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Photochemical Reaction is a chemical reaction that occurs due to the absorption of light energy. As a result of the molecules absorbing the light, transient excited states are created whose physical and chemical properties differ greatly from the original molecules. Photochemistry is the branch of chemistry that deals with rates and mechanisms of reactions that result from the exposure of reactants to light radiations. The photochemical reaction is therefore a thermal reaction of the electronically excited state of the molecule. For example, ozone is formed through a photochemical reaction that involves nitrogen dioxide and reactive organic compounds.
Key Terms: Photochemical, Photosynthesis, Photochemical decomposition reaction, Light Energy, Photochemical process
What are Photochemical Reactions?
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| “Photochemical Reaction is the chemical reaction that starts with light absorbed as a form of energy. It is triggered when the light energy is absorbed by the molecules of a substance.” |
- The reaction proceeds with the molecules separated, modified, and mixed with the reactants to form the desired products.
- This reaction takes place with the transfer of hydrogen atoms and electron charges to separate molecules, protons and electrons.
- Sunlight plays a crucial role in the Photochemical Reaction.
- In these reactions, large activation barriers are crossed in a short time.
- Some photochemical reactions are destructive such as the photodegradation of plastics.
Photochemical Reactions
- Photochemical reactions are controlled by the number of photons that can activate molecules to cause the desired reaction.
- In a photochemical reaction, these molecules tend to form a new structure.
- They can combine with each other or with other molecules and transfer electrons, atoms, protons or other excitation energies to other molecules.
- It further causes a long chain of chemical reactions.
- Photochemical reactions can take place in solid, liquid or gaseous states.
Read More:
| Photochemical Reactions – Important Topics | ||
|---|---|---|
| Exothermic reactions | Standard Enthalpy of Formation | Endothermic reactions |
| Combustion Reaction | Balanced Chemical Equations | Types of Chemical Reactions |
Types of Photochemical Reactions
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Here are some different types of photochemical reactions-
- Photo-dissociation: AB + hν → A* + B*
- Photo induced rearrangements, isomerization: A+ hν → B
- Photo-Addition: A + B + hν → AB + C
- Photo-substitution: A + BC + → AB + C
- Photo-Redox Reaction: A + B + hν → A- + B+
Read More: Group 18 Elements
Examples of Photochemical Reaction
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One of the most common examples of photochemical reactions is photosynthesis. In photosynthesis, plants use water and sunlight to convert carbon dioxide into glucose and oxygen. Photosynthesis reaction is given below:
- Photosynthesis is a photochemical process by which algae, green plants, seaweeds, and certain bacteria absorb solar energy.
- Plants convert sunlight energy into the chemical energy being stored using photosynthesis and thereby form carbohydrates using carbon dioxide and water.
- It releases oxygen as a by-product of the reaction.
- With the help of these carbohydrates and oxygen, animal life is sustained.
Read More: Properties of Matter
Other Examples of Photochemical Reactions
Some of the other examples of photochemical reactions are –
- The ozone layer that protects the earth from harmful ultraviolet radiation is formed by the Photochemical dissociation of molecular oxygen into oxygen atoms. These atoms react with molecules to form ozone molecules.
- Different types of commercial equipment are influenced by Photochemical Reactions and work upon its principle.
- Different activities that we come across in our day-to-day life like Xerography, and Photography work on the principle of Photochemical processes.
- The UV rays that are harmful to Human DNA and skin cancer are caused by Photochemical Reactions.
Important Photochemical Reactions
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Applications of Photochemical Reactions
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Some basic applications of Photochemical Reactions are:
- They are used for the production of Benzyl Chloride.
- They are used for the production of various synthetic organic molecules.
- Photochemical Reactions are used for the preparation of the anti-malarial drug.
Read More: Oxidation and Reduction
Laws of Photochemistry
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The photochemical reactions or processes follow the laws of photochemistry as well. There are two laws of photochemistry.
- Grotthuss-Draper law
- Stark–Einstein law.
Grotthuss-Draper Law
- The Law states that light must be absorbed by a chemical substance in order to take place in a chemical reaction.
- Grotthuss-Draper law was given by chemists, Theodor Grotthuss and John W. Draper.
Stark-Einstein Law
- This law states that for each photon of light absorbed by a chemical system, no more than one molecule is activated for a photochemical reaction, as defined by the quantum yield.
- Stark-Einstein Law was given by physicists, Johannes Stark and Albert Einstein.
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Things to Remember
- Sunlight plays a vital role in the photochemical reaction as it helps the chemical to decompose.
- Light contains photons that combine with molecules resulting in excitation of the molecules.
- These reactions require photons to interact with chemical compounds.
- Each molecule absorbs only one photon to get itself excited.
- Major Photochemical oxidants are O3, H2O2, Nitrogen Oxide, and Peroxyacetyl nitrate (PAN).
Check: Displacement Reaction
Photochemical Reaction: Sample Questions
Ques 1: Why do Coordination Complex and Organometallic compounds show Photochemical Reactions? (1 Mark)
Ans: The complex and compounds show Photochemical Reactions because they are photoreactive.
Ques 2: (i) Write a balanced chemical equation for the process of photosynthesis.
(ii)When do desert plants take up carbon dioxide and perform photosynthesis? (2 Marks)
Ans: (i) The chemical reaction for the process of photosynthesis is given as-
(ii) In desert plants the stomata are open at night. They take CO2 at night and stored it in the form of acid. During the daytime, they use it for the process of photosynthesis.
