What is molecularity

The rate constant for forward and backward reaction of hydrolysis of ester are 1. Equilibrium constant for the reaction is? When the temperature of a reaction system is increased by 10 o C from 25 o C to 35 o C , at constant volume, the reaction rate doubles. It is due to. If the concentration of reactants is increased by 'X', the rate constant K becomes:.

The rate of reaction that does not involve gases, is not dependent on:. The constant R is. Reaction with molecularity equal to one, two, three etc; are called unimolecular, biomolecular, trimolecular etc. A complicated reaction has no molecularity of its own but molecularity of each of the steps elementary reactions involved in the mechanism. The molecularity of each step in the mechanism is two, so that we say that the reaction takes in the sequence of three steps each of which is bimolecular.

There is another way also. According to which molecularity of a complicated reaction is taken to be equal to the molecularity of the slowest step i. There are 2 molecules of hydrogen iodide take part in this reaction, and hence, it is an example of bimolecular reaction.

Another example of bimolecular reaction is:. Trimolecular Reaction. In this case, there are three molecules of reactant that take part in the chemical reaction. There are 2 molecules of sulfur dioxide and one molecule of oxygen that collide and take part in the chemical reaction.

Hence, it is called trimolecular reaction. Other examples of this type are mentioned as follow:. Complex Example of Molecularity of Chemical Reaction. The decomposition of hydrogen peroxide H 2 O 2 is a complex reaction and it involves more than one step. This reaction takes place in two different steps. The overall decomposition reaction can be represented as:. This produces a molecule of water. This reaction can be divided in to following two steps. From these two steps, first step is slow whereas the second step is fast.

Now, as we know, the slowest step of complex reaction is a rate-determining step. In this case, the first step is rate-determining step. This step involves only one molecule of hydrogen peroxide. According to the above-mentioned definition of molecularity, this reaction is an example of unimolecular reaction.

Facts About Molecularity:. The concept of molecularity of the chemical reaction is theoretical. The value of molecularity cannot be zero, negative, fractional, infinite, and imaginary. So, it can only be positive integer. The value of molecularity cannot be greater than 3 as more than three molecules may not mutually collide or come closer during the course of the chemical reaction.

There are several examples of chemical reaction whose molecularity seems to be more than three from the stochiometric equations. Examples of these reactions are:. Reaction between hydrogen bromide and oxygen. This reaction can be written as:. At the first attempt, the molecularity of this reaction seems to be five. However, its molecularity is two.