Difference Between Photochemical and Thermal Reaction

The key difference between photochemical and thermal reaction is that the photochemical reaction initiates when the reactants get energy from the photons whereas thermal reactions initiate when the reactants get heat energy.

A chemical reaction is a process of rearranging the molecular or ionic structure of a substance other than a physical or a nuclear alteration. Photochemical and thermal reactions are two forms of chemical reactions that differ from each other according to the energy source they get in order to initiate the chemical reaction.

CONTENTS

1. Overview and Key Difference
2. What is Photochemical Reaction
3. What is Thermal Reaction
4. Side by Side Comparison – Photochemical vs Thermal Reaction in Tabular Form
5. Summary

What is Photochemical Reaction?

A photochemical reaction is a form of chemical reaction in which the reactants get energy as photons. There, the reaction initiates with the absorption of light where light is consisting of photons. When the reactant molecules absorb energy in this way, it causes the molecule to move to an excited state in which the chemical and physical properties of the molecule are different from that of the original molecule. We call this “excitation”. This new excited state can convert into new structures via combination with other molecules or by altering its structure.

The forms of light that can initiate a photochemical reaction includes UV light, visible light and IR light. Some examples of this form of reactions are as follows:

• Photosynthesis
• Bioluminescence
• Photo-degradation
• Vision
• Photo-alkylation

What is Thermal Reaction?

A thermal reaction is a form of chemical reaction in which the reactants get energy as heat. We name these reactions as “thermolysis” or “thermal decomposition reactions”. It mainly involves the chemical decomposition of a substance when we apply heat energy. The temperature at which this chemical reaction initiates is the “decomposition temperature”. Usually, these reactions are endothermic. This is because the reactants require heat energy in order to break down the chemical bonds between the atoms of the substance which undergo the decomposition.

Moreover, these reactions, most of the times involves a single reactant. Some examples of thermal reactions are as follows:

• Calcium carbonate decomposition into calcium oxide and carbon dioxide
• Decomposition of water molecules at 2000◦C

What is the Difference Between Photochemical and Thermal Reaction?

A photochemical reaction is a form of chemical reaction in which the reactants get energy as photons while a thermal reaction is a form of chemical reaction in which the reactants get energy as heat. This is the key difference between photochemical and thermal reaction. Both these are very important chemical reactions in chemistry. These two reactions differ from each other according to the energy source. Moreover, photochemical reactions directly affected by light while thermal reactions are not. However, the temperature has a direct effect on thermal reactions while there is no necessity of temperature for photochemical reactions.

The below infographic presents more details on the difference between photochemical and thermal reaction in tabular form.

Summary – Photochemical vs Thermal Reaction

Photochemical and thermal reactions, both are two forms of chemical reactions. The difference between photochemical and thermal reaction is that the photochemical reactions initiate when the reactants get energy from photons whereas the thermal reactions initiate when the reactants get heat energy.

Reference:

1. Fleming, Graham R., et al. “Photochemical Reaction.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 21 Oct. 2016. Available here 
2. “Thermal Decomposition.” Wikipedia, Wikimedia Foundation, 23 Aug. 2018. Available here  

Image Courtesy:

1.”Anoxygenic Photosynthesis in Green Sulfur Bacteria” By Lithium byproduct – Own work, (CC BY-SA 4.0) via Commons Wikimedia 
2.”ThermiteReaction” By User:Nikthestunned (Wikipedia) – Own work (CC BY-SA 3.0) via Commons Wikimedia