The concept of chemical equilibrium arises from the realization that many reactions are reversible, meaning they can proceed in both the forward and reverse directions. When the rates of the forward and reverse reactions become equal, a state of dynamic equilibrium is established. At this point, the concentrations of the reactants and products remain constant, although the individual molecules continue to interconvert.

According to Le Chatelier’s principle, a system at equilibrium responds to changes in its conditions by attempting to counteract the disturbance and restore the equilibrium. One such change that can be introduced is altering the concentration of the reactants or products. By selectively increasing the concentration of a specific ion involved in the reaction, we can investigate the subsequent equilibrium shift and its impact on the reaction.

Chemical equilibrium is the state of a system in which temperature, pressure, volume, and composition are constant over time. Chemical reactions are classified into two types:

1. Irreversible reactions: The reaction which proceeds to completion and the products fail to recombine to give back reactants. For example

AgNO₃ + NaCl → AgCl + NaNO₃

2. Reversible reactions: The reactions never go to completion, and products recombine to give back reactants. For example

PCl_5(g) ⇌ PCl_3(g) + Cl_2(g)

Every reversible reaction comprises one pair of forward and backward reactions. When forward and backward reactions occur simultaneously, at the same rate during reversible reactions, the reaction is said to be in equilibrium.

Law of chemical equilibrium
The law states that the rate of an elementary reaction is proportional to the product of the concentration of the reactants. At a constant temperature, the rate of a chemical reaction is directly proportional to the product of the molar concentration of the reactants; each is raised to a power equal to the corresponding stoichiometric coefficients as represented by the balanced chemical equation. Let us consider the reaction,

A + B ⇌ C + D
r_f = K_f[A][B]
r_b= K_b [C][D]
At equilibrium r_f = r_b
K_f [A][B] = K_b[C][D]

K_c is called the equilibrium constant, [A],[B],[C], and [D] → denotes active masses of reactants A, B, C and D, respectively.

For the general reversible reaction

aA + bB ⇌ cC + dD

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