Le Chatelier's Principle and Equilibrium Shifts

Le Chatelier's Principle

When a system at equilibrium is disturbed, it will shift to partially counteract the disturbance and establish a new equilibrium.

Concentration Changes

| Disturbance | Shift | Effect on K | |-------------|-------|-------------| | Add reactant | Forward → | No change | | Remove reactant | ← Reverse | No change | | Add product | ← Reverse | No change | | Remove product | Forward → | No change |

K does not change when concentrations change (at constant T).

Pressure/Volume Changes (gases only)

| Disturbance | Shift Direction | |-------------|----------------| | Decrease volume (increase P) | Toward fewer moles of gas | | Increase volume (decrease P) | Toward more moles of gas | | Add inert gas (constant V) | No shift |

If $\Delta n_{gas} = 0$, pressure changes have no effect.

Temperature Changes

Temperature is the only factor that changes K.

| Reaction Type | Increase T | Decrease T | |--------------|------------|------------| | Exothermic ($\Delta H < 0$) | Shift left, K decreases | Shift right, K increases | | Endothermic ($\Delta H > 0$) | Shift right, K increases | Shift left, K decreases |

Think of heat as a product (exothermic) or reactant (endothermic).

Catalyst Effects

A catalyst:

• Does NOT shift equilibrium
• Does NOT change K
• Reaches equilibrium faster
• Lowers activation energy equally for forward and reverse reactions

Common Ion Effect

Adding an ion already present in solution shifts the equilibrium away from that ion.

$\text{AgCl}(s) \rightleftharpoons \text{Ag}^+(aq) + \text{Cl}^-(aq)$

Adding NaCl (source of Cl⁻) shifts left → less AgCl dissolves.

AP Chemistry Tip: Always state both what happens (shift direction) AND why (Le Chatelier's principle counteracts the stress).