J Unit 4. Kinetics+Equilibrium 4.3 What is the LeChateliers Principle? March 14, 2011 Do Now: Imagine a situation where there are 4 people on the right 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? AIM: How do various stresses on a chemical system affect the system at equilibrium?

LeChateliers Principle

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

CONCENTRATION CHANGES: + Situation: Consider the following equation CH 4 (g) + H 2 O (g) 3H 2 (g) + CO (g) THE STRESS: adding more methane (CH 4 ), 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

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.

CONCENTRATION CHANGES: - Situation: Consider the following equation CH 4 (g) + H 2 O (g) 3H 2 (g) + CO (g) What if the concentration of CH 4 is reduced? WHICH WAY WILL THE REACTION GO? Reaction will go towards the LEFT, STRESS:Less CH 4 so rate of reverse reaction will and CH 4 will be formed. increase more

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 NH 3 will be formed. + -

EXAMPLE 2) 2NO 2 (g) + 7H 2 (g) 2NH 3 (g) + 4H 2 O (g) kJ Stress: Add more NH 3 Response: Stress: Less NH 3 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 NH 3 will be formed. +-

EXAMPLE 3) What stress would cause the equilibrium to shift right in the following reaction? Zn (s) + HCl (aq) ZnCl 2 (aq) + H 2 (g) That means we want rate of FWD reaction to increase So we need less of ZnCl 2 (aq) or H 2 (g) OR We need more of Zn (s) or HCl (aq)

EXAMPLE 4) What stress would cause the equilibrium to shift left in the following reaction? Zn (s) + HCl (aq) ZnCl 2 (aq) + H 2 (g) That means we want rate of RVS reaction to increase So we need MORE of ZnCl 2 (aq) or H 2 (g) OR We need LESS of Zn (s) or HCl (aq)

TEMPERATURE INCREASE Situation: Consider the production of ammonia: N 2 (g) + 3H 2 (g) 2NH 3 (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

TEMPERATURE DECREASES Situation: Consider the production of ammonia: N 2 (g) + 3H 2 (g) 2NH 3 (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

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

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 + -

EXAMPLE 2POCl 3 (g) kJ 2PCl3 3 (g) + O 2 (g) Rate of FORWARD reaction INCREASES Rate of REVERSE reaction INCREASES +- Stress: More heat Response: Stress: Less heat Response:

EXAMPLE 3) H 2 (g) + Cl 2 (g) 2HCl (g) ΔH = 90kJ Rate of FORWARD reaction INCREASES Rate of REVERSE reaction INCREASES + - Stress: More heat Response: Stress: Less heat Response:

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

EXAMPLE 1) N 2 (g) + 3H 2 (g) 2NH 3 (g) If pressure is increased, which way will the reaction go? 4 particles The rate of forward reaction will increase If pressure is decreased, which way will the reaction go? The rate of reverse reaction will increase 2 particles

EXAMPLE 2SO 2 (s) + O 2 (g) 2SO 3 (g) 1 particles2 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

EXAMPLE 2NO 2 (s) + 7H 2 (g) 2NH 3 (g) + 4H 2 O (g) kJ 7 particles6 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

INDEPENDENT PRACTICE 1.To be collected at the end of CLASS! 2.COME SEE ME AFTER SCHOOL IF YOU NEED TO MAKE UP WORK! 1.I WILL BE IN THE ROOM 3:10-3:50PM 2.If you want to boost your grade, please conduct online EXTRA CREDIT LABS!