18.3: Ozone

Discussion Questions

• What is UV?
• How is ozone produced in the atmosphere?
• How much ozone is in the atmosphere, and where is the ozone layer?
• What is the interaction of ozone and UV?
• What is ozone depletion?
• What is ozone hole and how does it vary over time?
• What are CFCs?
• How do CFCs help depleting ozone?
• How is ozone depletion in the polar region different from other regions?
• What has been done and what can be done to reduce ozone depletion?

Most of the ozone in the atmosphere is in the stratosphere of the atmosphere, with about 8% in the lower troposphere. As mentioned there, the ozone is formed due to photo reaction. The ozone level is measured in Dobson Unit (DU), named after G.M.B. Dobson, who investigated the ozone between 1920 and 1960. One Dobson Unit (DU) is defined to be 0.01 mm thickness of ozone at STP when all the ozone in the air column above an area is collected and spread over the entire area. Thus, 100 DU is 1 mm thick.

What is UV?

In the electromagnetic radiation spectrum, the region beyond the violet (wavelength ~ 400 nanometer nm) invisible to eye detection is called ultraviolet (UV) rays. Its wavelength is shorter than 400 nm.

Figure \(\PageIndex{1}\): (a) This diagram shows the wavelength and frequency ranges of electromagnetic radiation. The visible portion of the electromagnetic spectrum is the narrow region with wavelengths between about 400 and 700 nm. (b) When white light is passed through a prism, it is split into light of different wavelengths, whose colors correspond to the visible spectrum.

UV is divided into three regions:

• UV A, wavelength = 400 - 320 nm
• UV B, wavelength = 320 - 280 nm
• UV C, wavelength = < 280 nm

Obviously, photons of UV C are the most energetic. UV-A radiation is needed by humans for the synthesis of vitamin-D; however, too much UV-A causes photoaging (toughening of the skin), suppression of the immune system and, to a lesser degree, reddening of the skin, and cataract formation. Ozone strongly absorbs UV B and C, but the absorption decreases as the wavelength increases to 320 nm. Very little UV C reaches the Earth surface due to ozone absorption.

How is Ozone produced in the Atmosphere?

When an oxygen molecule receive a photon (h\nu), it dissociates into monoatomic (reactive) atoms. These atoms attack an oxygen molecule to form ozone, O3.

\[\ce{O2 + h\nu \rightarrow O + O}\label{1}\]

\[\ce{O2 + O \rightarrow O3} \label{2}\]

The last reaction requires a third molecule to take away the energy associated with the free radical \(O^{\cdot}\) and \(O_2\), and the reaction can be represented by

\[\ce{O2 + O + M \rightarrow O3 + M*} \label{3}\]

The over all reaction between oxygen and ozone formation is:

\[\ce{3 O2 \rightleftharpoons 2 O3} \label{4}\]

The absorption of UV B and C leads to the destruction of ozone

\[\ce{O3 + h\nu \rightarrow O + O2} \label{5}\]

\[\ce{O3 + O \rightarrow 2 O2} \label{6}\]

A dynamic equilibrium is established in these reactions. The ozone concentration varies due to the amount of radiation received from the sun.

\(\PageIndex{1}\)

The enthalpy of formation of ozone is 142.7 kJ / mol. The bond energy of O2 is 498 kJ / mol. What is the average O=O bond energy of the bent ozone molecule O=O=O?

Solution

The overall reaction is

\[\ce{3 O2 \rightarrow 2 O3} \;\;\; \Delta H = 286 kJ\]

Note that 3 O=O bonds of oxygen are broken, and 4 O-O bonds of ozone are formed. If the bond energy of ozone is E, then

\[ \begin{align*} E &= (3*498 + 286) kJ / 4 mol \\[4pt] &= 445 kJ / mol \end{align*} \]

DISCUSSION
The ozone bonds are slightly weaker than the oxygen bonds. The average bond energy is not the bond energy for the removal of one oxygen from ozone.

\[\ce{O3 + h\nu \rightarrow O + O2}\]

Can the energy to remove one oxygen be estimated from the data given here?

The techniques used in this calculation is based on the principle of conservation of energy.

Example \(\PageIndex{2}\)

The bond energy of O2 is 498 kJ / mol. What is the maximum wavelength of the photon that has enough energy to break the O=O bond of oxygen?

Solution

The energy per O=O bond is:

\[(498000 J/mol) / (6.022x1023 bonds/mol) = 8.27x10-19 J/bonds\]

The wavelength \(\lambda\) of the photons can be evaluated using

\[E = \dfrac{h c}{\lambda}\]

\[ \begin{align*} \lambda &= \dfrac{(6.626 \times 10^{-34}\, J \cdot s)*(3 \times 10^8\, m/s)}{8.27 \times 10^{-19} J} \\[4pt] &= 2.403 \times 10^{-7} \,m = 240 nm \end{align*} \]

DISCUSSION

The visible region range from 300 nm to 700 nm, and radiation with a wavelength of 240 nm is in the ultraviolet region (Figure \(\PageIndex{1}\)). Visible light cannot break the O=O bond, and UV light has enough energy to break the O=O bond.

Chlorofluorohydrocarbons (CFCs)

Chemist Roy J. Plunkett discovered tetrafluoroethylene resin while researching refrigerants at DuPont. Known by its trade name, Teflon, Plunkett's discovery was found to be extremely heat-tolerant and stick-resistant. After ten years of research, Teflon was introduced in 1949. His continued research led to the usage of chlorofluorohydrocarbons known as CFCs or freon as refrigerants.

