Chemical Equilibrium

Equilibrium exhibits the state of a process at which the measurable properties of the system do not show any change with time. The term chemical equilibrium is used for the chemical reaction when the concentration of the reactants and the products does not change with time.

We consider the following reactions:

(i)     $BaCl_2 + H_2SO_4 \to BaSO_4 \downarrow + 2HCl$

ii)      $2KClO_3 \to 2KCl + 3O_2 \uparrow$

(iii)    $2HI \leftrightharpoons H_2 + I_2$(Closed vessel)

(iv)     $N_2 + 3H_2 \rightleftharpoons 2NH_3$(Haber’s process)

Since the reactions (i) and (ii) proceed in one direction and the reaction is completed with time hence these are known as irreversible reactions. On the other hand, reactions (iii) and (iv) in which the products recombine under a similar set of conditions, to give back the reactants in significant quantities are called reversible reactions. Such reactions are represented by the sign of reversibility $(\rightleftharpoons)$

The reactions (ii) and (iii) are carried out an heating, but (ii) is irreversible reaction hence it is also called thermal decomposition whereas reaction (iii) is reversible hence it is also called thermal dissociation.

Some more examples of Irreversible and Reversible reactions are:

Irreversible reactions:

$2Na + 2H_2O \to 2NaOH + H_2$ [un - reactive products]

$NaCl + AgNO_3 AgCl \downarrow + NaNO_3$  [precipitation reaction]

$SnCl_2 + 2FeCl_3 \to SnCl_4 + 2FeCl+2$ [Redox]

$I_2 + 2Na_2S_2O_3 \to 2NaI + Na_2S_4O_6$ [Redox]

• Decomposition of $CaCO_3$ in open
• Evaporation of $H_2O$ in open.

Reversible Reactions

1. Decomposition of $CaCO_3$ and evaporation of $H_2O$ in closed container.

$3Fe + 4H_2O \leftrightharpoons Fe_3O_4 + 4H_2$ [Reactive products]

$PCl_5 \rightleftharpoons PCl_3 + Cl_2$ $N_2 + 3H_2 \rightleftharpoons 2NH_3$

It is evident that chemical equilibrium is possible in reversible reactions only. In fact, the reaction does not stop but proceeds in both directions with equal rate, therefore, it is said that equilibrium is dynamic. Hence the state of chemical equilibrium may be defined as, “The state of reversible reaction in which the concentrations of the reactants and products do not change with time is called chemical equilibrium”.

Characteristics of Chemical Equilibrium are listed below:

(i)                  It can be achieved from any side.

(ii)                It is dynamic in nature.

(iii)               The rate of forward reaction is equal to that of backward reaction.

(iv)               The ratio between the concentration of the reactants and the products is constant in the mixture.

(v)                It is affected by the change of measurable properties of the system such as concentration, temperature, pressure, mass, volume etc.

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