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Ozone (O3) - Definition, Structure, Preparation, Physical and Chemical Properties, Uses, Significance of Ozone Layer, Ozone Layer Depletion

Ozone (O3) - Definition, Structure, Preparation, Physical and Chemical Properties, Uses, Significance of Ozone Layer, Ozone Layer Depletion

One of nature’s greatest gifts to mankind is the ozone layer. It is a blanket made of millions of molecules of ozone. One can find the ozone layer in the Earth's lower atmosphere. It can absorb around 97-99 % of the sun's damaging UV radiation, which can destroy life on Earth. 

In addition, most people get skin disorders, and their immune systems would be impaired if the ozone layer disappeared. A major concern right now for scientists is the gradually increasing ozone hole. They have detected a hole in the ozone layer above Antarctica. It has elevated their awareness of different environmental concerns and the measures that may be taken to address them. Methyl bromide, chlorofluorocarbons, carbon tetrachloride, and hydrochlorofluorocarbons are the significant causes of the ozone hole.

Before we delve into that particular issue, let’s first understand the chemistry of ozone!

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TABLE OF CONTENTS

What is Ozone?

Ozone is an allotropic molecular form of oxygen containing three atoms of oxygen (O3). Ozone is generated through the passage of oxygen O2 through a high voltage potential resulting in the attachment and formation of a third oxygen atom. Ozone and oxygen are gaseous allotropes.

The molecular formula for ozone (O3) was established by Soret (1863) by considering the ratio of the changes in volumes when O3/O2 mixtures were either heated with turpentine or cinnamon oil. 

The word ‘ozone’ comes from the Greek word ‘ozein’ which means ‘to smell’. This meaning comes from ozone at the ground level, which gives off a pungent, unpleasant odour.

Structure of Ozone

Ozone is a polar molecule comprising two resonating structures. The central oxygen atom has a formal charge of +1 and the atoms at the terminals have a formal charge of -1. Hence, it has bent geometry and has polarity.

Ozone has a bent geometry as is evident from microwave spectroscopy. The O – O bond length in ozone molecule is 127.2 pm. The O – O – O bond angle in ozone is 116.7. The central atom of the ozone molecule is sp2 hybridised and has one lone pair. It is a polar molecule and has a dipole moment of 0.53 D. It has C2v symmetry. 

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Preparation of Ozone

Preparation of O3 via electrical discharge:  Pure and dry stream of oxygen is passed through a silent electrical discharge. Silent electrical discharge is used in the preparation of ozone to prevent its decomposition. 

  • The mixture obtained contains 5-10% ozone by volume, and this mixture is called ozonised oxygen. A higher concentration or pure ozone can be obtained by fractional liquefaction of the mixture.

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Physical Properties of Ozone 

  • Ozone is composed of three oxygen atoms covalently bonded. It is a compound of an element corresponding to the p-block of the modern periodic table.
  • Ozone in the gaseous state is pale blue, in the liquid state it is dark blue, and in the solid state it is violet.
  • It has a strong disturbing smell.
  • It has the ability to absorb the UV rays which ranges between 220-290 nm of the atmospheric spectra.
  • This form of oxygen boils at 161.2 K and forms violet-blue crystals when solidified. It melts at 80.6 K

Chemical Properties of Ozone 

  • Ozone easily undergoes dissolution in water producing hydrogen peroxide.

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  • Ozone oxidises lead sulphide forming lead sulphate.

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  • O3 is thermodynamically unstable with respect to O2. Hence, a high concentration of ozone can be dangerous explosives. The following reaction has G= -ve and is highly exothermic.

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  • Ozone is a powerful oxidising agent due to the ease with which it liberates the atoms of nascent oxygen.

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  • Ozone oxidises iodide ion to iodine.

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Uses of Ozone

  • Used as a germicide, disinfectant and for sterilising water.
  • It absorbs strong UV rays and protects us from harmful UV radiation from the sun.
  • Ozone is used for purification at water treatment plants without using filtration systems.
  • Ozone may also be formed by commonly used equipments like photocopiers, laser printers, and other electrical devices.
  • Ozone can be used to minimise the effects of bacteria, viruses, fungi, yeast, and protozoa.
  • Ozone therapy is used to disinfect and treat diseases.
  • Ozone therapy is used in hair-loss treatments as well.
  • The ozone layer protects our earth from the harmful UV radiation of the sun.

Ozone Layer Depletion

The primary source of ozone layer depletion is chlorofluorocarbons (CFCs). The UV radiations in the stratosphere break down chlorofluorocarbon molecules, releasing chlorine atoms. These chlorine atoms degrade the ozone layer by reacting with it.

According to studies, the unregulated firing of rockets depletes the ozone layer far more than chlorofluorocarbon does. If this is not addressed, this may significantly deplete the ozone layer by 2050. There is a possibility that nitrogen oxides (NO2, NO, N2O) emitted from the exhaust systems of supersonic jet aeroplanes might be slowly depleting the concentration of the ozone layer in the upper atmosphere. 

Another threat to the ozone layer is probably posed by the use of freons, which are used in aerosol sprays and as refrigerants. 

