Chemical Kinetics

Chemical Kinetics

Question

What do you understand by the rate law and rate constant of a reaction?

Identify the order of a reaction if the units of its rate constant are:

(i) L-1 mol s-1

(ii) L mol-1 s-1

Answer

The rate law can be defined as an expression containing the stoichiometric coefficients of reactants and products. It is an expression in which the rate of reaction is given in terms of the molar concentration of the reactants, with each term raised to some power, which may or may not is the stoichiometric coefficient of the reacting species in a balanced chemical equation. The rate constant can be defined as the rate of the reaction when the concentration of each of the reactant is taken as unity.

Example: 2NO(g) + O(g)---> 2NO2(g)

The rate expression for the above reaction can be written as follows:

Rate = k [NO]2 [O2] (Experimentally determined)

Now, if the concentration of NO and O2 is taken to be unity, then the rate constant is found to be equal to the rate of the reaction.

(i) Comparing power of mole in L-1 mol s-1 and (mol L-1)1-n s-1,

We get

 1 = l – n              => n = 0 i.e., zero order reaction

(ii) Again comparing power of mole in L mol-1 s-1 and (mol L-1)1-n s-1

 We get

–1 = 1 – n            => n = 2, i.e., second order reaction.

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