the right.
  • Fe + SCN \rightleftharpoons FeSCN
    • Adding Fe: [Fe] \uparrow, [SCN] \downarrow, [FeSCN] \uparrow
  • K_{eq} remains constant despite concentration changes.
  • Adding a product (e.g., FeSCN^{2+}) increases the reverse reaction rate, shifting the equilibrium to the left.
    • [FeSCN] \uparrow, [SCN] \uparrow, [Fe] \uparrow
  • Removing a substance shifts the equilibrium to counteract the removal.
    • Removing SCN: [FeSCN] \downarrow, [SCN] \downarrow, [Fe] \uparrow

Changes in Pressure & Volume

  • Applicable only to systems involving gases.
  • Increasing pressure (reducing volume) shifts equilibrium towards fewer gas moles.
  • Decreasing pressure shifts equilibrium towards more gas moles.
  • Equal gas moles on both sides means pressure change has no effect.
  • Example: N2O4 (g) \rightleftharpoons 2NO_2 (g)
    • Increased pressure shifts equilibrium to the left.

Changes in Temperature

  • Two consequences:
    • Increased temperature favors endothermic reaction.
    • K_{eq} changes.
  • N2O4 (g) + 58.0 kJ \rightleftharpoons 2NO_2 (g)
    • Adding heat shifts equilibrium to the right, increasing NO_2.
  • If forward reaction is favored, K{eq} increases; if reverse, K{eq} decreases.
  • Removing heat favors exothermic reaction.

Addition of a Catalyst

  • Increases both forward and reverse reaction rates equally.
  • Does not shift equilibrium; helps achieve equilibrium faster.

Example

  • H2O2 (g) \rightleftharpoons H2 (g) + O2 (g), \Delta H = 187 kJ/mol
    • Adding H_2: Shifts left
    • Removing O_2: Shifts right
    • Increasing temperature: Shifts right
    • Reducing volume: Shifts left