Le Châtelier’s Principle

1. Le Châtelier’s Principle • System starts at equilibrium. • A change/stress is then made to system at equilibrium. • Change in concentration • Change in volume • Change in pressure • Change in Temperature • Add Catalyst • System responds by shifting to reactant or product side to restore equilibrium.

2. N2(g) + 3H2(g) 2NH3(g) Equilibrium shifts left to offset stress Add NH3 Le Châtelier’s Principle If an external stress is applied to a system at equilibrium, the system adjusts in such a way that the stress is partially offset as the system reaches a new equilibrium position. • Changes in Concentration 14.5

3. Remove Remove Add Add aA + bB cC + dD Le Châtelier’s Principle • Changes in Concentration continued Change Shifts the Equilibrium Increase concentration of product(s) left Decrease concentration of product(s) right Increase concentration of reactant(s) right Decrease concentration of reactant(s) left 14.5

4. Changes in pressure • Pressure is caused by gas molecules hitting the sides of their container. • The more molecules you have in the container, the higher the pressure will be. • Pressure can increase or decrease by changing the volume of the container • Decreasing the volume of the container _______ the pressure; increasing the volume _____ the pressure • The system can reduce the pressure by reacting in such a way as to produce fewer molecules.

5. 2NO2(g) ↔ N2O4(g)Brown colourless Chemistry; The Science in Context;by Thomas R. Gilbert, Rein V. Kirss, and Geoffrey Davies, Norton Publisher, 2004, p 752

6. A (g) + B (g) C (g) Le Châtelier’s Principle • Changes in Volume and Pressure Change Shifts the Equilibrium Increase pressure Side with fewest moles of gas Decrease pressure Side with most moles of gas Increase volume Side with most moles of gas Decrease volume Side with fewest moles of gas 14.5

7. Changes in temperature If we consider heat as a product in the following reaction: A + B  C + heat and increasing the temperature of a system as increasing “concentration” of that product, then increasing the temperature will shift the equilibrium to the ________ and decreasing the temperature will shift the equilibrium to the _______. What happens if the heat term is on the reactant side?

8. Le Châtelier’s Principle; Temperature Effect Chemistry; The Science in Context;by Thomas R. Gilbert, Rein V. Kirss, and Geoffrey Davies, Norton Publisher, 2004, p 764

9. Keq is temperature dependent • What effect does an increase in temperature have on Keq? • It depends on which side the heat term is written. • Consider A + B  C + heat Increasing temperature causes [C] to ____ and [A] and [B] to _____ What effect does this have on Keq? What effect does a decrease in temperature have on Keq? How do these changes in temperature effectKeq for the reaction: A + B + heat  C?

10. uncatalyzed catalyzed Le Châtelier’s Principle • Adding a Catalyst • does not change K • does not shift the position of an equilibrium system • system will reach equilibrium sooner Catalyst lowers Ea for both forward and reverse reactions. Catalyst does not change equilibrium constant or shift equilibrium. 14.5

11. Change Equilibrium Constant Le Châtelier’s Principle Change Shift Equilibrium Concentration yes no Pressure yes no Volume yes no Temperature yes yes Catalyst no no 14.5