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 chemical reaction. In other.
words, it is the number of concentration terms that determine the dependence of the rate of reaction.
For a general reaction numerical value of the 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 we can see that ongoing from experiment 1 to experiment 2, the [NO] doubles ([O 3] are held constant) and the rate increased fourfold, so 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 that participate in the step leading to the chemical change.
- Molecularity is always a whole number.
- It is zero value, or negative value is not possible.
- It can be known by writing chemical reactions.
- It is always equal to the number of molecules taking part 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. Every single step has its own molecularity.
- It provides no information about the 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 a fractional, whole number, or zero or negative.
- It is determined by experiment.
- It is not always equal to the number of molecules taking part in the reaction.
- It can vary with conditions such as temperature, pressure, or concentration.
- It can change its magnitude 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.