1. Le Chatelier's Principle
This principle is used along with reaction kinetics to help industry predict product formation and maximize $$$ by manipulating the yield of the desired product.

2. Include: Interpret concentration versus time graphs.
OUTCOME QUESTION(S):
C LeCHATELIER’S PRINCIPLE
Use Le Chatelier’s principle to predict and explain shifts in equilibrium.
Include: Interpret concentration versus time
graphs.
Describe some practical applications of Le Chatelier’s principle.
Vocabulary & Concepts

3. Le Chatelier's Principle (1884)
When a system at equilibrium is subjected to a stress, the system will adjust to relieve the stress and return to equilibrium.
Remember: Kc value is constant, BEFORE the stress and AFTER the reaction adjusts
stress:
A change in conditions that causes a related change in collision rates that upsets the reaction equilibrium.

4. 1. Concentration stress A + B C Kc = Kc = 1.35
Stress: a change in concentration by adding or
removing products or reactants.
Adjustment: change in collision rates and redistribution of particles.
Increase [C]:
Kc =
[C]
[A][B] A + B C
We say “shifts left”
Increased rate of reverse collisions
Kc = 1.35
Reverse reaction is favoured
Re-establish ratio; same Kc

5. A + B C A + B C Kc = Kc = 1.35 Kc = Kc = 1.35 Increase [B]: [C] [A][B]
We say “shifts right”
Increased rate of forward collisions
Forward reaction is favoured
Re-establish ratio; same Kc
Kc = 1.35
Decrease (removing) [A]:
Kc =
[C]
[A][B] A + B C
We say “shifts left”
Decreased rate of forward collisions
Reverse reaction is favoured
Re-establish ratio; same Kc
Kc = 1.35

6. 2 NO2 (g) N2O4 (g)
Notice equilibrium
re-established after adjusting to the stress
smog
smog
Spike indicates that concentration was added or removed.

7. 2 NH3 (g) N2(g) + 3H2(g) H2(g) + I2(g) 2 HI(g)
Which way will the system shift IF the concentration of ammonia is increased?
2 NH3 (g) N2(g) + 3H2(g)
Increased rate of forward collisions
“shifts right”
Which way with the system shift IF the concentration of iodine is decreased?
H2(g) + I2(g) HI(g)
Decreased rate of forward collisions
“shifts left”

8. *Equilibrium constant changes with new temperature – NEW Kc*
2. Temperature stress
Stress: a change in temperature by adding or removing heat.
Adjustment: change in collision rate and redistribution of particles.
Put the energy into the equation and treat the change like a concentration question
Exothermic: A  B (- ∆H )
Endothermic: A  B (+ ∆H)
+ HEAT
HEAT +
*Equilibrium constant changes with new temperature – NEW Kc*

9. + A B heat + A B heat = Kc = Kc Increase temperature: [B] [A]
Increased rate of reverse collisions
Endothermic reaction is favoured
New ratio created; new Kc
We say “shifts left”
Decrease temperature: + A B
heat =
[B]
[A] Kc
Decreased rate of reverse collisions
Exothermic reaction is favoured
New ratio created; new Kc
We say “shifts right”

10. 1 2 NO2 + N2O4 heat 2 ∆H = –58 kJ smog smog
A curved change indicates it is NOT a concentration stress (it could be either temperature or pressure/volume)
To clarify which stress:
Use concentrations to find the value of the Kc before(1) and after(2) stress
(2) 1 2
(1)
(1)
A change in Kc value indicates temp stress
(2)

11. 2 NH3 (g) N2(g) + 3H2(g) H2(g) + I2(g) 2 HI(g)
Which way with the system shift IF the temperature is decreased for this exothermic reaction?
2 NH3 (g) N2(g) + 3H2(g)
+ HEAT
Decreased rate of reverse collisions
“shifts right”
Which way with the system shift IF the temperature is increased for this endothermic reaction?
H2(g) + I2(g) HI(g)
HEAT +
Increased rate of forward collisions
“shifts right”

12. Remember: pressure changes only affect gases
3. Pressure stress
Stress: a change in pressure by increasing or
decreasing the volume of the system.
Remember: pressure changes only affect gases
Adjustment: change in collision rate and redistribution of particles.
Equilibrium expressions for pressure would be calculated using partial pressure values.
Kp value would still be constant
before and after pressure stress =
{Productsp}
{Reactantsp} Kp

13. A + 2 B C B A C B C = Kp Increased Pressure (↓V ): 1 + 2 : 1 {Cp}: C B =
{Cp}
{Ap}{Bp} Kp C
Increased rate of forward collisions
We say “shifts right”
Forward reaction is favoured
Re-establish ratio; same Kp
The reaction will shift to the side with fewer molecules
(reduce the number of molecules, reduce the pressure)

14. A + 2 B C B A B B A B C = Kp Decrease Pressure (↑V ): 1 + 2 : 1 {Cp}: =
{Cp}
{Ap}{Bp} Kp B B A B C
Decreased rate of forward collisions
We say “shifts left”
Reverse reaction is favoured
Re-establish ratio; same Kp
The reaction will shift to the side with more molecules
(increase the number of molecules, increase the pressure)

15. Same number on both sides – no shift will affect pressure
Which way with the system shift IF the size of the container is cut in half ?
2 NH3 (g) N2(g) + 3H2(g)
Increased rate of reverse collisions
“shifts left”
Which way with the system shift IF the pressure is decreased?
H2(g) + I2(g) HI(g) :
No change in collisions rates
“no shift”
Same number on both sides – no shift will affect pressure

16. Factors that do not affect Equilibrium Systems
1. Catalysts:
Lowers activation energy for both forward and reverse reaction equally.
Same equilibrium positions
established, just more quickly
2. Inert gases:
Unreactive – therefore can not affect equilibrium of a concentration-based equation.
Catalysts, inert gases, pure solids, pure liquids do NOT appear in the Equilibrium expression – so are not a stress if altered

17. Include: Interpret concentration versus time graphs.
CAN YOU / HAVE YOU?
C LeCHATELIER’S PRINCIPLE
Use Le Chatelier’s principle to predict and explain shifts in equilibrium.
Include: Interpret concentration versus time
graphs.
Describe some practical applications of Le Chatelier’s principle.
Vocabulary & Concepts