Le Châtelier’s Principle - Science at Yorkdale with ...€¦ · Le Châtelier’s Principle Effects of Volume and Pressure • The system shifts to remove gases and decrease pressure

Le Châtelier’s PrincipleWhat happens when we disturb equilibrium?

Thursday, September 22, 2011

Refresh your memory!

The above reaction has an equilibrium constant of 25.0 at 1100 K. In a 1.00 L reaction vessel, 2.00 mol H2(g) and 3.00 mol I2(g) react. What is the equilibrium concentration of each gas?

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ReactantsReactants Products

Concentration (mol/L) H2(g) I2(g) HI(g)

Initial 2.00 3.00 0

Change -x -x +2x

Equilibrium 2.00 - x 3.00 -x 2x

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Doesn’t make sense

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ReactantsReactants Products

Concentration (mol/L) H2(g) I2(g) HI(g)

Initial 2.00 3.00 0

Change -1.7 -1.7 +2(1.7)

Equilibrium 2.00 – 1.7= 0.3

3.00 -1.7= 1.3

3.4

So…

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Applications of Equilibrium Constants

Predicting the Direction of Reaction• We define Q, the Reaction Quotient, for a reaction at conditions

NOT at equilibrium

as

where [A], [B], [P], and [Q] are molarities at any time.

• Q = K only at equilibrium.

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Applications of Equilibrium ConstantsPredicting the Direction of Reaction• If Q > K then the reverse reaction must occur to

reach equilibrium (go left)• If Q < K then the forward reaction must occur to

reach equilibrium (go right)

Large K means lots of products!

Small K means few products!

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Le Chatelier’s Principle: if you disturb an equilibrium, it will shift to undo the disturbance.

Remember, in a system at equilibrium, come what may, the concentrations will always arrange themselves to multiply and divide in the Keq equation to give the same number (at constant temperature).

Le Châtelier’s Principle

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Le Châtelier’s PrincipleChange in Reactant or Product Concentrations• Adding a reactant or product shifts the equilibrium

away from the increase.• Removing a reactant or product shifts the equilibrium

towards the decrease.

• To optimize the amount of product at equilibrium, we need to flood the reaction vessel with reactant and continuously remove product (Le Châtelier).

•

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Le Châtelier’s PrincipleChange in Reactant or Product Concentrations• Consider the Haber process

• If H2 is added while the system is at equilibrium, the system must respond to counteract the added H2 (by Le Châtelier).

• That is, the system must consume the H2 and produce products until a new equilibrium is established.

• Therefore, [H2] and [N2] will decrease and [NH3] increases.

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Le Châtelier’s PrincipleChange in Reactant or Product Concentrations

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Le Châtelier’s PrincipleEffects of Volume and Pressure• The system shifts to remove gases and decrease pressure.• An increase in pressure favours the direction that has fewer moles

of gas.• In a reaction with the same number of product and reactant moles

of gas, pressure has no effect.• Consider

• An increase in pressure (by decreasing the volume) favours the formation of colourless N2O4.

• The instant the pressure increases, the system is not at equilibrium and the concentration of both gases has increased.

• The system moves to reduce the number moles of gas (i.e. the forward reaction is favoured).

• A new equilibrium is established in which the mixture is lighter because colourless N2O4 is favoured.

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Le Châtelier’s PrincipleEffect of Temperature Changes• The equilibrium constant is temperature dependent.• For an endothermic reaction, ΔH > 0 and heat can be

considered as a reactant.

C(s) + H2O (g) +113 kJ CO(g) + H2 (g)

• For an exothermic reaction, ΔH < 0 and heat can be considered as a product.

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4H2O (g) + 2043 kJ

• Adding heat (i.e. heating the vessel) favours away from the increase:– if ΔH > 0, adding heat favors the forward reaction,– if ΔH < 0, adding heat favors the reverse reaction.

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Le Châtelier’s PrincipleEffect of Temperature Changes• Removing heat (i.e. cooling the vessel), favors towards

the decrease:– if ΔH > 0, cooling favors the reverse reaction,– if ΔH < 0, cooling favors the forward reaction.

• Consider

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Temperature Change • CoCl4 + 6H2O ⇔ Co(H2O)6 +2 + 4Cl-

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Catalyst• A catalyst speeds up the rate of a reaction.• Overall effect is to lower the activation energy, which

increases the rate of reaction, of both the forward and reverse reactions.

• Catalysts do not affect the position of equilibrium, only the time taken.

Changes that do not affect the position of Equilibrium Systems

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Inert Gases (N2 & Noble Gases)• Inert gases do not react with other gases.

This will not be part of the equilibrium system.• The equilibrium position of the system will not

change.– The presence of the inert gas changes the

probability of successful collisions for both the reactants and products equally, resulting in no shift in the equilibrium system.

Changes that do not affect the position of Equilibrium Systems

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Chickens and Equilibrium

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Le Châtelier’s Principle - Science at Yorkdale with ...€¦ · Le Châtelier’s Principle Effects of Volume and Pressure • The system shifts to remove gases and decrease pressure