Simple Chemical Reactions

Chemical reactions are evidence a chemical change is occurring. The starting materials change into new products or chemical species. How do you know a chemical reaction has taken place? If you observe one or more of the following, a reaction may have occurred:

While there are millions of different reactions, most can be classified as belonging to one of 5 simple categories. Here is a look at these 5 kinds of reactions, with the general equation for each reaction and examples.

Synthesis Reaction or Direct Combination Reaction

This is the general form of a synthesis reaction.
This is the general form of a synthesis reaction. Todd Helmenstine

One of the main types of chemical reactions is a synthesis or direct combination reaction. As the name implies, simple reactants make or synthesize a more complex product. The basic form of a synthesis reaction is:

A + B → AB

A simple example of a synthesis reaction is the formation of water from its elements, hydrogen, and oxygen:

2 H2(g) + O2(g) → 2 H2O(g)

Another good example of a synthesis reaction is the overall equation for photosynthesis, the reaction through which plants make glucose and oxygen from sunlight, carbon dioxide, and water:

6 CO2 + 6 H2O → C6H12O6 + 6 O2

Decomposition Chemical Reactions

This is the general form of a decomposition reaction.
This is the general form of a decomposition reaction. Todd Helmenstine

The opposite of a synthesis reaction is a decomposition or analysis reaction. In this type of reaction, the reactant breaks down into simpler components. A telltale sign of this reaction is that you have one reactant, but multiple products. The basic form of a decomposition reaction is:

AB → A + B

Breaking water into its elements is a simple example of a decomposition reaction:

2 H2O → 2 H2 + O2

Another example is the decomposition of lithium carbonate into its oxide and carbon dioxide:

Li2CO3 → Li2O + CO2

Single Displacement or Substitution Chemical Reactions

This is the general form of a single displacement reaction.
This is the general form of a single displacement reaction. Todd Helmenstine

In a single displacement or substitution reaction, one element replaces another element in a compound. The basic form of a single displacement reaction is:

A + BC → AC + B

This reaction is easy to recognize when it takes the form of:

element + compound → compound + element

The reaction between zinc and hydrochloric acid to form hydrogen gas and zinc chloride is an example of a single displacement reaction:

Zn + 2 HCl → H2 + ZnCl2

Double Displacement Reaction or Metathesis Reaction

This is the general form for a double displacement reaction.
This is the general form for a double displacement reaction. Todd Helmenstine

A double displacement or metathesis reaction is just like a single displacement reaction, except two elements replace two other elements or "trade places" in the chemical reaction. The basic form of a double displacement reaction is:

AB + CD → AD + CB

The reaction between sulfuric acid and sodium hydroxide to form sodium sulfate and water is an example of a double displacement reaction:

H2SO4 + 2 NaOH → Na2SO4 + 2 H2O

Combustion Chemical Reactions

This is the general form of a combustion reaction.
This is the general form of a combustion reaction. Todd Helmenstine

A combustion reaction occurs when a chemical, usually a hydrocarbon, reacts with oxygen. If a hydrocarbon is a reactant, the products are carbon dioxide and water. Heat is released, too. The easiest way to recognize a combustion reaction is to look for oxygen on the reactant side of a chemical equation. The basic form of a combustion reaction is:

hydrocarbon + O2 → CO2 + H2O

A simple example of a combustion reaction is the burning of methane:

CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(g)

More Types of Chemical Reactions

Although there are 5 main types of chemical reactions, other kinds of reactions also occur.
Although there are 5 main types of chemical reactions, other kinds of reactions also occur. Don Bayley, Getty Images

In addition to the 5 main types of chemical reactions, there are other important categories of reactions and other ways to classify reactions. Here are some more kinds of reactions:

  • acid-base reaction: HA + BOH → H2O + BA
  • neutralization reaction: acid + base → salt + water
  • oxidation-reduction or redox reaction: one atom gains an electron while another atom loses an electron
  • isomerization: structural arrangement of a molecule changes, although its formula remains the same
  • hydrolysis: AB + H2O → AH + BOH