Classification of rates based on the order of the reaction

Classification of rates based on the order of the reaction

The rate law for a reaction must be determined by experiment. Usually the order of the reaction determined experimentally does not coincide with the stoichiometric coefficients of the reactants or products in the balanced chemical equation. Each reaction proceeds by a rate value determined by the rate constant and initial concentrations of the reacting species. Rate constant values differ for different `order' reactions even if concentrations are maintained the same. Therefore chemical reactions are classified according to its rate of chemical transformation which inturn depend on the order of the reaction. Let us consider a general rate equation such as

rate = k[A]^p [B]^q

Total order is p + q and order with respect to A is p and with respect to B in q respectively.

Zero order reaction

A reactant whose concentration does not affect the reaction rate is called as zero order reaction,

rate law is,

rate = k[A]⁰

-d[A]/dt = k or k =[A]₀-[A]t / t

Examples of zero order reaction is

H ₂ (g) + Cl₂ (g)  -hv- > < -- 2HCl(g)

The first order reaction

when aqueous solution of NH4NO2 is warmed it decomposes rapidly to H₂O and N₂.

NH₄NO₂ -- >  2H₂O + N₂

This reaction goes by first order manner rate constant k is given by

K = 2.303/t . log(V_inf/V_inf-V_t) sec^-1

V_¥ and V_t are volume of N₂ collected at room temperature and 1 atm when a fixed amount of NH4NO2 decomposes at t = ¥ (after completion of reaction) and at any time `t'.

Second order reaction

A reaction is said to be second order if its reaction rate is determined by the variation of two concentration terms or square of a single concentration term.

Third order reactions   

A reaction is said to be third order if its rate is determined by the  variation of three concentration terms.

A second (or) third (or) any other high order reaction can be experimentally followed in an easy way by reducing the overall order to first order type by adopting pseudo order conditions. In this method,  excluding the concentration of one of reactants, concentrations of all other reactant are kept in excess (at least 10 times) of the concentration of one of the reactant whose concentrations are to be varied to study the changes in the rate.