1. Le Chatelier’s principle and more... 7.2.3-7.2.5

2. But first, a review of Topic 7.1

3. Feature of Equilibrium State
Explanation
Equilibrium is dynamic
The reaction has not stopped but the forward and reverse reaction are still occurring
Equilibrium is achieved in a closed system
A closed system prevents exchange of matter with the surroundings
The concentrations of reactants and products remain constant
They are being produced and degraded at an equal rate
The macroscopic properties do not change
Color and density are properties that depend upon concentrations
Equilibrium can be reached from either direction
The same equilibrium mixture will result under the same conditions, regardless of the starting point.

4. Catalyst
the same process still has to happen, catalysts just help out by lowering the activation energy
increase the RATE of a reaction….and therefore the decrease the time in which equilibrium is reached
they speed up the forward and reverse reactions equally
therefore decreases the time required for the system to achieve equilibrium
less time equals $$$ when making chemicals

5. aA + bB  cC + dD [C]c[D]d Kc = [A]a[B]b Kc =
Write the equilibrium expression for the following reaction:
4NH3(g) + 7O2(g)  4NO2(g) + 6H2O(g)
[NO2]4[H2O]6
[NH3]4 [O2]7
Kc =

7. Le Chatellier’s principle and more...
Nice video- 20 minutes
Another good one- 15 minutes
states when a system in chemical equilibrium is disturbed by a change, the system shifts in a way that tends to counteract this change of variable
a change imposed on an equilibrium system is called a stress
a stress usually involves a change in the temperature, pressure, or concentration
the equilibrium always responds in such a way so as to counteract the stress

8. Stress 1. Temperature change
McGraw Hill Flash animation
Stress 1. Temperature change
Haber Process again
DH = + 92 kJ N2 (g) + 3H2 (g)  2NH3 (g) DH = - 92 kJ
this is the ONLY stress that would actually change Kc
increasing temperature
favors the “cold side”/endothermic/the reaction that needs heat
adding heat favors the reaction to the left since it needs +92 kJ
Kc decreases

9. decreasing temperature favors the “hot side”/exothermic
DH = + 92 kJ N2 (g) + 3H2 (g)  2NH3 (g) DH = - 92 kJ
decreasing temperature
favors the “hot side”/exothermic
the reaction is already giving off heat it doesn’t need so cooling down is good
Kc increases

10. Stress 2. Pressure change
McGraw Hill Flash animation
Stress 2. Pressure change
Haber Process again
N2 (g) + 3H2 (g)  2NH3 (g) DH = - 92 kJ
an increase in pressure causes the equilibrium to shift in the direction that has the fewer number of moles
results in a decrease in N2 and H2 and an increase in NH3
an decrease in pressure causes the equilibrium to shift in the direction that has the most number of moles
results in a an increase in N2 and H2 and an decrease in NH3
does NOT affect the equilibrium constant Kc

11. Stress 3. Concentration change
McGraw Hill Flash animation
Haber Process again
N2 (g) + 3H2 (g)  2NH3 (g) DH = - 92 kJ
Kc =
[NH3]2
[N2] [H2]3
the equilibrium responds in such a way so as to diminish the increase or equalize the ratio
increasing concentration of reactants shifts the reaction to the right (forward, more product)
increasing concentration of products shifts the reaction to the left (reverse, more reactants)
does NOT affect the equilibrium constant Kc

12. Practice Problem
Predict the effect of the following changes on the reaction in which SO3 decomposes to form SO2 and O2.
2 SO3(g)  2 SO2 (g) + O2 (g) Ho = kJ
increasing the temperature of the reaction
shifts right
increasing the pressure on the reaction
shifts left
adding more O2 when the reaction is at equilibrium
removing O2 from the system when the reaction is at equilibrium

13. Le Chatelier’s Principle – Summary
Change
Effect on Equilibrium
Change in Kc?
Increase concentration
Shifts to opposite side No
Decrease concentration
Shifts to same side
Increase pressure
Shifts to side with least moles of gas
Decrease pressure
Shifts to side with most moles of gas
Increase temperature
Shifts in endothermic direction
Yes
Decrease temperature
Shifts in exothermic direction
Add a catalyst
No change

14. Le Châtelier’s Principle
Changes in Concentration
Remove
Add
Add
Remove
aA + bB cC + dD
Change
Shifts the Equilibrium
Increase concentration of product(s)
left
Decrease concentration of product(s)
right
Increase concentration of reactant(s)
right
Decrease concentration of reactant(s)
left

15. Le Châtelier’s Principle
If an external stress is applied to a system at equilibrium, the system adjusts in such a way that the stress is partially offset as the system reaches a new equilibrium position.
Changes in Concentration
N2 (g) + 3H2 (g) NH3 (g)
Add
NH3
14.5

16. Le Châtelier’s Principle
Changes in Volume and Pressure
(Only a factor with gases)
A (g) + B (g) C (g)
Change
Shifts the Equilibrium
Increase pressure
Side with fewest moles of gas
Decrease pressure
Side with most moles of gas
Increase volume
Side with most moles of gas
Decrease volume
Side with fewest moles of gas

17. N2(g) + O2(g)  2NO(g) ∆H = +kJ mol-1
For each of the following reactions, predict how the equilibrium will shift as the temperature is increase
N2(g) + O2(g)  2NO(g) ∆H = +kJ mol-1
2SO2(g) + O2(g)  2SO3(g) ∆H = -kJ mol-1
Right
left

18. Position of Equilibrium
Effect of:
Position of Equilibrium
Value of Kc
Concentration
Changes
No change
Pressure
Changes if reaction involves a change in the number of gas molecules
Temperature
Changes (depends on if exothermic or endothermic)
Catalyst