Unit 9.5 Le Chatellier’s
Principle
Teacher: Dr. Van Der Sluys
Objectives
• Le Chatellier’s Principle
– Concentration effects
– Temperature effects
– Pressure effects
Equilibrium
An equilibrium system is balanced in terms of the forward and
reverse reaction rates. Any "stress" that alters one of these rates
makes the system "shift" . The "shift" occurs so that the two rates
eventually equalize. The reaction rate for the forward reaction is
typically high because the reactants are present in the mixture at
the start. There normally are no products around at the start of a
reaction. The reverse reaction will become more important as the
amanita of product increases.
Le Chatelier's Principle:
A (g)
B (g)
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Le Chatellier’s Principle
• When a stress is applied to a chemical
equilibrium, the reaction will respond in
such a ways as to relieve the stress to
what ever extend possible.
Equilibrium - Adding a Stress
Increasing the concentration of a substance in the equilibrium mixture creates a
temporary increase the reaction rate on that side of the equilibrium. The
illustration shows the effect of additional reactant on the forward rate. The
forward reaction rate "jumps". The reverse reaction rate gradually climbs up to
reestablish balance. The new equilibrium has an increased amount of product.
The system shifted to relieve the stress. Addition of a reactant has resulted in an
increase in the amount of product.
Balance Beam Model
An equilibrium can be viewed like a balance beam. The reactants are on the right hand
side and the products are on the left hand side. When the system is balanced the forward
and reverse rates are equal. The picture shows an equilibrium where K = 1. The balance
arms are equal length.
If reactants are added to the system the "balance" will be lost temporarily. The balance
can be recovered when the system forms more product to bring back an equilibrium
condition.
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Effect of pressure changes
Usually only reactions involving gases are altered by pressure
changes. Gas pressure results from gas molecules banging
against the walls of the container. If you handle gases you know
that gas molecules "seek" low pressure. Compressed gas
"escapes" high pressure in a balloon or out of a tire valve stem.
Low pressure also goes with fewer molecules of gas. If the
pressure is on an equilibrium is increased the system will shift to
lower the pressure. The lower number of gas molecules uses less
space or volume. This favors the side of the reaction with fewer
gas molecules.
What would happen to the equilibrium if the pressure
was raised?
CO (g) + 2 H2 (g) <---> CH3 OH (g)
Pressure and Volume Effects
The reactants side has three molecules of gas while the products
side has only one molecule of gas. Increasing the pressure
favors the side that occupies less volume. The products are
favored because the fewer gas molecules occupy less volume.
The forward rate would be greater than the reverse rate. The
formation of methanol would occur faster than its decomposition.
CO (g) + 2 H 2 (g) <---> CH3OH (g)
Effect of temperature changes
Temperature changes can shift an equilibrium balance just as
concentration changes. Raising the temperature for an exothermic
reaction favors reactants.
A (g) + B (g) <--> C (g) + Heat
Raising the temperature for an endothermic reaction favors
formation of products.
Heat + A (g) + B (g) <--> C (g)
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Example - Heat
What happens to the ammonia equilibrium if the temperature is
raised?
3 H 2 (g) + 1 N 2 (g) <---> 2 NH 3 (g) + heat
The equilibrium balance can be reestablished if the
amount of reactants increase. Raising the temperature
for the ammonia reaction favors the reverse reaction
and the formation of reactants.
Summary:
1. An increase in concentration on one side of an equation favors
or drives the reaction to the opposite side.
adding reactants favors products
adding products favors reactants
2. An increase in temperature favors or drives an
endothermic reaction forward to products.
3 . An increase in temperature drives an exothermic
reaction backwards to reactants.
4. An increase in pressure drives a reaction toward the side with fewer molecules
(moles) of gas. Increased pressure "forces" the reaction into a smaller volume.
The gas volume is smaller with fewer gas molecules.
5. Adding a catalyst does not alter the relative amounts of reactant and
product. Forward reaction happens more easily and so does the reverse
reaction. Equilibrium is only reached faster.
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