In most of the reaction carried out in closed vessels, reaction does not go to completion under given set of conditions of temperature and pressure. Initially, in the vessel, only reactants are present, and as the reaction proceeds, the concetration of reactants will decrease and that of products will increase.
After some time a stage is reached when no further change in concetrations of reactants and products is observed. This state is called equilibrium state and some of the important questions regarding this phenomenon are:
1. why do reactions seem to stop before they reach completion?
2. What is the extent to which a reaction proceed?
3. Can we modify the conditions to improve the yield of products?
Equilibrium - The phenomenon
Equilibrium is the state at which the concentrations of reactants and products do not change with time.
It is important to remember that equilibrium is achieved in closed vessel reactions only.
The important aspect of reaction equilibrium is the reversibility. The products combine and form reactants. At equilibrium, both the forward and backward reactions are taking place. The rates of forward and backward reactions are same or equal at the equilibrium. As a result, the concentration of each species becomes constant.
The equilibrium is termed as dynamic reaction equilibrium. Dynamic means at a microscopic level, the system is in motion. But at macroscopic level, concentrations are not changing.
Chemical reactions may be classified as reversible reactions and irreversible reactions.
Example of irrereversible reaction
Decomposition of potassium chlorate into potassium chloride and oxygen. Even in a closed vessel this reaction is not reversible.
Example of reversible reaction
1. Decompositon of calcium carbonate. When solid calcium carbonate is heated in a closed vessel at 1073 K, it decomposes into solid calcium oxide and gaseous carbon dioxide. Due to gaseous CO2 there is pressure of gas in the vessel which can be measured. At a constant temperature it can be observed that pressure becomes constant after some time, which means no further CO2 is being produced even though calcium carbonate is still there in the vessel. The constant pressure indicates to us that reaction equilibrium is reached.
Characteristics of chemical equilibrium
1. Chemical equilibrium is dynamic in nature (already explained).
2. the properites of the system become constant at equilibrium and remain unchanged thereafter unless external or internal conditions are changed.
3. The equilibrium is attained only if the system is closed one.
4. As the reactions are reversible and happen under the same conditions, equilibrium can be attained from either direction.
5. A catalyst does not alter the equilibrium point. The catalyst increases the rate of reaction, and at equilibrium it increases both forward and backward reaction rates. But it does not alter equilibrium point, the concentrations of products and reactants at a given set of conditions. But the equilibrium is reached earlier in the presence of a catalyst.