j unit 4. kinetics+equilibrium 4.3 what is the lechateliers principle? march 14, 2011 do now:...
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J
Unit 4. Kinetics+Equilibrium
4.3 What is the LeChatelier’s 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?
LeChatelier’s Principle
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
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 formincrease
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
CH4 (g) + H2O (g) ↔ 3H2 (g) + CO (g)
What if the concentration of CH4 is reduced?
WHICH WAY WILL THE REACTION GO?
Reaction will go towards the LEFT,
STRESS: Less CH4
so rate of reverse reaction will and CH4 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 increaseMore Products will form
Reaction will go to the left.Rate of Rvse Reaction will increaseMore NH3 will be formed.
+-
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 increaseMore Reactants will form
Reaction will go to the right.Rate of Fwd Reaction will increaseMore NH3 will be formed.
+-
EXAMPLE3) 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)
EXAMPLE4) 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)
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
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
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 heatResponse:
Stress: Less heatResponse:
Rate of REVERSE reaction INCREASES
Rate of FORWARD reaction INCREASES
+-
EXAMPLE
2POCl3 (g) + 1127kJ↔ 2PCl33 (g) + O2 (g)
Rate of FORWARD reaction INCREASES
Rate of REVERSE reaction INCREASES
+-
Stress: More heat
Response:
Stress: Less heat
Response:
EXAMPLE
3) H2 (g) + Cl2 (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) N2 (g) + 3H2 (g) ↔ 2NH3 (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
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
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
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!