Module J: Equilibria Aim of Change Nov 10

Equilibrium & Free Energy

Recall:

If G = H TS = 0, then the process is at equilibrium.

But what is meant by equilibrium?

Equilibrium Consider the generic reaction:

+ +

The reaction may also proceed in the reverse direction: + +

Dynamic Equilibrium is reached when the rate of the forward reaction is equal to the rate of the backward reaction.

It may appear as though no change is occurring. However, products are being converted into reactants at the same rate reactants are converted into products.

Equilibrium Constant (K) A state of equilibrium does NOT mean that concentrations of

reactants and products are equal.

For the reaction: + +

Equilibrium may be expressed through the equilibrium constant, Kc

= [][]

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Where [ ] represents the molar concentration

Equilibrium Constant (K)

Example: Write the equilibrium constant expression for the following reaction:

Equilibrium Constant (K)

For gas phase reactions:

is used equilibrium constant in terms of partial pressures of the gases

= ((3)

()(2)2

Solids and liquids are NOT included in the equilibrium constant expression.

Example

In the industrial synthesis of hydrogen, mixtures of CO and H2 are enriched in H2 by allowing the CO to react with steam. The chemical equation for this so-called water-gas shift reaction is

What is the value of Kp at 700 K if the partial pressures in an equilibrium mixture at 700 K are 1.31 atm of CO, 10.0 atm of H2O, 6.12 atm of CO2, and 20.3 atm of H2?

Equilibrium Constant (K)

Equilibrium constants are specific for each reaction at a particular temperature.

If K >> 1, then the products (or forward reaction) is favoured.

If K < 1, then reactants (or backward reaction) is favoured.

Reaction Quotient (Qc)

The reaction quotient is used to determine if equilibrium has been reached.

It can also help to predict in which direction the reaction will proceed.

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Reaction Quotient (Qc) If Qc = Kc, the reaction is at equilibrium, no net change occurs.

If Qc > Kc, then the reaction proceeds in the backward direction ( from right to left) since the mixture contains more products than reactants.

If Qc < Kc, then the reaction proceeds in the forward direction ( from left to right) since the mixture contains more reactants than products.

Reaction Quotient (Qc)

Le Chateliers Principle

If a stress is applied to a reaction mixture at equilibrium, a net reaction occurs in the direction that relieves the stress.

That stress can be:

Change in concentration

Change in temperature

Change in pressure (volume)

A possible fourth factor, addition of a catalyst, increases only the rate at which equilibrium is reached. A catalyst does not affect the equilibrium concentrations.

Le Chateliers Principle

In general, when an equilibrium is disturbed by the addition or removal of any reactant or product, Le Chteliers principle predicts that:

The concentration stress of an added reactant or product is relieved by net reaction in the direction that consumes the added substance.

The concentration stress of a removed reactant or product is relieved by net reaction in the direction that replenishes the removed substance.

Le Chateliers Principle

Le Chateliers Principle