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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!