What is the Order of Reaction? Define Molecularity of a Reaction.
Order of reaction :
Order of reaction is defined as the total number of reacting species (atom, ions or
molecules) whose concentration actually alters during the course of chemical reaction. In other
words, it is the number of concentration terms which determines the dependence of rate of reaction.
For a general reaction numerical value of order of a reaction is the sum of all the exponents to which
the concentrations in the rate equation are raised.
Thus, when the rate of a reaction,
x, y , z = Reaction order of individual reactants A, B, C respectively.
Then order of the reaction as whole = x + y + z.
The sum of the powers to which the concentration terms are raised in the rate law is known as the order of the reaction.
The order of reaction can be known only experimentally and chemical equations totally fail to supply information about the order of a reaction.
Example : For the reaction NO ( g ) + O 3( g )—> NO 2( g ) + O2( g ) , the following experimental data was collected :
The general rate law for this reaction would be :
Rate = K[NO]m [O 3 ]n
In this example it can be seen that ongoing from experiment 1 to experiment 2, the [NO] doubles ( [O 3 ] is held constant) and the rate increased four‐fold, which means that the reaction is second order
with respect to NO i.e. m = 2. If experiments 1 and 3 are compared, it can be seen that the [O 3 ] doubles ([NO] is held constant) and the rate doubled. Therefore, the reaction is first order with respect to O3 i.e. n = 1 and the rate equation is,
Rate = K[NO] 2 [O 3 ]
Hence, the overall order = m + n = 2 + 1 = 3 .
Difference between molecularity and order of reaction :
- It is the total number of atoms, ions or molecules which participate in the step leading to the chemical change.
- Molecularity is always a whole number.
- Its zero value or negative value are not possible.
- It can be known by writing chemical reaction.
- It is always equal to numbers of molecules participating in the reaction.
- It does not vary with conditions such as temperature, pressure or concentration.
- Its magnitude cannot be changed during the reaction.
- The overall molecularity of a complex reaction has no significance. Each single step has its own molecularity.
- It provides no information about reaction mechanism.
Order of reaction
- It is the sum of the powers to which the concentration terms are raised in the rate equation for the reaction.
- Order of reaction may be fractional, whole number or zero or negative.
- It is determined by experiment.
- It is not always equal to the number of molecules participating in the reaction.
- It can vary with conditions such as temperature, pressure or concentration.
- Its magnitude can be changed during the reaction.
- For a complex reaction, the overall order is the order of the slowest step involved in it.
- The slowest step in the reaction can be judged by the order of the reaction and this gives further information about the mechanism. Order of Reaction Order of Reaction