To understand the third order reaction, first, we should understand what the term order of a reaction means. 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 molecule. 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 the rate formula,
Rate, R= k [A]ˣ [B]ʸ
Here, the order with respect to A is given by x, and the 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 summarise, 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 concentration.
(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
The 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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