Chemical reactions occur due to the rearrangement of molecules of two or more substances (reactants) to form newly formed substances called products.
The rearrangement of these molecules leads to the breaking or formation of bonds that causes changes in the heat absorbed or released.
Based on the energy released, chemical reactions can be classified as exothermic, endothermic, exergonic or endogenic.
Key Takeaways
- Exothermic reactions release heat and energy to the surroundings.
- Exergonic reactions release energy and can do work.
- Exothermic reactions can be exergonic, but not all exergonic reactions are exothermic.
Exothermic vs Exergonic
The difference between exothermic reaction and exergonic reaction is that exothermic reaction deals with enthalpy change in any chemical process that is measured in terms of heat in a closed system, while exergonic reactions deal with free energy change of any chemical reaction called Gibbs free energy. Both are releasing reactions; however, the type of energy differs.
In thermodynamics, an exothermic reaction is an energy-releasing reaction. During the process of an exothermic reaction, energy is released in the form of heat.
Heat is released as the enthalpy (internal energy in given pressure and volume or simply total heat of a system) of reactants is more than the products. This energy is released in the form of heat for chemical stability.
In thermodynamics, an exergonic reaction is also an energy-releasing reaction. During the process of an exergonic reaction, energy is released in the form of Gibbs free energy.
Thus the energy released is also measured in terms of change in entropy (energy not available to do work). Thus energy released helps to get some work done and gives stability to the reaction.
Comparison Table
| Parameters of Comparison | Exothermic | Exergonic |
|---|---|---|
| Meaning | It is a heat-releasing reaction. | It is an energy-releasing reaction. |
| Form of Energy | The form of energy released is heated. | The form of energy released is measured in terms of Gibbs free energy or change in entropy. |
| Effect on Surrounding | The energy of the surrounding is increased by heating. | It has nothing to do with the heating of the surrounding. Until energy is available for doing work, the reaction is feasible. |
| Energy of Reactants | It is higher than products. | It is also higher than that of products. |
| Energy of Products | It is lower than the reactants. | It is also lower than that of the reactants. |
| Overall Change in Energy | Overall there is the release of energy in the reaction. All exothermic reactions are naturally exergonic as energy is released. | Energy is released, but the reaction continues only until work is done with the free energy. |
| Gibbs Free Energy | ∆G is negative (energy is released). | ∆G is negative as well. Usually, exothermic reactions have a larger ∆G. |
| Work Done | Work is not done. | Work is done in the form of entropy change. |
| Example | Combustion of fossil fuel, lighting a candle etc. | Respiration in plants and animals. (Mostly bioenergetics reactions) |
What is Exothermic?
An exothermic reaction is an energy-releasing reaction in which two or more reactants rearrange their molecules, forming and breaking chemical bonds, releasing energy ( there is a change in enthalpy ∆H is also negative) to its surrounding in the form of heat or even light.
This is measured in terms of Joule (the unit of heat). This implies that the reactants have higher energy than the products and keep the reaction thermodynamically stable. Energy has to be released to the surroundings in the form of heat.
The energy thus released lowers the Gibbs free energy of the system (∆G is negative), but the energy is released as a result of the reaction and is dissipated in the surrounding.
The only difference is that the surrounding gets heated. Classification of reactions on the basis of exothermic and endothermic reactions measures only the heat released or required for a reaction.
In exothermic reactions, no energy is required at the beginning of the reaction. The reactants have the energy to react on their own.
The best example of an exothermic reaction is the combustion of any material. When any material, let’s say wood, is burnt. Wood reacts with oxygen in the surrounding air to form carbon dioxide and water vapour that we view as smoke.
Fire is in the form of energy released by the reactants (wood and oxygen) from the products. The fire provides us with heat and light. This chemical energy is successfully transformed into mechanical energy.
What is Exergonic?
An Exergonic is an energy-releasing reaction in which two or more reactants rearrange their molecules, forming and breaking chemical bonds and releasing energy to their surrounding in the form of energy that is used to get work done.
It is also measured in joules as work done is also the same as the amount of energy used to do the work.
The energy thus released lowers the Gibbs free energy of the system (∆G is negative), but the energy released is used to do some work spontaneously (meaning there is a change in entropy as well). ∆H remains negative.
No external energy is required whatsoever to start the reaction.
The best example of exergonic reactions is found in bioenergetic reactions like cellular respiration, catabolism, metabolism of food substances and such.
On average, during the process of cellular respiration, glucose is broken down into water and carbon dioxide with the help of oxygen.
This releases energy which is used to form ATP molecules that drive the body’s functioning. Thus it is a spontaneous energy-releasing process.
Main Differences Between Exothermic and Exergonic
- Exothermic reactions are mainly thermodynamic reactions, while exergonic reactions are mostly bioenergetic ones
- .Exothermic reaction release energy in the form of heat that is dissipated in its surrounding in contrast to exergonic reaction, which uses this energy to get work done.
- Exothermic reactions are a subtype of exergonic reactions, but all exergonic reactions are not exothermic due to the spontaneity of their nature.
- Exothermic reactions are measured only in terms of enthalpy change, while exergonic reactions are measured in terms of both enthalpy and entropy change.
- Lighting a fire, reactions between metal and water, cement and water etc., are examples of exothermic reactions, while catabolism, metabolism, anabolism, respiration, ATP formation are examples of exergonic reactions.
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