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# What is the difference between the order and molecularity of a reaction? Explain.

Last updated date: 20th Jun 2024
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Hint: Order and molecularity of a reaction are not the same. For example, the order of the given reaction is 1 whereas its molecularity is 2.
${{C}_{2}}{{H}_{4}}(g)+{{H}_{2}}(g)\to {{C}_{2}}{{H}_{6}}(g)$
$Rate=k\left[ {{C}_{2}}{{H}_{4}} \right]$

Order and molecularity of a reaction are different because of the following reasons:
 Order of reaction Molecularity of reaction Order of a reaction is the total number of the concentration terms on which the reaction rate actually depends. We can only deduce the order of a reaction from the rate law expression. Rate law for a general reaction is given as: $aA+bB\to cC+dD$ $Rate=k{{\left[ A \right]}^{\alpha }}{{\left[ B \right]}^{\beta }}$ $Order=\alpha +\beta$.Here, k is rate constant and $\alpha$ and $\beta$ are not necessarily equal to stoichiometric constants $a$ and $b$. For a first order reaction, $\alpha +\beta =1$. The total number of ions, atoms or molecules that collide simultaneously with one another to bring about a chemical reaction is the molecularity of the reaction.One step reactions are simple or elementary reactions whereas complex reactions involve more than one step. For elementary reactions, molecularity can be obtained by adding the stoichiometric constants in the balance chemical equation. For example: Dissociation of HI: $2HI\to {{H}_{2}}+{{I}_{2}}$ Here, the molecularity is two and the reaction is bimolecular in nature. Order of a reaction can be a whole number, zero or fractional.$C{{H}_{3}}CHO(g)\to C{{H}_{4}}(g)+CO(g)$$Rate=k{{\left[ C{{H}_{3}}CHO \right]}^{3/2}}$ Molecularity is the sum of species involved in the collisions. So it is always a whole number. Order is applicable to both elementary and complex reactions. It is given for the overall reaction. Complex reactions involve a number of ions, atoms or molecules and the chances of all the molecules coming together and colliding are very less. These reactions, thus, take place in a series of one step (elementary reaction) reactions. The slowest step is the rate determining step and only its molecularity is significant for the overall complex reaction. It is an experimental quantity and cannot always be obtained from the balanced chemical equation. It may or may not be equal to the sum of reactant molecules represented by the balance chemical equation. It can simply be calculated by adding the species involved in the slowest step of a complex reaction. For elementary reactions, the sum of all the stoichiometric coefficients in the balanced chemical equation gives the molecularity. It can change with change in physical conditions like temperature, pressure, pH, etc. It does not change with any change in reaction conditions.

Note: It has to be kept in mind that molecularity of an overall complex reaction has no significance as such. Molecularity of the slowest step involved in a complex reaction is the same as the order of the reaction. However, order and molecularity are the same for an elementary reaction.