Synthesis Reaction Description Plus Examples

Two or more simple substances combine to form more complex products

chemistry synthesis beaker
In a synthesis reaction, simple reactants combine to form a more complex product. Rafe Swan / Getty Images

While there are many types of chemical reactions, they all fall into at least one of four broad categories: synthesis reactions, decomposition reactions, single displacement reactions, and double displacement reactions.

A synthesis reaction or direct combination reaction is a type of chemical reaction in which two or more simple substances combine to form a more complex product. The reactants may be elements or compounds, while the product is always a compound.

General Form of Synthesis Reactions

The general form of a synthesis reaction is:

A + B → AB

Examples of Synthesis Reactions

Here are some examples of synthesis reactions:

  • Water:
    2 H2(g) + O2(g) → 2 H2O(g)
  • Carbon dioxide:
    2 CO(g) + O2(g) → 2CO2(g)
  • Ammonia:
    3 H2(g) + N2(g) → 2 NH3(g)
  • Aluminum oxide:
    4 Al(s) + 3 O2(g) → 2 Al2O3(s)
  • Iron sulfide:
    8 Fe + S8 → 8 FeS
  • Potassium chloride:
    2 K(s) + Cl2(g) → 2 KCl(s)

Recognizing Synthesis Reactions

The hallmark of a synthesis reaction is that a more complex product is formed from the reactants. One easy-to-recognize type of synthesis reaction occurs when two or more elements combine to form a compound. The other type of synthesis reaction happens when an element and a compound combine to form a new compound.

Basically, to identify this reaction, look for a product that contains all the reactant atoms. Be sure to count the number of atoms in both the reactants and the products. Sometimes when a chemical equation is written, "extra" information is given that might make it hard to recognize what is going on in a reaction. Counting numbers and types of atoms makes it easier to identify reaction types.