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Difference Between Exothermic and Endothermic Reactions


Chemical reactions take place all around us, enabling various processes in the biome and ecosystem. Exothermic and endothermic are two primary classes of chemical reactions. They are categorized based on intake or release of heat, for example photosynthesis and cellular respiration. Whenever a chemical reaction takes place, it absorbs or releases energy. That energy can be measured through heat emission or absorption when you know if the process is exothermic or endothermic.

Keep reading to learn all the differences between exothermic and endothermic reactions and their occurrences in the environment.

Comparison Table

FactorsExothermic ReactionEndothermic Reaction
Type of RxnEnergy-releasing reactionsEnergy-absorbing reaction
Change in
Lower energyHigher energy
Surrounding Temp.IncreasesDecreases
Form of EnergyLight, heat or soundHeat
Enthalpy ChangeNegativePositive
Entropy ChangePositiveNegative
Type of ProductsMore stable than reactantsLess stable than reactants
ExamplesNuclear fission, rusting iron,
and neutralization
Photosynthesis, melting ice,
and thermal decomposition

What are Exothermic Reactions?

The word exothermic combines ‘exo,’ meaning release, and ‘thermal,’ meaning heat. Thus, exothermic reactions are chemical changes that release energy. Sometimes the heat or light energy released by exothermic reactions may be negligible. On the other hand, you may feel the heat evidently in other cases.

Reactants Products + energy

The release of energy during an exothermic reaction is attributed to the formation of new chemical bonds. As the energy for breaking the bonds is lower, the enthalpy changes are also lower at the completion of the reaction. The energy released at the end of the reaction was used by bonds to hold the molecules together. Thus, the reaction between two molecules results in energy release.

Exothermic reactions lower the internal energy by releasing energy. You can find out exothermic reactions by feeling a higher temperature around the working station.

The enthalpy changes in an exothermic reaction are denoted by:

ΔH    =     H (products)   –   H (reactants)      =    A negative value

Examples of Exothermic Reactions

  • Fuel combustion
  • Neutralization reaction between acid and base
  • Dissolution of acid in water
  • Nuclear fission

What are Endothermic Reactions?

Opposed to exothermic, endothermic is a fusion of ‘endo’ meaning ‘within’ and ‘thermic’ translating to ‘heat.’ Endothermic reactions absorb energy instead of releasing it. Organic and inorganic endothermic reactions take place by absorbing heat from the surroundings to complete the chemical reaction.

Reactants + Energy Products

The reaction absorbs energy from the outside, which makes the energy changes in the system positive. The system’s enthalpy is equal to the sum of enthalpies of the reactants in the beginning. Towards the end of the natural or lab experiment, the enthalpy of the product formed is higher due to the energy absorbed from the surroundings. The enthalpy change during the process can be expressed through this equation:

ΔH    =     H (products)   –   H (reactants)      =    A positive value

Examples of Endothermic Reactions

  • Photosynthesis
  • Melting ice
  • Thermal decomposition
  • Dissolution of ammonium chloride in water

Difference Between Exothermic and Endothermic Reactions

Literal Meaning

Exothermic Reactions

The word exothermic forms from ‘exo,’ meaning release, and ‘thermic,’ meaning heat.

Endothermic Reactions

Similarly, endothermic combines ‘endo,’ meaning absorb, and ‘thermic,’ translating to heat.


Exothermic Reactions

Exothermic reactions are chemical reactions that release energy in the form of heat or light.

Endothermic Reactions

Opposed to exothermic reactions, endothermic reactions absorb energy from their surroundings during the chemical reaction.


Exothermic Reactions

Exothermic reactions are denoted by: Reactants Products + energy

Endothermic Reactions

Conversely, endothermic reactions are expressed by the equation: Reactants + Energy → Products

Energy Change

Exothermic Reactions

Energy is lost during exothermic reactions in the form of heat or light as studied in different branches of chemistry.

Endothermic Reactions

Alternatively, endothermic reactions have higher energy at the end resulting from energy absorption.

Form of Energy

Exothermic Reactions

Exothermic reactions may release energy in the form of light, heat, or sound energy.

Endothermic Reactions

On the contrary, endothermic reactions absorb heat from the surrounding in applied chemistry and other occurrences.

Enthalpy Change

Exothermic Reactions

The enthalpy change in an exothermic reaction is negative at the end of the reaction due to the release of energy.

Endothermic Reactions

At the same time, endothermic reactions have a positive enthalpy change due to energy absorption during the chemical reaction.


Exothermic Reactions

Exothermic reactions include rusting iron, nuclear fission, cellular respiration and neutralization reaction between acid and base.

Endothermic Reaction

On the contrary, photosynthesis, melting ice, and thermal decomposition are examples of endothermic reactions.

The Bottom Line

Exothermic and endothermic reactions are the main types of chemical reactions based on the absorption or release of energy. Exothermic reactions release energy in the form of heat, light, or sound. On the other hand, endothermic reactions take place by absorbing energy from the surroundings. The fundamental difference between exothermic and endothermic reactions is the enthalpy change. Exothermic reactions exhibit a negative enthalpy, whereas endothermic reactions show positive change. They are of critical importance in nature and in chemistry, biochemistry and molecular biology.


How can you tell the difference between exothermic and endothermic reactions?

The main difference between exothermic and endothermic reactions is the release of energy in the former and absorption of energy in the latter.

How do you tell if a reaction is endothermic or exothermic by looking at the equation?

The chemical equation can help detect if the reaction is exothermic or endothermic by looking at the enthalpy changes. Equations exhibiting a higher enthalpy show an endothermic reaction. Contrarily, a higher sum of reactants than the products shows an exothermic reaction.

How are exothermic or endothermic reactions used in daily life?

Lighting a stove, fuel combustion, corrosion, and transpiration are common exothermic reactions in our daily life. Unlike transpiration, evaporation and melting are everyday endothermic reactions.

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