Short Answer

Photochemical reactions are chemical reactions that take place in the presence of light. In these reactions, light energy acts as the driving force that helps molecules absorb energy and undergo chemical change. Sunlight or artificial light can start these reactions by providing the required energy.

These reactions are important in both nature and everyday life. Examples include photosynthesis in plants, the formation of ozone in the atmosphere, and photochemical reactions used in photography. Light plays a key role in breaking or forming chemical bonds during these reactions.

Detailed Explanation :

Photochemical Reactions

Photochemical reactions are chemical processes that occur when light energy triggers a chemical change. The term “photo” refers to light, and “chemical reaction” refers to the change in substances. Unlike normal chemical reactions that depend mainly on heat, pressure, or catalysts, photochemical reactions specifically require light of a certain wavelength, such as ultraviolet (UV), visible, or infrared light.

When light falls on a molecule, it is absorbed by the molecule, giving the particles extra energy. This energy can cause chemical bonds to break, new bonds to form, or the molecule to rearrange itself into a new structure. In this way, light acts as a source of energy that helps reactions happen faster or in a way that would not normally occur in the dark.

Photochemical reactions are essential in nature, industry, and many scientific processes. They play a major role in plant growth, atmospheric changes, and several technological applications.

1. How Photochemical Reactions Occur

A photochemical reaction begins when molecules absorb light energy:

1. a) Absorption of Light

Light energy of a specific wavelength is absorbed by atoms or molecules.
Not all wavelengths can start a reaction; only suitable ones give the required energy.

1. b) Excited State Formation

Once the molecule absorbs light, it becomes “excited,” meaning its electrons move to a higher energy level.

1. c) Chemical Change

The excited molecule becomes more reactive. It may:

• Break down into simpler substances
• Split into two or more particles
• Combine with other molecules
• Undergo rearrangement

This leads to the formation of new products.

2. Examples of Photochemical Reactions

Photochemical reactions occur widely in nature and technology. Some of the most important examples are:

1. a) Photosynthesis

Plants use sunlight to convert carbon dioxide and water into glucose and oxygen.
This is the most important photochemical reaction on Earth because it supports life.

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (in presence of sunlight and chlorophyll)

1. b) Formation of Ozone

In the upper atmosphere, oxygen molecules absorb UV light and split into atoms.
These atoms combine with oxygen molecules to form ozone (O₃).
This ozone layer protects Earth from harmful UV rays.

1. c) Photographic Reactions

Old-style photographic films work by photochemical reactions.
Light causes silver halide salts on the film to decompose and form a latent image.

1. d) Vitamin D Formation

Human skin produces vitamin D when exposed to sunlight through a photochemical reaction involving cholesterol.

1. e) Photodecomposition

Some compounds break down when exposed to light.
Example: Hydrogen peroxide decomposes faster in light than in darkness.

3. Importance of Photochemical Reactions

Photochemical reactions have great significance in both natural processes and human activities.

1. a) Essential for Plant Life

Photosynthesis depends completely on light-driven reactions.

1. b) Atmospheric Protection

The ozone layer formed through photochemical reactions helps protect life on Earth.

1. c) Industrial Use

Photochemical reactions are used in:

• Photography
• Printing and developing images
• Manufacturing plastics
• Production of certain medicines

1. d) Environmental Processes

Many pollutants break down under sunlight through photochemical reactions, helping clean the atmosphere.

1. e) Biological Importance

Formation of vitamin D in the human body is a photochemical process essential for healthy bones.

4. Conditions Required for Photochemical Reactions

1. a) Light Source

Sunlight or artificial UV/visible light is necessary.

1. b) Suitable Wavelength

Only specific wavelengths are absorbed by the molecule.

1. c) Photosensitive Substance

The reacting substance must be capable of absorbing light energy.

1. d) Presence of Catalyst (Sometimes)

Some photochemical reactions need a photocatalyst like titanium dioxide.

5. Photochemical Reactions vs Normal Reactions

• Photochemical reactions require light; normal reactions do not.
• They often occur at lower temperatures because energy comes from light.
• They can form products that are not possible through heat-based reactions alone.

Light therefore expands the possibilities of chemical changes.

Conclusion

Photochemical reactions are chemical processes that start when substances absorb light energy. These reactions are crucial in nature, especially in photosynthesis and ozone formation. They are also widely used in photography, industrial manufacturing, environmental cleanup, and human health. Light makes molecules more energetic and reactive, allowing unique chemical changes that would not happen otherwise. Understanding photochemical reactions helps us appreciate the role of light in chemical and biological systems.