An endothermic reaction requires energy while an exothermic reaction releases energy. This classification of chemical reactions takes into account the participation of energy either as a reactant or as a product.
Energy is the ability to do work or to produce heat. Remember that chemical reactions involve a rearrangement of atoms between substances with breakage or formation of chemical bonds. In general, this formation or breaking of chemical bonds is accompanied by changes in the energy of the system.
| Endothermic reaction | Exothermic reaction | |
|---|---|---|
| Definition | Chemical reaction where energy is absorbed. | Chemical reaction where energy is released in the form of heat. |
| Origin of energy | Of the environment | Of the system |
| Potential energy | Lower in reactants than in products. | Higher in reactants than in products. |
| Production | Not spontaneous | Spontaneous |
| Internal energy change | ΔE> 0; internal energy change greater than zero. | ΔE <0; internal energy change less than zero. |
| Temperature | Decreases | Increases |
| Examples | Reactions in photosynthesis and synthesis in general. | A burning match, combustion reactions. |
Index
What is an endothermic reaction?
A chemical reaction is endothermic when it absorbs energy from the environment . In this case, the heat is transferred from the outside to the inside of the system. When we place a thermometer while the endothermic reaction is taking place, the temperature drops.
The word “endothermic” derives from the Greek endon which means “inside” and therme which means “heat”. Endothermic reactions do not proceed spontaneously.
Where does the energy come from in endothermic reactions?
In endothermic reactions the energy comes from the environment outside the system. The amount of potential energy of the products is greater than the potential energy of the reactants. Therefore, it is required to add energy to the reactants for the reaction to proceed. This energy comes from the heat of the environment.
For example, photosynthesis is an endothermic process , where plants capture solar energy to produce glucose from carbon dioxide and oxygen:
The products of the photosynthesis reaction, glucose and oxygen, have a greater amount of potential energy with respect to the reactants, carbon dioxide and water. Other examples of endothermic chemical reactions with the amount of energy used:
Examples of endothermic reaction
It is not only in the chemistry laboratory that reactions occur. On a day-to-day basis, we also find situations where endothermic reactions occur.
Cooking food
Although it may not seem like it, the process of cooking food is endothermic. In order to consume certain foods, we must provide heat.
Instant cold bag
Cold packs used to treat bumps or sprains are filled with water, but when shaken or knocked, a capsule containing ammonium nitrate breaks inside. Mixing ammonium nitrate with water is an endothermic reaction, causing the bag to cool.
You may be interested in seeing the Difference between heat and temperature .
What is an exothermic reaction?
An exothermic reaction is one where energy flows out of the system . This energy is released in the form of heat, so placing a thermometer in the reaction system increases the temperature.
The word “exothermic” is formed by exo which means “outward” and thermes, which means “heat”. Exothermic reactions can occur spontaneously and, in some cases, be explosive, such as the combination of alkali metals and water.
Where does the energy come from in exothermic reactions?
In a chemical reaction, reactants are the compounds that transform and give rise to products. For example, when sodium Na reacts with chlorine Cl, these are the reactants and the product is sodium chloride NaCl:
Both reactants and products have stored potential energy. We know from the energy conservation law that energy is neither lost nor gained, so the energy of the reactants must be equal to that of the products.
In an exothermic reaction, reactants have more potential energy compared to products , so excess energy is released as heat. In this case, energy is also considered as part of the products:
In any exothermic process, part of the potential energy stored in chemical bonds is converted into thermal energy through heat.
Examples of exothermic reaction
Laundry detergent
When we dissolve a little powder detergent with water in our hands we can feel a slight heating.
Domestic gas combustion
The combustion of gases for domestic use, such as methane or butane, involves the chemical reaction with oxygen with the formation of carbon dioxide and water, and the release of energy. This is a typical exothermic reaction in everyday use:
The energy released in the combustion process is used to cook food.
You may also be interested in knowing what are Open system, closed system and isolated system .
