Exothermic Reaction: Process, Types and Examples

One of the most common chemical reactions is the exothermic reaction , in which an energy transfer occurs, mainly in the form of release of heat or light, to the surroundings. The name comes from the Greek prefix exo , which means"abroad"; and the term"thermal", which refers to heat or temperature.

In this sense, exothermic reactions can transfer other types of energy to the environment where they are generated, such as occurs with explosions and their way of transferring kinetic and sonic energy when substances that are in the gas phase at high temperatures are expanded. violent way.

In the same way, in the case of the use of batteries, an exothermic type reaction is also carried out, only that in this case electrical energy is transported.

Process

Previously, it was mentioned that when an exothermic reaction occurs an energy release occurs, which can be visualized more easily in the following equation:

Reagents (s) → Product (s) + Energy

So, to quantify the energy absorbed or released by a system, a thermodynamic parameter called enthalpy (denoted by"H") is used. If in a system (in this case, a chemical reaction) there is energy release to the surroundings, then the variation of the enthalpy (expressed as ΔH) will have a negative value.

Otherwise, if the variation of this measure is positive, it reflects the heat absorption of the surroundings. Also, the magnitude of the enthalpy variation of the system is an expression of the amount of energy that is transferred to or from the environment.

The larger the magnitude of ΔH, the greater the release of energy from the system to the surrounding medium. This is because in these reactions the net energy that is released when new links are created is greater than the net energy used in the fragmentation of the links.

From the above it can be inferred that this kind of reactions is very common, because the products of the reaction have an amount of energy stored in the bonds that is greater than what is contained in the reactants initially.

Types

There are different kinds of exothermic reactions in the various fields of chemistry, whether in the laboratory or in industry; Some are carried out spontaneously and others need specific conditions or some type of substance such as a catalyst to be produced.

The following are the most important types of exothermic reactions:

Combustion reactions

The combustion reactions are those of the redox type that occur when one or more substances react with oxygen, generally to result in the release of light and thermal energy - that is, light and heat - when a flame is produced.

Neutralization reactions

Neutralization reactions are characterized by the interaction between an acidic species and an alkaline substance (base) to form a salt and water, which manifest an exothermic nature.

Oxidation reactions

There are many reactions of this type that show an exothermic behavior, because the oxidation of oxygen causes the release of a large amount of energy, as in the oxidation of hydrocarbons.

Termite reaction

This reaction can produce a temperature of about 3000 ° C, and due to the high affinity of aluminum powder with a large number of metal oxides, it is used in the welding of steel and iron.

Polymerization reaction

This type of reaction is the one that originates when a certain number of chemical species called monomers reacts, which are units that when combined are repeated in chains to form macromolecular structures called polymers.

Nuclear fission reaction

This process refers to the division of the nucleus of an atom considered as heavy - that is, with a mass number (A) greater than 200 - to produce fragments or nuclei of smaller size with an intermediate mass.

In this reaction, where one or more neutrons are formed, a large amount of energy is released because the core with a higher weight has a lower stability than its products.

Other reactions

There are also other exothermic reactions of great relevance, such as the dehydration of some carbohydrates when reacting with sulfuric acid, the absorption of water presented by sodium hydroxide exposed to the open air or the oxidation of metallic species in many corrosion reactions.

Examples

Following are some examples of exothermic reactions, which produce a variation of enthalpy that has a negative value due to the fact that they release energy, as mentioned above.

For example, the combustion of propane is a spontaneous exothermic reaction:

C ₃ H ₈ (g) + 5O ₂ (g) → 3CO ₂ (g) + 4H ₂ O (l)

Another case of exothermic behavior is shown by the neutralization reaction between sodium carbonate and hydrochloric acid:

NaHCO ₃ (ac) + HCl (ac) → NaCl (ac) + H ₂ O (l) + CO ₂ (g)

It also presents the oxidation of ethanol to acetic acid used in the breathalyzers, whose complete reaction is shown in the following equation:

3CH ₃ CH ₂ OH + 2K ₂ Cr ₂ OR ₇ + 8H ₂ SW ₄ → CH ₃ COOH + 2Cr (SO ₄ ) ₃ + 2K ₂ SW ₄ + 11H ₂ OR

Another class of exothermic reaction is the so-called thermite reaction, in which aluminum is combined with a metal oxide, as exemplified below:

2Al (s) + Faith ₂ OR ₃ (s) → Al ₂ OR ₃ (s) + Fe (l)

In addition to the examples explained above, there is a wide variety of reactions that are also considered exothermic, such as the decomposition of certain organic waste substances for composting.

It also highlights the oxidation of luciferin pigment by the action of the enzyme luciferase to produce the bioluminescence characteristic of fireflies, and even breathing, among many other reactions.

References

1. Wikipedia. (s.f.) Exothermic reaction. Retrieved from es.wikipedia.org
2. BBC (s.f.) Energy changes and reversible reactions. Retrieved from bbc.co.uk
3. Chang, R. (2007). Chemistry, Ninth edition. (McGraw-Hill)
4. Walker, D. (2007). Chemical Reactions. Recovered from books.google.co.ve
5. Saunders, N. (2007). Exploring Chemical Reactions. Retrieved from books.google.co.ve