Lecture 28 : Elementary Reactions and Reaction Mechanisms
28.1
Introduction
We have seen in the lecture on the integrated rate laws (lecture 26) that the order of a reaction has to be obtained experimentally and it can not be inferred from the stoichiometric coefficients of the chemical reaction. This can be understood as follows.
If the reaction is of the type 2A + 3B
products it is highly unlikely that 2 molecules of A and three molecules of B simultaneously collide to give the products. Even if the reaction is of the type A
products, this does not imply that it is first order with respect to A. It is possible that A has to be energized to an activated molecule A* either by collision with other molecules or by the absorption of light. For reactions in solution media, the participation of the solvent molecules in the reaction needs to be considered. These considerations have lead to the identification of elementary reactions in which the order of the reaction is the same as molecularity which is expressed in the stoichiometric equation for the chemical reaction. The overall mechanism(sequence of steps or elementary reactions involved in the reaction of the reaction is then built up in terms of elementary reactions. Unravelling the reaction mechanism is almost like detective work. Many times a prevailing mechanism of a well known reaction may be replaced by a more accurate mechanism when additional intermediates or pathways are detected and illustrated experimentally.
products it is highly unlikely that 2 molecules of A and three molecules of B simultaneously collide to give the products. Even if the reaction is of the type A
products, this does not imply that it is first order with respect to A. It is possible that A has to be energized to an activated molecule A* either by collision with other molecules or by the absorption of light. For reactions in solution media, the participation of the solvent molecules in the reaction needs to be considered. These considerations have lead to the identification of elementary reactions in which the order of the reaction is the same as molecularity which is expressed in the stoichiometric equation for the chemical reaction. The overall mechanism(sequence of steps or elementary reactions involved in the reaction of the reaction is then built up in terms of elementary reactions. Unravelling the reaction mechanism is almost like detective work. Many times a prevailing mechanism of a well known reaction may be replaced by a more accurate mechanism when additional intermediates or pathways are detected and illustrated experimentally. 
