Ozone in Chemistry: Formula, Structure, Molecular Mass and Example

Q: What is ozone, and where does it occur?

The molecular composition of three atoms of oxygen, O₃. It is formation takes place in both the stratosphere and troposphere of the Earth. Tropospheric O₃ is considered a pollutant. It is formation from reaction of pollutants, particularly in the presence of sunlight. The strontospheric O₃ forms the ozone layer, and this shieldlife from damaging UV radiations. One by the splitting of oxygen molecules into single atoms of oxygen; under UV light, they recombine to form ozone with O₂. The other is through the reaction of mainly nitrogen oxides and volatile organic compounds pollutants from vehicles and industrial sources in sunlight to form tropospheric ozone.

Q: How does ozone protect us from UV radiation?

It means that shortly, stratospheric ozone absorbs and scatters high energy ultraviolet radiation from the sun, UV-B and UV-C, which is harmful to living organisms. Hence these powerful rays do not strike the surface of the Earth. Therefore, living organisms are saved from the harmful effects caused by them. Most of the harmful UV-B and UV-C radiations from the sun would get absorbed by stratospheric ozone and would not reach the surface. Therefore, most of the UV energy turns into heat, baring the very penetrative ability of the rays into the atmosphere to reach the surface and cause harm to life on Earth.

Q: What are the reasons for the reduction of ozone?

It is mainly the man-made chemicals that cause depletion of the ozone: chlorofluorocarbons, halons, and other substances depleting the ozone. These chemicals allow, upon their decomposition in the stratosphere, atoms of chlorine and bromine to split molecules of ozone and hence make the layer thin. International agreements have been in place for the Phasing Out of production and use of ozone-depleting substances in Protecting the ozone layer from depletion. The efforts made in this direction have resulted in its gradual recovery; this vividly shows that global cooperation is a must for environmental protection.

Q: Ground-level ozone health effects

Ground-level ozone forms a primary component of air pollution, which is always responsible for triggering respiratory diseases in human beings, such as asthma and bronchitis, among many more lung-related diseases. Other health-related effects attributed to it include the aggravation of health conditions of people and reduced lung functions, especially in children, old people, and those who are suffering from any pre-existing respiratory disease. Exposure to ground-level ozone can only be reduced by monitoring air quality, reducing outdoor activities when the levels of ozone are high, and with policies that reduce emissions of nitrogen oxides and volatile organic compounds from transport and industrial sources. Another way is through public awareness and health advisories; these two methods are very effective at minimizing the health effects of ozone pollution.

Q: How is ozone used in water purification?

Having the strong oxidation ability characterized by it, it is this nature that makes ozone effective in destroying pathogenic microorganisms—bacteria, and viruses in water. Besides that, it contributes to the degradation of a diversity of organic and inorganic compounds, hence improving the quality and safety of the water without hazardous residues.

Ozone in Chemistry: Formula, Structure, Molecular Mass and Example

Edited By Shivani Poonia | Updated on Oct 17, 2024 11:16 PM IST

Imagine a moment for a second what it would be like if stepping out under the sun was sure to bring about nasty sunburns within just a few minutes—or even worse, to considerably increase the risk of contracting a dangerous form of skin cancer. That would be our world if there was no ozone—an extremely simple molecule made up of three oxygen atoms: O₃. There is an ozone layer embedded in Earth's atmosphere that is appropriately called so because it goes about its work of absorption and diffusion for most of the harmful ultraviolet radiations from the sun.

JEE Main 2025: Chemistry Formula | Study Materials | High Scoring Topics | Preparation Guide

JEE Main 2025: Syllabus | Sample Papers | Mock Tests | PYQs | Study Plan 100 Days

NEET 2025: Syllabus | High Scoring Topics | PYQs

This Story also Contains

Ozone
Varying Dimensions of Ozone
Tropospheric Ozone
Relevance and Applications of Ozone
Some Solved Examples
Summary

Ozone in Chemistry: Formula, Structure, Molecular Mass and Example

Ozone

Ozone is a light blue gas with a sharp irritating smell and actually an allotrope of oxygen.

