Unit 4. Kinetics+Equilibrium 4.3 What is the LeChatelier’s Principle?

1. AIM: How do various stresses on a chemical system affect the system at equilibrium? Unit 4. Kinetics+Equilibrium4.3 What is the LeChatelier’s Principle? Do Now:Imagine a situation where there are 4 people on theright side of the room and 2 people on the left side of the room 1. Which side has the most people?2. If you add two more people on the right side, how will we have to move the people in order balance the room out?3. What does this situation remind you of? J March 14, 2011

2. LeChatelier’s Principle

3. DEFINITION Any change in temperature, concentration, or pressure on an equilibrium system is called a stress. Le Chatelier’s principle explains how a system at equilibrium responds to relieve any stress on the system

4. CONCENTRATION CHANGES: + Situation: Consider the following equation CH4 (g) + H2O (g) ↔ 3H2 (g) + CO (g) THE STRESS: adding more methane (CH4), which way will the reaction want to go in order to respond to relieve the stress? Reaction will go towards the RIGHT, so rate of forward reaction will and products will form increase more

5. KEY IDEA If the concentration of one substance is increased, how would a system in an equilibrium want to respond? *response would be to have a reaction (forward or reverse) that will reduce the amount of the added substance.

6. CONCENTRATION CHANGES: - Situation: Consider the following equation CH4 (g) + H2O (g) ↔ 3H2 (g) + CO (g) What if the concentration of CH4 is reduced? WHICH WAY WILL THE REACTION GO? STRESS: Less CH4 Reaction will go towards the LEFT, so rate of reverse reaction will and CH4 will be formed. increase more

7. EXAMPLE • The following example shows how a change in concentration affects equilibrium. A plus sign (+) means the concentration increases, and a minus sign (-) means that the concentration decreases • 1) 4NH3 (g) + 5O2(g) ↔ 4NO (g) + 6H2O (g) + heat • Stress: Add more NH3 • Response: • Stress: Less NH3 • Response: + - Reaction will go to the right. Rate of Fwd Reaction will increase More Products will form Reaction will go to the left. Rate of Rvse Reaction will increase More NH3 will be formed.

8. EXAMPLE + - • 2) 2NO2 (g) + 7H2 (g) ↔ 2NH3 (g) + 4H2O (g) + 1127 kJ • Stress: Add more NH3 • Response: • Stress: Less NH3 • Response: Reaction will go to the left. Rate of Rvse Reaction will increase More Reactants will form Reaction will go to the right. Rate of Fwd Reaction will increase More NH3 will be formed.

9. EXAMPLE 3) What stress would cause the equilibrium to shift right in the following reaction? Zn (s) + HCl (aq) ↔ ZnCl2 (aq) + H2 (g) That means we want rate of FWD reaction to increase So we need less of ZnCl2 (aq) or H2(g) OR We need more of Zn (s) or HCl (aq)

10. EXAMPLE 4) What stress would cause the equilibrium to shift left in the following reaction? Zn (s) + HCl (aq) ↔ ZnCl2 (aq) + H2 (g) That means we want rate of RVS reaction to increase So we need MORE of ZnCl2 (aq) or H2(g) OR We need LESS of Zn (s) or HCl (aq)

11. TEMPERATURE INCREASE • Situation: Consider the production of ammonia: N2 (g) + 3H2 (g) ↔ 2NH3 (g) + heat Is heat on the reactant or product side? If temperature is raised: Reaction will go to the LEFT, so rate of reverse reaction will increase and more reactants will form

12. TEMPERATURE DECREASES • Situation: Consider the production of ammonia: N2 (g) + 3H2 (g) ↔ 2NH3 (g) + heat Is heat on the reactant or product side? If temperature is lowered: Reaction will go to the RIGHT, so rate of forward reaction will increase and more products will form

13. KEY IDEA If heat is on one side and you increase temperature, rate of reaction will increase in the opposite side.

14. EXAMPLE - + 1) 4NH3 (g) + 5O2(g) ↔ 4NO (g) + 6H2O (g) + heat Stress: More heat Response: Stress: Less heat Response: Rate of REVERSE reaction INCREASES Rate of FORWARD reaction INCREASES

15. EXAMPLE - + 2POCl3 (g) + 1127kJ↔ 2PCl33 (g) + O2 (g) Stress: More heat Response: Stress: Less heat Response: Rate of FORWARD reaction INCREASES Rate of REVERSE reaction INCREASES

16. EXAMPLE - + 3) H2 (g) + Cl2 (g) ↔ 2HCl (g)ΔH = 90kJ Stress: More heat Response: Stress: Less heat Response: Rate of FORWARD reaction INCREASES Rate of REVERSE reaction INCREASES

17. PRESSURE CHANGES An increase in pressure will favor the reaction toward the side with fewer particles OF GAS!!  WE LOOK AT # MOLES

18. EXAMPLE 1) N2 (g) + 3H2 (g) ↔ 2NH3 (g) 4 particles 2 particles If pressure is increased, which way will the reaction go? The rate of forward reaction will increase If pressure is decreased, which way will the reaction go? The rate of reverse reaction will increase

19. EXAMPLE 2SO2 (s) + O2 (g) ↔ 2SO3 (g) 1 particles 2 particles If pressure is increased, which way will the reaction go? The rate of reverse reaction will increase If pressure is decreased, which way will the reaction go? The rate of forward reaction will increase

20. EXAMPLE 2NO2 (s) + 7H2 (g) ↔ 2NH3 (g) + 4H2O (g) + 1127 kJ 7 particles 6 particles If pressure is increased, which way will the reaction go? The rate of forward reaction will increase If pressure is decreased, which way will the reaction go? The rate of reverse reaction will increase

21. INDEPENDENT PRACTICE • To be collected at the end of CLASS! • COME SEE ME AFTER SCHOOL IF YOU NEED TO MAKE UP WORK! • I WILL BE IN THE ROOM 3:10-3:50PM • If you want to boost your grade, please conduct online EXTRA CREDIT LABS!