Equilibrium

In all reactions, there are in fact two processes occurring, a forward reaction where the reactants produce the products, and a reverse reaction where the products react to form the reactants.

• Many chemical reactions are reversible and never go to completion. Equilibrium can be approached from both directions.
• For a system in equilibrium the rate of the forward reaction equals the rate of the reverse reaction and the concentrations of all reactants and products remain constant.
• The system is closed and macroscopic properties remain constant.
• Use phase equilibrium as an example of dynamic equilibrium involving physical changes.

Dynamic equilibrium is when there is change within a system in opposite directions and, as the change occurs at the same rate, then the concentrations of the two components remains constant.

A liquid in equilibrium with its own vapour is a physical example of a dynamic equilibrium as there is no change in chemical identity. This is also called a Phase Equilibrium as the particles are changing between phases.

liquid vapour

In a chemical equilibrium the forward reaction is happening at the same rate as the reverse reaction and the concentration of the reactants and products does not change (note: the concentrations are NOT equal)

reactants products

The equilibrium constant

Kc is a constant which represents how far the reaction will proceed at a given temperature.

• When Kc is greater than 1, products exceed reactants (at equilibrium).
• When much greater than 1, the reaction goes almost to completion. When Kc is less than 1, reactants exceed products.
• When much less than 1 (Kc can never be negative...so when it is close to zero) the reaction hardly occurs at all.
• The only thing which can change the value of Kc for a given reaction is a change in temperature.
• The position of equilibrium, however, can change without a change in the value of Kc.

Effect of Temperature

• The effect of a change of temperature on a reaction will depend on whether the reaction is exothermic or endothermic.

• When the temperature increases, Le Chatelier's principle says the reaction will proceed in such a way as to counteract this change, ie lower the temperature.
• Therefore, endothermic reactions will move forward, and exothermic reactions will move backwards (thus becoming endothermic).

• The reverse is true for a lowering of temperature.

Effect of Concentration

·       When the concentration of a product is increased, the reaction proceeds in reverse to decrease the concentration of the products.

·       When the concentration of a reactant is increased, the reaction proceeds forward to decrease the concentration of reactants.

Effect of Pressure

·       In reactions where gases are produced (or there are more mols of gas on the left), and increase in pressure will force the reaction to move to the left (in reverse).

·       If pressure is decreased, the reaction will proceed forward to increase pressure. If there are more mols of gas on the left of the equation, this is all reversed.

Effect of catalysts on equilibrium

·       A catalyst does not effect either Kc or the position of equilibrium, it only effects the rate of reaction.

·       As the rate of forward reaction and reverse reaction is affected equally then the equilibrium cannot be affected.

 The Haber process

N_2(g) + 3H_2(g) 2NH_3(g) Δ-H = -92.4 kJ mol^-1

·       There are more moles of gas on the left than the right, so a greater yield will be produced at high pressure. (The equilibrium position will lie further to the right)

·       The reaction is exothermic, therefore it will give a greater yield at low temperatures. (The equilibrium position lies further to the right)

·       In practice, if low temperatures are used the time taken for the reaction to attain equilibrium becomes unfeasably long.

·       An intermediate temperature is chosen (450ºC) which allows the reaction to get to an equilibrium in a reasonable time and still has enough of the products in the equilibrium mixture.

·       A catalyst of finely divided iron is also used to help speed the reaction (finely divided to maximise the surface area).

·       To make the process more efficient the ammonia produced at equilibrium is removed by first cooling the mixture when the ammonia turns into a liquid which can be tapped off.

·       The unreacted gases in the process are then mixed with fresh reactants and returned to the reaction chamber to reestablish the equilibrium again and the cycle is repeated continuously.