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