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Le Chatelier’s Principle
Disrupting Equilibrium
Equilibrium can be disrupted by:• changing concentrations of reactants
and/or products•changing the temperature•changing the pressure (gaseous reactions)
When equilibrium is disrupted, the rate of the forward rxn no longer equals the rate of the reverse rxn
Le Chatelier’s Principle•a way of predicting what happens to the
relative rates of the forward and reverse reactions when equilibrium is disrupted
The Law:
•when a system at equilibrium is upset, the system responds by changing in a way which counteracts ( undoes) the disturbance
•eventually equilibrium is restored
Changing Concentration
Type of Change Response
increase shifts to consume (use up) the added reactant or product
decrease shifts to replace (produce) the removed reactant or product
Changing Temperature
Type of Change Response
increase shifts to favour the endothermic change
decrease shifts to favour the exothermic change
Changing Volume
NOTE: only impacts reactions involving gases
Type of Change Response
increase (decrease in pressure) shifts towards the side with the greater number of moles of gas
decrease (increase in pressure) shifts towards the side with the fewest number of moles of gas
Adding a Catalyst
Type of Change Response
No effect but equilibrium is established faster
Adding an Inert Gas
Type of Change Response
No effect.
Examples
1) N2O4 (g) 2 NO2 (g) ΔH = + 24kJ/mol
colourless brown Upset Observation Direction of Shift
T T V V
Examples
1) N2O4 (g) 2 NO2 (g) ΔH = + 24kJ/mol
colourless brown Upset Observation Direction of Shift
T Darker brown T V V
Example
1) N2O4 (g) 2 NO2 (g) ΔH = + 24kJ/mol
colourless brown Upset Observation Direction of Shift
T Darker brown T Lighter brown V V
Examples
1) N2O4 (g) 2 NO2 (g) ΔH = + 24kJ/mol
colourless brown Upset Observation Direction of Shift
T Darker brown T Lighter brown V Darker brown V
Examples
1) N2O4 (g) 2 NO2 (g) ΔH = + 24kJ/mol
colourless brown Upset Observation Direction of Shift
T Darker brown T lighter brown V Darker brown V Lighter brown
Examples
1) N2O4 (g) 2 NO2 (g) ΔH = + 24kJ/mol
colourless brown Upset Observation Direction of Shift
T Darker brown T lighter brown V Darker brown V Lighter brown
Showing LCP Graphically
Initial equilibrium
T
rate of forward greater than rate of reverse
new equilibrium
Impact of Temperature on K
Ex.From the previous graph, calculate K for
the initial equilibrium and for the new equilibrium.
On board
Impact of Temperature on KType of Rxn in Forward Direction
Temperature Value for K
Endothermic Increases
Exothermic Decreases
Endothermic Decreases
Exothermic Increases
Example[Co(H2O)6 ]2+ (aq) + 4Cl- (aq) CoCl4 2- (aq) + 6
H2O (l)
Pink Blue
Upset Observation Direction of Shift
Add conc. HCl
T
T
Add H2O
Example[Co(H2O)6 2+ (aq) + 4Cl- (aq) CoCl4 2- (aq) + 6
H2O (l)
Pink Blue
Upset Observation Direction of Shift
Add conc. HCl blue
T
T
Add H2O
Example[Co(H2O)6 2+ (aq) + 4Cl- (aq) CoCl4 2- (aq) + 6
H2O (l)
Pink Blue
Upset Observation Direction of Shift
Add conc. HCl blue
T blue
T
Add H2O
Example[Co(H2O)6 2+ (aq) + 4Cl- (aq) CoCl4 2- (aq) + 6
H2O (l)
Pink Blue
Upset Observation Direction of Shift
Add conc. HCl blue
T blue
T pink
Add H2O
Example[Co(H2O)6 2+ (aq) + 4Cl- (aq) CoCl4 2- (aq) + 6
H2O (l) Pink Blue
Based on the observations, is the forward rxn endo or exothermic?
Upset Observation Direction of Shift
Add conc. HCl blue
T blue
T pink
Add H2O pink
Example[Co(H2O)6 2+ (aq) + 4Cl- (aq) CoCl4 2- (aq) + 6
H2O (l) Pink Blue
Based on the observations, is the forward rxn endo or exothermic? ENDO
Upset Observation Direction of Shift
Add conc. HCl blue
T blue
T pink
Add H2O pink
