Broadly speaking, there are five types of chemical reactions:
- Combination
- Decomposition
- Combustion
- Single replacement
- Double replacement
Understanding these different categories can help predict the likely outcome of particular chemical reactions.
Chemical reactions take place all around us. The most frequent types of chemical reactions are those that take place inside the cells of living organisms. In the human body, for instance, there are about 37 trillion cells. In each cell, around a billion biochemical reactions occur per second. That means around 37 billion trillion biochemical reactions are taking place in our bodies every single second!
With numbers like this, it’s difficult to comprehend the complexity and enormity of chemical reactions in nature. That being said, chemical reactions tend to fall into one or more of the five basic categories outlined above.
In this post:
What is a chemical reaction?
Put simply, a chemical reaction is a process that leads to a change in the atomic bonds of a particular substance. It can involve the breaking of bonds, the formation of bonds, or in some cases both. The outermost electrons, known as the valence electrons, are directly involved in chemical reactions.
Chemical reactions also change enthalpy or energy dynamics, with energy either being absorbed or released. This alteration is typically measured in terms of temperature change. Other common indications of a chemical reaction include a change in the state of matter, a change in colour, the evolution of gases, and the formation of precipitates.
How to identify different types of chemical reactions
You can identify the type of chemical reaction based on the reactants and their products. In other words, if you know what the reactants are, then it’s possible to predict which products will be formed.
For instance, if there’s only one reactant but two products are produced, then it’s clear a decomposition reaction has occurred. An example of this is the evolution of hydrogen and oxygen gases from water through electrolysis.
It’s also possible to calculate chemical reactivity based on the reactivity of the elements involved. Reactivity is dependent on an element’s ability to ionise (gain or lose electrons) or share electrons, with the most reactive elements tending to form positive ions more easily. Understanding reactivity allows you to work out which metals can displace other metals. For example, copper can be easily replaced by zinc because the latter is more reactive.
Types of chemical reactions
Chemical reactions are classified into five main categories based on the reactants and products. These reactions can also be described in terms of the way the bonds are created or broken.
1. Combination reactions
Combination reactions are chemical reactions that involve the formation of a product through the joining of two or more different substances to form a single substance. The generic formula for a combination reaction is:
A + B → AB
Example: 2H2 + O2 → 2H2O
2. Decomposition reactions
Decomposition reactions are the simplest type of chemical reaction. The opposite of combination reactions, they start with a compound that breaks down into its constituent elements or simpler compounds. The reaction may occur spontaneously or under special conditions such as in the presence of a catalyst. The generic formula for this type of reaction is:
AB → A + B
Example: Na2CO3 → Na2O + CO2
3. Combustion reactions
Combustion reactions are always exothermic. Generally, these reactions involve oxygen reacting with a substance to produce an oxide and other products. When hydrocarbons combust, the products are water and carbon dioxide.
Fuel + O2 → CO2 + H2O
4. Single replacement reactions
In this type of reaction, one element is replaced by another more reactive element. Single replacement reactions follow a reactivity-based hierarchy, with the less reactive element being replaced by a more reactive element. The generic formula is:
A + BC → AC + B
Example: F2 + 2NaBr → 2NaF + Br2
5. Double displacement reactions
In this type of reaction, the elements or groups in the reactant exchange partners. The generic formula for a double displacement reaction is:
AB + CD → AD + BC
Example: HCl + NaOH → NaCl + H2O
How many different types of chemical reactions are there?
As previously mentioned, there are five main types of chemical reactions. There are, however, enumerable subsets and specific types of chemical reactions within each of these basic categories. For example, acid-base or neutralisation reactions are a subtype under double displacement reactions. 
Another broad sub-category of chemical reactions is oxidation-reduction or redox reactions. This type of reaction can either occur spontaneously such as in a galvanic cell, or non-spontaneously, as is the case in electrolysis. Spontaneous reactions don’t require energy input to occur, whereas non-spontaneous reactions do.
Chemical reaction examples in everyday life
Chemical reactions take place every second of every day, both in the biotic and abiotic realms. Many of them occur naturally while others are artificially induced. Here are a few examples of some of the most important chemical reactions in everyday life.
1. Photosynthesis
Photosynthesis is a highly complex biochemical process that includes the Calvin cycle and light-dependent reactions. Plants produce food (primarily in the form of glucose) through this process, making it crucial for sustaining life and the ecosystem. 
2. Combustion
Combustion, particularly the type that happens in internal combustion engines, is responsible for moving the economy (quite literally). However, despite its many benefits to humankind, combustion is causing serious problems when it comes to climate change. 
3. Washing with soap
Washing with soap is another example of a common chemical reaction that occurs in everyday life. When you wash with soap, the hydrophobic and hydrophilic components of soap molecules interact with oily substances and effectively dissolve them. They also break molecular bonds.
Summary
Various types of chemical reactions take place every second. Most of these occur naturally as part of systems. Chemical reactions play different roles, especially in biological processes. Despite the complexity of many chemical reactions, they can be broadly divided into five basic categories – combination, decomposition, combustion, single replacement, and double replacement.
