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Basic concepts in Exothermic and Endothermic Reaction

 

  • A chemical reaction leads to the formation  of new substances by making and breaking of chemical bonds. The newly formed substances are called products and the substances that involve them are called reactant molecules. 

 

  • A chemical reaction involves certain energy change as making a chemical bond releases a certain amount of energy whereas breaking of a chemical bond requires some energy.

 

  • On the basis of energy change, the reactions can be classified as Exothermic and endothermic reactions. An exothermic reaction releases a certain amount of energy in the form of heat or light during the chemical reaction. On the contrary, an endothermic reaction furnishes with the absorption of energy. The energy profile diagrams are used to explain the exothermic and endothermic reactions.  

 

  • The energy profile diagrams for exothermic and endothermic reactions are shown below. The curve for exothermic reactions indicates lesser energy of product compared to reactant. It proves that certain energy is released in the form of heat or light during exothermic reactions. 
     

                                 Energy profile diagrams for exothermic and endothermic reactions

 

  • In an endothermic reaction, reaction proceeds with absorption of energy in the form of heat/light, therefore the energy level of product molecules is always greater than reactant molecules in the energy profile diagram.  

 

  • Heat or enthalpy of reaction (H) can be defined as the energy change during the reaction. The mathematical expression of enthalpy of reaction shows the difference between the enthalpy of product and reactant molecules.   

              Hreaction= Hproduct - Hreactant
 

  • Activation energy is the least amount of energy that is required to initiate the chemical reaction from the given product molecules. The transition state theory proposed the formation of transition state from those reactant molecules who achieve the activation energy.

 

 

Sample Exothermic and Endothermic Reaction:

 

Q.1 With the use of a catalyst, which of the following energy values decreases?

(a) Enthalpy of reaction 

(b) Heat of reaction 

(c) Activation energy

(d) Collision energy 

Answer: (c)  In the presence of a catalyst, the activation energy for a reaction decreases that increases the rate of reaction.
 

Q. 2 Why does the enthalpy of reaction signify a negative for exothermic reaction? 

Answer: The enthalpy of reaction is the difference in the enthalpy of product and reactant molecules. 

Hreaction=Hproduct - Hreactant

For an exothermic reaction, the enthalpy of the product is lesser than reactant as some of the energy is released in the form of heat or light. Thus, the difference that is enthalpy of reaction will be negative.  


Q.3 Identify one of the most common exothermic reactions in our surroundings. 

(a) Burning of candle 

(b) Cooking of food 

(c) Distillation of crude oil  

(d) Water sedimentation 

Answer: (a) Burning of candle is an exothermic reaction as it releases a certain amount of energy in the form of heat and light.

 

 

Frequently Asked Questions

 

 

What are three examples of endothermic reactions? 


Three examples of endothermic reactions are:

 

  • Evaporation of water to water vapor 
  • Melting of solid to liquid 
  • Dissolution of salt in water

 
Is exothermic hot or cold?


Exothermic reactions release a certain amount of energy in the form of heat or light whereas endothermic reactions absorb a certain amount of energy so the temperature of the reaction system decreases whereas it increases during exothermic reactions.  So exothermic reactions are hot. 


What is the difference between an exothermic and endothermic reaction?

 

Exothermic reaction Endothermic reaction
It occurs with release of energy It absorbs some energy.
Temperature of the reaction system will increase.  Temperature of the reaction system will decrease. 
∆H = -ve ∆H = +ve

 

 

Do endothermic reactions feel hot or cold?

 
An endothermic reaction can be defined as the chemical reaction that occurs with absorption of a certain amount of energy in the form of heat. For example; dissolution of salt in water is an endothermic process as it decreases the temperature of the system.  So endothermic reactions are cold. 

 

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