Equilibrium reactions

It is a common observation that most of the reactions when carried out in a closed vessel do not go to completion under given set of conditions of temperature and pressure. In fact in such cases, in the initial state, only the reactants are present but as the reaction proceeds, the concentration of reactants decreases and that of products increases. Finally a stage is reached when no further change in concentration of reactants and products take place. This state at which the concentration of reactants and products do not change with time is called a state of chemical equilibrium.

The equilibrium can be established in physical processes as well as in chemical processes. Hence, equilibrium can be classified into following two categories:

Physical equilibrium

Equilibrium set up in physical processes is called physical equilibrium. For example, when liquid water is placed in a closed vessel at room temperature, it starts evaporating. As the reaction continues, more and more molecules of water escape to the vapor state and the level of water decreases continuously. But after sometime the level of water stops decreasing because the molecules of water vapours collected over the liquid water strike with the surface of liquid water and get condensed. This process of condensation acts in opposite direction to the process of evaporation. Finally a stage comes when the rate of evaporation of water becomes equal to the rate of condensation of vapours, and this stage is called equilibrium stage. So at equilibrium,

Rate of evaporation = Rate of condensation

or

H₂O (l)            H₂O (g)

Chemical equilibrium

The equilibrium set up in a chemical process is called a chemical equilibrium. For example- nitrogen gas can react with hydrogen gas to produce ammonia. This reaction can be shown as:

N₂ (g) +3H₂ (g)            2NH₃ (g)

This reaction is forward reaction as the reactants are converted into products.  However it is found that as soon as ammonia gas is produced in the above reaction it starts breaking into nitrogen gas and hydrogen gas in the reverse direction as given below:

2NH₃ (g)            N₂ (g) +3H₂ (g)

This reaction is exactly opposite the first reaction so it is called backward reaction. As this reaction is taking place in both the directions so equilibrium state will be attained in this reaction when the rate of forward reaction will become equal to the rate of backward reaction i.e. at equilibrium,

Rate of formation of ammonia = Rate of decomposition of ammonia

Now we can represent the forward reaction and backward reaction in equilibrium with each other as:

N₂ (g) +3H₂ (g)            2NH₃ (g)

Characteristics of chemical equilibrium

1) Chemical equilibrium is dynamic in nature

The term ‘dynamic’ means continuous activity. it means that after attaining the state of equilibrium the reaction does not stop though it may appear as if it has stopped. In fact, both the forward and backward reactions continue even after attaining the equilibrium. But the rate the forward reaction becomes equal to the rate of backward reaction. This means that if some products are formed from the reactants than an equal amount of products are converted back into reactants. As a result of this the concentration of both the reactants and products become constant. Since both the forward reaction and backward reaction are still going on so it is concluded that equilibrium is dynamic in nature.

2) The equilibrium can be attained only if the reaction is taking place in a closed vessel

For the establishment of equilibrium it is necessary that the vessel in which the reaction is taking place should be closed, otherwise the products formed during the forward reaction will escape into the environment and the backward reaction will not take place. Consequently it will not be possible to attain the equilibrium. Let us understand it with the help of an example.

Suppose liquid water is present in an open vessel which starts evaporating. Now as the vessel is open thus, the vapours formed will escape out of the vessel into the environment. Hence there will be no condensation of water vapours, and it will not be possible to attain the equilibrium.

3) The equilibrium can be attained from either the reactant side or from the product side.

4) At equilibrium the concentration of reactants and products do not change.

Test your understanding and answer these questions:

1. What are equilibrium reactions?
2. What is chemical equilibrium? Give characteristics of chemical equilibrium.