What is the Difference Between Photochemical and Thermal Reaction

The main difference between photochemical and thermal reaction is that photochemical reactions are initiated by light, typically ultraviolet or visible light, whereas an increase in temperature drives thermal reactions.

Photochemical reactions and thermal reactions are two fundamental types of chemical processes that drive transformations in matter. Each operates under distinct conditions, utilizing different sources of energy to initiate and propagate chemical changes.

Key Areas Covered

1. What are Photochemical Reactions
      – Definition, Features 
2. What are Thermal Reactions
      – Definition, Features 
3. Similarities Between Photochemical and Thermal Reaction
      – Outline of Common Features
4. Difference Between Photochemical and Thermal Reaction
      – Comparison of Key Differences
5. FAQ: Photochemical and Thermal Reaction
      – Frequently Asked Questions

Key Terms

Photochemical Reaction, Thermal Reaction

What are Photochemical Reactions

Photochemical reactions involve the transformation of chemical substances through the absorption of light energy. At the core of these reactions is the interaction between molecules and photons. The chemistry of photochemical reactions is governed by the principles of photochemistry, a branch of chemistry that explores the effects of light on chemical systems.

One key concept in photochemistry is the absorption of light by molecules. When a molecule absorbs light, it undergoes an electronic transition, moving from a lower energy state to a higher one. This excitation is often associated with the promotion of electrons to higher energy orbitals. The absorption of light is selective and depends on the molecular structure, leading to the formation of excited states with specific energy levels.

Once in the excited state, molecules can undergo various processes. One common route is fluorescence, where the molecule returns to its ground state by emitting a photon. Alternatively, molecules may undergo non-radiative decay, dissipating the absorbed energy as heat. In photochemical reactions, however, the excited state can facilitate chemical transformations.

Photochemical reactions are often initiated by the absorption of ultraviolet or visible light, as these wavelengths provide sufficient energy to drive molecular changes. Photoreactions can involve bond cleavage, isomerization, or the formation of new chemical species. For example, the photodissociation of ozone in the upper atmosphere is a crucial photochemical reaction that influences atmospheric chemistry.

Moreover, photosynthesis, a fundamental process in biology, relies on photochemical reactions to convert light energy into chemical energy. Chlorophyll, the pigment responsible for capturing light in plants, undergoes photochemical reactions that ultimately produce glucose from carbon dioxide and water.

What are Thermal Reactions

Thermal reactions in chemistry involve the transformation of substances through the absorption or release of heat energy. The concept is rooted in thermodynamics, where changes in temperature play a crucial role in determining the direction and feasibility of chemical reactions. These reactions are classified into two main categories: endothermic and exothermic.

Endothermic reactions absorb heat from their surroundings, causing a decrease in the surrounding temperature. An example is the process of photosynthesis in plants, where carbon dioxide and water absorb energy from sunlight to produce glucose and oxygen. On the other hand, exothermic reactions release heat into the surroundings, often leading to an increase in temperature. Combustion reactions, such as the burning of fossil fuels, exemplify this category as they generate heat and light during the process.

The study of thermal reactions is essential for various applications, including industrial processes and environmental considerations. Industries utilize this knowledge to optimize reaction conditions for maximum efficiency and yield. Additionally, understanding the thermal aspects of reactions is crucial for safety protocols, ensuring that processes are controlled to prevent unwanted temperature spikes.

In environmental contexts, thermal reactions are key considerations in climate change discussions. The combustion of fossil fuels, for instance, not only releases energy but also contributes to the rise in global temperatures due to the release of greenhouse gases.

Similarities Between Photochemical and Thermal Reaction

• The absorption of energy initiates both photochemical and thermal reactions.
• In both cases, reactants undergo changes in molecular structure, leading to the formation of new products.

Difference Between Photochemical and Thermal Reaction

Definition

A photochemical reaction is a chemical process initiated by the absorption of light, typically ultraviolet or visible light, while a thermal reaction is a chemical reaction that occurs due to an increase in temperature.

Initiated By

Photochemical reactions are initiated by absorption of light energy, typically ultraviolet or visible light, whereas thermal reactions are initiated by an increase in temperature, leading to higher kinetic energy of molecules.

External Source

While photochemical reactions require light as an external energy source, thermal reactions depend on heat energy from an external source.

Rate

Photochemical reactions generally occur at faster rates compared to thermal reactions due to the higher energy input from photons.

FAQ: Photochemical and Thermal Reaction

How do you know if a reaction is photochemical or thermal?

Thermal reactions utilize the HOMO (highest occupied molecular orbital) from the ground-state electron configuration of the molecular orbital diagram, while photochemical reactions utilize the HOMO in the excited-state electron configuration.

What is the difference between thermal and photochemical processes?

Photochemical reaction initiates when the reactants get energy from the photons whereas thermal reactions initiate when the reactants get heat energy.

What is an example of a thermal reaction?

When heated, calcium carbonate decomposes into calcium oxide and carbon dioxide. This is an example of a thermal reaction. 

Conclusion

A photochemical reaction is a chemical process initiated by the absorption of light, typically ultraviolet or visible light, while a thermal reaction is a chemical reaction that occurs due to an increase in temperature. Thus, this is the main difference between photochemical and thermal reaction.

Reference:

1. “Thermal Reaction – An Overview.” Science Direct.
2. “Photochemical Reaction.” Byju’s.

Image Courtesy:

1. “Photosynthesis overview” By ELaurent – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Fire – an exothermic reaction of wood with oxygen from the air” By Hi-Res Images of chemical Elements – (CC BY 3.0) via Commons Wikimedia