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Synthesis Reaction (Combination Reaction)

What is a Synthesis Reaction?

A synthesis reaction is a reaction in which two or more reactants chemically bond and combine to form a product. A synthesis reaction is also known as a combination reaction. Most synthesis reactions are exothermic reactions, i.e., heat is released during the reaction.

General Equation

The general chemical equation for a synthesis reaction is given by the following equation.

A + B → AB

When writing an actual reaction, the reaction must be balanced.

Synthesis (Combination) Reaction

How to Identify a Synthesis Reaction?

A synthesis reaction combines all the reactants of the reaction to form a product. To recognize a synthesis reaction, look for a product that contains all the reactant atoms.

Example of Synthesis Reaction

An example of a combination reaction is the combination of aluminum (Al) and oxygen (O2) to aluminum oxide (Al2O3).

4 Al (s) + 3 O2 (g)  → 2 Al2O3 (s)

Different Types of Synthesis Reaction

There are three types of synthesis reaction.

1. Reaction between two elements


  • The reaction between hydrogen (H2) and nitrogen (N2) to form ammonia (NH3)

N2 (g) + 3 H2 (g) → 2 NH3 (g)

  • The reaction between carbon (C) and oxygen (O2) to form carbon dioxide (CO2)

C (s) + O2 (g) → CO2 (g)

  • When sodium (Na) metal reacts with chlorine (Cl2) gas, the reaction results in sodium chloride (NaCl), also known as common salt

2 Na (s) + Cl2 (g) → 2 NaCl (s)

  • The reaction between iron (Fe) and oxygen (O2) results in iron (III) oxide (Fe2O3), commonly known as rust. Rusting is a naturally occurring phenomenon.

4 Fe (s) + 3 O2 (g) → 2 Fe2O3 (s)

2. Reaction between two compounds


  • When magnesium oxide (MgO) and carbon dioxide (CO2) combine, the resulting product is magnesium carbonate (MgCO3)

MgO (s) + CO2 (s) → MgCO3 (s)

3. Reaction between an element and a compound


  • The reaction between carbon monoxide (CO) and oxygen (O2) yields carbon dioxide (CO2).

2 CO (g) + O2 (g) → 2 CO2 (g)

Synthesis (Combination) Reaction Examples

Examples of Synthesis Reactions in Everyday Life

There are a few examples of the synthesis reaction in real and daily life. Almost all real-life examples are seen in the industry. During industrial production, the synthesis reaction plays a significant part in synthesizing new compounds.

  • Synthesis of ammonia
  • Commercial production of slaked lime (calcium hydroxide)
  • Production of sodium chloride or common salt
  • Preparation of hydrochloric acid and ammonium chloride

Other examples include

  • Rusting
  • Photosynthesis


Q.1. Why is a decomposition reaction called the opposite of a combination reaction?

Ans. A decomposition reaction is one when a substance decomposes into two or more products. This reaction is the opposite of what a combination does.

Q.2. Can a combination reaction be a redox reaction?

Ans. Yes. In fact, most common oxidation-reduction (redox) reactions are combination reactions.

Q.3. Can a combination reaction be an oxidation reaction?

Ans. Yes, a combination reaction can be an oxidation reaction if one of the reactants is oxygen.

Q.4. Can the combination reaction be endothermic?

Ans. While all combination reactions are exothermic, there can be an exception. The production of nitric oxide (NO) from nitrogen and oxygen is an endothermic reaction.

Q.5. What is a dehydration synthesis reaction?

Ans. Dehydration synthesis is the formation of larger molecules from smaller reactants, followed by the loss of a water molecule.

Q.6. How do hydrolysis reactions compare to dehydration synthesis reactions?

Ans. Dehydration synthesis reactions build up the molecules and generally require energy, while hydrolysis reactions break down the molecules and generally release energy. The two are opposite to one another.