Chemistry - A Molecular Science

Chapter 9 Reaction Energetics

Example 9.8

Express the rate law for each of the following one-step processes:

a) O

(g) + O(g) 3

→

2O

(g) 2

The rate law is simply the rate constant (the

fraction of collisions resulting in the transition

state) times the product of t

he concentrations of all reactants (the number of collisions).

Rate = k[O

][O] 3

b) 2NOCl(g)

→

2NO(g) + Cl

(g) 2

Two NOCl particles (2NOCl = NOCl + NOCl) collide in this reaction.

Rate = k[NOCl][NOCl] = k[NOCl]

2

The rate of a reaction can be increased by

increasing the concentrations of the

reactants or by increasing the temperature.

Increasing the concentrations increases the

frequency of collisions, while increasing th

e temperature increases both the rate of

collisions (the molecules are moving faster) and the fraction of collisions with sufficient energy to attain the transition state. However,

the rate of a reaction can also be increased

by reducing the activation energy by altering

the mechanism and the transition state of the

reaction. This is done with the addition of a

catalyst,

which is a material that speeds the

rate of reaction without being changed by the reaction.

Biochemical reactions occur at the low temperatures and concentrations found in the

human body due to the presence of catalysts called

enzymes

. The

catalytic converter

in an

automobile aids the conversion

of unwanted pollutants into CO

, H 2

O and N 2

. As an 2

example of unwanted catalysis, consider the s

ituation of the industrially important class of

compounds known as chlorofluorocarbons or CFC’s. These compounds were used in aerosol sprays and refrigeration units and were once produced at levels exceeding one billion kilograms a year. Then it was discovered th

at they absorbed high energy light in the

upper atmosphere to create chlorine atoms, wh

ich react with ozone molecules as follows:

O^3

(g) + Cl(g)

→

OCl(g) + O

(g) 2

OCl molecules then react with oxygen at

oms and regenerate chlorine atoms.

OCl(g) + O(g)

→

Cl(g) + O

(g) 2

Summing the two reactions yields the net reaction for the depletion of the ozone layer.

O^3

(g) + O(g)

→

2O

(g) 2

The Cl atoms do not appear in the net reac

tion because they are formed in the second

reaction after being consumed in the first; that is, there is no

net

change in the chlorine

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State

University