Photochemical Reaction - Laws, Principles, Examples Applications and FAQs

This article contains the important sections of the chapter on photochemistry. Here we will have a discussion on the laws of photochemistry with its different types and examples. Application of photochemistry, such as photosensitisation, and the difference between photochemical and thermal reactions. The study of chemistry that deals with the absorption of light energy is known as photochemistry. Molecules retain energy and jump into an excited state when they absorb light, usually UV or visible light. Regaining energy leads to a more excited state, and reactions occur that may not occur otherwise.

This Story also Contains

1. Photochemistry
2. Photochemical Reaction
3. Principles of Photochemical Reaction
4. Laws of photochemistry involve all these laws.
5. Examples of Photochemical Reaction
6. Exploring the Applications of Photochemical Reactions
7. Comparing Photochemical and Thermal Reactions
8. Photochemical and Electrochemical Reactions: A Comparative Study

Photochemistry

Photochemistry is a branch of chemistry that deals with the chemical reactions that are triggered by the absorption of light radiation, primarily in the ultraviolet (UV) or visible spectrum. These reactions differ from thermal reactions as they rely on photons rather than heat to carry out molecular changes. Light absorption causes molecules to jump into their higher-energy excited electronic states, which causes different reaction pathways that cannot be achieved by traditional thermal activation. The properties of the molecules when they are in this state are completely different from the previous state

Photochemical Reaction

Developments in the field of photochemistry took place in the 1800s. In the year 1817, German physicist Theodor von Grotthuss put forward theoretical ideas about the photochemical process.

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Importance of Photochemical Reaction

Photochemical reactions are of great importance to support life on Earth. Chemical changes in the atmospheric gases are done by solar radiation and modified by the particles present. The photochemical reactions of the upper atmosphere helps us to study about the depletion of the ozone layer, acid rain, and global warming.

Photochemical reactions have many advantages when compared to other types of reactions. Photochemical reactions need sunlight and it is present abundantly. Complex organic molecules such as proteins and nucleic acid are synthesized by the simple gaseous molecules like methane, ammonia, and carbon dioxide after undergoing photochemical reaction.

Principles of Photochemical Reaction

The principles of photochemical reaction are based on photochemistry. When a molecule absorbs photons, it gets excited to a higher energy state. This process is known as photoexcitation. The photochemical reaction depends on two laws:

• Grotthuss-Draper Law: This law states that all light radiations do not produce chemical reaction. Some increase the kinetic energy of molecules while some are reemitted.

• Stark-Einstein Law: This law states that each molecule of absorbing substance absorb one photon or quantum of the radiation in the primary process.

• Beer-Lambert’s Law: This law gives a linear relationship between absorbance and concentration of the species. According to this law, if a monochromatic light is passed through a solution of an absorbing substance then the rate of decrease in intensity of radiation is directly proportional to the thickness of the tube and concentration of the solution.

Laws of photochemistry involve all these laws.

Quantum yield

The efficiency of a particular reaction taking place by photochemical process is given by the term called quantum yield. Quantum yield is defined as “the ratio of the number of molecules reacting in a given time to the number of quanta absorbed in the same time.” Many photochemical reactions are complex and occur differently, so the quantum yield is usually specific for each reaction.

Different types of photochemical reactions

The different types of photochemical reactions are as given below:

• Photo-dissociation reaction:

$\mathrm{X}_2+h \nu \rightarrow 2 \mathrm{Y}^{\bullet}$

• Isomerization reaction:

$\mathrm{X}+h \nu \rightarrow \mathrm{Y}$

• Photo-addition reaction:

$\mathrm{X}+\mathrm{Y}+h \nu \rightarrow \mathrm{XY}$

- Photo-substitution reaction:

$\mathrm{X}-\mathrm{Y}+\mathrm{Z}+h \nu \rightarrow \mathrm{X}-\mathrm{Z}+\mathrm{Y}$

• Photo-redox reaction:

$\mathrm{X}+\mathrm{Y}+h \nu \rightarrow \mathrm{X}^{+}+\mathrm{Y}^{-}$

Photosensitization

Photosensitization involves two processes in which a donor species absorbs light to form an excited species followed by energy transfer to a suitable acceptor species. The excited species, thus excited indirectly, can undergo various processes known as photosensitized reactions or photosensitization. There are two types of photosensitized reaction: One is electron transfer and second is energy transfer reaction.

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Examples of Photochemical Reaction

Photochemical reaction examples are:

• Photosynthesis: During this process, the chlorophyll pigment in plants takes up the energy (hν) from the sun. Solar energy along with water convert carbon dioxide to glucose and oxygen. Artificial light is also used to carry out this process.

$6 \mathrm{CO}_2+6 \mathrm{H}_2 \mathrm{O}+\mathrm{hv} \rightarrow \mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6+6 \mathrm{O}_2$

• An example of a photochemical decomposition reaction is seen in photography. When light falls on silver chloride (AgCl) or silver bromide (AgBr) it produces an image. During this reaction Silver halides (AgX) decomposes into silver (Ag) and halogen (X₂).

$\begin{aligned} & 2 \mathrm{AgCl}+\mathrm{hv} \rightarrow 2 \mathrm{Ag}+\mathrm{Cl}_2 \\ & 2 \mathrm{AgBr}+\mathrm{hv} \rightarrow 2 \mathrm{Ag}+\mathrm{Br}_2\end{aligned}$

• Solar cells release the energy in the form of electricity by using the light energy from the sun.

• Formation of vitamin D when skin is exposed to sunlight.

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Photochemical Reaction in Atmosphere

In the atmosphere, there are some gaseous substances that change the chemical composition of air. From the kinetic molecular theory of gases, it is observed that the molecules present in the atmosphere move and collide with each other continuously. During day time, the atmosphere gets continuous solar radiation. As a result, these molecules absorb the light and photochemical reactions take place. These reactions have an important role in studying the nature of chemical species in the atmosphere. The oxidation reactions in the atmosphere are due to the reaction with solar energy.

Exploring the Applications of Photochemical Reactions

Some applications of photochemical reactions are:

• Photochemical reactions are used for the synthesis of vitamins, drugs, and fragrances.

• It is used for free-radical chlorination, nitration etc.

• It is used for the formation of anti-malaria drug

• It is used for the preparation of benzyl chloride

• It is used for the synthesis of various synthetic organic compounds

• It is used for the development of optical bleaches.

Comparing Photochemical and Thermal Reactions

Photochemical and Electrochemical Reactions: A Comparative Study

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