Photochemical Reaction - Laws, Principles, Examples Applications and FAQs
Have you ever wondered how sunlight helps plants prepare food, why silver chloride turns grey in sunlight, or how photographic films work? What causes certain chemical reactions to occur only in the presence of light? The branch of chemistry that studies the interaction of light with matter and the chemical changes produced by absorbed light energy is known as photochemistry. It deals with photochemical reactions, their mechanisms, and the principles governing them.
This Story also Contains
- Photochemistry
- Photochemical Reaction
- Principles Of Photochemical Reaction
- Types Of Photochemical Reactions
- Examples Of Photochemical Reaction
- Applications Of Photochemical Reactions
- Comparison of Photochemical And Thermal Reactions
- Comparison of Photochemical And Electrochemical Reactions
- Some Solved Examples
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 may occur that would not otherwise. In this article, we will discuss Photochemical reactions, laws, principles, examples, applications, and the comparison of Photochemical and Thermal reactions that reveal the remarkable role of light in chemistry, and we also provide some solved examples. To know more, scroll down.
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 those in the previous state
Photochemical Reaction
A photochemical reaction is a chemical reaction that is initiated or accelerated by the absorption of light energy (usually ultraviolet or visible light) by the reactants. Photochemical reactions occur when molecules absorb light and are excited to a higher energy state, making them more reactive.
Examples
1. Chlorination of methane
$\mathrm{CH}_4+\mathrm{Cl}_2 \rightarrow \mathrm{CH}_3 \mathrm{Cl}+\mathrm{HCl} \text { (in the presence of sunlight) }$
2. Photosynthesis
$6 \mathrm{CO}_2+6 \mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6+6 \mathrm{O}_2 \text { (in the presence of sunlight and chlorophyll) }$
3. Decomposition of silver chloride
$2 \mathrm{AgCl} \rightarrow 2 \mathrm{Ag}+\mathrm{Cl}_2 \text { (sunlight) }$
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 behavior of photochemical reactions is explained by the following laws:
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Grotthuss-Draper Law: This law states that only the light absorbed by a substance can bring about a photochemical change. Light that is reflected, transmitted, or not absorbed does not produce a chemical reaction.
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Stark-Einstein Law: This law states that each molecule of absorbing substance absorbs one photon or quantum of the radiation in the primary process.
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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.
Types Of Photochemical Reactions
Photochemical reactions can be classified into different types depending on the chemical change produced after the absorption of light.
Photo-dissociation reaction:
In these reactions, a molecule absorbs light and breaks into smaller fragments, atoms, or free radicals.
$\mathrm{X}_2+h \nu \rightarrow 2 \mathrm{Y}^{\bullet}$
Photo Addition reaction:
Two or more molecules combine under the influence of light to form a single product.
$\mathrm{X}+\mathrm{Y}+h \nu \rightarrow \mathrm{XY}$
Photoisomerisation Reaction:
Light causes a molecule to convert from one isomeric form to another without changing its molecular formula. Example: cis-2-Butene ⇌ trans-2-Butene
Photochemical Substitution Reaction:
In these reactions, one atom or group is replaced by another under the influence of light.
$\mathrm{CH}_4+\mathrm{Cl}_2 \rightarrow \mathrm{CH}_3 \mathrm{Cl}+\mathrm{HCl}$ (sunlight)
Photo-redox reaction:
A photoredox reaction is a photochemical reaction in which oxidation and reduction (redox) processes occur under the influence of light. Light energy excites a photocatalyst or reactant, enabling the transfer of electrons between chemical species.
Examples
1. Photosynthesis: In plants, light energy drives a series of redox reactions in which water is oxidised and carbon dioxide is reduced to carbohydrates.
2. Reduction of Metal Ions: Certain metal ions can be reduced by light in the presence of suitable electron donors.
Examples Of Photochemical Reaction
Photochemical reaction examples are:
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Photosynthesis: During this process, the chlorophyll pigment in plants takes up the energy (hν) from the sun. Solar energy, along with water, converts carbon dioxide to glucose and oxygen. Artificial light is also used to carry out this process.
$6 \mathrm{CO}_2+6 \mathrm{H}_2 \mathrm{O}+\mathrm{hv} \rightarrow \mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6+6 \mathrm{O}_2$
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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) decompose into silver (Ag) and halogen (X2).
$\begin{aligned} & 2 \mathrm{AgCl}+\mathrm{hv} \rightarrow 2 \mathrm{Ag}+\mathrm{Cl}_2 \\ & 2 \mathrm{AgBr}+\mathrm{hv} \rightarrow 2 \mathrm{Ag}+\mathrm{Br}_2\end{aligned}$
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Solar cells release the energy in the form of electricity by using the light energy from the sun.
