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The energy of chemical reactions


No atom "comes out of nowhere" or "disappears" during a chemical reaction.

If chemical reactions are just a rearrangement of atoms, then where does the energy involved in, for example, a burning come from?

When a sheet of paper burns, we can see that its paper changes because a chemical reaction occurs. We also realize that in this reaction there is release of light and heat, which are forms of energy.

  • Where did this energy that was released come from?
  • Why was she released?
  • Can there be a reaction that, instead of releasing, absorbs heat?

Chemical energy

Substances have a certain amount of stored energy called chemical energy. This energy comes from its chemical bonds and the forces of attraction and repulsion between the atoms that compose it.

Since each substance has a specific amount of chemical energy stored in it, there is a difference between the energy content of its reagents and its products.

Depending on the chemical energy of reagents and products, the reaction can occur in two different ways:

The energy of reagents is lower than that of products.

In this case the reagents will have to gain energy to turn into products.

The energy of reagents is greater than that of products.

In this case the reagents will have to release energy to turn into products.

Energy absorption and release usually occurs accompanied by heat absorption or release.

Reagents gain energy to become higher energy products or may lose energy to become lower energy products.

Energy-absorbing chemical reactions are called endotherms and those that release energy are called exotherms.

When products have more energy than reagents, we know that these reactions have absorbed energy, that is, they usually gain heat to occur. An example is the burning of sugar to make the pudding syrup. When it gets hot, sugar turns into syrup, and its appearance and taste change. For this reaction to occur it is necessary to give energy to the system.

Already in exothermic reactions, the energy of reagents is greater than that of products. Typically, reagents lose heat for the reaction to occur, such as burning paper. It is easy to see that the system is releasing energy in the form of heat and light.

It is important to note that the released and absorbed energy does not always occur in the form of heat, an example is photosynthesis, where the absorption of energy occurs by the presence of light (light energy).