Chemical Equilibrium , Law of mass action

Introduction:

It is an experimental fact that most of the process including chemical reactions, when carried out in a closed vessel, do not go to completion.

Under these conditions, a process starts by itself or by initiation, continues for some time at diminishing rate and ultimately appears to stop. The reactants may still be present but they do not appear to change into products any more.

A reaction is said to be reversible if the composition of reaction mixture on the approach of equilibrium at a given temperature is the same irrespective of the initial state of the system, i.e. irrespective of the fact whether we start with reactants or the products.

Some examples are:

$ A + B \rightleftharpoons C + D $

$ H_2(g) + I_2(g) \rightleftharpoons 2HI(g) $

$ Ag^+(aq) + Fe^{2+}(aq) \rightleftharpoons Fe^{3+}(aq) + Ag(s)$

$ 2NH_3(g) \rightleftharpoons N_2(g) + 3H_2(g) $

Characteristics of Chemical Equilibrium

∎ The equilibrium is dynamic i.e. the reaction continues in both forward and reverse directions.

∎ The rate of forward reaction equals to the rate of reverse reaction.

∎ The observable properties of the system such as pressure, concentration, density remains invariant with time.

∎ The chemical equilibrium can be approached from either side.

A catalyst can hasten the approach of equilibrium but does not alter the state of equilibrium.

Types of Equilibria

There are mainly two types of equilibria

(a) Homogeneous: Equilibrium is said to be homogeneous if reactants and products are in same phase.

$ H_2(g) + I_2(g) \rightleftharpoons 2HI(g) $

$ N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g) $

$ N_2 O_4(g)\rightleftharpoons 2NO_2(g) $

$ CH_3COOH(l) + C_2H_5OH(l) \rightleftharpoons CH_3COOC_2H_5(l) + H_2O(l)$

(b) Heterogeneous: Equilibrium is said to be heterogeneous if reactants and products are in different phases

$ CaCO_3(s) \rightleftharpoons CaO(s) + CO_2(g) $

$ NH_4HS(s) \rightleftharpoons NH_3(g) + H_2S(g) $

$ NH_2CO_2NH_4(s) \rightleftharpoons 2NH_3(g) + CO_2(g)$

Law of mass action

Guldberg and Waage proposed that ” The rate at which a substance reacts is directly proportional to its active mass and rate of a chemical reaction is directly proportional to product of active masses of reactants each raised to a power equal to corresponding stoichiometric coefficient appearing in the balanced chemical equation ” .

For dilute solutions active mass is equal to concentration. Taking the example of the reaction.

$ N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$

We can write,

Rate of forward reaction r_f ∝ [N₂] [H₂]³ rate of reverse reaction r_r ∝ [NH₃]²