until they all become constant when equilibrium is reached (Fig. . ). If total number of H and I atoms are same in a given volume, the same equilibrium mixture is obtained whether we start it from pure reactants or pure product.
equilibrium is attained, these two mixtures (H , N , NH and D , N , ND ) are mixed together and left for a while. Later, when this mixture is analysed, it is found that the concentration of ammonia is just the same as before. However, when this mixture is analysed by a mass spectrometer, it is found that ammonia and all deuterium containing forms of ammonia (NH , NH D, NHD and ND ) and dihydrogen and its deutrated forms (H , HD and D ) are present. Thus one can conclude that scrambling of H and D atoms in the molecules must result from a continuation of the forward and reverse reactions in the mixture.
If the reaction had simply stopped when they reached equilibrium, then there would have been no mixing of isotopes in this way. Use of isotope (deuterium) in the formation of ammonia clearly indicates that chemical reactions reach a state of dynamic equilibrium in which the rates of forward and reverse reactions are equal and there is no net change in composition. Equilibrium can be attained from both sides, whether we start reaction by taking, H (g) and N (g) and get NH (g) or by taking NH (g) and decomposing it into N (g) and H (g). N (g) + 3H (g) 2NH (g) Fig.
. Chemical equilibrium in the reaction H (g) + I (g) 2HI(g) can be attained from either direction Fig. . Depiction of equilibrium for the reaction .
LAW OF CHEMICAL EQUILIBRIUM AND EQUILIBRIUM CONSTANT A mixture of reactants and products in the equilibrium state is called an equilibrium mixture. In this section we shall address a number of important questions about the composition