Chemical equilibrium

The chemical equilibrium has the following characteristics:

• Chemical equilibrium is dynamic in nature. The constancy of observable property in an equilibrium system does not mean that the reaction has stopped altogether. After the attainment of equilibrium the reaction does not stop although, it appears to have been stopped. The rates of two opposing reactions become equal. This means that if products are formed from the reactants, exactly equivalent amount of reactants being formed from the products.

For example, for the reaction between H₂ and I₂ to form HI,at equilibrium if certain amount of hydrogen iodide is formed by the combination of hydrogen and iodine, then exactly same amount of hydrogen iodide gets decomposed to give back H₂ and I₂.

The rate of formation of HI at the equilibrium becomes equal to the rate of decomposition of HI keeping the concentrations of the reactants and products constant. Since both the opposing reactions continue (at the same rate), even after the equilibrium, this chemical equilibrium is known as dynamic equilibrium.

• The properties of the system become constant at equilibrium and remain unchanged with time so long as the external factors remain the same. Fox example, in the decomposition of CaCO₃ and evaporation of water in closed vessel, pressure becomes constant at equilibrium.

• The equilibrium can be attained only if the system is a closed one. If the system is not closed, some of the products may escape from the container and therefore, the backward reaction will not take place. Consequently, it will not be possible to attain the equilibrium.

• The equilibrium can be approached from either direction. For example, the equilibrium between nitrogen tetraoxide N₂O₄ (colourless gas) and nitrogen dioxide, NO₂ (a reddish brown gas).

It is observed that at ordinary temperature, say 298 K the two gases exist as an equilibrium mixture having pale brown colour. At very low temperature, (273 K) N₂O₄ is stable and exists as colourless gas. When the temperature is increased (373 K), it decomposes to NO₂, which is dark brown in colour.

Fig: 7.6 - Equilibrium between N2O4and NO2

• A catalyst does not alter the equilibrium point. In a reversible reaction, a catalyst increases the rate of forward as well as backward reaction to equal extent. Therefore, the equilibrium point is not altered. However, the equilibrium may be attained faster in the presence of a catalyst.