Dynamic Nature of Equilibrium

The observable properties of the system become constant at equilibrium, which leads us to think that reaction stops altogether at equilibrium. But it is not true, actually at equilibrium the rate of forward reaction becomes equal to rate of backward reaction and hence there is no net change in concentration of various species. This can be illustrated by considering the reaction between H₂ and I₂ to form hydrogen iodide (HI).

When H₂ and I₂ are taken in a closed vessel maintained at 717K, hydrogen molecules combine with iodine molecules to form hydrogen iodide.

H₂ (g)   +   I₂ (g)    →  2HI (g).

The molecules of HI formed, begins to dissociate back to H₂ and I₂.

2HI (g)  →   H₂(g)    +   I₂ (g).

As the reaction progresses, the concentration of H₂ and I₂ decreases and hence rate of forward reaction slows down. On the other hand, concentration of hydrogen iodide increases and therefore rate of backward reaction increases. A stage is reached when rate of forward reaction and rate of backward reaction becomes equal and the system attains equilibrium. At equilibrium no change in concentration occurs provided the temperature of reaction mixture remains constant. Thus, at equilibrium the reaction does not stop but the system attains constant observable properties because of equal rates of forward reaction and backward reaction. Thus, equilibrium is Dynamic in Nature.

The dynamic nature of chemical equilibrium can be demonstrated by adding small amount of radioactive iodine (I₂*) to above reaction in equilibrium. It is observed that after some time hydrogen iodide also contains some radioactive iodine (I₂*). It indicates that reaction is going on even at equilibrium. If the equilibrium being static then hydrogen iodide should not contain radioactive iodine.

H₂   +    I₂*  ↔ 2HI*

Thus, it demonstrates the dynamic nature of chemical equilibrium.