LeChâtelier’s Principle

Regaining Equilibrium6-2

LeChâtelier’s Principle

This states that when a change is imposed on a system at equilibrium, the position of the equilibrium shifts in a direction that tends to reduce the effect of that change.

• Three types of stress are applied:• Concentration• Pressure and volume• Temperature

Concentration

• When a reactant or product is added to a system at equilibrium, the system shifts away from the added component. (moves in a direction that uses up the excess component)

• But if a reactant or product is removed, the system shifts toward the removed component.

Changing Concentration

• If you add reactants (or increase their concentration).

• The forward reaction will speed up.• More product will form.• Equilibrium “Shifts to the right”• Reactants products

Changing Concentration

• If you add products (or increase their concentration).

• The reverse reaction will speed up.• More reactant will form.• Equilibrium “Shifts to the left”• Reactants products

Changing Concentration

• If you remove products (or decrease their concentration).

• The forward reaction will speed up.• More product will form.• Equilibrium “Shifts to the right”• Reactants products

Changing Concentration

• If you remove reactants (or decrease their concentration).

• The reverse reaction will speed up.• More reactant will form.• Equilibrium “Shifts to the left”.• Reactants products• Used to control how much yield you get

from a chemical reaction.

Changing Temperature

• As you raise the temperature the reaction proceeds in the endothermic direction.

• (in order to use up the heat)• As you lower the temperature the reaction

proceeds in the exothermic direction.• ( to create more heat)Reactants + heat Products at high TReactants + heat Products at low T

• Addition of heat

• Will cause this reaction to proceed to the left, creating more reactants.

• Will cause this reaction to proceed to the right, creating more products.

N2 (g) + 3H2 (g) 2NH3(g) + 92 kJ

CaCO3 (s) + 556 kJ CaO (s) + CO2(g)

Changes in Pressure

• As the pressure increases the reaction will shift in the direction of the least gases, (the number of moles).

• At high pressure 2H2(g) + O2(g) 2 H2O(g)

• At low pressure 2H2(g) + O2(g) 2 H2O(g)

3 moles verses 2: will go to the side with fewer moles

volume

When the volume of a gaseous system at equilibrium is decreased, the system shifts in the direction that gives the smaller number of gas molecules

• opposite to pressure.

CaCO3 (s) CaO (s) + CO2(g)

decreasing the volume of the following reaction will:

shifts the equilibrium to the left ( 1 vs 2)

N2 (g) + 3H2 (g) 2NH3(g)

PCl3 (g) + 3NH3(g) P(NH2)3 (g) + 3HCl (g)

shifts the equilibrium to the right

has no effect on equilibrium position

Take the equilibrium between 2NO2 (g) N2O4 (g) brown gas clear gas

Which direction is favored by increasing the volume?

The right!

Products favored

For each of the following predict whether the given change will cause the equilibrium to shift in favour

of the products or the reactants, or neither.

a) 2 SO2 (g) + O2 (g) 2 SO3 (g)

• Increase SO2

• Decrease total pressure• Inject more O2 at constant pressure• Add a catalyst• Decrease volume of the container

rightleftright

No changeright

b)Ag2CrO4(s) + heat 2 Ag+(aq) + CrO4

2- (aq)

• Add more Ag2 CrO4 (s) • Increase temperature• Add AgNO3 (aq)• Increase pressure• Add NaCl(aq)

(which precipitates AgCl)

rightleftno changeright

No change

c)BaCO3 (s) + heat BaO(s) + CO2

(g)• Add BaCO3 (s)• Increase temperature• Add BaO(s)• Add CO2 (g) at constant volume.• Increase volume of the container.

No changeright

No changeleftright

N2O4 (g) + energy 2 NO2 (g)

• Addition of N2O4

• Addition of NO2

• Removal of N2O4

• Removal of NO2

• Decrease volume• Increase temperature• Decrease temperature• Addition of a catalyst

rightleftleftrightleftrightleftNo change

• Text book questions• P: 568 # 5• P: 586 #4, 6,7,11