Le Chatelier’s Principle

When a system at equilibrium is subjected to a
stress, the system “shifts” to relieve effects of the
stress and restore equilibrium

“Shift” = forward or reverse reaction speeds up
“Stress” = something that causes a reaction
(forward or reverse) to speed up or slow down.

http://www.learnerstv.com/animation/anim
ation.php?ani=120&cat=chemistry

Possible Stresses:



Change in Conc.
Change in Pressure
Change in Temperature
https://www.youtube.com/watch?v=dIDgP
FEucFM (2 min)
Effects of Stresses on Equilibrium
Ex: A + B ↔ C + D

If forward rate increases

C + D produced at greater rate
 amounts

increase
A + B used up faster
 amounts
decrease
Change in Concentration

Will affect only gas and aqueous
INCREASE Concentration
 Reaction
shifts in direction that consumes extra amount
A + B ↔ C + D (Increase [A], shifts
)
DECREASE Concentration
 Reaction
shifts in direction that produces more of what
was taken away
A + B ↔ C + D (Decrease [B], shifts
)
Note:
 Changing amounts of a pure liquid or solid
will not cause a shift

That will not change their concentrations, just
their amounts.
Change in Temperature
Increase Temp.
 Favors ENDOTHERMIC
 Direction

that consumes excess heat
A + B ↔ C + D + energy
Decrease Temp.
 Favors EXOTHERMIC
 Direction

that produces heat
A + B ↔ C + D + energy
Pressure Changes
Affect gases only
 Count total MOLES of gas
on each side of equation


Increase pressure = Decrease volume
 Shifts

in direction that forms fewer moles of gas
Decrease pressure = Increase volume
 Shifts
in direction that forms greater moles of gas
NOTE: If moles of gas equal on both sides NO SHIFT
http://www.kentchemistry.com/links/Ki
netics/LeChatelier.htm
N2(g) + 2O2(g) ↔ N2O4(g)
3 moles gas
1 mole gas

Ex:

Increase Pressure: Shifts

Decrease Pressure: Shifts

How might you optimize the production of
ammonia in the Haber reaction?
N2(g) + 3H2(g) ↔ 2NH3(g) + 91.8kJ
What could we do to:
concentration, temp. and pressure?
To shift right
Increase [N2], [H2], Decrease [NH3]
Decrease Temp
Increase Pressure
Haber reaction
http://www.youtube.co
m/watch?v=NWhZ77Q
m5y4&safe=active
Common Ion Effect
 Substance is added to an equilibrium system that
has a “common ion” with the reaction.
 This will increase the concentration of that aqueous
ion and will cause a shift.

Ex: Saturated solution of AgCl
AgCl(s) ↔ Ag+1(aq) + Cl-1(aq)





Stress: Add NaCl (s).
It breaks apart in water forming Na+1 and Cl-1
Common Ion = Cl-1
Shift
This causes a decrease in the solubility of AgCl
Effect of Catalyst on an Equilibrium System
 Increases rate of the forward and reverse
reactions equally.
 There is no overall “shift”

Lowers activation energy the same for forward
and reverse reaction
Le Chat’s Overview Part 1
 http://www.youtube.com/watch?v=7zuUV4
55zFs&safe=active


Le Chat’s Overview Part 2
http://www.youtube.com/watch?v=XhQ02e
gUs5Y&safe=active
Practice Regents Questions
Given the a system at equilibrium:
N2(g) + 3H2(g) ↔ 2NH3(g) + energy
Which changes occur when the temperature of this
system is decreased?
(1) The conc. of H2(g) increases and the conc. of N2(g) increases.
(2) The conc. of H2(g) decreases and the conc. of N2(g) increases.
(3) The conc. of H2(g) decreases and the conc. of NH3(g) decreases.
(4) The conc. of H2(g) decreases and the conc. of NH3(g) increases.
Given the equation representing a reaction at
equilibrium:
N2(g) + 3H2(g)↔2NH3(g) + energy
Which change causes the equilibrium to shift to
the right?
(1) decreasing the concentration of H2(g)
(2) decreasing the pressure
(3) increasing the concentration of N2(g)
(4) increasing the temperature
Given the system at equilibrium:
2POCl3(g) + energy ↔ 2PCl3(g) + O2(g)
Which changes occur when O2(g) is added?
(1) The equilibrium shifts to the right and the concentration of
PCl3(g)increases.
(2) The equilibrium shifts to the right and the concentration of
PCl3(g)decreases.
(3) The equilibrium shifts to the left and the concentration of
PCl3(g) increases.
(4) The equilibrium shifts to the left and the concentration of
PCl3(g) decreases.
Given the reaction at equilibrium:
N2(g) + 3H2(g) -->2NH3(g) + 91.8 kJ
What occurs when the conc. of H2(g) is increased?
(1) The rate of the forward reaction increases and the
concentration of N2(g) decreases.
(2) The rate of the forward reaction decreases and the
concentration of N2(g) increases.
(3) The rate of the forward reaction and the concentration of
N2(g) both increase.
(4) The rate of the forward reaction and the concentration of
N2(g) both decrease.
Given the system at equilibrium:
N2(g) + O2(g) + energy ↔ 2 NO(g)
Which changes will result in a decrease in the
amount of NO(g) formed?
(1) decreasing the pressure.
(2) decreasing the concentration of N2(g).
(3) increasing the concentration of O2(g) .
(4) increasing the temperature
Given the equilibrium reaction in a closed system:
H2(g) + I2(g) + heat ↔ 2 HI(g)
What will be the result of an increase in temperature?
(1) The equilibrium will shift to the left and [H2] will increase.
(2) The equilibrium will shift to the left and [H2] will decrease.
(3) The equilibrium will shift to the right and [HI] will increase.
(4) The equilibrium will shift to the right and [HI] will decrease.
Given the reaction at equilibrium:
the concentration of A(g) can be increased by
A. lowering the temperature
B. adding a catalyst
C. increasing the concentration of AB(g)
D. increasing the concentration of B(g)
Le Chats: (Honors)
 http://www.mhhe.com/physsci/chemistry/essentialchemis
try/flash/lechv17.swf
Crash Course: Equilibrium
http://www.youtube.com/watch?v=g5wNg_dKsYY&safe=active
Le Chat’s (University of Surrey)
http://www.youtube.com/watch?v=dIDgPFEucFM&safe=active