Types of Chemical Reactions

4 Types of Reactions — The four main types of chemical reactions are synthesis, decomposition, single displacement, and double displacement reactions.

A chemical reaction is a process or chemical change that transforms one set of substances (the reactants) into another set of substances (the products). The process involves breaking chemical bonds between atoms and forming new bonds, so the number and type of atoms are the same for both reactants and products. The chemical change is described by a chemical equation. There are several types of chemical reactions. Here is look at the four main types of chemical reactions, plus additional key reaction types.

4 Main Types of Chemical Reactions

Keep in mind, there are different names for the reaction types. The four main types of chemical reactions are:

Synthesis or combination reactions
Decomposition or analysis reactions
Single replacement, single displacement, or substitution reactions
Double replacement, double displacement, or metathesis reactions

There are many other types of reactions, though. This table summarizes some key points of the main types of chemical reactions:

Type of Reaction	General Reaction	Explanation
Synthesis or Combination	A + B → AB	Two or more elements or compounds combine to form one compound.
Decomposition or Analysis	AB → A + B	A complex molecule breaks into simpler ones.
Displacement, Replacement, or Substitution	A + BC → AC + B	One element replaces another in a compound.
Double displacement, Double replacement, or Metathesis	AB + CD → AD + CB	Cations and anions exchange partners.
Combustion	A + O₂ → H₂O + CO₂	A compound combines with oxygen to form an oxide. Often, carbon dioxide and water form.
Acid-Base or Neutralization	Acid + Base → Salt + Water	An acid and base react to form a salt and water.
Precipitation	A + Soluble salt B → Precipitate + soluble salt C	Two solutions of soluble salts react to form an insoluble salt (a precipitate).

Main types of chemical reactions

Let’s take a closer look at the types of reactions and get examples:

Synthesis or Direct Combination Reaction

In a synthesis, direct combination, or composition reaction, two (or more) reactants combine to form a more complex product. The general form of the reaction is:

A + B → AB

An example of a synthesis reaction is the combination of iron and sulfur to form iron(II) sulfide:

8 Fe + S₈ → 8 FeS

Here are other examples of synthesis reactions:

2 Na(s) + Cl₂(g) → 2 NaCl(s)
C(s) + O₂(g) → CO₂ (g)
S(s) + O₂(g) → SO₂(g)
2 Fe(s) + O₂(g) → 2 FeO(s)
2 SO₂ + O₂ → 2 SO₃
6 C + 3 H₂ → C₆H₆
4 Na + 2 C + 3 O₂ → 2 Na₂CO₃

Decomposition or Analysis Reaction

A compound breaks or decomposes into smaller pieces in a chemical decomposition or analysis reaction. The general form of the reaction is:

AB → A + B

An example of a decomposition reaction is the electrolysis of water to form oxygen and hydrogen:

2 H₂O → 2 H₂ + O₂

Here are additional examples of decomposition reactions:

CaCO₃ → CaO + CO₂
2 KClO₃ → 2 KCl + 3 O₂
Na₂CO₃ → Na₂O + CO₂

Single Replacement, Single Displacement, or Substitution Reaction

A single replacement, single displacement, or substitution reaction is when one element is displaced from a reactant to form a compound with another element. The reaction has the general form:

A + BC → AC + B

An example of a single replacement reaction is when zinc combines with hydrochloric acid to form zinc chloride and hydrogen. The zinc replaces or displaces the hydrogen in hydrochloric acid:

Zn + 2 HCl → ZnCl₂ + H₂

Double Replacement, Double Displacement, or Metathesis Reaction

When the reactant cations and anions “swap partners” the reaction is called a double replacement, double displacement, or metathesis reaction. The general form for a double replacement reaction is:

AB + CD → AD + CB

An example of a double replacement reaction is the reaction between sodium chloride and silver nitrate to yield sodium nitrate and silver chloride:

NaCl(aq) + AgNO₃(aq) → NaNO₃(aq) + AgCl(s)

Combustion Reaction

A combustion reaction is the reaction between a fuel and oxygen to form one or more oxides. Because many fuels are carbon-based, carbon dioxide (CO₂) is often a product. Sometimes water (H₂O) is a product.

Here are examples of combustion reactions and their balanced equations:

C₁₀H₈ + 12 O₂ → 10 CO₂ + 4 H₂O
H₂ + O₂ → 2 H₂O
C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O
2 Fe₂S₃ + 9 O₂ → 2 Fe₂O₃ + 6 SO₂
2 Al₂S₃ + 9 O₂ → 2 Al₂O₃ + 6 SO₂
P₄ + 5 O₂ → 2 P₂O₅

Acid-Base or Neutralization Reaction

An acid-base reaction is a type of double replacement reaction that occurs between an acid and a base. The H⁺ ion in the acid reacts with the OH^– ion in the base to form water and an ionic salt:

HA + BOH → H₂O + BA

The reaction between hydrobromic acid (HBr) and sodium hydroxide is an example of an acid-base reaction:

HBr + NaOH → NaBr + H₂O

Oxidation-Reduction or Redox Reaction

Redox stands for reduction and oxidation. The two processes occur together. This type of reaction involves electron transfer between reactants and a change in oxidation number. An example is the reduction of I₂ to form I^– and oxidation of S₂O₃^2- (thiosulfate anion) to form S₄O₆^2-:

2 S₂O₃²⁻(aq) + I₂(aq) → S₄O₆²⁻(aq) + 2 I⁻(aq)

Isomerization Reaction

In an isomerization reaction, the structural arrangement of a compound is changed but its net atomic composition remains the same.

For example:

CH₃CH₂CH₂CH₃ (n-butane) → CH₃CH(CH₃)CH₃ (i-butane)

Hydrolysis Reaction

A hydrolysis reaction is a reaction in which one or more water molecules is added to a substance. In some cases, this causes both the substance and water molecule to split. The general form of a hydrolysis reaction is:

X^–(aq) + H₂O(l) ↔ HX(aq) + OH^–(aq)

The reverse reaction is a condensation reaction. In a condensation reaction, water is removed from a substance.

How Many Types of Chemical Reactions Are There?

Technically, there are hundreds or even thousands of different types of chemical reactions. However, chemistry students usually learn to classify them as 4 main types, 5 main types, or 6 main types. The four main types of chemical reactions are synthesis, decomposition, single displacement, and double displacement. But, remember, some people use different names for these reactions. Other important types of reactions are combustion, acid-base, and redox reactions. It gets even more complicated in organic chemistry, where many reactions have special names. However, these other types of reactions also fit into one of the four main categories!