Third Order Reaction

To understand the third-order reaction, first, we should know what does it mean by order of a reaction. The order of a reaction can be defined as the addition of the power of all the reactants taking part in that chemical reaction. The reaction is said to be third order when the minimum number of molecules required for the chemical reaction to take place is 3.

What is Third Order Reaction?

A third-order reaction is a chemical reaction where the rate of reaction is proportional to the concentration of each reacting molecules. In this reaction, the rate is usually determined by the variation of three concentration terms.

For example, let us consider a chemical reaction where,

A+2B ⇢ C+D

According to rate formula,

Rate, R= k [A]ˣ [B]ʸ

Here, the order with respect to A is given by x and order with respect to B is given by y. So the complete order of the reaction is the sum of x and y.

For a third-order reaction, the order of the chemical reaction will be 3.

Let aA+bB+cC ⇢ Product

By rate formula, R= k [A]ˣ [B]ʸ [C]□

The third-order reaction for the above chemical reaction is given by,

Order = x+y+z

To summarize, the order of reaction can be defined as the sum of the exponents of all the reactants present in that chemical reaction. If the order of that reaction is 3, then the reaction is said to be a third-order reaction.

Different Cases in Third Order Reaction

When we are dealing with a third-order reaction there can be different cases involved. It could be;

(i) All three reactants have the same level of concentrations.

(ii) Two reactants have equal concentrations but one has a different concentration.

(iii) All three reactants have different or unequal concentrations.

Third Order Reaction Examples

● Let us consider the reaction between nitric oxide and chloride

2 NO +Cl2 ⇢ 2 NOCl

Rate, R= k[NO]2 [Cl2]

Order of above reaction =Sum of exponent of nitric oxide and chloride Order = 2 + 1 = 3

● Let us consider the reaction between nitric oxide and oxygen

2 NO + O2 ⇢ 2 NO2 R= k[NO]2 [O2]

Order = 2 + 1 = 3

The unit of third-order reaction when a rate is constant is given by, Rate of reaction= k[Reactant]³

Unit of rate is given by,

R = mol/ Ls = mol L⁻¹ s⁻¹ mol L⁻¹ s⁻¹ = k ( mol L⁻¹ )³ k =L² mol⁻² s⁻¹


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