Topic 7.9 Notes – Introduction to Le Châtelier's Principle

1. Le Châtelier’s Principle

At dynamic equilibrium, the forward and reverse reaction rates are equal. Concentrations stay constant, but particles are still reacting.

Le Châtelier’s Principle:
When a system at equilibrium experiences a stress, it shifts in the direction that reduces that stress and establishes a new equilibrium.

Keep these straight:

• The system shifts to reduce what you changed.
• Equilibrium does not mean equal amounts.
• K only changes if temperature changes.
  (Concentration, pressure, dilution, catalyst → K stays the same.)

You always reason from the balanced equation. Ask yourself, “What did I change, and which direction helps undo that?”

2. The Four Types of Stress

a. Changes in Concentration

Applies to gases and aqueous species. Pure solids and liquids don’t count in the equilibrium expression.

If you disturb concentration:

Example:

$$\ce{2SO2(g) + O2(g) <=> 2SO3(g)}$$

Add $\ce{O2}$ → shift right to consume it.

A common AP twist:
If you add something that reacts with a species and removes it, that counts as removing it.

Example: If something binds $\ce{SO3}$, lowering its concentration → shift right to make more $\ce{SO3}$.

K does not change. Only the equilibrium concentrations shift.

b. Temperature Changes

Temperature is different because it changes K.

First, treat heat as a reactant or product.

• Exothermic (ΔH < 0)
  $$\text{Reactants} \rightleftharpoons \text{Products} + \text{heat}$$
• Endothermic (ΔH > 0)
  $$\text{Reactants} + \text{heat} \rightleftharpoons \text{Products}$$

Then apply the same logic as concentration.

If temperature increases:

• Shift away from heat
• Favor the endothermic direction

If temperature decreases:

• Shift toward heat
• Favor the exothermic direction

Effect on $K$:

• Exothermic: ↑T → $K$ decreases
• Endothermic: ↑T → $K$ increases

On tests, they love asking whether $K$ changes. Only temperature affects it.

c. Pressure and Volume (Gas Systems Only)

Only matters if gases are involved.

Pressure ↑ (volume ↓) → shift toward fewer moles of gas
Pressure ↓ (volume ↑) → shift toward more moles of gas

Example:

$$\ce{N2O4(g) <=> 2NO2(g)}$$

Left side has 1 mole gas. Right side has 2.

Increase pressure → shift left (fewer moles).

If both sides have equal gas moles → no shift.

Inert gas rule:

• Added at constant volume → no shift
  (Partial pressures of reacting gases don’t change.)

d. Dilution

Dilution means adding water to an aqueous system.

All aqueous concentrations decrease.

System shifts toward the side with more dissolved particles to increase total concentration again.

Solids and pure liquids still don’t matter.

3. What Does Not Shift Equilibrium

Catalyst

• Speeds up forward and reverse reactions equally
• Does not change:
  • $K$
  • Equilibrium position
• Only reduces time to reach equilibrium

Adding More Solid or Pure Liquid

If it’s not in the equilibrium expression, it won’t cause a shift.

Students often overthink this. If it’s not in $K$, ignore it for shifting.

4. What You Actually Observe

AP questions rarely stop at “Which way does it shift?” They ask what changes physically.

pH

If equilibrium involves $\ce{H+}$:

• Shift toward $\ce{H+}$ → pH decreases
• Shift away from $\ce{H+}$ → pH increases

Color

If different species have different colors:

• Shift toward that species → color intensifies
• Shift away → color fades

Classic example:

$$\ce{Co(H2O)6^{2+}(aq) + 4Cl^{-}(aq) <=> CoCl4^{2-}(aq) + 6H2O(l)}$$

Pink ⇌ Blue system. Changing conditions visibly shifts equilibrium.

Temperature of the System

• Shift in exothermic direction → temperature rises
• Shift in endothermic direction → temperature drops

5. How to Tackle Equilibrium Shift Questions

When you see one on a quiz or FRQ:

1. Write the balanced equation.
2. Identify the stress.
3. Apply the correct rule.
4. Decide direction of shift.
5. Predict what increases/decreases.
6. Translate that into pH, color, pressure, or temperature change.

Most mistakes happen when students skip step 1 and try to guess.