Theories of Reaction Rate

Abstract

The feasibility (i.e. direction and extent of the chemical change) and mechanism (i.e. study of rate and factors effecting it) are two important aspects, which should be distinguished clearly while considering a chemical reaction. There is no simple connection between the two aspects. For a general reaction

$$ aA + bB \rightleftharpoons c C + d D $$

The equilibrium constant is defined as

$$ K_c = \frac{{[C]^c [D]^d }} {{[A]^a [B]^b }} $$

K_c does not depend on concentrations but depends on temperature. At the given temperature, if the equilibrium concentrations of C and D are higher than those of A and B and indicates high value of K_c, then A and B have reacted to a considerable extent. On the other hand, if K_c is small, there will be little of C and D at equilibrium. Thus, the extent of chemical reaction is determined by equilibrium constant and is not related in any simple way to the rate or velocity of reaction with which the chemical change takes place. The reaction between two reactants may occur to almost completion, but the time for even very small fraction of the molecules to react may be extremely long.