Synthesis Reaction Definition and Examples

Overview of a Synthesis or Direct Combination Reaction

In a synthesis reaction, two or more reactants combine to form a more complex product.
In a synthesis reaction, two or more reactants combine to form a more complex product. shapecharge /Getty Images

A synthesis reaction or direct combination reaction is one of the most common types of chemical reaction.

In a synthesis reaction, two or more chemical species combine to form a more complex product: A + B → AB.

In this form, a synthesis reaction is easy to recognize because you have more reactants than products. Two or more reactants combine to make one larger compound.

One way to think of synthesis reactions is that they are the reverse of a decomposition reaction.

Synthesis Reaction Examples

In the simplest synthesis reactions, two elements combine to form a binary compound (a compound made of two elements). The combination of iron and sulfur to form iron (II) sulfide is an example of a synthesis reaction:

8 Fe + S8 → 8 FeS

Another example of a synthesis reaction is the formation of potassium chloride from potassium and chlorine gas:

2K(s) + Cl2(g) → 2KCl(s)

As in these reactions, it's common for a metal to react with a nonmetal. One typical nonmetal is oxygen, as in the everyday synthesis reaction of rust formation:

4 Fe (s) + 3 O2 (g) → 2 Fe2O3 (s)

Direct combination reactions aren't always just simple elements reacting to form compounds: Another everyday synthesis reaction, for example, is the reaction that forms hydrogen sulfate, a component of acid rain. Here, the sulfur oxide compound reacts with water to form a single product:

SO3 (g) + H2O (l) → H2SO4 (aq)

Multiple Products

So far, the reactions you have seen have only one product molecule on the right-hand side of the chemical equation. Let's take a look at more complex reactions with multiple products. For example, the overall equation for photosynthesis:

CO2 + H2O → C6H12O6 + O2

The glucose molecule is more complex than either carbon dioxide or water.

Remember, the key to identifying a synthesis or direct combination reaction is to recognize two or more reactants form a more complex product molecule.

Predictable Products

Certain synthesis reactions form predictable products. For example:

  • Combining two pure elements will form a binary compound.
  • A metallic oxide and carbon dioxide will form a carbonate.
  • Binary salts combined with oxygen form a chlorate.