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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