Four differencies between order and molecurity of reaction?
Answers
The molecularity of a reaction is defined as the number of molecules or ions that participate in the rate determining step. A mechanism in which two reacting species combine in the transition state of the rate-determining step is called bimolecular. If a single species makes up the transition state, the reaction would be called unimolecular. The relatively improbable case of three independent species coming together in the transition state would be called termolecular.
The order of a reaction is an experimentally derived number. Let's consider the general reaction: A+B⟶C+D The reaction rate=k[A]n⋅[B]m where the rate constant k is a proportionality constant that reflects the nature of the reaction, [A] is the concentration of reactant A, [B] is the concentration of reactant B, and n & m are the exponents used to fit the rate equation to the experimental data. Chemists refer to the sum n+m as the kinetic order of a reaction.
In a simple bimolecular reaction n and m would both be 1, and the reaction would be termed second order, supporting a mechanism in which a molecule of reactant A and one of B are incorporated in the transition state of the rate-determining step. A bimolecular reaction in which two molecules of reactant A (and no B) are present in the transition state would be expected to give a kinetic equation in which n=2 and m=0 (also second order).
(ii) Order concerns with kinetics (rate-law) while molecularity concerns with mechanism.
(iii) Order may be any number, fractional, integral or even zero whereas molecularity is always an integer excepting zero.
Note that a zero order reaction is one in which concentration of reactant remains throughout constant and as such rate of reaction remains throughout constant equal to the rate constant.
(iv) Order may change with change in experimental condition while molecularity