Equilibrium - Le Chatelier's Principle

Le Chatelier's Principle

  • Le Chatelier’s Principle: The principle states that if a change in conditions (stress) is applied to a system at equilibrium, the system will adjust to counteract that change.

Conditions That Affect Equilibrium:

  • Concentration

  • Temperature

  • Pressure (volume)

  • Catalyst

Effect of Concentration on Equilibrium

  • If the concentration of a reactant is increased:

    • The rate of the forward reaction will increase, while the reverse reaction will not be affected.

    • Forward and reverse reaction rates will be unequal until equilibrium is reestablished.

    • The equilibrium will shift towards products to counteract the increase in reactant concentration.

Concentration Change Examples:

  1. As4O6(aq) + 6C(s) ⇌ 4(g) + 6CO(g)

    • Adding more As4O6(aq): shifts right (towards products).

    • Removing As4(g): shifts right (towards products).

    • Adding more C(s): no effect (pure solid).

  2. N2(g) + 3 H2(g) ⇌ 2NH3(g)

    • Adding more NH3(g): shifts left (towards reactants).

    • Removing NH3(g): shifts right (towards products).

Effect of Pressure on Equilibrium

  • Pressure changes significantly impact gaseous equilibria, favoring sides with fewer gas molecules.

    • An increase in pressure favors the side with the smaller number of molecules.

    • A decrease in pressure favors the side with the larger number of molecules.

Pressure Change Examples:

  1. PCl3(g) + Cl2(g) ⇌ PCl5(g)

    • Increase pressure: shifts right (2 mol → 1 mol).

  2. PCl3(g) + 3 NH3(g) ⇌ P(NH2)3(g) + 3 HCl(g)

    • Increase pressure: shifts right (4 mol → 4 mol — no effect).

  3. N2(g) + 3 H2(g) ⇌ 2NH3(g)

    • Increase pressure: shifts right.

    • Decrease pressure: shifts left.

Effect of Volume on Equilibrium

  • Volume changes affect gases since they directly change pressure (inversely proportional).

    • If volume increases, pressure decreases, and vice versa.

Volume Change Example:

  • N2(g) + 3H2(g) ⇌ 2NH3(g)

    • Volume reduction (2 L to 1 L): shifts to the right (fewer gas particles).

Effect of Adding a Noble Gas

  • Adding a noble gas does not alter the equilibrium reaction because noble gases are unreactive.

  • It increases the total pressure, but partial pressures of reacting gases remain unchanged, causing no shift.

Effect of Temperature on Equilibrium

  • Kc (equilibrium constant) is temperature-dependent

    • Exothermic Reactions: Release heat, shifting left if temperature increases (moving back to reactants).

    • Endothermic Reactions: Absorb heat, shifting right if temperature decreases (favoring products).

Temperature Change Examples:

  1. N2 + O2 ⇌ 2NO (ΔH0 = +181 kJ):

    • Increase temperature: moves right (products).

  2. N2 + O2 ⇌ 2NO (ΔH0 = -181 kJ):

    • Increase temperature: moves left (reactants).

Catalysts and Equilibrium

  • Catalysts speed up reactions by providing a pathway with lower activation energy (Ea).

    • They increase the rates of both forward and backward reactions alike but do not affect equilibrium position.

    • Application areas include biochemical reactions and green chemistry.

Conclusion

  • Changes in concentration, pressure, volume, temperature, and presence of catalysts significantly influence the position of chemical equilibria. Understanding these principles is essential for predicting system behavior under different conditions.