Synthesis Reaction

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. The product is always a compound.

General Form of a Synthesis Reaction

The general form of a synthesis reaction is:

A + B → AB

Examples of synthesis reactions:

    Generic Formula
  • metal + oxygen → metal oxide
  •        2 Mg (s) + O2 (g) → 2 MgO (s)

  • metal chloride + oxygen → metal chlorate
  •        2 MgCl (s) + O2 (g) → 2 MgClO3 (s)
  • nonmetal + oxygen → nonmetallic oxide
  •        C (s) + O2 (g) → CO2 (g)
  • metal oxide + water → metallic hydroxide
  •        MgO (s) + H2O (l) → Mg(OH)2 (s)
  • metal oxide + carbon dioxide → metallic carbonate
  •        MgO (s) + CO 2 (g) → MgCO3 (s)

  • nonmetallic oxide + water → acid
  •        CO2 (g) + H2O (l) → H2CO3 (aq)

  • metal + nonmetal → salt
  •        2 Na(s) + Cl2 (g) → 2 NaCl (s)

  • a few nonmetals combine with each other.
  •        2 P (s) + 3 Cl2 (g) → 2 PCl3 (g)


    Remember to use the Criss-crossing Method, taking into account the valence of each element, to determine the correct formula of your products.


       Mg (s) + O2 (s) →
    reaction of metal with oxygen will produce a metal oxide
    Mg has a valence of 2+ and O has a velence of 2-, criss cross and get MgO
       2 Mg (s) + O2 (g) → 2 MgO (s)
       CaO (s) + CO2 (g)
    reaction of metal oxide with carbon dioxide will produce a metal carbonate
    Ca has a velence of 2+ and CO3 has a valence of 2-, criss cross and get CaCO3
       CaO (s) + CO2 (s) → CaCO3 (s)
       CaO (s) + H2O (l)
    reaction of metal oxide with water will produce metal hydroxide
    calcium has a valence of +2 and hydroxide has a valence of -2, criss cross and get Ca(OH)2
       CaO (s) + H2O (l)→ Ca(OH)2 (s)
       P2O5 (s) + H2O (l)
    reaction of non-metal oxide with water will produce acid
    acid produced will most likely be phosphoric acid
       P2O5 (s) + 3 H2O (l) →2 H3PO4 (aq)
        Fe (s) + O2 (g)
    reaction of multivalent metal with oxygen will produce one product for each valences of the metal;
    iron as a cation is either +2 or +3
    therefore the two possible products are iron (II) oxide or iron (III) oxide
       2 Fe (s) +3 O2 (g) →2 Fe2O3 (s)

       2 Fe (s) + O2 (g) →2 FeO (s)