Ques 3: What is the colour of ferrous sulphate crystals? How does this colour change after heating? (2 Marks)
Ans: The colour of ferrous sulphate is pale green. The colour of ferrous sulphate changes from pale green to reddish-brown on heating due to the formation of iron (III) oxide.
Ques 4: What role does Photochemical Reaction play in the environment? (2 Marks)
Ans: Photochemical Reactions play a vital role in the environment that includes a wide range of reactions in the atmosphere, Natural waters, soil, and living organisms. The Photochemical Reactions can take place in solid, liquid, and gas.
Ques 5: What are the radiations that are emitted in the Photochemical Reaction? Also, give their wavelengths. (2 Marks)
Ans: The radiations that are emitted in the photochemical reaction and their wavelength are:
- Ultraviolet- 100-400nm
- Visible- 400-750 nm
- Infrared- 750-2500 nm
Ques 6: Give an example each for thermal decomposition and photochemical decomposition reactions. Write relevant balanced chemical equations also. (2 Marks)
Ans: Thermal decomposition reaction:
Photochemical decomposition reaction:
Ques 7: Explain Photochemical Reaction. Give an example of a photochemical reaction with the balanced equation. (2 Marks)
Ans: Chemical reactions that take place in the presence of light are called photochemical reactions.
Example: Reaction of Photosynthesis process
Photosynthesis is a photochemical reaction in which the leaves of green plants prepare glucose and oxygen from carbon dioxide and water in the presence of chlorophyll and sunlight.
Ques 8: The following diagram displays a chemical reaction. Observe carefully and answer the following questions:
(a) Identify the type of chemical reaction that will take place and define it. How will the colour of the salt change?
(b) Write the chemical equation of the reaction that takes place.
(c) Mention one commercial use of this salt. (3 Marks)
Ans: (a) The reaction that will take place is the Photochemical Reaction. The reactions in which a compound breaks down into simple substances in presence of light are called photochemical decomposition reactions. The colour of salt will change from white to grey.
(b) The chemical reaction that will take place can be given as-
(c) Silver chloride is used in photography.
Ques 9: (a) Write the essential condition for the following reaction to take place:
2AgBr →2Ag + Br2
Write one application of this reaction.
(b) Complete the following chemical equation of a chemical reaction:
(c) What happens when water is added to the quick line. Write the chemical equation. (3 Marks)
Ans: (a) Sunlight is required to carry out the chemical reaction-
This reaction is used in photography.
(b) The complete reaction can be given as-
(c) Slaked lime is formed with a hissing sound and a lot of heat is evolved during the chemical reaction.
Ques 10: What is the difference between Photochemical and thermal Reaction? (5 Marks)
Ans: Differences between Photochemical and Thermal Reactions are:
| Parameters | Photochemical Reactions | Thermal Reactions |
|---|---|---|
| Definition | Photochemical reactions are the reactions that take place due to the absorption of radiation | Thermal reactions are the reaction that takes place due to the absorption of heat energy, generally by the increase in temperature |
| Source | Light | Heat |
| Effect of Light | Appropriate | In the Absence of Light |
| Effect of Temperature | No effect | Direct effect |
Ques 11. What are thermal or dark reactions? (1 mark)
Ans. The reactions in absence of light caused by heat are known as thermal or dark reactions.
Ques 12. Why are medicine bottles often made with darkened glass? (2 marks)
Ans. Medicine bottles are often made with darkened glass in order to prevent drugs or chemicals from undergoing photodegradation. The brown color glass is generally used due to its ability to prevent UV light from degrading potentially photosensitive contents through photochemical reactions.
Ques 13. How is vision initiated? (1 mark)
Ans. Vision is initiated by a photochemical reaction of rhodopsin which is a primary photoreceptor molecule of vision.
Ques 14. How are Mercaptans produced? (2 marks)
Ans. Mercaptans can be produced by the photochemical addition of hydrogen sulfide to alpha olefins.
- In this method, there is a provision to increase the total conversion of reactants to mercaptans.
- This is done by including carbon disulfide in the reaction mixture.
Ques 15. What are photosensitized reactions? (2 marks)
Ans. Photosensitized reactions are the reactions in which the molecules that absorb light don’t react themselves but induce other molecules to react.
Ques 16. How is ozone formed? (2 marks)
Ans. The molecular oxygen undergoes photochemical dissociation to form individual oxygen atoms. These atoms then react with the molecular oxygen to form ozone.
Ques 17. What is the range of radiations used for photochemical reactions? (1 mark)
Ans. The range of radiations used for the photochemical reaction is 2000–8000 Å.
Ques 18. What is a thermopile? (1 mark)
Ans. Thermopile is an instrument used to measure the energy of monochromatic radiation in a most accurate manner.
Ques 19. What are the factors affecting photochemical reactions? (2 marks)
Ans. A photochemical reaction is initiated by light. In addition to the light, the two important factors contributing to the formation of photochemical products are –
- Photosensitizer
- Oxygen
Ques 20. What are the primary and secondary processes in a photochemical reaction? (2 marks)
Ans. A primary photochemical reaction is an immediate consequence that takes place after the absorption of light.
The subsequent chemical changes are known as secondary processes.
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