The Chemical and Photochemical Reactions in the Atmosphere: Photochemistry is a special branch of chemistry that deals with the reactions that occur on exposure to light radiations. The chemical reactions that occur due to light radiations are known as photochemical reactions. They occur due to the excitation of an electron to the excited state on gaining energy. Photochemical reactions are the basis of many important processes which are responsible for sustainable life on the Earth. In this article, you will also get to know the consequences of photoexcitation.

Photosynthesis in plants with the help of sunlight is a very good example of photochemical reaction in nature. In the atmosphere, there are numerous photochemical reactions taking place all the time. Some of the famous examples include the formation of ozone in the atmosphere, photochemical smog, etc. There are many gases present in the atmosphere like oxygen, carbon dioxide, nitrogen, aerosols, etc.

What are Photochemical Reactions? 

Those chemical reactions which take place due to the absorption of light energy are called photochemical reactions. Most photochemical reactions occur by the absorption of ultraviolet radiations, visible light, or infrared radiation.

Apart from thermal reactions in the presence of heat energy, photochemical reactions are light-driven reactions. High energy intermediates are formed in paths of photochemical reactions which cannot be formed thermally. In these reactions, large activation energy barriers are crossed in a short time span. Some photochemical reactions are very destructive to the atmosphere, such as the photodegradation of plastics.

Thermochemical and Photochemical Reaction in the Atmosphere with Examples

Thermochemical Reactions

The chemical reactions that involve the absorption or evolution of heat are known as thermochemical reactions. They can occur in the absence of light. In this type of reaction, temperature plays a significant role in the rate of the reaction. Gibbs energy change \(\Delta {\rm{G}}\) and the enthalpy change \(\Delta {\rm{H}}\) of a thermochemical reaction vary for different chemical reactions. For example, sulphur dioxide in the atmosphere reacts with oxygen to form sulphur trioxide with an enthalpy change of \( – 98.9\,{\rm{kJ}}/{\rm{mol}}\).

\(2{\rm{S}}{{\rm{O}}_{2\left( {\rm{g}} \right)}} + {{\rm{O}}_{2\left( {\rm{g}} \right)}} \to 2{\rm{S}}{{\rm{O}}_{3\left( {\rm{g}} \right)}}\,\left( {\Delta {\rm{H}} = \, – 98.9\,{\rm{kJ}}/{\rm{mol}}} \right)\)

Another example of the thermochemical reaction is the reaction between carbon and oxygen to form carbon dioxide gas.

\({\rm{C}}\left( {\rm{S}} \right){\rm{ + }}{{\rm{O}}_{2\left( {\rm{g}} \right)}} \to {\rm{C}}{{\rm{O}}_{2\left( {\rm{g}} \right)}}\)

Photochemical Reactions

Photosynthesis

Photosynthesis is the most common example of a photochemical reaction. In photosynthesis plants, plants prepare their food using sunlight and water to convert carbon dioxide into glucose and oxygen. The chemical reaction is given below:

Photochemical Smog

Photochemical smog, or ‘summer smog,’ is formed when ultraviolet \(\left( {{\rm{UV}}} \right)\) rays coming from the sun react with nitrogen oxides and volatile organic compounds \(\left( {{\rm{VOCs}}} \right)\) present in the atmosphere. It is noticed as a visible haze during the morning and in the afternoon. Photochemical smogs are likely to occur more often on dry summer days when a particular region experiences maximum sunlight.

It is commonly found in densely populated urban cities such as Los Angeles, Sydney, New Delhi, Beijing, etc.

The chemical reactions involved in the formation of Photochemical smog are:

1. In the morning, nitrogen gas is oxidized when released from automobiles to form nitric oxide. It is an oxidation step.
   \({{\rm{N}}_2} + {{\rm{O}}_2} \to 2{\rm{NO}}\)
2. After some time, nitric oxide (NO) undergoes another oxidation reaction and combines with more oxygen to form nitrogen dioxide.
   \({\rm{2}}{{\rm{N}}_2} + {{\rm{O}}_2} \to 2{\rm{N}}{{\rm{O}}_2}\)
3. This Nitrogen dioxide absorbs sunlight to break down and forms nitric oxide (NO) and oxygen radical (O) as carried in a reduction reaction.
   \({\rm{N}}{{\rm{O}}_2} + {\rm{Sunlight}} \to {\rm{NO}} + {\rm{O}}\)

Formation of Ozone

Ozone can be defined as a pale blue gas that forms a protective shield against the harmful UV radiations coming from the sun. It is also known as trioxygen because of the presence of three oxygen molecules having chemical formulas as \({{\rm{O}}_3}\)

Ozone in the Stratosphere

Ozone is formed naturally by the series of photochemical reactions between the oxygen molecules and ultraviolet light in the stratosphere layer of the atmosphere. The concentration of stratospheric ozone is \(21\% \) higher than the tropospheric ozone.

