Chemical EquilibriumChemical Equilibrium

Unit 2Unit 2

One way reactionsOne way reactions

Reactants react forming products.Reactants react forming products. The reaction continues until one of The reaction continues until one of

the reactants is finished.the reactants is finished. The products are not allowed to The products are not allowed to

react to reform reactants.react to reform reactants. The reaction stops proceeding any The reaction stops proceeding any

further.further.

Reversible ReactionsReversible Reactions

Some reactions proceed in both Some reactions proceed in both directions -reactants to products and directions -reactants to products and products to reactants.products to reactants.

Example: Example: CuSO CuSO44 .5H2O (s) CuSO4(s) + 5H2O(g)

When heated the blue crystals change to white powders which change to blue when water is added.

A reaction in equilibriumA reaction in equilibrium

The reactants react forming products.The reactants react forming products. Products react forming reactants.Products react forming reactants. Finally the rates of the forward and Finally the rates of the forward and

reverse reactions are equal. reverse reactions are equal. The reaction is proceeding in both The reaction is proceeding in both

directions, but at equal rates.directions, but at equal rates. There is no overall change.There is no overall change. The reaction is now in a state of dynamic The reaction is now in a state of dynamic

equilibrium.equilibrium.

Dynamic Equilibrium -Dynamic Equilibrium -characteristicscharacteristics

There is no overall change ie the There is no overall change ie the concentrations of the reactants and concentrations of the reactants and products overall remain the same.products overall remain the same.

Rates of the forward and reverse Rates of the forward and reverse reactions are equal.reactions are equal.

The equilibrium can be achieved from The equilibrium can be achieved from either direction.either direction.

The equilibrium can be achieved in a The equilibrium can be achieved in a closed systemclosed system

How to describe a reaction How to describe a reaction in equilibriumin equilibrium

Use two concepts: The Equilibrium Use two concepts: The Equilibrium constant and the Position of the constant and the Position of the Equilibrium.Equilibrium.

The Equilibrium ConstantThe Equilibrium Constant

This can be described in terms of This can be described in terms of the (I) concentrations of the (I) concentrations of each substance in the reaction each substance in the reaction mixture - Kmixture - Kcc, or (ii) The partial pressures of each in the reaction mixture - Kp as the partial pressure of a gas is proportional to its concentration.

The position of the The position of the equilibriumequilibrium

This describes the relative proportions This describes the relative proportions of the reactants and products at a of the reactants and products at a given moment in time in the reaction given moment in time in the reaction mixture.mixture.

When the proportion of products is When the proportion of products is more than the reactants, the position more than the reactants, the position is said to be to the right.is said to be to the right.

When the proportion of the reactants is When the proportion of the reactants is more the position is to the left.more the position is to the left.

Factors affecting KFactors affecting Kcc or Kp

Only temperature affects KOnly temperature affects Kcc or Kp. Concentration, pressure or the

presence of a catalyst does not affect equilibrium constant.

Kc or Kp is constant at a given temperature.

Factors affecting the Factors affecting the positionposition

TemperatureTemperature ConcentrationConcentration Pressure Pressure CatalystCatalyst

Le Chatelier’s PrincipleLe Chatelier’s Principle

If a system in equilibrium is If a system in equilibrium is subjected to a change, the position subjected to a change, the position of the equilibrium will shift to of the equilibrium will shift to minimise the imposed change.minimise the imposed change.

Effect of Temperature Effect of Temperature

Exothermic reaction - An increase in Exothermic reaction - An increase in temperature will shift the position of temperature will shift the position of the equilibrium to the left to absorb the equilibrium to the left to absorb heat. The concentrations of products heat. The concentrations of products will decrease Kwill decrease Kcc will decrease.

Exothermic reaction - An increase in temperature will shift the position to the right to increase heat. Kc will increase.

Effect of concentration Effect of concentration

An increase in concentration of one of An increase in concentration of one of the reactants will shift the position to the reactants will shift the position to the right to use up some of the the right to use up some of the reactant added. Kreactant added. Kcc will remain the same.

An increase in concentration of one of the products will shift the position to the left to use up some of product added. Kc will remain the same.

Effect of pressureEffect of pressure

If there are more moles of reactants If there are more moles of reactants than products, an increase in pressure than products, an increase in pressure will shift the position to the right to will shift the position to the right to reduce pressure. There will be no reduce pressure. There will be no change in the value of Kchange in the value of Kcc

If the moles of reactants and products are equal, pressure has no effect on the position of the equilibrium.

Effect of a CatalystEffect of a Catalyst

A catalyst has no effect either on A catalyst has no effect either on the position of the equilibrium or the position of the equilibrium or the value of Kthe value of Kcc or Kp.

A catalyst will affect the forward and the reverse reaction equally. The equilibrium will be achieved quicker.

Writing an expression for Writing an expression for KKcc

For the reaction : aA + bB For the reaction : aA + bB cC + dD KcC + dD Kcc=[C]c [ D] D]dd/[A]a[B]b

The unit for Kc depends on the expression for Kc for a specific reaction.

Some reactions may not have a unit for Kc.