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Molecularity Of Reaction

The molecularity of a reaction simply refers to the number of molecules involved in the rate limiting rate of that reaction. The rate limiting step is the slowest step to take place in the intermediate pathway between the reactants and final products. 

For example, suppose compounds A and B react to form compound C. The reaction does not necessarily take place in one step. It may be necessary for compound A to first ionize before it can react with compound B, so that the reaction mechanism is: 

1. A => A(-)

2. A(-) + B => C

There are 2 major steps in the overall reaction of A + B => C. The molecularity of this reaction depends on which of these is the rate limiting step, or which one has a greater energy of activation. 
(1) If the first step, ionization of A, is rate-limiting (ie it is the slowest of the 2 reactions), then the reaction is unimolecular, as there is only one molecule involved. The concentration of B will not have an effect on reaction rate, because it is not involved in the rate limiting step. The equation for the rate of this unimolecular reaction: rate=k[A], where k is an experimentally derived constant for the specific reaction. The reaction is then said to follow first order kinetics. 
(2) On the other hand, if the second step is rate limiting, then the reaction is considered to be bimolecular. The rate of the reaction depends on the concentrations of both A and B and the reaction rate will be written as: rate=k[A][B]. Again, k is an experimentally derived constant for this specific reaction and will be different if the reaction is of different molecularity. 
 
It is possible to determine the molecularity of a reaction experimentally. By carrying out the reaction multiple times and varying the concentrations of the reactants each time, you can find the equation that best represents the reaction. Of course you will have to measure the reaction rate as you go along so that you have the three variables [A], [B], and rate. When you put these variables into one of the previous equations and get the same value for k for each experiment, then you have found the molecularity. 

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