In simple terms, ‘Exothermic’ means ‘to give off heat’. The word is used in chemistry, typically when describing a reaction or process. An exothermic reaction is a chemical reaction that gives off heat.
The word itself is a compound word that supports this. The prefix, ‘exo’ refers to ‘out’ or ‘outward’ (because heat energy is released ‘out’ to the surroundings). ‘Thermic’ is the root word, and means ‘relating to heat’.
Often, an exothermic reaction occurs between two chemicals reacting with each other. However, it can also occur with a single substance that is changing state. An everyday example of this would be steam. As boiling water (a liquid) turns into steam (a gas), that steam may collect on a surface. If the steam then releases the heat energy it holds, and begins to turn back into a liquid… this is an exothermic reaction.
Determining whether or not something is an exothermic reaction requires us to think critically about what goes on around us all the time. Is it really a chemical reaction that produces heat? Or is it just releasing heat? Hot food cooling on the table may be releasing heat, but it is not a chemical reaction, and the word would not really be appropriate if used to describe this. One thing that may help to determine this is to know that, by default, if a chemical reaction occurs, you don’t come out of it with the same product/s you started with (though they may be similar).
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In this article:
- How Does an Exothermic Reaction Happen?
- Is THIS an Exothermic Reaction…?
- Exothermic vs. Endothermic
- More About Reactions that Release Heat…
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How Does an Exothermic Reaction Happen?
What causes the release of heat in an exothermic reaction? This has to do with the bonds that hold elements together (all elements are held together with bonds). These bonds require various amounts of energy to maintain. The creation of or destruction of these bonds are ultimately what determine where the energy is moved.
In a chemical reaction, as products come together and different products are formed, the bonds that hold them together are changing and rearranging. If the bonds that are formed hold less energy than the bonds that were originally there, you will get an exothermic reaction.
Another way of saying this may be to say that, in the process of a heat-releasing chemical reaction, high-energy bonds are broken and lower-energy bonds are created. The lower-energy bonds may be stable, but the energy released in the reaction creates heat. An exothermic reaction doesn’t occur unless bonds are broken.
Is THIS an Exothermic Reaction…?
- Freezing: Freezing something (for example, water) is, in its most accurate sense, mildly exothermic. It is a single product changing state (from a liquid to a solid). The water must give up the heat energy that allows it to stay liquid.
- Boiling: If you are boiling water, water is changing from a liquid to a gas because you are pouring heat into that water to take it to a new physical state. This is not an exothermic reaction. However, as mentioned before, when the steam collects on a surface and condenses back into water, it has to release the heat energy that keeps it a gas. This IS an exothermic reaction.
- Refrigerator: Refrigerators work to collect and extract heat energy and push it out the back end of the fridge. This is not a ‘chemical reaction that releases heat’.
- Food: As discussed above, hot food cooling is not.
- Microwave: A microwave creates heat inside food, and is not exothermic.
- Stoves: Stoves also pour heat energy into food, and are not.
- Fireworks: Chemical compounds are blended, to make fireworks. These ARE exothermic.
- Fire: Fire is a combination of a wood carbon source, and oxygen. This IS a chemical reaction that releases heat.
- Gasoline: Burning gas in your gas engine, yes. This is a chemical reaction.
- The Human Body: An example of an exothermic reaction in the human body is shivering. Our bodies are breaking down the food molecules we consumed. Energy is released through rapid muscle movements, and heat is released. This is not an obvious chemical reaction, but one that takes place on a microbiological level.
Exothermic vs. Endothermic
The opposite of an exothermic reaction is an endothermic one, and persons that are not familiar with the terms could easily confuse them. In an endothermic reaction, heat is taken in. Certain chemical products and/or solutions actually take up heat energy when they are created – they get cold. There are chemical solutions that drop 40 degrees when you blend them.
Like exothermic reactions, you can get an endothermic reaction when a single chemical changes state, as well as when a reaction takes place between two or more substances. If the bonds that are created require more energy for the creation of those bonds than the bonds that are destroyed, generally this is an endothermic reaction.
More About Reactions that Release Heat…
Here are some other good questions, and answers, about exothermic reactions:
- Are all exothermic reactions exergonic? Exergonic means that something is “accompanied by the release of energy”. The answer is yes, by default- heat energy is energy.
- Do they need activation energy? Not all of them do. It depends on the specific situation, but some don’t require a catalyst.
- Is an exothermic reaction combustion? No, not all of them are… but all combustion reactions are exothermic.
- Are they spontaneous? While there are certain products that could be spontaneously exothermic, most are not. Sodium metal is kept under hydrocarbons because it can spontaneously react with oxygen, but this is an anomaly.
- Are all exothermic reactions redox? No. This is a type of organic reaction.
- Are they all, combination reactions? No, a chemical changing state is not a combination reaction.
- Are all exothermic reactions decomposition? No, the reaction that happens could be building things up, and this would not be decomposition.
Ultimately, handling chemicals that may release heat should be done by persons that are trained. The giving off of heat does produce potential for hazard, and in some cases there is potential for a lot of hazard. Specific training and the appropriate PPE (personal protective equipment), as well as general awareness, are essential for the handling of these types of chemicals and reactions.
With industrial needs for exothermic products or chemicals, reach out to a representative. We’ll be happy to help!
