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EQUILIBRIUM IN PHYSICAL PROCESS

The different types of physical Equilibrium are briefly described below

(a) Solid – liquid Equilibrium

The equilibrium that exist between ice and water is an example of solid – liquid equilibrium. In a close system, at  0oC ice and water attain equilibrium. At that point rate of melting of ice is equal to rate of freezing of water. The equilibrium is represented as
H2O(s) ⇔ H2O(l)

(b) Liquid-Gas Equilibrium

Evaporation of water in a closed vessel is an example of liquid – gas equilibrium. Where rate of evaporation is equal to rate of condensation. The equilibrium is represented as
H2O(l) ⇔ H2O(g)
(c) Solid – solution equilibrium
If you add more and more salt in water taken in a container of a glass and stirred with a glass rod, after dissolving of some amount. You will find out no further salt is going to the solution and it settles down at the bottom. The solution is now said to be saturated and in a state of equilibrium. At this stage, many molecule of salt from the undissolved salt go into the solution (dissolution) and same amount of dissolved salt are deposited back (Precipitation).
Thus, at equilibrium rate of dissolution is equal to rate of precipitation.

Salt(Solid) ⇔ Salt(in solution)
(d) Gas –Solution equilibrium
Dissolution of a gas in a liquid under pressure in a closed vessel established a gas – liquid equilibrium. The best example of this type of equilibrium is cold drink bottles. The equilibrium that exists with in the bottle is

CO2(g) ⇔ CO2(in solution)

Equilibrium in Chemical Process (Reversible and irreversible reactions)
A reaction in which not only the reactants react to form the products under certain conditions but also the products react to form reactants under the same conditions is called a reversible reaction.

Examples are

(i)   3Fe(s) + 4H2O(g) ⇔ Fe3O4(s) + 4H2(g)
(ii) CaCO3(s) ⇔ CaO(s) + CO2(g)
(iii) N2(g) + 3H2(g) ⇔ 2NH3(g)
If a reaction cannot take place in the reverse direction, i.e. the products formed do not react to give back the reactants under the same condition it is called anirreversible reaction.

Examples are:

(i)         AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(g)
(ii)        2M(g) + O2(g) → 2MgO(s)

Note:

If any of the product will be removed from the system, reversible reaction will become irreversible one.
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