Difference Between Exothermic and Endothermic Reaction

Introduction

A chemical reaction can be defined as the process where two compounds (known as reactants) get converted to one or more different compounds (known as products) due to the rearrangement of the constituent atoms of the reactants. Some common examples of chemical reactions include the rusting of iron, the burning of fuels, etc.

A chemical reaction can be categorized into three categories on the basis of its way of formation:

Synthesis – Here, the reactants are synthesized to get converted into products.

Example: Reaction of Zinc and Oxygen

2Zn + O2 → 2ZnO

In the above equation, the plus sign indicates that zinc reacts with oxygen. The arrow sign implies that the reaction “forms” the new substance, Zinc oxide, as the product.

1. Energy considerations – Energy plays an important role in the chemical reactions. The bonds between the atoms of the reactants are broken, and these broken atoms are rearranged into products, forming new bonds.
2.  Kinetic considerations – Chemical reactions require an initial input of a certain amount of energy to bring out the changes and begin with the process. Physical factors like heat and light can provide the energy to initiate the chemical change.

Exothermic Reaction vs. Endothermic Reaction

Exothermic Reaction

The term “exo” means “outside” and “thermic” means heat.

An exothermic reaction is a reaction in which energy is released to the surroundings of the system (the place where the reaction occurs) in the form of heat. In simple words, it can be defined as a chemical reaction where the heat evolves while undergoing chemical changes. Some examples include burning fossil fuels, mixing sulphuric acid into water, etc.

The term “endo” means “inside” and “thermic” means heat.

An endothermic reaction is a reaction in which heat energy is absorbed from the surroundings by the system (the place where the reaction occurs). In simple words, it can be defined as a chemical reaction where, in an Endothermic Reaction, the heat is absorbed while undergoing the chemical changes. Some examples include the mixing of ammonium chloride in water, the reaction of nitrogen and oxygen, etc.

Difference Between Exothermic and Endothermic Reaction in Tabular Form

TOPIC OF COMPARISON	EXOTHERMIC REACTION	ENDOTHERMIC REACTION
Definition	Exothermic reactions are the chemical reactions in which energy is released in the form of heat to the surroundings to form products.	Endothermic reactions are the chemical reactions in which the energy is absorbed in the form of heat from the surroundings to form products.
Involvement of heat	The heat energy is released from the system to the surroundings.	The heat energy is absorbed by the system from the surroundings.
Effect on the temperature of the reaction	The temperature of the reaction increases as it progresses.	The temperature of the reaction decreases as it progresses.
Change in enthalpy	The change in enthalpy is a negative value.	The change in enthalpy is a positive value.
Change in entropy	The change in entropy is a positive value.	The change in entropy is a negative value.
Relation between enthalpy of the reactants and enthalpy of products	Enthalpy of reactants is higher than that of the enthalpy of products.	Enthalpy of reactants is lower than that of the enthalpy of products.
Supply of energy	Energy is released from the system.	Energy should be given to the system.
The spontaneity of the reaction	It is a spontaneous reaction.	It is a non - spontaneous reaction.
Effect on the surrounding temperature	The surrounding temperature increases.	The surrounding temperature decreases.
The stability of the products formed	The stability of the products formed during the reaction is higher.	The stability of the products formed during the reaction is less.
Nature of the reaction	The reaction is exergonic, i.e., the change in the free energy is negative.	The reaction is endergonic i.e., the change in the free energy is positive.
Examples	Burning of fossil fuels, mixing of sulphuric acid into water, etc.	Mixing of ammonium chloride in water, reaction of nitrogen and oxygen, etc.

What Is Exothermic Reaction?

As stated earlier, energy plays a very important role in chemical processes, and the chemical processes involve the breaking of bonds between the atoms and reassembling of these pieces of atoms to form new bonds, which results in the formation of products. Energy is evolved during the formation of the bonds. In some reactions, the energy that evolved to make new bonds is larger than the energy required to break the bonds, and the net result is the evolution of energy. Such a reaction is said to be exothermic if the energy released is in the form of heat. Many common reactions are exothermic. Almost all the reactions that result in the formation of compounds from the constituent elements are always exothermic. Generally, the evolution of heat from a chemical reaction favors the conversion of the reactants to products. A chemical reaction favors the formation of products if the sum of the changes in entropy for the reaction system and its surroundings is positive.

Examples include the combustion of fuels, the formation of water from molecular hydrogen and oxygen, and the formation of metal oxide (formation of calcium oxide from metal calcium and oxygen gas)

The formation of Calcium hydroxide [Ca(OH)2, Common name: Slaked lime] when water is added to Calcium oxide (CaO) is exothermic.

CaO (s) + H2O (l) → Ca(OH)2 (s)

In this reaction, the evolution of energy as heat is evident because the mixture eventually becomes warm.

What Is Endothermic Reaction?

The chemical processes involve the breaking of bonds between the atoms and reassembling of these pieces of atoms to form new bonds, which results in the formation of products. Energy is absorbed in order to break the bonds and initiate the process of rearrangement. In some reactions, the energy absorbed to break the bonds is larger than the energy required to break the bonds, and the net result is the absorption of energy. Such a reaction is said to be endothermic if the energy absorbed is in the form of heat. Although most of the reactions are exothermic, there are some reactions that require energy input when they are formed from the elements. Entropy is also important in determining the fact that whether a reaction is exothermic or endothermic. Entropy is a measure of the number of ways in which energy can be distributed in any system.

Examples include reactions involving compounds like nitric oxide (NO), hydrazine (N2H4), the decomposition of limestone (CaCO3)to make lime (CaO), and the decomposition of water into its constituent elements by the process of electrolysis.

The E of Calcium oxide [CaO, Common name: Lime] when limestone (CaCO3) is decomposed is endothermic.

CaCO3 (s) → CaO (s) + CO2 (g)

In this reaction, it is necessary to heat the limestone to a very high temperature to initiate the reaction and carry it out further.

Main Differences Between Exothermic and Endothermic Reaction in Points

• An exothermic reaction is a chemical reaction that involves the release of heat, whereas an endothermic reaction is a chemical reaction that involves the absorption of heat.
•  In exothermic reactions, the reactants have more energy than the products, whereas in endothermic reactions, the products have more energy than the reactants.
•  The energy is released as the products are formed in an exothermic reaction, whereas the energy is absorbed as the products are formed in an endothermic reaction.
•  The atmosphere where the exothermic reaction takes place will feel warm, whereas the atmosphere where the endothermic reaction takes place will feel cold.
•  The internal energy change of an exothermic reaction is less than zero i.e., negative, whereas the internal energy change of an endothermic reaction is greater than zero, i.e., positive.
•  The origin of energy of an exothermic reaction is within the system, whereas the origin of an endothermic reaction is from the surroundings.
•  An exothermic reaction is a spontaneous reaction, whereas an endothermic reaction is a non-spontaneous reaction.
•  The products formed in the exothermic reaction are more stable, whereas the products formed in the endothermic reaction are less stable.

Conclusion

We can conclude by saying that exothermic reactions are the reactions in which more energy is released during the formation of new bonds in the products than is required to break the bonds in the reactants as it requires more energy in order to break the bonds in the reactants than it is released when the new bonds form in the products in an endothermic reaction. An endothermic reaction requires a constant input of energy, mainly in the form of heat. Endothermic reactions are those reactions in which less energy is produced when the new bonds form in products than is required to break the bonds in the reactants. An exothermic reaction is just the opposite of an endothermic reaction, where the energy required to break the bonds in the reactants is less than the energy released when the formation of new bonds takes place in the products.