Is Combustion Endothermic or Exothermic? (And Why?)

Combustion is an exothermic process. It releases energy in the form of heat and light. ¹ During combustion, a fuel (such as wood or gasoline) reacts with oxygen to produce carbon dioxide, water, and heat, resulting in a release of energy. ²

Well, this was just a simple answer. But there are few more things to know about this topic which will make your concept super clear.

So let’s dive right into it.

Why is combustion an exothermic process?

Combustion is an exothermic process because it releases energy in the form of heat and light. ³ It involves the rapid chemical reaction between a fuel and an oxidizer (usually oxygen) in the presence of heat or an ignition source. ⁴ This reaction produces new chemical compounds and releases energy in the process.

During combustion, the fuel molecules break apart and combine with oxygen molecules from the air to form new compounds, such as carbon dioxide, water, and other combustion byproducts. This chemical reaction involves the rearrangement of atoms and the formation of new chemical bonds.

The breaking of existing bonds in the fuel molecules requires an input of energy, which is typically provided as heat or an ignition source. However, the formation of new bonds in the products releases a larger amount of energy.

The energy released in the form of heat and light during the combustion process is greater than the energy needed to initiate and sustain the reaction, resulting in an overall release of energy.

This net release of energy makes combustion an exothermic process. The excess energy is released into the surroundings as thermal energy, causing an increase in temperature. This is why combustion is commonly associated with the generation of heat and flames.

The specific amount of energy released depends on the fuel and the reaction conditions, such as the amount of fuel, the availability of oxygen, and the reaction efficiency.

It’s important to note that not all chemical reactions are exothermic. Some reactions, such as endothermic reactions, require an input of energy and absorb heat from the surroundings instead of releasing it.

Why is combustion not an endothermic process?

Combustion is not an endothermic process because it releases energy in the form of heat and light, rather than absorbing it. When a fuel combines with an oxidizer, new chemical compounds are formed, and this reaction releases more energy than it requires to initiate and sustain the combustion process.

During combustion, the fuel undergoes oxidation, which involves the breaking of existing chemical bonds and the formation of new ones.

Breaking bonds requires an input of energy, but the formation of new bonds releases a larger amount of energy. This energy release is what makes combustion exothermic.

In an endothermic process, the reaction would require more energy than it releases, resulting in a net absorption of heat from the surroundings.

This is not the case in combustion, as it involves the release of energy in the form of heat and light.

The excess energy is transferred to the surroundings, causing an increase in temperature and supporting the exothermic nature of the process.

Further reading

Is Melting Ice Endothermic or Exothermic?
Is Sublimation Endothermic or Exothermic?
Is Combustion a Physical or Chemical Change?
Is Mass a Physical or Chemical Property?
Is Color a Physical or Chemical Property?

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2. Human Wonder Research: Carbon Dioxide and Fire. (n.d.). Human Wonder Research: Carbon Dioxide and Fire. http://www.appstate.edu/~goodmanjm/rcoe/hwr/science/co2_and_fire/co2_and_fire.html
3. Foundation, C. (n.d.). Welcome to CK-12 Foundation | CK-12 Foundation. Welcome to CK-12 Foundation | CK-12 Foundation. https://www.ck12.org/book/ck-12-physical-science-concepts/section/5.23/
4. Combustion. (n.d.). Combustion. https://www.grc.nasa.gov/www/k-12/airplane/combst1.html