Environmental Chemistry

Ozone

All of us are aware of how the concern related to depletion of ozone layer and the ozone hole, which also led to the ban of many chemicals such as the chlorofluorocarbon (CFC). Depletion of O3 can increase UV radiation entering Earth surface. This can not only lead to different type of severe diseases like cancer but it can also threaten the entire life on earth. Thankfully, the latest evidence suggests that the ozone depletion has come to a halt due to global initiatives were taken everywhere. Let us know more about this compound and its importance.

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Ozone

Have you seen the air or water purifier? One of the types of air or water purifier utilizes ozone for removal of unwanted toxic substances. The chemical compound can trap the bacteria and kill them. Hence, ozone is not only present in earth’s atmosphere but it is also possible to artificially produce this chemical substance.

Can you name another planet other than Earth that has ozone layer?  Venus has a thin ozone layer near at an altitude of 100 kilometres from the surface of the planet. Thus, large concentrations of Omolecules are present in the upper layer of atmosphere, specifically troposphere.

It is a type of gaseous compound. It is made from the combination of three oxygen atoms. Therefore, the chemical formula of this compound is O3. The formation of O3 is a two-step reaction. In the first step, the oxygen molecule is broken down into oxygen atoms with the help of sunlight. In the second step, the collision of oxygen atoms with another oxygen atom leads to the formation of ozone.

Ozone

Structure of ozone (Source: Wikipedia)

We can also produce ozone artificially with the help of oxygen molecules in the laboratory. The principle is similar to natural process. However, voltage electric current is applied in place of sunlight. Ozone formation takes place by placing dry oxygen in ozonizer and passing it through high voltage electric current by silent electric discharge. The electricity will transform the oxygen molecule into ozone. The most commonly known ozonizers for laboratory purposes are Siemens and Brodie ozonizer’s reaction:

3O2 + energy = 2O3

Ozone Layer

French Physicist Charles Fabry and Henri Buisson discovered ozone in the year 1913. Ozone layer/ shield is a region present in the stratosphere of the earth’s atmosphere and this region absorbs majority sun’s UV radiation. Mainly found in the lower region of stratosphere almost 20 to 30 kilometres above Earth.

The thickness of the layer differs according to the season and geographic region. The Earth’s stratosphere contains high concentrations of this compound. However, it is still a small portion in comparison to other gases present in Earth’s atmosphere. United Nations General Assembly declared International Day for the Preservation of the Ozone Layer on September 16th.

Importance of Ozone Shield

The stratosphere of the earth’s atmosphere contains a significant amount of O3. Thus, this gaseous compound protects living organisms including humans from the harmful UV radiations (λ = 255 nm). Excessive exposure to the UV radiation for a longer period of time can cause melanoma or skin cancer in humans.

Furthermore, it can cause many other forms of cancer and other diseases. Overall excessive UV radiation is a threat to any living organism. Hence, it is essential to maintain and protect the ozone layer.

Formation of Ozone

The reaction of UV radiation with O2 or dioxygen molecules results in the formation of O3. The UV radiation splits the oxygen molecule into the free oxygen or O atoms. These O atoms combine with molecular form of oxygen to form ozone (O3). It is thermodynamically an unstable compound and has a tendency to decompose into molecular oxygen. Therefore, a constant dynamic equilibrium is present between the production and decomposition of O3 molecules.

O2(g) → O(g) + O(g)

(in presence of UV Radiation)

O(g) + O2(g) ↔ O3(g)

(in presence of UV Radiation)

Depletion of Ozone Layer

In the past decade, we have come across constant news about depletion of the ozone layer. The seriousness of the situation made global regulatory bodies like the UN and the countries to work together to bring this constant depletion to a halt. So, what is the reason for depletion of ozone?

The depletion of the protective O3 layer is because of the presence of particular chemicals in the stratosphere of earth’s atmosphere. The constant release of compounds like carbon tetrachloride, carbon tetrafluoride, CFCs (chlorofluorocarbon) or freons and other chlorine or bromine containing halogens in the atmosphere is the main reason for the depletion.

