Types Of Reactions Based On Reaction's Order

Order of a reaction is an experimental term. It can not be directly determined by the observed rate law. In order to determine the reaction order, the rate expression must be known. The term 'Reaction Order' (or sequence of reaction) refers to how the concentration of one or more reactants (chemicals) affects the rate of the reaction.

The order of reaction refers to the relationship between the rate of a chemical reaction and the concentration of the species participating in it. Order of a reaction is defined as, "The summation of exponents of all the reactant's concentration term present in the rate law obtained from the rate determining step is called as an order of reaction". 

The rate equation can help you determine the order of the reaction. This equation shows the increase or decrease of a particular substance over time. Depending on the concentration of the reactants present in rate equations, the reactions may be different types: zero order reactions, first order reactions, second order reactions & pseudo first order reactions, etc. 

In this article, we will learn all these types of reactions based on different orders with the help of various examples.

Types of Reaction Based On Reaction's Order

Based on the effect of reactant's concentration on rate of reaction, Reactions are classified into different types and these are: Zero order, First order, Second order, Third order, Fractional order and Pseudo First order Reaction, etc.

(A)Zero Order Reaction 

....."As the name suggests that the order of a reaction is zero, means the rate of reaction is not depend on the reactant's concentration"....

In other words, a zero order reaction refers to those chemical reactions whose rate of reaction does not depend on the concentration of the reactants i.e. the concentration of the reactants does not affect the rate of the reaction. Thus, if we change the concentration of the reactant, there is no effect on the rate of the reaction. Hence, the zero order reaction is completed under the condition of constant rate.

(No matter what reactants are present and the extent to which their concentrations are changed, the rate of the reaction will remain the same)

For understanding purpose let's take a general reaction, 

                           A + B ➝ R 

       Rate law for the above reaction;

                     ➩ -r = k [A]°[B]° 

                                     ∵ [A]°=[B]°=1 

                     ➩ { -r = k } ,only

                                   -------------------(1)

Unit of rate constant for zero Order reactions,

                      k = "concentration/time"

Therefore,

                ➩ r = k = "concentration/time"

                                 Or

                ➩ k = "mol/(liter-time)"

According to Equation (1), the rate of a zero order reaction depends only on the rate constant (k) for a specific reaction and is independent of the concentration of the reactant. Therefore, as long as the temperature remains constant, the rate of the zero order reaction will also remain constant. 

Thus, when we draw a graph between concentration data and time, the result of this graph is a straight line.

Examples Of A Zero Order Reactions Are

(a) Photochemical reaction in presence of light,

            H₂(gas) + Cl₂ (gas) ➝ 2HCl(gas) 

(b) Nitrogen oxide decomposition over a hot platinum surface (exothermic in nature) , 

           2N₂O ➝ 2 N₂(gas)+ O₂(gas)

(c) Reverse of Haber process is an example of zero order, 

                2NH₃ ➝ N₂ + 3H₂

(B)First Order Reaction

........"The order of this reaction is equal to one which means that the rate of the reaction depends only on the concentration of one reactant"......... 

In this reaction, more than one reactant may be present but only one of them will be affecting the rate of the reaction. Therefore, the other reactants are assumed to be in zero order with respect to this particular reaction.

For understanding purpose, consider a general reaction;

                      A + B + C ➝ R 

   rate law for the above reaction,

                 ➩ - r = k [A]¹[B]°[C]°

                             ∵ [B]°=1 & [B]°=1;

                 ➩ { - r = k[A]¹ } ,only

                               --------------------(1)

Since, the rate of reaction is proportional to the concentration of only one reactant whose exponent is equal to one as shown in the above equation. Therefore, when we draw a graph between concentration data and time, the result of this graph is a curved line.

Here, unit of reaction rate is, 

               ➩ r = "concentration/ time"

Unit of reaction rate constant,

                 ➩ k= "1/sec" Or "sec⁻¹ "

For Examples

(a) Decomposition of hydrogen peroxide is an example of first order kinetics;

                 2H₂O₂ ➝ 2H₂O + O₂

                  ➩ rate = k [H₂O₂]¹ 

(b) The decomposition of dinitrogen pentoxide (N₂O₅) is an example of first order kinetics;

              2N₂O₅ ➝ 4NO₂ + O₂

                ➩ rate = k[N₂O₅]¹

(C)Second Order Reaction

The reaction is said to be second order reaction if it's order is equal to '2'. Hence, the rate of reaction depends either of one reactant's concentration whose exponent is equal to '2' or two separate reaction's concentrations.

