### Le Chatelier`s Principle

```SCH 4U
What happens when we disturb
chemical systems at equilibrium?
[Co(H2O)6]Cl2 + heat
pink
[ CoCl2(H2O)2] + 4 H2O
blue
When a chemical system at
equilibrium is disturbed by a
change in a property, the
systems adjusts in a way
that opposes the change
...eventually establishing a
new equilibrium!
1.
Concentration Change
2. Temperature Change
3.
Total Pressure Change
aA + bB
cC + dD
Adding a reactant
= more available to react
= favours forward rxn (“equil shifts right”)
With time, product concentration(s) increase
Eventually new equilibrium established...
... with different reactant/product
concentrations from initial equilibrium
NOTE:
Initial and final K values are the same!
aA + bB
cC + dD
Removing (decreasing conc of) a product
also favours forward rxn (shifts equil right)
Remember: system shifts to try to
counteract the disturbance!
e.g.
2 CO2(g)
2 CO(g) + O2(g)
aA + bB
cC + dD
What about...
....decreasing a reactant concentration?
....increasing a product concentration?
Add an ion to solution that already contains that
...equilibrium shifts away
ion...
from added ion
3 NO2(g) + H2O(l)
2 HNO3(aq) + NO(g)
desired product
what about
removing
water?
remove
Hb(aq) + O2
Decreases
Increases
as
asblood
blood
circulates
goes into
through
lungs
body
HbO2(aq)
Endothermic:
Exothermic:
reactants + E
products
reactants
products + E
If we heat/ cool the system, equilibrium
shifts to minimize the change...
If we cool the system...
Equilibrium shifts in direction
that produces heat (favours
exothermic)
If we heat up the system...
Equilibrium shifts in direction
that absorbs heat (favours
endothermic)
NOTE: The value of K changes with temp.
(only constant at specific temperature)
Predict how K will change if...
...we heat up an endothermic (fwd) rxn?
→ Equil. shifts right, K increases
...we cool down an endothermic rxn?
→ Equil. shifts left, K decreases
And what about exothermic (fwd) reaction?
N2O4(g) + energy
2 NO2(g)
Recall Boyle’s Law:
Remember, pressure is caused by gas
molecules striking the walls of a container, so
you change pressure by changing the number
of collisions (change volume)

ONLY affects GASES!

Increasing total pressure of system shifts
equilibrium to side of balanced equation
showing fewest number of gas molecules

If P (↑), then V (↓)
and concentrations change (i.e. increase)
If both sides of the equation have the same
number of molecules of gas, any change in
pressure or volume will have no effect on
equilibrium
i.e. H2(g) + I2(g)
2 HI(g)
Addition of a catalyst
 A catalyst speeds up the rate of a reaction
 Lower the activation energy, which
increases the rate of reaction, of both the
forward and reverse reactions
 Catalysts do NOT affect position of
equilibrium, only time taken
Adding Inert Gases
 Inert gases do NOT react with other
gases (i.e. NOT part of the
equilibrium system)
Presence of inert gas changes the
probability of successful collisions
for both the reactants and products
equally, resulting in NO shift in
equilibrium


Pg 457 #1 – 4

Pg 459 #2 – 4

Read pgs 461 – 462 on the Haber Process
Haha, just kidding!
```