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: $N<em>2O</em>4 (g) \rightleftharpoons 2NO_2 (g)$
  • Increased pressure shifts equilibrium to the left.

Changes in Temperature

• Two consequences:
  • Increased temperature favors endothermic reaction.
  • $K_{eq}$ changes.
• $N<em>2O</em>4 (g) + 58.0 kJ \rightleftharpoons 2NO_2 (g)$
  • Adding heat shifts equilibrium to the right, increasing $NO_2$.
• If forward reaction is favored, $K<em>{eq}$ increases; if reverse, $K</em>{eq}$ 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

• $H<em>2O</em>2 (g) \rightleftharpoons H<em>2 (g) + O</em>2 (g)$, $\Delta H = 187 kJ/mol$
  • Adding $H_2$: Shifts left
  • Removing $O_2$: Shifts right
  • Increasing temperature: Shifts right
  • Reducing volume: Shifts left