Chemical Kinetics: Unit of rate constant

Unit of rate constant

In general, rate expression for the reaction,

pA + qB -- >  cC + dD

Rate  =       k [A]^p [B]^q

K= Rate / [A]^p [B]^q

The unit for the rate constant `k' depends upon the rate of the reaction, the concentration of the reactants and the order of the reaction.

In the case of the first order reaction.

K= Rate / [A]¹ [B]²

K= mol dm ^-3 sec^-1 / mol dm^-3

Unit of

k = sec^-1 for first order reaction.

Similarly unit of k = mol^-1 dm³ sec^-1 for second order reaction unit of k = mol^-(n-1) dm^3(n-1) sec^-1 for n^th order reaction

Following are the important differences between rate and rate constant of a reaction

Rate of reaction

1.      It represents the speed at which the reactants are converted into products at any instant.

2.      At any instant of time, the rate depends upon the concentration of reactants at that instant.

3.      It decreases as the reaction proceeds.        

4.      Rate of rate determining step determines overall rate value.

Rate constant of reaction

1.      It is the constant of proportionality in the rate law expression.

2.      It refers to the rate of a reaction at the specific point when concentration of every reacting species is unity.

3.      It is constant and does not depend on the progress of the reaction.

4.      It is an experimental value. It does not depend on the rate determining step.