Thermal Changes In Chemical Reaction: Exothermic, Endothermic & Photochemical Reaction

Usually whenever a chemical reaction occurs, then either energy is released or energy is absorbed. And this energy is either emitted or absorbed in the form of heat or light. There are three types of chemical reactions based on energy conversion. And these are-
  1. Exothermic reaction 
  2. Endothermic reaction
  3. Photochemical reaction
Let us now look at the above chemical reactions one by one in details, 
(1) EXOTHERMIC REACTION
"Chemical reactions that emit energy in the form of heat, light, electricity & sounds, are called Exothermic reactions". 
During these reactions, the temperature of the system decreases while the surrounding temperature increases. 
In these types of reactions, the energy of the products is less than the energy of the reactants. And thus the heat of reaction (∆H) is negative for exothermic reactions. In this, the product molecules are more stable than reactant molecules. 
For Example; 
All combustion processes (like lighting a match, burning a piece of paper etc.), Rusting of iron, Reaction between water and calcium chloride, Condensation of water vapour into rain, Respiration and Neutralization, etc. 

Let's look at a diagram to clearly understand exothermic reactions, 
Or
Fig:- Shows Reaction-Energy Diagram
For Exothermic Reaction

As shown in the above figure, the enthalpy of reaction or heat of reaction (which is defined as the difference between the energy of products and the energy of reactants) is negative for exothermic reactions, i.e., energy is released during the reaction. Let's look at some of the chemical reactions, 
(a) Methane combustion, 
        CH₄ + 2O₂ ➝ CO₂ + 2H₂O + Heat (🔥) 
(b) Thermite reaction, 
    Fe₂O₃ + 2Al ➝ 2Fe + Al₂O₃ + Heat (🔥) 
(c) Salt with water, 
     CaCl₂ + 2H₂O ➝ Ca(OH)₂ + 2HCl + Heat (🔥) 
(d) Lime reaction,
      CaO + H₂O ➝ Ca(OH)₂ + Heat (🔥) 


(2) ENDOTHERMIC REACTION
"The reaction in which energy is absorbed in the form of heat from the surrounding, is called Endothermic reaction". 
During these reaction, the temperature of the system increases while the temperature of the surrounding decreases as it absorbs energy from the surrounding. 
In this type of reaction, the energy of the reactant molecules is lower than the energy of the product molecules. Thus, the heat of reaction (∆H) for endothermic reactions are positive and the reactant molecules are more stable than the product molecules. 
For Example; 
Melting, Boiling, Evaporation all are examples of endothermic reactions like Melting of an ice, evaporating liquid water, Photosynthesis, Frying eggs etc. 

Let's look at a diagram to clearly understand endothermic reactions, 
Fig:- Shows Reaction-Energy Diagram
For Endothermic Reaction

As shown in the above figure, the enthalpy of reaction or heat of reaction (which is defined as the difference between the energy of products and the energy of reactants) is positive for endothermic reactions, i.e., energy is absorbed during the reaction. Let's look at some of the chemical reactions, 
(a) When nitrogen and oxygen are heated to extremely high temperatures , 
           N₂ + O₂ ➝ 2NO - Heat (🔥) 
(b) Decomposition reaction, 
       CaCO₃ ➝ CaO + CO₂ - Heat (🔥) 
(c) Photosynthesis in plants 🌱🌱🌱. 
         6CO₂ + 6H₂O ➝ C₆H₁₂O₆ + 6O₂
(d) Salt in water, 
   NH₄NO₃ + H₂O ➝ NH₄OH + HNO₃ - Heat(🔥) 


(3) PHOTOCHEMICAL REACTION
Chemical reactions that occur in the presence of light are called photochemical reactions. In these chemical reactions, light is absorbed in the form of energy. We can say that it is a type of endothermic reaction as it absorbs energy from the sun. In general,
Ground state molecules + light energy ➝ excited state molecules ➝ products
The best example to understand these photochemical reaction is the Photosynthesis In Plants in which plants 🌱🌱 make their own food in the presence of Sunlight And Chlorophyll. Photosynthesis reaction is defined as a reaction in which green plants 🌱🌱 absorb light energy from the sun and used it to convert the mixture of carbon dioxide & water into oxygen & some energy rich organic compounds where Oxygen Produced As A By-product Or As A Waste. 
  6CO₂ + 6H₂O + light energy ➝ C₆H₁₂O₆ + 6O₂↑↑

Photosynthesis is mainly responsible for the production and maintenance of oxygen content on the earth's atmosphere and supplies most of the oxygen needed for life on earth.
( Sunlight represent by the symbol "hv") 
And some other examples are, 
(a) Chlorination of methane, 
       CH₄ + Cl₂ + light energy ➝ CH₃Cl + HCl
(b) In photography, 
       2AgBr + light-energy ➝ 2Ag + Br₂
     Similarly, 
       2AgCl + light-energy ➝ 2Ag + Cl₂
(c)Decomposition of 'NO₂' into 'NO', 
       NO₂ + light-energy ➝ NO + O₂



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