When two boys balance each other on a see-saw we say that the balanced see-saw is a system that is in equilibrium. The weight of one boy tends to move the see-saw in a clockwise direction, while that of the other tends to move it in an anticlockwise direction. Since these two opposing processes are equal, there is no change in the position of the see-saw. In a broad sense, we can define chemical equilibrium as a state of a chemical system where there is no observable change in the properties’ of the system with respect to time.
Another example of a system in equilibrium is a saturated solution. To make a saturated sodium chloride solution, we dissolve the salt in water until some salt is left undissolved in the solution. In this system, at any given moment, undissolved sodium chloride particles are dissolving while the same numbers of dissolved sodium chloride particles are left undissolved. Here two opposing processes, dissolution and deposition, are taking place at the same rate. As a result, the net number of dissolved particles in the solution remains the same, i.e., there is no observable change in the ‘properties’ of the system.
The balanced see-saw is an example of static equilibrium, since the see-saw is at rest or stationary. The saturated solution is an example of dynamic equilibrium since the salt particles are dissolving and depositing at the same rate, i.e., there is constant motion in the system.
A volatile liquid in an air-tight container is also an example of a system that can exist in a state of dynamic equilibrium.
In general, dynamic equilibrium can occur during a physical change or a chemical change that is reversible. Dynamic equilibrium involving a physical change is referred to as physical equilibrium while that involving a chemical change is known as chemical equilibrium. In a physical equilibrium, there is no change in the composition of the substances involved. In a chemical equilibrium, the reactions undergo a change in composition to form the products, which in turn are reconverted to the original reactions at the same rate.
Properties Of A System Existing In Chemical Equilibrium
The following are characteristics of a system existing in chemical equilibrium.
1. A chemical equilibrium can only be achieved in a closed system. In other words no substance can enter or leave the system. For example the equilibrium between hydrogen, iodine and hydrogen iodide can be achieved in a sealed glass vessel but not in the open air. This is applicable to this esterification reaction below
CH3COOHaq) + CH3CH2OH4 = CH3CO2CH3L) + H2OL)
2. A chemical equilibrium can be approached from the point of view of either forward or backward reaction (i.e. either reactions or productions can be used as the initial source for the establishment of chemical equilibrium) for instance, equilibrium can be started from a mixture of alcohol and carboxylic acid or from a mixture of ester and water.
3. A chemical equilibrium is dynamic in nature. Chemist describes the situation as a system having macroscopic constant properties but with continually occurring microscopic changes. In other words, the amount of reactants or products remains constant but infact, both the forward and backward reactions go on the same rate.
4. The concentrations of all chemical species present in a system at dynamic equilibrium remain constant as long as the reaction conditions are unchanged.
5. A catalyst can be used to alter the time required to reach the equilibrium however it does not change the yield of the reaction because the addition of catalyst increase both the forward and backward reaction rates to the same extent.