Rate Law Expression

Ask a Question, Get an Answer!

Type your question here

Hundreds of tutors are online and ready to help you right now!

Experimentally, it is found that the rate of a reaction is proportional to the concentrations of the reactants raised to some power, provided that other factors, which also affect the rate of the reaction are kept constant.

For e.g., for the hypothetical reaction,

the rate of reaction might depend on the concentrations of A and B in the following manners.

the expressions, k [A][B] and k' [A]² [B] are the rate law expressions for the rate of the reaction and k is the rate coefficient or specific rate constant of the reaction. This constant is independent of concentration but is usually dependent on temperature.

There can be other rate law expression for the same hypothetical reaction given above such as

rate = k'' [A] [B]^o

rate = k''' [A]^1/2 [B]¹

Theoretically, there are many possibilities to rate law expression of a reaction. However, only one expression will fit the experimentally determined rate data.

In general, for any hypothetical reaction,

the rate law expression can be written as

rate (r) = k [A]^m [B]ⁿ …..(8)

Rate Law

It is an equation that relates the true rate of reaction to the concentrations of the reactants raised to some power.

Rate law expression serves three purposes.

(i) If the composition of the reaction mixture and the rate constant are known, then the rate of the reaction can be predicted.

(ii) It is useful in postulating the mechanism of a reaction.

(iii) It is useful in classifying the reactions into various orders

Experimental determination of rate law

This involves the determination of the values of the exponents 'm' and 'n' as given in the expression (equation 8). The units of the rate constant k depend on the order of the reaction.