In contrast to the usual diatomic oxygen we breathe every day, ozone is formed from three atoms of oxygen. It exists both in the stratosphere of the Earth and near its surface in the troposphere. It is said to be the ozone layer when it is found within the stratosphere. It is formed when the ultraviolet light from the sun reacts with the diatomic 8 oxygen, that is, O₂. The UV breaks down the diatomic oxygen molecule into two; each atom combines with another oxygen molecule, that is, O₂, to form a molecule of ozone, that is, O₃. This layer of O₃ protects all life on Earth from the dangerous, high-energy ultraviolet light from the sun. Ground-level ozone, on the other hand, is a pollutant formed because of reactions between pollutants emitted by vehicles, industries, and many other sources in the presence of sunlight.

Preparation

When a slow dry stream of oxygen is passed through a silent electrical discharge, conversion of oxygen to ozone (10%) occurs. The product is known as ozonized oxygen.

$3 \mathrm{O}_2 \rightarrow 2 \mathrm{O}_3 \Delta \mathrm{H}^{\Theta}(298 \mathrm{~K})=+142 \mathrm{kJmol}^{-1}$
Since the formation of ozone from oxygen is an endothermic process, it is necessary to use a silent electrical discharge in its preparation to prevent its decomposition. If a concentration of ozone greater than 10 percent is required, a battery of ozonizers can be used, and pure ozone (b.p. 101.1K) can be condensed in a vessel surrounded by liquid oxygen.

Properties

Pure ozone is a pale blue gas, dark blue liquid, and violet-black solid. Ozone has a characteristic smell and in small concentrations it is harmless. However, if the concentration rises above about 100 parts per million, breathing becomes uncomfortable resulting in headache and nausea.
Ozone is thermodynamically unstable with respect to oxygen since its decomposition into oxygen results in the liberation of heat ($(\Delta H$ is negative) and an increase in entropy ($\Delta S$ is positive). These two effects reinforce each other, resulting in a large negative Gibbs energy change ($\Delta G$) for its conversion into oxygen. It is not surprising, therefore, that high concentrations of ozone can be dangerously explosive.

$2 \mathrm{O}_3 \rightleftharpoons 3 \mathrm{O}_2 ; \Delta \mathrm{H}<0 ; \Delta \mathrm{S}>0 ; \Delta \mathrm{G}<0$
Due to the ease with which it liberates atoms of nascent oxygen $\left(\mathrm{O}_3 \rightarrow \mathrm{O}_2+\mathrm{O}\right)$, it acts as a powerful oxidizing agent. For example, it oxidizes lead sulphide to lead sulphate and iodide ions to iodine.

\begin{aligned} P b S (s) + 4 O_{3} (g) \to {P b S O}_{4} (s) + 4 O_{2} (g) \\ 2 I^{-} (a q) + H_{2} O (l) + O_{3} (g) \to 2 {O H}^{-} (a q) + I_{2} (s) + O_{2} (g) \end{aligned}

$
When ozone reacts with an excess of potassium iodide solution buffered with a borate buffer (pH 9.2), iodine is liberated which can be titrated against a standard solution of sodium thiosulphate. This is a quantitative method for estimating O₃ gas.
Experiments have shown that nitrogen oxides (particularly nitrogen monoxide) combine very rapidly with ozone and there is, thus, the possibility that nitrogen oxides emitted from the exhaust systems of supersonic jet airplanes might be slowly depleting the concentration of the ozone layer in the upper atmosphere.

$\mathrm{NO}(\mathrm{g})+\mathrm{O}_3(\mathrm{~g}) \rightarrow \mathrm{NO}_2(\mathrm{~g})+\mathrm{O}_2(\mathrm{~g})$

Another threat to this ozone layer is probably posed by the use of freons which are used in aerosol sprays and as refrigerants.
The two oxygen-oxygen bond lengths in the ozone molecule are identical (128 pm) and the molecule is angular as expected with a bond angle of about 117^o. It is a resonance hybrid of two main forms:

Uses

It is used as a germicide, disinfectant, and for sterilizing water.
It is also used for bleaching oils, ivory, flour, starch, etc.
It acts as an oxidizing agent in the manufacture of potassium permanganate.

