[Chemistry Class Notes] Order of Reaction Pdf for Exam

The order of reaction of a chemical reaction refers to the relation between its rate and the concentration of the elements taking part in the chemical reaction. The reaction order is obtained by calculating the rate equation of the chemical reaction. From the rate equation, the entire composition of the mixture consisting of all the elements or species of the reaction can be known. In this article, definition of order of reaction, characteristics, determination and the different orders of reactions are discussed. Let’s first understand what is meant by the order of reaction.

What is the Order of Reaction?

The order of reaction is defined as the dependence of the concentration of all reactants in a chemical reaction on the rate law expression. For example, in a first order chemical reaction, the rate of reaction is entirely dependent on the concentration of one reactant in the reaction.

Order of a chemical reaction can be defined as the sum of power of concentration of reactants in the rate law expression is called the order of that chemical reaction. Reactions can be first order reaction, second order reaction, pseudo first order reaction etc. depending on the concentration of the reactants. Order of a reaction is an experimental value. It means it is an experimentally determined parameter. It can have fractional value as well.

Characteristics

There are some characteristics of order of reaction that are enumerated as follows:

• Order of reaction represents the number of reactant entities whose concentration affects the rate of reaction directly.

• The value of the order of reaction can be obtained by the summation of exponents of concentration terms in the expression of the rate of reaction.

• It does not depend on stoichiometric coefficients of the reactants in the balanced equation.

• Only the concentration of the reactants are considered and not that of the products formed in the chemical reaction.

• The value of reaction order can take an integer value or a fraction. It can even be zero.

If experimental rate law expression is given for a reaction, then we can deduce the order of that reaction as well. For example, consider a reaction – 

aA + bB [rightarrow] P

and rate law is given as – 

rate = k(Ax)(By)

order of reaction for the above reaction on the basis of given rate law can be written as follows – 

order of reaction = x + y

How to Find Order of Reaction?

Order of reaction is determined by experiment. Although if we know rate law expression determined experimentally then we can determine order of reaction using rate law. Order of reaction can be an integer or fractional value. Following orders of reactions are possible – 

• Order of reaction can be zero – In zero order reaction the concentration of reactant/s doesn’t affect the rate of a reaction. 

• Order of reaction can be negative integer – Negative integer value of order of reaction indicates that the concentration of the reactants inversely affects the rate of a reaction. 

• Order of reaction can be positive integer – Positive integer value of order of reaction indicates that the concentration of the reactants directly affects the rate of a reaction. 

• Order of reaction can be fractional value – Fractional value of order of reaction indicates a more intricate relationship between concentration of reactants and rate of reaction. Generally, complex reactions possess fractional values of order of reaction.

Following Methods Can be Used For Determination of Order of Reaction –

• Differential Method – It is also called the initial rates method. In this method concentration of one reactant varies while others are kept in constant concentration and initial rate of reaction is determined. Suppose if three reactants A, B and C are taking part in the reaction then in this method we keep varying concentration of one reactant (for example reactant A) while concentration of other reactants such B and C constant. 

• Graphical Method – This method is used when only one reactant takes part in the reaction. In this method if we draw a graph between logA (where A is concentration of reactant) and t (time) and it’s a straight line then reaction follows a first order.  In the same way if we draw a graph between 1/A and t and get a straight line then reaction follows second order. While if we draw a graph between 1/A² and t and get a straight line then the reaction is a third order reaction. 

• Integral Method – In this method concentrations of the reactants are compared with the integral form of the rate law. It is used for verification of initial rate methods.

Difference Between Molecularity and Order of Reaction 

Molecularity and order of reaction both give information about the chemical reaction but are very different from each other as one tells about the number of molecules taking part in reaction while another one tells about the relationship between rate of reaction and concentration of reactants. For your better understanding we are providing you here pointwise difference between molecularity and order of reaction- 

Zero Order Reaction 

In these reactions the rate of reaction doesn’t depend upon the concentration of reactants. It means change in concentration of reactants doesn’t affect the rate of reaction. 

Example –

[ 2NH_{3}(g) overset{text{Fe or W as catalyst }} rightarrow N_{2}(g) + 3H_{2}(g) ]

First Order Reaction 

In these reactions the rate of reaction depends on the concentration of one reactant only. There can be many reactants in the reaction but concentration of only one reactant will affect the rate of reaction. Concentration of other reactants will have no effect on order of reaction. 

Example – [ N_{2}O_{5} rightarrow N_{2}O_{3} + O_{2} ]

Rate = k[N2O5]

Second Order Reaction 

In these reactions the rate of reaction depends on the concentration of two different reactants or square of concentration of one reactant. 

Example – [ 2NO_{2} rightarrow 2NO + O_{2} ]

Rate = k[NO2]2

[ CH_{3}COOC_{2}H_{5} + OH^{-}] [rightarrow] [CH_{3}COO^{-} + C_{2}H_{5}OH ]

Rate = k[CH3COOC2H5] [OH–]

Pseudo First Order Reaction 

Those reactions which are not of 1st order but approximated or appear to be of 1st order due to higher concentration of the reactant/s than other reactants are known as pseudo first order reactions. 

Example – Hydration of alkyl halide

 

[ CH_{3}I + H_{2}O ] [rightarrow] [CH_{3}OH + H^{+}+ I^{-} ]

Rate of reaction = k [CH3I] [ H2O]

As methyl iodide is also used in aqueous solution form so the concentration of water is far higher than methyl iodide. 

[CH3I] <<< [ H2O]

So, concentration of water doesn’t change much and can be approximated as no change or constant. 

Now we can write – Rate of reaction = k’ [CH3I]

Where k’ = k [H2O]

Thus, the reaction appears to be first order, but it is actually of second order that’s why it is known as pseudo first order reaction.