7.10 Reaction Quotient and Le Châtelier's Principle

Reaction Quotient Q vs. K

The reaction quotient (Q) has the same expression as the equilibrium constant K, but uses current (non-equilibrium) concentrations or pressures. Comparing Q to K predicts reaction direction: if Q < K, the reaction shifts forward (toward products); if Q > K, it shifts reverse (toward reactants); if Q = K, the system is at equilibrium.

Le Châtelier's Principle basics

Le Châtelier's Principle states that when a system at equilibrium is disturbed (by changing concentration, pressure/volume, or temperature), it shifts to relieve the stress and restore equilibrium. Adding a reactant shifts the reaction forward; removing product also shifts forward. These stresses momentarily change Q relative to K, driving the shift.

Temperature and volume effects

Changing temperature actually changes the value of K itself: raising T favors the endothermic direction, lowering T favors the exothermic direction. Changing volume/pressure shifts equilibrium toward the side with fewer moles of gas (compression) or more moles of gas (expansion), but does not change K. Adding an inert gas at constant volume has no effect on equilibrium.

Key terms

Reaction Quotient (Q)

An expression with the same form as K, but calculated using current (non-equilibrium) concentrations or pressures; compared to K to predict shift direction.

Q < K

System shifts forward, producing more products until Q rises to equal K.

Q > K

System shifts in reverse, producing more reactants until Q falls to equal K.

Le Châtelier's Principle

A system at equilibrium responds to a disturbance (concentration, pressure/volume, temperature change) by shifting to relieve the stress and restore equilibrium.

Effect of temperature change on K

Increasing T favors the endothermic direction (increases K for endothermic-forward reactions); decreasing T favors the exothermic direction. Unlike concentration/volume changes, temperature change alters K itself.

Try an AP-style question

For 2NO₂ ⇌ N₂O₄, the volume is halved. What happens to Q right after?

1. K doubles
2. Q increases above K, so it shifts left
3. Q stays equal to K
4. Q decreases below K, so the reaction shifts right