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A chemical reaction is a process of change of matter in which one or more chemical substances are transformed into other different substances . There are thousands of chemical reactions and many of them are constantly happening around us and even inside our bodies. These reactions can be very simple, involving as few as two, three, or four chemicals, or they can become very complex, involving a multitude of reactants and producing a wide variety of different products.
In the sections of this article we will see how chemical reactions are classified according to different criteria, as well as examples of representative simple chemical reactions of each type of reaction.
Types of chemical reactions according to the number of reactants and/or products
The easiest way to classify chemical reactions is according to the number of reactants and products involved and according to the type of exchange that occurs between the reactants. Following this criterion, chemical reactions can be:
- Combination or synthesis reactions.
- Decomposition reactions.
- Replacement or substitution reactions.
- Double substitution or metathesis reactions.
- Isomerization reactions.
Combination or synthesis reactions
They are those reactions in which two or more chemical substances combine to form a single product. They are recognized because there is more than one reactant but only one product. The simplest general reaction looks like this:
Examples of Combination Reactions
- The formation reaction of ferric oxide
- combustion of carbon
- The hydration of ethylene
decomposition reactions
They are essentially the opposite of combination reactions. They consist of a reaction in which a substance breaks down or decomposes to form other different molecules. The general reaction is the opposite of the previous one:
Examples of Decomposition Reactions
- electrolysis of water
- The decomposition of nitrogen triiodide
- The decomposition of potassium chlorate
Simple displacement, replacement, or substitution reactions
In this type of reaction, two reactants participate and two products are produced; however, one of the two reactants can be seen to replace one of the two halves of the other reactant. The general reaction has the following form:
In this reaction, reactant C replaces B in compound AB.
Examples of displacement reactions
- displacement of iodine by bromine
- Displacement of hydrogen by zinc
- displacement of silver by copper
Double substitution or metathesis reactions
These reactions are similar to the previous ones, with the difference that in this case there are two compounds each made up of pairs and, during the reaction, one half of one of the pairs replaces half of the other and vice versa. They are recognized because there are two compounds as reactants and two as products. These reactions are similar to a change of partners during a dance. The overall reaction is:
Examples of Double Displacement Reactions
- Neutralization reaction between sodium hydroxide and hydrochloric acid
Notice in this example that sodium replaces hydrogen in HCl, and at the same time hydrogen replaces sodium in NaOH.
- Metathesis reaction between silver nitrate and sodium chloride
- Hydrolysis of ethyl acetate
Rearrangement or isomerization reactions
These are the simplest reactions to recognize, since they involve only two chemical species: a reactant molecule and a product molecule. Since the law of conservation of matter must be satisfied, all the atoms that are present in the reactant must still be present in the products, so both molecules have the same molecular formula. However, both compounds are different because changes occur in the structure, the order in which the atoms are attached, or both.
This type of reaction can be represented in a general way as follows:
Examples of isomerization reactions
- Isomerization of methyl thiocyanate
In this reaction it can be seen that there was a change in the atom that is attached to the methyl group (CH 3 ). At first it is sulfur, while in the product methyl is attached to nitrogen.
- Isomerization of unsaturated fatty acids
This reaction shows the isomerization of oleic acid, which is a cis fat, to elaidic acid, which is a trans fat.
- Isomerization of pent-4-enoic acid
Types of chemical reactions according to heat flow
Depending on the direction in which the heat of a reaction flows, these can be classified into two large groups:
exothermic reactions
They are those that, when they occur, release energy in the form of heat. They are characterized by having a negative reaction enthalpy and because the chemical bonds in the products are stronger and more stable than in the reactants.
Examples of exothermic reactions
- Combustion of cooking gas, butane
In this reaction it can be seen that there was a change in the atom that is attached to the group
- Reaction of metallic sodium with water
- The dissolution of copper by hydrochloric acid
endothermic reactions
They are just the opposite of exothermic reactions. In this case, the reactants absorb heat as they transform into the products and cause a cooling of their surroundings.
Examples of endothermic reactions
- Photosynthesis
- Conversion of molecular oxygen to ozone
- electrolysis of water
Types of reactions according to the type of chemical change that occurs
A good part of the thousands of chemical reactions that exist fall into one of these three categories, depending on the type of chemical change or exchange of electrons that occurs:
- precipitation reactions
- Acid-base reactions
- Oxidation-reduction reactions
precipitation reactions
These are reactions involving two ionic compounds in solution that react with each other to form a solid compound that precipitates.
Examples of precipitation reactions
- The precipitation of silver ions with bromide ions
- Calcium carbonate precipitation from hard water
- The precipitation of mercury ions with hydrogen sulfide
Acid-base reactions
Also called neutralization reactions, they consist of the reaction between an acid or substance capable of donating protons and a base or substance capable of accepting protons. In many cases of simple acid-base reactions, the products are a salt and water.
Examples of acid-base reactions
- Neutralization of hydroiodic acid with sodium hydroxide
- Neutralization of sulfuric acid with lithium hydroxide
- Neutralization of phosphoric acid with aluminum hydroxide
Oxidation-reduction or redox reactions
These are reactions in which one reactant removes one or more electrons from another, causing both to undergo a change in their valence, or oxidation state. The one that receives the electrons is called the oxidizing agent, while the one that gives them up is called the reducing agent.
Examples of redox reactions
- The oxidation of iron to ferrous oxide
- The formation of sodium chloride from chlorine and elemental sodium
- The oxidation of isopropyl alcohol to acetone
References
Brown, T. (2021). Chemistry: The Central Science (11th ed.). London, England: Pearson Education.
Chang, R., Manzo, Á. R., Lopez, PS, & Herranz, ZR (2020). Chemistry (10th ed.). New York City, NY: MCGRAW-HILL.
Classification of chemical reactions. (2020, October 30). Retrieved from https://espanol.libretexts.org/@go/page/1820
