Chemical Kinetics

Rate of Reaction:

Consider the general reaction

P + 2Q 3R.

In the above reaction, two moles of Q disappear for every mol of P reacting and three moles of R are formed for each mol of P disappears.

rate of reaction =


Note: Negative sign indicates the consumption of the reactants and positive sign indicates the production of the product.

The rate of reaction at any time t is determined in the following way,

i) Concentration of any of the reactants or products which ever may be convenient is determined at various time intervals.

ii) Then concentration Vs time curve is drawn.

iii) A tangent is drawn at the point p of the curve which corresponds to the time t at which rate is to be determined.

iv) The slope of the tangent gives the rate of reaction at the required time as shown below.



(C_Rand C_P denote concentration of reactant and product respectively)

Unit of rate = = Concentration time^-1 i.e. mole L^-1 S^-1

The molecularity of an elementary reaction is defined as the minimum number of molecules, atoms or ions of the reactants(s) types required for the reaction to occur.

The mathematical expression showing the dependence of rate on the concentration(s) of reactant(s) is known as rate-law or rate-expression of the reaction and sum of the indices

(powers) of the concentration terms appearing in the rate law as observed experimentally is called order of reaction.


Difference between Order and Molecularity

i) Order is an experimental property while molecularity is the theoretical property.

ii) Order concerns with kinetics (rate-law) while molecularity concerns with mechanism.

iii) Order may be any number, fractional, integral or even zero whereas molecularity is always an integer excepting zero.

. Rate of a reaction at unit concentration of reactants is called rate constant

Rate =

. Unit of rate constant =

where n = order of reaction.


. Integrated rate equations

. For a zero order reaction

K = and t_1/2 =



1. For a first order reaction

K = and t_1/2 = .

Amount of substance left after n half lives of a first order reaction

=



2. For a second order reaction

(i) when concentration of both reactants are equal

k =

(ii) When the initial concentrations of the two reactions are different

k = and t_1/2 =



3.For a third order reaction

K = and t_1/2 =



4. General expression for an nth order reaction

K =

And t_1/2 =


Arrhenius Equation:

In 1889, Arrhenius suggested a simple relationship between the rate constant k for a reaction and the temperature T.

k = Ae^-Ea/RT

i) log k = log A

ii) log



Some important graphs for different orders of reactions