Certain natural phenomena, including stratospheric winds, and sunspots have been shown to degrade or deplete the ozone layer. However, it only contributes to a 1-2 per cent reduction in ozone layer depletion.

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Effects of Ozone Layer Depletion

The depletion of the ozone layer has negative consequences for the environment. Let us look at the primary consequences of ozone depletion on the environment and humans.

Human health effects: Due to the ozone layer's depletion, people will be directly or indirectly exposed to UV radiation. Humans may have significant health problems as a result, including skin disorders, cancer, weakened immune system, sunburns, cataracts, and rapid ageing.

Animal reactions:  The depletion of the ozone layer has resulted in epidermal damage in whales due to high UV radiation. In addition, because of ozone layer depletion, more solar damage has been observed in many aquatic species.

Environmental implications:  Plants may have reduced growth, blooming, and photosynthesis as a result of solid UV radiation. The forests are also subjected to the damaging impacts of UV radiation.

Marine life effects:  Exposure to damaging UV radiation has a significant impact on the life of planktons. The species in the food chain are also harmed if the planktons are destroyed or eliminated.

Ways to Control Ozone Layer Depletion

The ozone layer's depletion is a significant problem, and governments have initiated numerous programs worldwide. However, individual actions should be taken to protect the ozone layer from being depleted. The following are some suggestions for preventing this problem on a worldwide scale.

  • Use of chlorofluorocarbons and other related freons and aerosols to be reduced.

Reduce the usage of chemicals that deplete the ozone layer. Avoiding the use of CFCs in refrigerators and air conditioners, as well as substituting halon-based fire extinguishers, are just a few examples.

  • Reduce the reliance on automobiles. 

Cars generate many chances for the causes of greenhouse effects by generating greenhouse gases, which contribute to ozone depletion and global warming. As a result, automobiles like cars should be minimized to the greatest extent feasible.

  • Make use of eco-friendly cleaning supplies.

The cleaning products contain bromine-releasing compounds and chlorine, which enter the atmosphere and deplete the ozone layer. To safeguard our environment, these compounds should be restricted and replaced with natural goods.

Practice Problems

Q1. The angular shape of ozone molecule (O3) consists of

A. 1σ and 1π bond

B. 2σ and 1π bond

C. 1σ and 2π bond

D. 2σ and 2π bond

Answer: A double bond consists of one sigma and one π bond. Thus, the angular shape of ozone contains two sigma and one π bonds, So, option B) is the correct answer.

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Q2 Ozone is obtained from oxygen:

A. By oxidation at high temperature

B. By oxidation using a catalyst

C. By silent electric discharge

D. By conversion at high pressure

Answer: Pure and dry stream of oxygen is passed through a silent electrical discharge. Silent electrical discharge is used in the preparation of ozone to prevent its decomposition. 

The mixture obtained contains 5-10% ozone by volume, and this mixture is called ozonised oxygen. A higher concentration or pure ozone can be obtained by fractional liquefaction of the mixture.

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So, option C) is the correct answer.

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Q3. The correct statements regarding ozone is/are:

A. Ozone is thermodynamically less stable with respect to oxygen.

B. It acts as a powerful oxidising agent.

C. It rapidly reacts with NO to form NO2 and O2

D. All of these

Answer: 

  • Ozone is thermodynamically less stable because it decomposes into oxygen and this decomposition results in the liberation of heat. So, its entropy is positive and free energy is negative.
  • Ozone is a powerful oxidising agent due to the ease with which it liberates the atoms of nascent oxygen. 

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  • Ozone readily react with NO and form NO2 and O2.

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So, option D) is the correct answer.

Q4. Ozone dissolves in water forming

A. Hydrogen peroxide

B. Dihydrogen gas

C. Nitric acid

D. Nitrogen dioxide

Answer: Ozone easily undergoes dissolution in water producing hydrogen peroxide.

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Frequently Asked Questions - FAQ

Question 1. What happens when ultraviolet light strikes the ozone layer?
Answer:
When UV rays strike the ozone layer, they get absorbed and ozone molecules split into an oxygen molecule and a free oxygen atom. Later, these two components combine and reform the ozone molecule (O3). By absorbing UV rays in the stratosphere, the ozone layer prevents harmful radiation from reaching Earth’s atmosphere.

Question 2. Which gas is responsible for ozone layer depletion?
Answer:
Chlorofluorocarbons and hydrochlorofluorocarbons (CFCs and HCFCs) are the majorly responsible gases for ozone depletion. Basically, when chlorine and bromine free radicals break apart from these CFCs and HCFCs and come into contact with ozone in the stratosphere, they destroy ozone molecules by initiating a chain reaction.

Question 3. Is ozone toxic to breathe?
Answer:
Ozone if inhaled can be dangerous. It causes severe damage to the lungs. Lower levels of ozone cause pain in the throat, coughing, shortness of breath and inflammation in the lungs.

Question 4. The structure of ozone is bent or linear?
Answer:
Ozone has a bent molecular geometry owing to the presence of lone pair of electrons present on the central oxygen atom.

Related Topics

Oxygen

Alkali Metals

Potassium

Phosphorus

Diagonal Relationship

Metallic and Non-metallic character

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