Varying Dimensions of Ozone

Stratospheric Ozone

Most of the sun's ultraviolet radiation "UV-B and UV-C" that would otherwise tend to cause human skin cancer, cataracts, and immune deficiencies, as well as other adverse biological effects on wildlife and vegetation, is absorbed by it. The normal ozone layer is rather dynamic and is constantly being replenished and depleted through interactions involving man-made chemicals, such as chlorofluorocarbons, which lead to problems like the Antarctic ozone hole. Indeed, international efforts set in by the Montreal Protocol have been able to reduce consumption of ozone-depleting substances and allow slow healing of the ozone layer.

Tropospheric Ozone

Tropospheric ozone is another of the important constituents of air responsible for smog formation.

It belongs to the class of photochemical oxidants whose members are byproducts of photochemical reactions of NOx with VOCs, their major emitting sources related to vehicles, power plants, and industrial engines. Ground-level ozone is a very serious air pollutant damaging to human respiratory health and promoting lung problems such as asthma, bronchitis, and other kinds of lung diseases. Moreover, it damages vegetation, food crops, forests, and construction materials. Current and future practices in reducing air pollution by offering a clean environment to man and plants emphasize the use of cleaner forms of energy and the employment of stricter controls on emissions.

Relevance and Applications of Ozone

The importance of ozone goes way beyond environmental conservation; it has so many other applications. Administration of ozone therapy medicinally emanates from the antimicrobial property that is contained in the compound, which assists in treatments against infections and wounds. Other industrial uses include application in the bleaching of paper industries and textiles, where it normally substitutes for bleaches whose basic component is chlorine because it is friendly to the environment.

The studies on the ozone provide information on atmospheric chemistry and climate change.

Amongst others, most of these researchers have information on how depleted ozone is affecting ecosystems and human health, investigating mechanisms occurring in the processes of formation and destruction of the ozone. With this information, it's basic to design ways meant for the protect of the ozone layer and reduction of air pollution. Additionally, ozone level monitoring and modeling are very instrumental in weather forecasting, as well as rating the effectiveness of measures employed to regulate air quality.

Some Solved Examples

Example 1
Question: Which of the following is the wrong statement?
1. Ozone is a diamagnetic gas.
2. $(\text{ONCl})$ and $(\text{ONO}^{-})$ are isoelectronic.
3. $(\mathrm{O_3})$ molecule is bent.
4. Ozone is violet-black in solid state.

Solution: Isoelectronic species are those which have the same number of electrons. $(\text{ONCl})$ and $(\text{ONO}^{-})$ have different numbers of electrons.

$[\text{ONCl} = 8 + 7 + 17 = 32 \text{ electrons}]$

$[\mathrm{ONO}^{-} = 8 + 7 + 8 + 1 = 24 \text{ electrons}]$

Therefore, $(\text{ONCl})$ and $(\text{ONO}^{-})$ are not isoelectronic.

Additionally, $(\mathrm{O_3})$ is a bent-shaped molecule. The structure of ozone is bent and it is a diamagnetic gas that exists as a pale blue gas, dark blue liquid, and violet-black solid.

Hence, the correct answer is option (2).

Example 2
Question: When $(\mathrm{O_3})$ reacts with $(\mathrm{HCl})$, it converts $(\mathrm{HCl})$ into:
1. $(\text{HClO})$
2. $(\mathrm{Cl_2})$
3. $(\mathrm{Cl^{-}})$
4. $(\mathrm{ClO_3^{-}})$

Solution: When ozone reacts with hydrochloric acid, it oxidizes the $(\mathrm{HCl})$ to form chlorine gas.

$[\mathrm{2HCl + O_3 \rightarrow Cl_2 + H_2O + O_2}]$

Thus, ozone converts $(\mathrm{HCl})$ into $(\mathrm{Cl_2})$.

Hence, the correct answer is option (2).