Consider general reaction, 

                       A + B ➝ R 

     rate law for the above reaction,

                 ➩ - r = k [A]² [B]°

                    ➩ -r = k [A]²

                               OR

                 ➩ -r = k [A]¹[B]¹

Unit of reaction rate, 

               ➩ r = "concentration/ time"

Unit of reaction rate constant, 

             ➩ k = "1/ concentration-time"                                                  Or

               ➩ k = "litre/ (mol-sec)"

For Example

       (a)  NO₂ + CO  ➝ NO + CO₂

                  ➩ rate = k [NO₂]¹[CO]¹

       (b)  2HI  ➝ H₂ + I₂

                 ➩  rate =k [HI]²

(D)Third Order Reaction

When the order of a reaction is equal to 'three' then the reaction is called as third order reaction. In this, the rate of reaction depends on the three individuals reactant's concentration whose exponent is '1'. All possible cases for third order reactions are, 

             2A + B  ➝  product (rate=k[A]²[B]¹)

                    3A  ➝  products. (rate =k [A]³ )

          A + B + C ➝ products (rate =k[A][B][C] )

             A + 2B  ➝ products (rate =k [A]¹[B]² )

Unit of reaction rate constant, 

            ➩ k = "1/concentration²-time"

For Examples

     (a) 2NO + Cl₂  ➝  2NOCl

                 ➩ rate = k [NO]²[Cl₂]¹

     (b) 2FeCl₃ + Sn-Cl₂  ➝ 2FeCl₂ + Sn-Cl₄ 

                ➩ rate = k [Fe-Cl₃]² [Sn-Cl₂]¹

(E)Pseudo First Order Reaction

A pseudo first order reaction can be defined as a second order (or bimolecular) reaction which behaves like a first order reaction because in this type of reaction, one of the reactant is present in relatively high concentration and it's concentration changes only a little bit during the entire reaction and therefore it's concentration value assumed to be constant during the reaction. Thus, the reaction behaves like a first order reaction instead of the second order reaction. Pseudo first order reaction is used in the "Study Of Chemical Kinetics". 

Thus, the reaction is called as a pseudo first order reaction. 

For Example

(a) CH₃-COO-C₂H₅ + H₂O ➝ CH₃-COOH + C₂H₅OH

     The rate law for the above reaction is, 

       ➩ rate = k [CH₃-COO-C₂H₅]¹ [H₂O]º

Here, 

       water is in excess amount and due to that it's concentration doesn't change during the reaction.

(b) C₁₂H₂₂O₁₁ + H₂O ➝ C₆H₁₂O₆ + C₆H₁₂O₆

        ➩rate = k [C₁₂H₂₂O₁₁]¹[H₂O]º

        ➩ rate =k [C₁₂H₂₂O₁₁]¹

            (water is in excess amount) 

Unit of reaction rate is same as of first order reaction. 

(F)Fractional Order Reactions

In this, the order of a reaction have fractional values instead of integer values. Here, some examples are given below,

        (a)  CHCl₃ + Cl₂ ➝ CCl₄ + HCl

               ➩ rate = k[CHCl₃]¹[Cl₂]⁰⋅⁵

            Hence, overall order = 1+0.5 =1.5

        (b) CH₃CHO  ➝ CH₄ + CO₂

               ➩ rate = k[CH₃CHO]¹⋅⁵

Based on the order, unit of the reaction 

rate constant is different. 

The general unit of reaction rate constant, 

          k = "concentration / time-

                          (concentration)ⁿ"

Some Special Cases

FIRST CASE

Find the order of a reaction in which doubling both reactants leads to no change in rate??????

If a change in the concentration of a reactant does not change the rate of a reaction, then the reaction is said to be of zero order. This is because the rate of a zero order reaction is not proportional to the concentration of the reactant but to the rate of the constant. 

SECOND CASE

Determine the order of a reaction when doubling either reactant results in a doubling of the rate????

You should know that if the rate of a reaction is doubled by doubling the concentration of a reactant, then that reaction is a first order reaction. This is because the rate of a first order reaction is proportional to the first power of the reactant concentration.

THIRD CASE

Determine the order of a reaction in which doubling one reactant quadruples the rate?????

If the rate of a reaction is quadrupled by doubling the concentration of a reactant, then that reaction is a second order reaction. This is because the rate of a second order reaction is proportional to the second power of the reactant concentration.

Hope you have found this article helpful!!

Do you have suggestions? Please write in comment box!!!

Feel free to comment if you have any queries!!