Kinetics The study of rates of chemical reactions

Kinetics 2NO 2 2NO + O 2 Rate = change in reactants or products in a given time

Definition of rate of reaction

Change in quantity

Definition of rate of reaction Molar concentration Change in quantity

Definition of rate of reaction Molar concentration time Change in quantity

2NO 2 2NO + O 2

Figure 12.3 A Plot of the Concentr- ation of N 2 O 5 as a Function of Time for the Reaction 2NO 2  2NO + O 2

2NO 2 2NO + O 2 Time[NO 2 ] 0 s3.00 x M 2 s1.00 x M

2NO 2 2NO + O 2 Time[NO 2 ] 0 s3.00 x M 2 s1.00 x M

Rate Laws Rate α concentration of reactants (& sometimes products) - - also depends on temperature... more later For: wA + xB → yC + zD Rate α [A] a [B] b or Rate = k[A] a [B] b a, b usually whole numbers (called orders) k is proportionality constant ; rate constant

Rate = k[A] a [B] b Rate always has units of M/s Units of k depend of rest of rate law a or b may be 0, which means rate is not dependent on the concentration of that reactant [X] 0 = 1

To find actual rate law Rate = k[A] a [B] b Change [A] while keeping [B] constant and measure rate.

To find actual rate law Rate = k[A] a [B] b Change [A] while keeping [B] constant and measure rate. Separate experiments

Rate = k’[A] (k’ = k[B] b ) ( rate is directly proportional to [A] ) Rate = k’[A] 1

Rate = k[A] 1 [B] b Change [B] while keeping [A] constant

The data show that the rate is proportional to [B] 2, or: Rate = k’’[B] 2 when [A] is kept constant The combined rate law is Rate = k[A] 1 [B] 2 The value of k can be determined once a and b are known.