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Order of a reaction is the sum of the concentration of the reactant on which the rate of reaction depends primarily. Molecularity is the number of reacting molecules, such as atoms, which collide and result in a chemical reaction. Both order of reaction and molecularity are important terms in chemical kinetics.
We define the order of reaction as the relationship between the rate of a chemical reaction and the concentration of the reactants taking part in that chemical reaction. If we can find the order of a reaction, then the entire composition of the mixture in the reaction can be understood.
Molecularity is a theoretical quantity, which can be determined through reaction mechanisms.
Order of a reaction
In chemical kinetics, the order of a reaction concerning a certain reactant is defined as the power to which its concentration of the reactant term in the rate equation is increased.
Order of a reaction is used to determine the rate of the reaction with the help of the rate constant.
There are different values of the order of reaction like zero, first and second.
The order of reaction can be determined by adding the individual orders or powers of the concentration of the reactants. For example, if reactant A is first order (power of 1) and reactant B is first order (power of 1) then the overall reaction would be second order.
The order of reaction does not depend on the coefficients which are corresponding to each species present in the reaction. The order of the reaction is in the form of an integer. It can also have a value of zero (0).
Molecularity of a reaction
The molecularity of simple reactions is equal to the sum of the number of reacting molecules of reactants that are involved in the balanced stoichiometric equation. The molecularity of a reaction is the number of reacting molecules taking part in a single step of the reaction. The molecularity can only be in positive integer form. A mechanism in which two reacting molecules combine in the transition state of the determining step, which is the rate-determining step, is called bimolecular. In a unimolecular reaction, one reacting molecule makes up the transition state. The value of molecularity can be 3 or less than that.
Distinguishing between order and molecularity of a reaction
| Order of reaction | Molecularity of reaction | |
| Order is defined as the sum of the powers of the concentration of reactants taking part in a chemical reaction. | Molecularity is defined as the number of reacting molecules taking part in the rate-determining step. | |
| Order of reaction can only be determined experimentally and can not be calculated. | Molecularity is a theoretical quantity, which can be determined through reaction mechanisms. | |
| Order of reaction can be a whole number, zero, integer, or fraction. | Molecularity of reaction is always in a whole number or integer, it can never be fraction or zero. | |
| The value of the order in any reaction can be 0, 1 and 2. Order of reaction beyond 2 is not possible. | The value of molecularity is always 3 or less because more than three molecules usually don’t come close during a chemical reaction. | |
| We use order to know the number of molecules of the reactant whose concentration changes during a given chemical reaction. | We use molecularity in the chemical reaction to know how many molecules affect the rate of a given chemical equation. | |
| Order of reaction can be based on unbalanced chemical reactions. | The Molecularity of the reaction can only be based on a balanced chemical equation. |
Uses of order and molecularity of the reaction
| Order of reaction | Molecularity of reaction |
| Order of reaction is used in baking industries to identify the rate of reaction at which cookies would be baked. | Molecularity is used in different chemical industries. |
| It is used in the Haber process which produces ammonia from hydrogen and nitrogen gas. | The Molecularity of reaction is applicable only for elementary reactions. It is used to find molecularity of various reactions such as decomposition of dinitrogen peroxide, decomposition of phosphorous pentachloride etc. |
Conclusion
Order of reaction and molecularity of reaction are two different quantities used to identify different aspects of a chemical reaction.Molecularity is nothing but the sum of the number of molecules of the reactants involved in the equilibrium chemical equation. Although the molecular nature of the complete reaction does not matter, the overall kinetics of the reaction depends on the rate-determining step. The total number of molecules or atoms whose concentration determines the rate of a reaction is called the order of the reaction.
Reaction order = sum of concentration indices. Conditions of customs law for the reaction
xA+yB → product
The rate law is Rate= [A]x[B]y
Then comes the general sequence of reactions. n=x+y, where x and y are the order relative to the individual reactants.
Molecules cannot be larger than three, because more than three molecules cannot collide with each other.
