Le Châtelier’s Principle

Le Châtelier’s Principle

• If a stress is applied to a system at equilibrium, the system shifts in the direction that relieves the stress.

• Stress is anything that upsets equilibrium – concentration, pressure, or temperature.

Concentration

• Measure of molarity (moles/L)

• If you ↑concentration of a reactant, equilibrium will shift toward the products.

• If you ↓concentration of a reactant, equilibrium will shift toward the reactants.

• CO(g) + 3H2(g) ↔ CH4(g) + H2O(g)

• CO(g) + 3H2(g) ↔ CH4(g) + H2O(g)

• CO(g) + 3H2(g) ↔ CH4(g) + H2O(g)

• CO(g) + 3H2(g) ↔ CH4(g) + H2O(g)

CO(g)

H2O(g)

Pressure

• Ideal Gas Law: PV=nRTPV=nRT

• If ↑P then ↑n, which means more number of atoms.

• If ↑P, then the equilibrium will shift toward the side with fewer moles of gas.

• CO(g) + 3H2(g) ↔ CH4(g) + H2O(g)

4 moles of gas 2 moles of gas

Note: If moles of gaseous reactant = moles of gaseous product, then no shift in equilibrium will occur from a change in pressure

Temperature

• Think of heat as a reactant or a product.

• CO(g) + 3H2(g) ↔ CH4(g) + H2O(g)

ΔH = -206.5kJ

Is this Exothermic or Endothermic?

Will heat be a reactant or product?

heat

CO(g) + 3H2(g) ↔ CH4(g) + H2O(g) + heat

CO(g) + 3H2(g) ↔ CH4(g) + H2O(g) + heat

Le Châtelier’s Principle

• If a stress is applied to a system at equilibrium, the system shifts in the direction that relieves the stress.– Concentration

– Pressure

– Temperature

Note: Only temperature affects K. The larger the value of K, the more product at equilibrium.