EXOTHERMIC REACTION: PROCESS, TYPES AND EXAMPLES - CHEMISTRY - 2025

The exothermic reaction is a type of chemical reaction in which a transfer of energy occurs, mainly in the form of the release of heat or light. For example, any combustion reaction, in which something burns, an exothermic reaction occurs.

In the course of daily life it is common to see how different exothermic reactions take place naturally or caused by causing changes in the temperature of different places. These temperature changes can be measured with the help of a thermometer.

Exothermic reaction with release of heat

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

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

Opposite to these reactions are endothermic ones, which absorb energy.

Process

The combustion of a match is an exothermic reaction

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

Reagent (s) → Product (s) + Energy

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

Otherwise, if the variation of this measure is positive, it reflects the absorption of heat from the surroundings. Likewise, the magnitude of the change in the enthalpy 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 environment.

This occurs because in these reactions the net energy that is released when new bonds are created is greater than the net energy used in the fragmentation of the bonds.

From the above it can be inferred that this class of reactions is very common, because the products of the reaction have an amount of energy stored in the bonds that is greater than that 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 kind of substance as a catalyst to be produced.

The most important types of exothermic reactions are listed below:

Combustion reactions

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 exothermic behavior, because the oxidation of oxygen causes the release of a large amount of energy, as occurs in the oxidation of hydrocarbons.

Thermite reaction

This reaction can produce a temperature of approximately 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 react, 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 nucleus with a greater weight has less 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 of exothermic reactions

Below are some examples of exothermic reactions, which produce a variation in enthalpy that has a negative value due to the fact that they release energy:

A lit candle

The process of combustion of the paraffin and the wick of the candle produces an exothermic reaction that generates heat and light.

Ignite a phosphorous

When a match is lit, a reaction is generated between the chemical substances that make it up and the oxygen present in the air. In this way, an exothermic reaction is generated that produces both light and heat.

Breathing

The respiration process produces an exothermic reaction within cells during gas exchange. In this way, glucose together with oxygen is transformed into carbon dioxide and heat.

Burning firewood

Burning wood produces an exothermic reaction in that the resulting product of this reaction manifests itself as heat and temperature.

Propane combustion

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)

Sodium carbonate - hydrochloric acid neutralization reaction

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

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

Oxidation of ethanol to acetic acid

The oxidation of ethanol to acetic acid used in breathalyzers is also presented, the complete reaction of which is shown in the following equation:

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

Thermite reaction

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

2Al (s) + Fe ₂ O ₃ (s) → Al ₂ O ₃ (s) + Fe (l)

Steel shavings + vinegar

This mixture works as a form of slow combustion where the steel undergoes an oxidation process thanks to the action of vinegar.

"Dog barking"

This reaction is given this name since it gives off a sound similar to that of the dog's barking.

This reaction takes place inside a laboratory tube where nitrous oxide and nitric oxide and carbon bisulfate are mixed.

Glass bottle + alcohol

Similar to the reaction generated by the aforementioned experiment, is that of rubbing a glass bottle with alcohol in such a way that a flame is produced.

Laundry detergent + water

When laundry detergent soap is dissolved, an exothermic reaction can be seen. This is one of the most easily observable examples of exothermic reactions in the home.

Elephant toothpaste

This is an experiment commonly used to explain the dynamics of exothermic reactions. It consists of the dissolution of hydrogen peroxide in some soapy medium, so that in this way a large amount of foam is produced.

A catalyst (potassium iodide) is added to this mixture, which helps the rapid decomposition of the peroxide.

Sulfuric acid + sugar

The dehydration process of sugar produces an obvious exothermic reaction. When you mix sulfuric acid with sugar, it is dehydrated and a column of black smoke appears, causing the environment to smell like burned bones.

Sodium + water

Sodium or any alkaline medium reacts strongly with water. When adding any alkali metal to the water (lithium, sodium, potassium, rubidium or cesium) it must react.

To the extent that the number of the element is higher in the periodic table, the reaction will be stronger.

Sodium acetate

Sodium acetate is known as hot ice. This material starts from the crystallization of frozen solutions that, instead of releasing cold, releasing heat.

Because of its appearance, it is called "ice," but actually crystallized sodium acetate is one of the most common materials used to make hand warmers.

Soda + vinegar

This mixture produces an exothermic reaction that generates large amounts of foam, therefore it is commonly used to resemble the explosion of a volcano.

The genie in the bottle

In this experiment, hydrogen peroxide (hydrogen peroxide) is mixed with potassium permanganate. In this way, permanganate breaks down hydrogen peroxide, causing a large amount of smoke and heat to be released.

Explosive gummy bears

Gummy bears are rich in sucrose (sugar), a substance that, when mixed with potassium chlorate at high temperatures, produces a violent explosion and movement of gummy bears.

Lightning in a tube

This reaction occurs when a corrosive acid is mixed with alcohol or acetone.

In this way, a vigorous chemical reaction can be seen that results in the generation of light inside a tube similar to that of lightning.

Freeze water

During this process the water releases energy in the form of heat, therefore when water cubes freeze, an exothermic reaction takes place.

Corrosion of metals

Pure metals, that is, in their natural state when they come into contact with air produce an oxidation reaction together with the generation of heat, therefore, this process is said to be exothermic.

Gas combustion process

The combustion process of any gas, such as methane or natural gas, produces an exothermic reaction that manifests itself in the generation of heat and on some occasions, when combustion occurs in a controlled manner, it can also produce light.

Other

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

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

References

1. Wikipedia. (sf). Exothermic reaction. Obtained from es.wikipedia.org
2. BBC. (sf). Energy changes and reversible reactions. Recovered 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. Obtained from books.google.co.ve