Class 12 Chemistry Chemical Kinetics Average rate of reaction

Average rate of reaction

The average rate of the reaction is the ratio of change in concentration of reactants to the change in time. It is determined by the change in concentration of reactants or products and the time taken for the change as well. As the reaction precedes forward the collisions between the molecules of the participating reactants reduces thereby decreasing the average rate of the reaction.

Mathematically, Average rate of reaction = Change in concentration / Time = (mol/litre)/time

PROBLEM.   For the reaction R → P, the concentration of a reactant changes from 0.03 M to 0.02 M in 25 minutes. Calculate the average rate of reaction using units of time both in minutes and seconds.

SOLUTION.   R2= 0.02 M

R1= 0.03M

t2 - t1= 25 minutes

∆[R]/∆t =∆[R2-R1]/ t2- t1=- (0.02-0.03)/25 = 6.67 X 10-6 Ms-1

=0.005ML-1 min-1

PROBLEM.   In a reaction, 2A → Products, the concentration of A decreases from 0.5 mol L-1 to 0.4 mol L-1 in 10 minutes. Calculate the rate during this interval?

SOLUTION.   -1/2 (∆[A]/∆t) =-1/2(∆[A2-A1]/∆t) =-1/2 (0.4-0.5/10)

=0.005ML-1 min-1

= 5 X 10-3M min-1

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