Le Châtelier’s Principle

Chapter 14.3

Chemical Equilibrium The point in a chemical reaction when

dynamic equilibrium has been achieved and the concentration of the reactants and products remains constant

What do you do when your stomach growls from hunger?

Feed it!

What do you do when your foot falls asleep? Adjust your legs so that the circulation will

be restored to your feet.

What do you do when your checking account is at a zero balance?

Have your mom deposit more money, of course!

All of these are examples of how a system at equilibrium responds to a stress in order to regain the state of equilibrium

Le Châtelier’s Principle The principle that states that a system in

equilibrium will oppose the change in a way that helps eliminate the change.

In other words Whenever a system at equilibrium is

disturbed, the system will shift in the direction which counteracts the disturbance

Huh? Chemical reactions respond to similar

stresses to the system

Note: when a system returns to a state of equilibrium, there is a new equilibrium point because the original conditions have been changed.

For Example In your classroom, as Mrs. Price is teaching, a

very loud and obnoxious student wanders in. Your classroom is at dynamic equilibrium when the teacher is

instructing and the students are engaged and learning (Insert laugh here!)

A student disturbs the peaceful instructive classroom Mrs. Price sends the student to Mr. Francis thereby restoring

order, hence equilibrium

Stress According to Le Châtelier Stresses including changes in

concentration, temperature and pressure

are subject to Le Châtelier’s Principle

Chemical Shift A chemical shift is when either the forward

or reverse reaction is favored by the introduction of a stress.

Equilibrium

So . . . A forward shift is to the right of the reaction

in response to a stress

A reverse shift is to the left of the reaction in response to a stress

Effect of ConcentrationN2 + 3H2 2NH3

If the [N2] is increased, in other words, we add more reactant,

Then, the reaction will shift to the right, forward shift, in order to remove any additional nitrogen

Forward Shift So, let’s 1add some nitrogen and the system will

become 2reactant heavy

As a result, the system will consume the nitrogen and the forward reaction will be favored; hence, the [NH3] increases and the [H2] decreases to once again 3achieve equilibrium

1. N2 + 3H2 2NH32. N2 +

3H2 2NH3

3N2 + 3H2 2NH3

Reverse Shift Suppose instead of nitrogen, the [NH3] is 1increased. The

reaction becomes 2product heavy

As a result, the system will decompose the ammonia and the reverse reaction will be favored; hence, the [NH3] decreases and the [N2] and [H2] decreases to once again 3achieve equilibrium

1. N2 + 3H2 2NH3

2. N2 + 3H

2 2NH3

3N2 + 3H2 2NH3

Common Ion Effect

In a saturated solution of an ionic compound, the ions are in equilibrium with it’s solid form

AgCl(s) Ag+(aq) + Cl-(aq)

If you add additional Cl- from a different ionic parent, more AgCl will be produced

Reduction of Solubility The common ion effect reduces the

solubility of slightly soluble compounds

Effect of Volume Change What happens when you reduce the

volume of a system? The pressure increases, and the particles are

closer together

The stress can be relieved by producing a smaller number of particles

Example Let’s look again at the Haber process

There are 4 moles of reactants and 2 moles of product

Reducing the volume would shift the reaction to the right where there are fewer particles

N2(g) + 3H2(g) 2NH3(g)

Another Example Let’s look at the reaction of Hydrogen and

Chlorine to form Hydrochloric Acid

H2(g) + Cl2(g) 2HCl(g)

There are 2 moles of reactant AND product so an increase or a decrease in volume would not cause the reaction to shift

Pressure Changes Pressure changes have almost no effect on

equilibrium reactions in solution

Pressure effects the equilibrium of gaseous species

Changing the Temperature We can raise the temperature of a system

by adding energy in the form of heat Adding heat to a system is endothermic Removing heat from a system is exothermic

Lower the Temperature Because an exothermic reaction releases

heat, it will favor a decrease in the temperature Lower the temperature in a system and the

reaction will shift to the exothermic side in order to replace some of the lost heat

Raise the Temperature Because an endothermic reaction absorbs

heat, it will favor an increase in the temperature Raise the temperature in a system and the

reaction will shift to the endothermic side in order to absorb the excess

Lower the Temperature in the Haber Process When we lower the temperature, the

reaction shifts toward the exothermic side

The [NH3] is increased and the [N2] and [H2] is decreased

N2(g) + 3H2(g) 2NH3(g) + 91.8kJ

Effects of a Catalyst At equilibrium, a catalyst increases the

forward and reverse reactions equally

However, if a system is NOT at equilibrium, a catalyst will shorten the time needed to achieve equilibrium

In General, here are the rulesDisturbance Reaction Shift

Add reactant consume reactant

Right

Remove reactant Replace reactant Left Add product Consume

product Left

Remove product Replace product Right

Increase temperature

Absorb heat Endothermic direction (the side without the energy)

Decrease temperature

Release heat Exothermic direction (the side with the energy)

Increase pressure (decrease volume)

Decrease pressure

To the side with the fewest # of gas molecules

Decrease pressure (increase volume)

Increase pressure

To the side with the most # of gas molecules

Now for the chemistry version . .

4NH3(g) + 5O2(g) --> 4NO(g) + 6H2O(g) + energy

What would happen if you . . . Added more product of NO(g)? Answer: The reaction would shift to the left in

order to consume some of the added NO

Now for the chemistry version . .

4NH3(g) + 5O2(g) --> 4NO(g) + 6H2O(g) + energy

What would happen if you . . . Removed an amount of the reactant O2(g)? Answer: the reaction would shift to the left to

replace the missing O2(g)

Now for the chemistry version . .

4NH3(g) + 5O2(g) --> 4NO(g) + 6H2O(g) + energy

What would happen if you . . . Increase the pressure by decreasing the volume? Answer: The reaction would shift toward the left

which is the side with fewest gas molecules.

Now for the chemistry version . .

4NH3(g) + 5O2(g) --> 4NO(g) + 6H2O(g) + energy

What would happen if you . . . Decreased the temperature of the system? Answer: The reaction would shift to the right

which is the side that produces energy in the form of heat.

Now for the chemistry version . .

4NH3(g) + 5O2(g) --> 4NO(g) + 6H2O(g) + energy

What would happen if you . . . Added a catalyst? Answer: There would be no change in the

equilibrium. A catalyst simply changes the rate of a reaction without being consumed or changed significantly.

Summary of EffectsDirection of Stress Result of Shift

[ ] decreases Some substance replaced

[ ] increases Some substance removed

Volume increases (pressure dec)

Move to the side with the most moles

Volume decreases (pressure inc)

Move to the side with fewer moles

Temperature increases Endothermic reaction favored

Temperature decreases Exothermic reaction favored

Catalyst is added No shift occurs