Photolysis of Oxygen to form Ozone

1. The photochemical reaction to form ozone is a two-step mechanism. The first step involves a short \({{\rm{UV}}}\)-light wavelength (\(240\) and \(160\,{\rm{nm}}\)), which splits molecular oxygen \(\left( {{{\rm{O}}_2}} \right)\) into two atomic oxygen molecules \(\left( {\rm{O}} \right)\).
2. The second step results in the combination of the highly reactive atomic oxygen with molecular oxygen to generate ozone \(\left( {{{\rm{O}}_3}} \right)\)
3. At last, the ozone splits off again into a highly reactive oxygen atom and one oxygen molecule due to solar \({{\rm{UV}}}\) radiation. This series of reactions is continuous. This means that the ozone is continuously generated and dissociated in the stratosphere layer of the atmosphere.

Good Ozone
The stratospheric ozone is a good zone as it shields the potent \({{\rm{UV}}}\) radiations coming to the surface of the Earth.

Tropospheric Ozone or Bad Ozone
In the Troposphere layer (ground layer) of the atmosphere, the gases released from the vehicles, factories, etc., contribute to the greenhouse gases accumulation, which then reacts with the heat or solar \({{\rm{UV}}}\)- radiations and turns out to be more toxic.

Compounds like \({{\rm{VOCs}}}\), nitrogen oxides and \({{\rm{CO}}}\) behave as the precursors of tropospheric ozone, which leads to the depletion of the ozone layer.

The oxides of nitrogen are predominantly formed via high-temperature combustion in power plants, nuclear reactors etc., while \({{\rm{CO}}}\) or carbon monoxide originates via fuel combustion.

Therefore, ground-level ozone is referred to as bad ozone as it can act as a toxic air pollutant, which can affect plants and animals; harmful health effects like respiratory disorders etc., occur due to this bad ozone. The concentration of tropospheric ozone is about \(10\%\) of the total atmospheric concentration.

Photochemical Reactions of Other Atmospheric Gases

1. Photochemical Reaction of Carbon Monoxide
   Hydroxyl radicals in the atmosphere react with carbon monoxide to yield carbon dioxide and hydrogen radicals.
   \({\rm{CO}} + \cdot {\rm{OH}}\,{\rm{light}} \to {\rm{C}}{{\rm{O}}_2} + \cdot {\rm{H}}\)
2. Photochemical Reaction of Formaldehyde
   Formaldehyde is found in trace quantities in the atmosphere as they are formed due to various atmospheric reactions and is oxidized by hydroxyl radical.
3. Photochemical Reaction with Methane
   Methane is naturally emitted from the Earth’s surface. In the atmosphere, methane is oxidized by hydroxyl radical yielding methyl radical and water.
   Methane undergoes a series of photochemical chemical reactions in the methane cycle to form acetic acid.

Summary

The atmosphere is defined as the gaseous surrounding around a star or planetary body held in place by gravity. It acts as a blanket that covers the Earth from the harmful radiations coming from the sun. The Earth’s atmosphere is composed of about \(78\%\) nitrogen, \(21\%\) oxygen, \(0.9\%\) argon, and other gases also. In the inside atmosphere, there are continuous chemical and photochemical reactions occurring.

A photochemical reaction is a chemical reaction triggered when light energy is absorbed by the molecules of a substance. We can take the examples of ozone formation in the atmosphere, formation of photochemical smog, etc. The other gases present in the atmosphere, include methane, oxides of nitrogen, oxides of sulphur, etc. When reacted with hydroxyl radical or other radicals present in the atmosphere, it causes intense effects on the atmospheric composition.

Photosynthesis in plants is a very good example of a photochemical reaction in nature. The major environmental issue that we are facing is ozone layer depletion. The ozone layer is formed by a series of photochemical reactions, which forms a layer of shield around the atmosphere as the high concentration of ozone in the stratosphere safeguards against the harmful \({{\rm{UV}}}\) light. Conversely, the high concentration of ozone in the troposphere layer contributes to greenhouse gases and causes various respiratory hazards. Thus, we can conclude that in normal chemical reactions, the source of energy is mostly heating, but in photochemical reactions, the source of energy is light.

FAQs on Chemical and Photochemical Reaction

Following are the frequently asked questions on chemical and photochemical reactions in the atmosphere:

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