Example 3
Question: In the following question, a statement of assertion is followed by a statement of reason. Mark the correct option:
- Assertion: Ozone is a toxic gas.
- Reason: Ozone reacts with and damages lung tissue.

1. Both assertion and reason are true, and the reason is the correct explanation of the assertion.

2. Both assertion and reason are true, but the reason is not the correct explanation of the assertion.

3. The assertion is true, but the reason is false.

4. The assertion is false, but the reason is true

Solution:

The assertion is true that ozone is a toxic gas. This is because it can react with and damage lung tissue, leading to respiratory problems. The reason provided for the assertion is also true, and it is the correct explanation for why ozone is toxic. Both Assertion and Reason are true, and the Reason is the correct explanation for the Assertion.

Hence, the answer is the option (1).

Summary

Ozone, a molecule made up of three atoms of oxygen, has played this dual role in our surroundings: from the first level stratosphere in shielding life by filtering off the bad ultraviolet radiations to this very ground level, where it becomes a pollutant and causes several other ill effects on health.

There is, therefore, much importance in learning about ozone: how it's formed, its properties, applications, and relevance in environmental protection and public health. This shall be instrumental in sustaining the delicate balance of this gas and eventually contributing to a healthier planet for the next generation through efforts taken to reduce substances depleting ozone and mitigate air pollution.

Frequently Asked Questions (FAQs)

1. What is ozone, and where does it occur?

The molecular composition of three atoms of oxygen, O₃. It is formation takes place in both the stratosphere and troposphere of the Earth. Tropospheric O₃ is considered a pollutant. It is formation from reaction of pollutants, particularly in the presence of sunlight. The strontospheric O₃ forms the ozone layer, and this shieldlife from damaging UV radiations.

One by the splitting of oxygen molecules into single atoms of oxygen; under UV light, they recombine to form ozone with O₂. The other is through the reaction of mainly nitrogen oxides and volatile organic compounds pollutants from vehicles and industrial sources in sunlight to form tropospheric ozone.

2. How does ozone protect us from UV radiation?

It means that shortly, stratospheric ozone absorbs and scatters high energy ultraviolet radiation from the sun, UV-B and UV-C, which is harmful to living organisms. Hence these powerful rays do not strike the surface of the Earth. Therefore, living organisms are saved from the harmful effects caused by them.

Most of the harmful UV-B and UV-C radiations from the sun would get absorbed by stratospheric ozone and would not reach the surface. Therefore, most of the UV energy turns into heat, baring the very penetrative ability of the rays into the atmosphere to reach the surface and cause harm to life on Earth.

3. What are the reasons for the reduction of ozone?

It is mainly the man-made chemicals that cause depletion of the ozone: chlorofluorocarbons, halons, and other substances depleting the ozone. These chemicals allow, upon their decomposition in the stratosphere, atoms of chlorine and bromine to split molecules of ozone and hence make the layer thin.

International agreements have been in place for the Phasing Out of production and use of ozone-depleting substances in Protecting the ozone layer from depletion. The efforts made in this direction have resulted in its gradual recovery; this vividly shows that global cooperation is a must for environmental protection.

4. Ground-level ozone health effects

Ground-level ozone forms a primary component of air pollution, which is always responsible for triggering respiratory diseases in human beings, such as asthma and bronchitis, among many more lung-related diseases. Other health-related effects attributed to it include the aggravation of health conditions of people and reduced lung functions, especially in children, old people, and those who are suffering from any pre-existing respiratory disease.

Exposure to ground-level ozone can only be reduced by monitoring air quality, reducing outdoor activities when the levels of ozone are high, and with policies that reduce emissions of nitrogen oxides and volatile organic compounds from transport and industrial sources. Another way is through public awareness and health advisories; these two methods are very effective at minimizing the health effects of ozone pollution.

5. How is ozone used in water purification?

Having the strong oxidation ability characterized by it, it is this nature that makes ozone effective in destroying pathogenic microorganisms—bacteria, and viruses in water. Besides that, it contributes to the degradation of a diversity of organic and inorganic compounds, hence improving the quality and safety of the water without hazardous residues.