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Formation of vitamin D when skin is exposed to sunlight.
Photochemical Reaction in the 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 the daytime, the atmosphere receives 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.
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Applications Of Photochemical Reactions
Some applications of photochemical reactions are:
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Photochemical reactions are used for the synthesis of vitamins, drugs, and fragrances.
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It is used for free-radical chlorination, nitration etc.
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It is used for the formation of an anti-malaria drug
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It is used for the preparation of benzyl chloride
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It is used for the synthesis of various synthetic organic compounds
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It is used for the development of optical bleaches.
Comparison of Photochemical And Thermal Reactions
|
Photochemical Reaction |
Thermal Reaction |
|
Takes place by the absorption of radiation (photons) by molecules |
Takes place by the absorption of heat energy, generally by increasing the temperature of the reaction medium |
|
A light source is used |
The heat source is used |
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An adequate light source is required |
Reaction can occur even in the absence of light |
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Temperature causes no effect |
Temperature causes a direct effect |
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Catalyst is not required to accelerate the reaction rate. But, a high intensity of light can increase the rate of reaction. |
Most reactions require a catalyst to increase the rate of the reaction |
Comparison of Photochemical And Electrochemical Reactions
|
Photochemical Reaction |
Electrochemical Reaction |
|
Takes place by the absorption of radiations (photons) by molecules |
Takes place by the passage of electric current |
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Light source is used |
Electricity is the source used |
|
Photosynthesis is an example |
Reactions in an electrical cell is an example |
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Some Solved Examples
Question.1 Which of the following laws governs the quantitative relationship between light absorbed and the chemical reaction occurring?
a) Boyle’s Law
b) Grotthuss-Draper Law
c) Hess’s Law
d) Henry’s Law
Solution:
The Grotthuss-Draper Law states that only the light absorbed by a substance can bring about a photochemical change. Unabsorbed light has no effect on the reaction.
Hence, the correct answer is option b) Grotthuss-Draper Law
Question.2 Which principle explains that one photon of light activates only one molecule in a photochemical reaction?
a) Kirchhoff’s Law
b) Beer-Lambert Law
c) Stark-Einstein Law
d) Le Chatelier’s Principle
Solution:
The Stark-Einstein Law of Photochemical Equivalence states that each absorbed photon activates one molecule, initiating the reaction (quantum yield may vary later).
Hence, the correct answer is option c) Stark-Einstein Law
Question 3: Which of the following is not an example of a photochemical reaction?
a) Photosynthesis in plants
b) Photography (formation of silver image)
c) Rusting of iron
d) Chlorine and hydrogen reaction in sunlight
Solution:
Rusting of iron is a slow electrochemical process, not a photochemical one. Photochemical reactions specifically require light energy to proceed.
Hence, the correct answer is option c) Rusting of iron
Question 4: According to the Stark-Einstein law, one molecule absorbs:
A) One mole of photons
B) One photon of radiation
C) Two photons of radiation
D) Infinite photons
Solution:
The Stark-Einstein Law of Photochemical Equivalence states that in the primary photochemical process, each molecule of the absorbing substance absorbs one photon (one quantum) of radiation.
Thus, one absorbed photon activates one molecule and initiates the photochemical reaction.
Hence, the correct answer is option (B)
Question 5: In a photochemical reaction, the primary process involves:
A) Emission of light
B) Absorption of a photon by a molecule
C) Increase in pressure
D) Collision between reactants
Solution:
A photochemical reaction begins when a reactant molecule absorbs light energy. The absorption of a photon excites the molecule from its ground state to a higher energy state. This initial step is called photoexcitation and is the primary process in photochemistry.
Molecule + hν ⟶ Excited Molecule
Hence, the correct answer is option (B)
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Frequently Asked Questions (FAQs)
Photochemical reactions require light absorption and often proceed via excited electronic states.
Thermal reactions rely on heat energy and follow the ground-state potential energy surface.
An example of a photochemical reaction involving water is the photolysis of water during photosynthesis.
Φ = 1: One photon leads to one reaction (follows the Stark-Einstein law).
Φ > 1: Chain reaction (e.g., radical reactions).
Φ < 1: Energy loss via fluorescence, phosphorescence, or non-radiative decay.
Fluorescence: Fast emission (ns timescale), same spin state (singlet→singlet).
Phosphorescence: Slow emission (ms-s), involves spin change (triplet→singlet).
The example of a photochemical reaction is the reaction between hydrogen and chlorine in the presence of sunlight
