CCEA ADVANCED SUBSIDIARY

CHEMISTRY

MODULE 2

2.7  Equilibrium

Some chemical reactions take place in one direction only i.e. they go to completion and are irreversible.

            Reactants                               products        

e.g.       2Mg (s)  +  O2 (g)                               2MgO (s)

 

In some reactions the products can combine and reform the reactants i.e. they are reversible. Such reactions will eventually reach a state of equilibrium in which both reactants and products are present.

            Reactants       ⇌        products

e.g.       CuSO4.5H2O (s)         ⇌        CuSO4 (s)  +  5H2O (l)

 

Consider a reaction in which

            A  +  B             ⇌   C  +  D

If A and B are mixed in a closed vessel, molecules of A and B combine to form C and D. Molecules of C and D can also combine to reform A and B. At first the rate of combination of A and B molecules is greater than the rate of combination of C and D molecules. After a time the amount of C and D builds up to a level where the rate of combination of A and B molecules is just equal to the rate of combination of C and D molecules. The system has reached a state of dynamic equilibrium. The molecules are still reacting but the macroscopic properties are constant.

If the concentration of one of the reactants and one of the products is measured over time a graph similar to the one below would be produced.

 


Text Box: concentration                                                                             [C]

 

 

 

 


                                                                             [A]

 

 

 

 


                                         Equilibrium                               Time

 

 

The dynamic nature of the equilibrium state.

 

Equilibrium reactions are dynamic. Both forward and reverse reactions still continue (at equal rates) at equilibrium although they appear to have stopped as there is no further change in the concentration of either reactants or products.

This can be shown by the fact that a saturated solution of lead (II) chloride becomes radioactive when solid radioactive lead chloride is added to it.

Pb*Cl2 (s)              ⇌      Pb2+ (aq)  +  2Cl- (aq)

 

For homogeneous reactions, the qualitative effects of changes of temperature, pressure and concentrations on the position of equilibrium.  Influence of these effects in industrial processes such as the synthesis stage of the Haber-Bosch process and the Contact process, precautions against SO2 emissions. In these processes candidates would be expected to know the conditions required but if a different process is used then information would be supplied for interpretation.