What Is a Synthesis Reaction? Definition and Examples

Synthesis Reaction Definition and Examples
A synthesis reaction or direct combination reaction reacts two or more simple elements or compounds to form a more complex product.

A synthesis reaction is one of the four main types of chemical reactions, along with decomposition, single replacement, and double replacement reactions. Here is the synthesis reaction definition, examples of the reaction using elements and compounds, a look at how many reactants are involved, and how to recognize a synthesis reaction.

Synthesis Reaction Definition

A synthesis reaction is a chemical reaction that combines two or more simple elements or compounds to form a more complex product.

A + B → AB

This type of reaction is also called a direct combination reaction or simply a combination reaction. It’s the type of reaction that forms compounds from their elements. Synthesis reactions also make large molecules from smaller ones. A synthesis reaction is the opposite of a decomposition reaction, which breaks complex molecules into simpler ones.

Synthesis Reaction Examples

There are many examples of synthesis reactions. Some involve elements. In others, an element reacts with a compound. In still other cases, compounds react with other compounds to form larger molecules.

Synthesis Reactions Between Elements

  • Iron and sulfur react to form iron sulfide.
    8 Fe + S8 → 8 FeS
  • Potassium and chlorine react to form potassium chloride.
    2K(s) + Cl2(g) → 2KCl(s)
  • Iron and oxygen react to form rust.
    4 Fe (s) + 3 O2 (g) → 2 Fe2O3 (s)
  • Hydrogen reacts with oxygen to form water.
    2 H2(g) + O2(g) → 2 H2O(g)

Synthesis Reactions Between an Element and a Compound

  • Carbon monoxide reacts with oxygen to form carbon dioxide.
    2 CO(g) + O2(g) → 2CO2(g)
  • Nitric oxide reacts with oxygen to form nitrogen dioxide.
    2NO + O2 → 2NO2
  • CH2CH2(g) + Br2(ℓ) → CH2BrCH2Br

Synthesis Reactions Between Compounds

  • Sulfur oxide reacts with water to form sulfuric acid.
    SO3 (g) + H2O (l) → H2SO4 (aq)
  • Calcium oxide reacts with water to form calcium hydroxide.
    2CaO (s) + 2H2O (l) → 2Ca(OH)2 (aq)
  • Iron oxide and sulfur oxide react to form iron sulfate.
    Fe2O3 + 3SO3 → Fe2(SO4)3

How Many Reactants Are There?

Usually, there are two reactants in a synthesis reaction. They could be two elements, an element and a compound, or two compounds. However, sometimes more reactants combine to form a product. Here are examples of synthesis reactions involving three reactants:

  • Sodium carbonate reacts with water and carbon dioxide to form sodium bicarbonate.
    Na2CO3 + H2O + CO2 → 2NaHCO3
  • Nitrogen reacts with water and oxygen to form ammonium nitrate.
    2N2(g) + 4H2O(g) + O2(g) → 2NH4NO3(s)

How to Recognize a Synthesis Reaction

The easiest way to recognize a synthesis reaction is to look for a reaction where multiple reactants produce a single product. However, sometimes a synthesis reaction equation includes multiple products and reactants. A good example is the overall reaction for photosynthesis, in which carbon dioxide and water combine to form glucose and oxygen.

CO2 + H2O → C6H12O6 + O2

But, even in this case, two simpler molecules react to form a more complex one. So, this is the key in synthesis reaction identification.

Some synthesis reactions form predictable products. If you recognize them, it’s easy to recognize the reaction type:

  • Reacting two elements forms a binary compound. For example, hydrogen and oxygen react to form water.
  • When two nonmetals react, more than one product is possible. For example, sulfur and oxygen react to form sulfur dioxide or sulfur trioxide.
  • Alkali metals react with nonmetals to form ionic compounds. For example, sodium and chlorine form sodium chloride.
  • Transition metals react with nonmetals to form more than one possible product. To predict the product, you need to know the oxidation state (charge) or the metallic cation.
  • Nonmetal oxides react with water to form acids. For example sulfur dioxide reacts with water to make sulfurous acid.
  • Metallic oxides react with water to form bases.
  • Nonmetal oxides react with one another to form salts.