Le Chatelier’s Principle
• If an equilibrium is disturbed by:1) Adding or removing reactants or products;
2) Changing the pressure;
3) Changing the temperature;
the equilibrium will shift to oppose the change.
1) Adding or Removing Reactants or Products
chromate / dichromate equilibrium
2_
CrO4
Yellow
(aq)
+
2_
+
2H
(aq)
Cr2O7
(aq)
+
H2O(l)
orange
+
•
•
•
Adding Acid disturbs the equilibrium, adding a reactant (H ions).
The equilibrium position will shift to the right to oppose the change.
This shift removes the excess H+ ions and converts chromate ions to
dichromate ions (the solution changes from yellow to orange)
•
•
•
Adding Alkali disturbs the equilibrium, removing a reactant ( H ions)
The equilibrium position will shift to the left to oppose the change.
This shift produces more H+ ions and converts dichromate ions to
chromate ions (the solution changes from orange to yellow).
+
2) Changing the Pressure
•
An increase in pressure will shift the equilibrium in the
direction that produces the least moles of gas.
•
e.g.
2NO2 (g)
N2O4 (g)
Brown
Colourless
2 moles
1 mole
This equilibrium would shift to the right if the pressure was
increased (turning colourless).
This would oppose the increase in pressure, because there
are less moles of gas on the right hand side of the
equation.
3) Changing the Temperature
• Increasing the temperature will shift the
equilibrium in the direction that absorbs
energy (endothermic).
exothermic
• e.g.
2NO2 (g)
N2O4 (g)
Brown
Colourless
endothermic
This equilibrium would shift to the left if the temperature
was increased (turning brown).
This would oppose the increase in temperature, because
the reverse reaction absorbs energy (endothermic).
Catalysts
• Catalysts don’t shift an equilibrium one
way or the other.
• They just increase the rate of attainment of
equilibrium (they speed up the forward and
reverse reactions).
CO2 and the Oceans
• CO2 (g)
• CO2 (aq)
+ aq
+ H2O(l)
CO2 (aq)
HCO3
_
(aq)
+ H+(aq)
Le Chatelier’s Principle
• If an equilibrium is disturbed by:1) Adding or removing reactants or products;
2) Changing the pressure;
3) Changing the temperature;
the equilibrium will shift to oppose the change.
The Oscillating Reaction
Br2
Mn2+
3CH2(CO2H)2 (aq) + 4BrO3-(aq)
4Br-(aq) + 9CO2(g) + 6H20(l)
catalyst