Kinetics: the Rate Lawand Effect of Concentration

Lecture 22

The rate law

for a chemical reaction is an equation which links the

reaction rate with concentrations or pressures of

reactants and constant parameters (normally rate coefficients and partial

reaction orders).

For aA+bB+… cC+dD+…

Rate = k[A]m[B]n…

Rate = k[A]m[B]n…

Herein k is the rate constant. It depends on temperature.m and n are reaction orders. They depend on the reaction mechanism.

Rate = k[A]m[B]n…

If the rate doubles when [A] doubles, m=1.If the rate quadruples when [A] doubles, m=2.If the rate does not change when [A] doubles, m=0.

Coefficients a and b in

aA+bB+… cC+dD+…may or may not be related in any way to the reaction

orders m and n:Rate = k[A]m[B]n…

The reaction mechanism

must conform to to the rate law.

The rate law is based on experimental fact.

Components of the rate law must be found experimentally:Measure concentrations to find the initial rate.Use initial rates from several experiments to find the reaction orders.Use reaction orders to find the rate constant.

How to define the reaction orders:

Rate=k[A] . First order overall.Rate=k[A]2 . Second order overall.Rate=k[A]0=k(1)=k . Zero order overall.

How to define the reaction orders:

NO(g)+O3(g)NO2(g)+O2(g)

Rate=k[NO][O3]First order with respect to NO.First order with respect to O3.Second order overall.

How to define the reaction orders:

2NO(g)+2H2(g)N2(g)+2H2O(g)

Rate=k[NO]2[H2]Second order with respect to NO.First order with respect to H2.Third order overall.

Reaction orders

cannot be deduced from the balanced equation.

Special cases:

If a reaction order is fractional, the rate depends on the square (cubic) root of the concentration: rate=k[A][B]1/2

If a reaction order is negative, the rate decreases when the concentration of that component increases.

A sample problem

on determining rate order from rate laws.

It we do not know the reaction law,

we have to find it from a series of experiments,

starting each one with a different set of reactant

concentrations and obtaining an initial rate in each case.

A sample problem

on determining reaction order from initial rate data.

THE END