Chemical Kinetics and Ozone Formation

Ozone's Enthalpy of Formation

In the back of most chemistry textbooks, you can find a table of enthalpy of formation values for many common molecules. There is a similar Hf table in our online material. Below is a Hf table for some molecules in the atmosphere.

Compound Hf (kJ/mol)
CO2 -394
CO -111
CH4 -75
H2O(g) -242
Compound Hf (kJ/mol)
O3 143
NO2 33
NO 90
SO2 -297

An enthalpy (or heat) of formation is the enthalpy of reaction for the synthesis of the compound from the element in its standard state. For ozone this would be:
    3/2 O2 O3 Hrxn = 143 kJ/mol O3

For nitrogen dioxide, this would be:
    1/2 N2 + O2 NO2 Hrxn = 33 kJ/mol NO2

However ozone is not formed directly from molecular oxygen and neither is nitrogen dioxide prepared from molecular oxygen and molecular nitrogen. The data in the table is calculated by adding together other reactions of these molecules and their heats of reaction.

How is ozone formed?

Ozone is formed by the reaction between molecular oxygen and an oxygen atom.

Oxygen atom can be formed by the photolysis of molecular oxygen in the upper regions of the stratosphere or by the photolysis of ozone.

Before the oxygen atom can react with an O2 molecule and make ozone, it must lose some energy and go to a more stable electronic configuration.

It loses energy by colliding with another molecule, usually a nitrogen molecule. The nitrogen molecule leaves with excess kinetic energy. The configuration of the oxygen atom goes from

[He]2s22px22py22pz0 ------- to ------- [He]2s22px22py12pz1

Although 2 electrons can be in the same orbital, there is electron-electron repulsion between them.

Enthalpy, like energy is a state function. The pathway is not important. So the total of the enthalpy changes of all the reactions that add up to the formation of 1 mole of ozone must equal the enthalpy of formation of ozone.

1/2 O2 O* Hrxn-1
O* + N2 O + N2 Hrxn-2
O2 + O O3

3/2 O2 O3 Hf = Hrxn-1 + Hrxn-2 + Hrxn-3 = 143 kJ/mol

Rate of Ozone Formation

Ozone is formed when an oxygen atom collides with a molecule of O2. The rate must depend on how much of each of these reactants is present.

rate = k'[O][O2]

Again, there is a constant k' that depends on the temperature and pressure in a specific altitude, the fraction of O2 and O likely to have the correct orientation for the reaction, and other constants specific to the reactants.

The concentration of molecular oxygen is relatively high but the concentration of oxygen atoms is always VERY low.

Second Order Reactions

Reactions that take place when two reactants collide have the same rate expression as does the formation of ozone. These are second order reactions. In general terms:

Professor Patricia Shapley, University of Illinois, 2012