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.
In some reactions, this reverse reaction is insignificant, but in others there comes a point where the rate of the two reactions is exactly equal and consequently the reactants and products remain in equal proportions, though both are continually being used up and produced at the same time. more...
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 right), an increase in pressure will force the reaction to move to the left (in reverse) to decrease the pressure.
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
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.
|Copyright: 2011 Isis Publication|