The term equilibrium in physical sense is defined as the “no change of state of the body.’’ The state of the body can be either the state of rest or the state of uniform motion. Such states of equilibrium can be further categorized into stable and unstable equilibriums.
In this article, we will be more concerned with the state of equilibrium attained in chemical reactions. Most of the chemical reactions are reversible in nature (i.e. occurs in both the directions). As soon as some product molecules are formed, theoretically the reverse process also begins to take place and reactant molecules are formed from product molecules.
Chemical equilibrium is achieved when the rate of forward and reverse reactions is equal and concentrations of the reactants and precuts remain constant. Chemical equilibriums are stable and dynamic in nature.
To understand chemical equilibrium, first we have to know about the reversible reactions. The reversible reaction occurs in forward as well as reverse directions. The forward and reverse directions are occurring in opposite direction. When the rates of two opposing reactions become equal, equilibrium is established. At equilibrium, no further change in the system is observed. This does not mean that the reaction has ceased, but a continuous cyclic situation result in which reactant gives products react to give original reactant. Such equilibriums are called dynamic equilibriums.
Every chemical reaction has a tendency to attain equilibrium but there are certain chemical reactions, which remain unidirectional only. Such reactions are called irreversible reactions. Thus, there must be some criterion for a chemical reaction to become irreversible. When a chemical reaction follows any of the given two criterions, the reaction would be irreversible:
A- If any off the product is insoluble (or gets precipitated)
B- If the reaction is carried out in an open vessel and any of the product is in gaseous state.
Characteristics of a Chemical equilibrium
a- At equilibrium, the rate of the forward reaction is equal to the rate of the reverse reaction.
b- Since both reactions take place at the same rate, the relative amounts of the reactants and products present at equilibrium will not change with time.
c- The equilibrium is dynamic, i.e. the reactions do not cease. Both the forward and reverse reactions continue to take place, although at equal rates.
d- Under the same conditions (temperature, pressure or concentration), the same state of equilibrium is reached.
e- If one of the conditions (temperature, pressure or concentration) under which equilibrium exists is altered, the equilibrium shifts and a new state of equilibrium is reached.
f- A catalyst does not alter the position of temperature, pressure or concentration. It accelerates both the forward and reverse reactions to the same extent and so the same state of temperature, pressure or equilibrium is reached but quickly. So a catalyst hastens the attainment of temperature, pressure or equilibrium.