CFC compounds are non-inflammable, non-toxic, nonreactive organic molecules. Hence, it is used in air conditioners, refrigerators, plastic foam production, cleaning computer parts, etc.

However, these chemicals mix with normal atmospheric gases and finally reach the stratosphere. Thus, these compounds break down into free chlorine radicals in the presence of powerful UV radiation in the stratosphere.

CF2Cl2 (g) → Cl(g) + CF2Cl(g)

(in presence of powerful UV Radiation)

The chlorine radicals combine with the stratospheric Othereby forming molecular oxygen and chlorine monoxide radicals.

Cl(g) + O3(g) → ClO(g) + O2(g)

Chlorine monoxide radicals will further react with atomic oxygen to form more chlorine radicals.

ClO(g) + O(g) → Cl(g) + O2(g)

This process will continue and constantly regenerate chlorine radicals. This, in turn, will lead to the breakdown of ozone. Hence, CFCs are transporting agents that are responsible for damaging the ozone layer.

Ozone Hole

The first report of depletion in Olayer came out in the 1980s by an atmospheric scientist in Antarctica. This was observed over the South Pole. Later it was found that certain specific set of conditions were responsible for the formation of the hole. During summer season a reaction between nitrogen dioxide and methane leads to the formation of chlorine monoxide.

ClO(g) + NO2(g) → ClONO2(g)

Moreover, methane will also react with chlorine atoms to form chlorine sinks.

Cl(g) + CH4(g) → CH3(g) + HCl(g)

This helps in prevention of excess ozone depletion. However, a special type of clouds “polar stratospheric clouds” formation occurs over Antarctica during the time of winters. These clouds provide a surface for the reaction of chlorine nitrate. Thus, the chlorine nitrate undergoes hydrolysis to form hypochlorous acid.

ClONO2(g) + H2O(g) → HOCl(g) + HNO3(g)

Chlorine nitrate also undergoes reaction with hydrogen chloride to produce molecular chlorine.

ClONO2(g) + HCl(g) → Cl2(g) + HNO3(g)

However, during the springtime when the sun returns to Antarctica again, the warmth emits sun and undergoes photolysis reaction with HOCl and Chlorine molecule.

HOCl(g) → OH(g) + Cl(g)

(in presence of light)

The above reaction leads to the formation of chlorine and begins a chain reaction thereby causing ozone depletion.

Cl2(g) → 2Cl(g)

(in presence of light)

Effects of Ozone Depletion

O3 depletion leads to the entry of more UV radiation into the troposphere. UV radiations are very harmful and can cause health issues such as skin cancer, ageing of skin, cataract, sunburn, etc. It not only affects human health but it is also a threat to plants and other living organisms.

Depletion in the O3 layer is responsible for killing the phytoplankton’s, decrease the productivity of fish, etc. It can cause mutation in plant cells by affecting the plant protein.

Furthermore, UV radiation increases the rate of evaporation from a surface through the surface of stomata. Therefore, it decreases the moisture content of the soil. It also affects constructions and materials. Thus, it damages paints and fibres which lead to fading of the colour faster.

Solved Examples for You

Question: Strangely ozone on Earth’s stratosphere serves to be a very beneficial compound and protects us from extreme ultraviolet radiation by absorbing most of it. However, if present in Earth’s troposphere, the same compound act as a toxic substance. Explain

Solution: The reason for this is ozone act as a toxic gas and it is a very strong oxidizing agent. Therefore, it can react with unburnt hydrocarbons and form compounds like formaldehyde, acrolein, and peroxyacetyl nitrate (PAN) thereby causing pollution. It is also one of the major components of photochemical smog. It is also a powerful eye irritant.

Moreover, it can cause damage to plant life. It can cause corrosion of stones, building materials, metals and other painted surfaces. If Ozone is present in the troposphere in excess quantity, it can cause many diseases like a headache, nose and throat irritation, cough, breathing problem, chest pain, and dryness of throat and so on.

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