Some Features of Physical Equilibria Process Conclusion Liquid Vapour H O (l) H O (g) p H O constant at given temperature Solid Liquid H O (s) H O (l) Melting point is fixed at constant pressure Solute(s) Solute (solution) Sugar(s) Sugar (solution) Concentration of solute in solution is constant at a given temperature Gas(g) Gas (aq) CO (g) CO (aq) [gas(aq)]/[gas(g)] is constant at a given temperature [CO (aq)]/[CO (g)] is constant at a given temperature . . General Characteristics of Equilibria Involving Physical Processes For the physical processes discussed above, following characteristics are common to the system at equilibrium: (i) Equilibrium is possible only in a closed system at a given temperature. (ii) Both the opposing processes occur at the same rate and there is a dynamic but stable condition.
(iii) All measurable properties of the system remain constant. (iv) When equilibrium is attained for a physical process, it is characterised by constant value of one of its parameters at a given temperature. Table . lists such quantities.
(v) The magnitude of such quantities at any stage indicates the extent to which the physical process has proceeded before reaching equilibrium. . EQUILIBRIUM IN CHEMICAL PROCESSES – DYNAMIC Analogous to the physical systems chemical reactions also attain a state of equilibrium. These reactions can occur both in forward and backward directions.
When the rates of the forward and reverse reactions become equal, the concentrations of the reactants and the products remain constant. This is the stage of chemical equilibrium. This equilibrium is dynamic in nature as it consists of a forward reaction in which the reactants give product(s) and reverse reaction in which product(s) gives the original reactants. For a better comprehension, let us consider a general case of a reversible reaction, A + B C + D With passage of time, there is accumulation of the products C and D and depletion of the reactants A and B (Fig.
. ). This leads to a decrease in the rate of forward reaction and an increase in the rate of