Ist order, IInd order, IIIrd order and zero order reactions

The order of reaction is determined with respect to each reactant in the reaction. The order of a reaction is the power to which the concentration of a reactant is raised. This is the order with respect to one reactant only. For the reaction

where there are more than one reactants, the order of the reaction is determined with A and then with B. As mentioned earlier in equation the rate law which is expressed as rate (r) = k [A]^m [B]ⁿ, the exponent 'm' denotes the order of the reaction with respect to reactant A and the exponent 'n' denotes the order of the reaction with respect to B. The overall order of the reaction is then (m + n). It should be noted that 'm' and 'n' do not necessarily represent the stoichiometric coefficients 'a' and 'b' of the reaction. Order of a reaction is experimentally determined.

Order of a reaction need not have a integral value. It can be a fraction and negative too (reactions with negative order will not be discussed here). Various reactions display different orders. A few examples are listed.

Zero order reaction

Example 4:

Ammonia (NH₃) gas decomposes over platinum catalyst to nitrogen gas (N₂) and hydrogen gas (H₂). The chemical reaction is as follows:

The reaction follows zero order kinetics. Therefore the rate law is rate r = k[NH₃]^o

For a zero order reaction the concentration versus time profile is linear and the rate of reaction versus time has the profile.

fig 6.4(a) - Concentration versus time profile for a zero order reaction

fig 6.4 (b) - Rate of reaction versus time for a zero order reaction

First order reaction

Example 5:

Cyclopropane (C₃H₆) at room temperature has a ring structure. When it is heated, the ring opens up and cyclopropane isomerizes to propylene.

The reaction takes place in a first order manner. Therefore, the rate law for the reaction is, rate (r) = k [C₃H₆]¹

fig 6.5 - First Order Reaction: Concentration versus time profile

The above figure is an example of the first order reaction. The rate of the reaction depends on the concentration of the reactant. Initially the rate is fast and then it slows down as the concentration of the reactant falls.

Second order reaction

Example 6:

Nitrogen dioxide (NO₂) gas reacts with fluorine gas (F₂) to give nitrosyl fluoride. The chemical reaction is given by the equation.

The reaction has the rate law as:

rate (r) = k [NO₂] [F₂]

The rate has order one with respect to nitrogen dioxide concentration and fluorine concentration and the overall order is (1 + 1) which is two.

This depicts the change of concentration of second order reaction with only one reactant, that is, a reaction of the type A P where the rate law is rate r = k [A]².