Equilibrium
and
Le Chatelier’s Principle
What is Equilibrium?
Equilibrium Crash Course
Chemical Equilibrium
Reversible Reactions:
A chemical reaction in which the products
can react to re-form the reactants
Chemical Equilibrium:
When the rate of the forward reaction
equals the rate of the reverse reaction
and the concentration of products and
reactants remains unchanged
2HgO(s)  2Hg(l) + O2(g)
Arrows going both directions (  ) indicates equilibrium in a chemical equation
Approaching Equilibrium
Many chemical reactions are reversible if the activation energy is low and the system is
closed.
Reactants
⇌
Products
Approaching Equilibrium
Many chemical reactions are reversible if the activation energy is low.
Reactants
⇌
Products
Forward Reaction is left to right
Reverse Reaction is right to left
Approaching Equilibrium
Reactant concentrations start high and decrease as the reaction proceeds. The
forward rate, which depends on collisions of the reactants, also decreases.
Product concentrations start at zero and increase as the reaction proceeds. The
reverse rate, which depends on collisions of the products, also increases.
Eventually the forward rate is equal to the reverse rate and the concentrations are
constant. This is equilibrium.
Graphing the Approach to Equilibrium
Forward rate
Concentration
Reverse rate
Overall rate
Time
LeChatelier’s Principle
When a system at
equilibrium is placed
under stress, the
system will undergo a
change in such a way
as to relieve that
stress.
Henry Le Chatelier
LeChatelier Translated:
When you take something away from a system at
equilibrium, the system shifts in such a way as to
replace what you’ve taken away.
When you add something to a system at
equilibrium, the system shifts in such a way as
to use up what you’ve added.
Video Clip
http://www.youtube.com/watch
?v=7zuUV455zFs
LeChatelier Example #1
A closed container of ice and water at
equilibrium. The temperature is raised.
Ice + Energy  Water
The equilibrium of the system shifts to the
right to use up the added energy.
_______
LeChatelier Example #2
A closed container of N2O4 and NO2 at
equilibrium. NO2 is added to the container.
N2O4 (g) + Energy  2 NO2 (g)
The equilibrium of the system shifts to the
left
_______ to use up the added NO2.
LeChatelier Example #3
A closed container of water and its vapor at
equilibrium. Vapor is removed from the system.
water + Energy
 vapor
The equilibrium of the system shifts to the
right to replace the vapor.
_______
LeChatelier Example #4
A closed container of N2O4 and NO2 at
equilibrium. The pressure is increased.
N2O4 (g) + Energy  2 NO2
(g)
The equilibrium of the system shifts to the
left
_______
to lower the pressure, because
there are fewer moles of gas on that side of
the equation.
Now you try…..
Predict what will happen in each
of the following situations:
The Forward and Reverse Rates and Shifting
Explain in terms of the immediate changes to the forward and reverse rates after
each stress why each shift occurs
2CO(g) +
O2(g)
⇄
2CO2(g)
+
energy
Some CO is added- more reactant collisions.
Shift
Forward
Reverse
Right
Increases
No change
The Forward and Reverse Rates and Shifting
Explain in terms of the immediate changes to the forward and reverse rates after
each stress why each shift occurs
2CO(g) +
O2(g)
.
CO is removed
Shift
Forward
Reverse
left
decreases
same
⇄
2CO2(g)
+
energy
The Forward and Reverse Rates and Shifting
Explain in terms of the immediate changes to the forward and reverse rates after
each stress why each shift occurs
2CO(g)
+
O2(g)
⇄
2CO2(g)
+
energy
We know it shifts left
Temperature is increased-
both forward and reverse rates
increase.
Shift
Forward
Reverse
left
increases
increases by more