CFCs are made up of carbon, hydrogen, fluorine, and chlorine. DuPont used a number system to distinguish their product based on three digits. The digits are related to the molecular formulas.

• The first digit is the number of carbon atoms minus 1.
• The second digit is the number of hydrogen atoms plus 1.
• The third digit is the number of fluorine atoms minus 1.

For example, CFC (or freon) 123 should have a formula C₂HF₃Cl₂. The number of chlorine atoms can be deduced from the structural formula of saturated carbon chains. CFC's containing only one carbon atom per molecule has only two digits. Freon 12 used for fridge and automobil air conditioners has a formula of CF₂Cl₂. The nontoxic and nonflammable CFCs have been widely used as refrigerants, in aerosol spray, and dry cleaning liquid, foam blowing agents, cleansers for electronic components in the 70s, 80s and early 90s.

In 1973, James Lovelock demonstrated that all the CFCs produced up to that time have not been destroyed, but spread globally throughout the troposphere. (Lovelock's report was later published: J. E. Lovelock, R.J.Maggs, and R.J. Wade, (1974); Nature, 241, 194) In the article, concentrations of CFCs at some parts per 10¹¹ by volume was measured, and they deducted that with such a concentration, CFCs are not destroyed over the years. In 1974, Mario J. Molina published an article in Nature describing the ozone depletion by CFCs. (see M.J. Malina and F.S. Rowland, (1974); Nature, 249, 810) NASA later confirmed that HF was present in the stratosphere, and this compound had no natural source but from the decomposition of CFCs. Molina and Rowland suggested that the chlorine radicals in CFCs catalyze the decomposition of ozone as discussed below.

How do CFCs help depleting ozone?

A relatively recent concern is the depletion of ozone, O3 due to the presence of chlorine in the troposphere, and eventually their migration to the stratosphere. A major source of chlorine is Freons: CFCl3 (Freon 11), CF2Cl2 (Freon 12), C2F3Cl3 (Freon 113), C2F4Cl2 (Freon 114). Freons decompose in the troposphere. For example,

\[\ce{CFCl3 \rightarrow CFCl2 + Cl}\]

\[\ce{CF2Cl3 \rightarrow CF2Cl + Cl^.}\]

The chlorine atoms catalyze the decomposition of ozone,

\[\ce{Cl + O3 \rightarrow ClO + O2}\]

and ClO molecules further react with O generated due to photochemical decomposition of ozone:

\[\ce{O3 + h\nu \rightarrow O + O2}\]

\[\ce{ClO + O \rightarrow Cl + O2}\]

\[\ce{O + O3 \rightarrow O2 + O2.}\]

The net result or reaction is

\[\ce{2 O3 \rightarrow 3 O2}\]

Thus, the use of CFCs is now a world wide concern. In 1987, one hundred and forty nine (149) nations signed the Montreal Protocol. They agreed to reduce the manufacturing of CFCs by half in 1998; they also agree to phase out CFCs.

Ozone depletion in the polar region is different from other regions. The debate of ozone depletion often involves the North and South Poles. In these regions when temperatures drop to 190 K, ice cloud is formed. The ice crystals act as heterogeneous catalyst converting HCl and ClONO₂ into \(HNO_3\) and \(Cl_2\),

\[\ce{Cl + ClONO2 \rightarrow HNO3 + Cl2}\]

\[\ce{H2O + ClONO2 \rightarrow HNO3 + HOCl.}\]

Both Cl₂, and HOCl are easily photolyzed to Cl atoms, which catalyze the depletion of ozone. This has just been discussed in the previous section.

What has been done and what can be done to reduce ozone depletion?

The U.S. and Canadian governments have banned the use of Freons in aerosol sprays, but their use in air conditioner and cooling machines continue. In order to eliminate Freon in the atmosphere, international concerted effort and determination is required. However, sound and reliable scientific information is required. The banning of CFCs opens a research opportunity for another invention to find its substitute. Who knows what other problems will the new product bring?

Questions

1. What is the unit used for measuring ozone layers?

   Skill - Define a unit you use.

2. What is the wavelength range of the UV radiation?

   Skill - Describe UV radiation.

3. How is ozone different from oxygen?

   Skill - Describe the formation of ozone.

4. When CFCs are exposed to UV or sun light, what species are produced?

   Skill - Explain a photodecomposition reaction.

5. What is the role of chlorine radical in the ozone formation or reactions.

   Skill - Explain the mechanism of the catalytic reaction.

6. What in the polar zone makes the depletion of ozone more serious?

Solutions

1. Dobson unit.
2. UV radiation is electromagnetic radiation with wavelength between 100 and 400 nm.
3. The ozone molecules consist of 3 atoms whereas the usual oxygen molecules 2.
4. Reactive radicals are produced, including monoatomic chlorine radical.
5. Chlorine radical catalyze the decomposition of ozone.
6. The ice clouds act as heterogeneous catalysts for the formation of chlorine gas.
   Discussion - The chlorine gas is photodissociated into Cl radicals that catalyze the decomposition of ozone.

Learning Guide

• Arrange the regions of the electromagnetic spectrum in increasing energies of their photons: X-rays, visible, gamma rays, ultraviolet, infrared, microwave, etc.
• Examples on this page can be testing questions.
• What are CFCs?
  What are freon 12, 123, and 114?
  Explain how CFC help destroy the atmosphere ozone layer, particularly the polar region?

Contributors and Attributions

• Chung (Peter) Chieh (Professor Emeritus, Chemistry @ University of Waterloo)