Photochemistry, Photochemical Reaction and Thermal Reaction

What is Photochemistry? 

Photochemistry is one of the parts of chemistry that deals with the effects of light on the chemical system.

What is a Photochemical reaction?

A reaction/the reaction that takes place by absorbing the visible and UV radiations is termed a photochemical reaction. Generally, the reaction can be divided into 2 classes. One is a photochemical reaction and the other is a thermal reaction.

• A photochemical reaction is a time-independent reaction.
• The range of Photochemical reaction is about 200 nm to 800 nm.

Characteristics of Photochemical Reaction:

1. It starts by absorbing radiation of light.
2. The temperature has little to no significance in a photochemical reaction.
3. Light is mandatory. It cannot occur in the dark
4. This type of activation is highly selective.
5. Change of free energy can be positive or negative.

Examples of Photochemical Reactions are:

1. Photosynthesis of Carbohydrates in plants

`6CO_2\+\6H_2O\+\h\uplison\(sunlight)\rightarrow C_6H_{12}O_{6\}+\6O_2`

2. Conversion of Oxygen into ozone

3. Formation of Vitamin-D with sunlight. 

4. Photography uses the action of light of grains of AgCl or AgBr. This reaction is also a photochemical reaction.

5. Carbonyl compounds undergo various photochemical reactions.

Types of photochemical reaction:

1. Dissociation Reaction:

`2HBr+h\upsilon\rightarrow H_2+Br_2`

2. Rearrangement Reaction

Fumaric Acid `\rightarrow` Maleic Acid

3.Subsitution Reaction

`C_6H_{12}+Br_2+h\upsilon\rightarrow C_6H_{11}Br+HBr`

4. Decomposition Reaction

`2O_3+h\upsilon\rightarrow O_2`

5. Combination Reaction:

`H_2+Cl_2+h\upsilon\rightarrow2HCl`

6. Polymerization Reaction:

`2(C_{14}H_{10})+h\upsilon\rightarrow C_{28}H_{20}`

7. Addition Reaction:

`CO+Cl_2+h\upsilon\rightarrow COCl_2`

All the reactions can be divided into 2 sides.

a) Photochemical Reaction

b) Thermochemical Reaction

The photochemical reaction is that type of reaction that takes place by absorbing the visible and Ultra Violate radiations. And the thermochemical reaction is that type of reaction where activation energy is provided to reactants as heat. The thermochemical reaction is often called a thermal reaction.

Differences between photochemical and thermochemical reaction:

Serial	Photochemical Reaction	Thermochemical Reaction
01	It occurs by absorbing light radiations	It occurs by absorbing heat
02	Temperature does not have a good significance in photochemical reaction	The temperature has great significance here.
03	The presence of light is important. It cannot occur in dark.	It can occur in both dark and light
04	Change of free energy can be both positive or negative.	Change of fee energy is always negative in a thermal reaction
05	This type is highly selective.	This is not selective in nature.
06	There are no needs of catalyst for almost all the photochemical reactions	Most of the thermal reactions need a catalyst to accelerate the reaction

Examples:

Photochemical Reaction

• `6CO_2\+\6H_2O\+\h\mu\(sunlight)\rightarrow C_6H_{12}O_{6\}+\6O_2`
• `2HBr+h\upsilon\rightarrow H_2+Br_2`
• `C_6H_{12}+Br_2+h\upsilon\rightarrow C_6H_{11}Br+HBr`
• `CO+Cl_2+h\upsilon\rightarrow COCl_2`
• `2O_3+h\upsilon\rightarrow O_2`

Thermochemical/Thermal reaction

• `C+O_2\rightarrow CO_2`
• `2H_2O(g)+2H_2(g)+O_2(g)`

See More:

• Laws of  Photochemistry