A combination reaction also recognized as a synthesis reaction, is a reaction where two or more elements or compounds (reactants) merge to form a single compound (product). Such reactions are represented by equations of the subsequent form: X + Y → XY. The combination of two or more elements to construct one compound is named a combination reaction. In alternative words, when two or more elements or compounds react so as to construct one single compound, then the chemical reaction that takes place is hailed a combination reaction.

  1. Between elements C + O2 → CO2. Carbon finally burnt in oxygen yields carbon dioxide.
  2. Between compounds CaO + H2O → Ca(OH)2. Calcium oxide (lime) mixed with water gives calcium hydroxide (slaked lime).
  3. Between elements and compounds 2CO + O2 → 2CO2. Oxygen combines with carbon monoxide, And carbon dioxide is formed

There is no definitive number of reactants in a combination reaction.

What is a Combination Reaction?

A combination reaction is a reaction in which two or more reactants chemically bond and merge to form a product. A synthesis reaction is also familiar with a combination reaction. Most synthesis reactions are exothermic reactions, i.e., heat is removed during the reaction.

Combination Reaction
Combination Reaction

General Equation

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

A + B → AB

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

What are the types of combination reactions?

Combination reactions are of three types.

Similarly, non-metals may perform with highly active metals to form covalent compounds. For example, sulphur reacts with oxygen gas to form gaseous sulphur dioxide

1.The reaction between two or more elements:

An example of this type of combination reaction is the reaction between a metal and a non-metal. Most metals react with non-metals to form ionic compounds. A good example of this would be:

2. The reaction between elements and compounds:

An element and a compound react to form another compound. For example, carbon monoxide reacts with oxygen gas to form carbon dioxide.

3. The reaction between two compounds:

Two compounds react with each other to form a new compound. For example, calcium oxide (quick lime) reacts with carbon dioxide gas to form calcium carbonate (limestone).

Most combination reactions are exothermic in nature. Why?

Combination reactions commit the formation of new bonds and this process releases a large amount of energy in the form of heat.

Let us consider some more examples of combination reactions.

  • Formation of Calcium Hydroxide: Reaction between quick lime (Calcium oxide, CaO) and water is a combination reaction. In this reaction, quick lime reacts with water to compose slaked lime (calcium hydroxide, Ca(OH)2). The reaction between quick lime and water is eminently vigorous as well as exothermic.
  • Combustion Reactions: Combustion of coal and combustion of hydrogen are illustrations of combination reactions. Coal burns in the air to form carbon dioxide gas.
  • Hydrogen burns in the existence of oxygen to form water in the form of steam. Upon cooling, it becomes liquid.
  • Formation of Ammonium chloride: Ammonium chloride is formed by combining vapours of ammonia with hydrogen chloride gas. It is a white-coloured solid.
  • Formation of Sulphuric acid: The formation of sulphuric acid from sulphur trioxide is also a combination reaction. Sulphur trioxide on hydration forms sulphuric acid. This reaction is highly exothermic in nature.
  • Formation of ferrous sulphide: It is formed by heating fine pieces of iron with sulphur powder.
  • Manufacture of Ammonia: The manufacture of ammonia gas from nitrogen and oxygen is also a combination reaction.
  • Combination of sodium oxide and water: Sodium oxide combines with water to form sodium hydroxide.

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 manufacturing, the synthesis reaction forms a significant part in the synthesis of 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

In a Nuclear Reaction, Energy is Released by the Combination:

A nuclear reaction refers to the process in which the nucleus of an atom undergoes a change, leading to the release of energy. One type of nuclear reaction is a combination reaction, where two or more atomic nuclei combine to form a heavier nucleus. This process is often accompanied by the release of a large amount of energy.

Example: One well-known combination nuclear reaction is the fusion of hydrogen nuclei (protons) to form helium in the core of the Sun. This process releases an enormous amount of energy in the form of light and heat, providing the Sun with its energy source.

Solution: Scientists have been exploring the potential of harnessing nuclear fusion reactions as a clean and abundant source of energy. However, achieving controlled fusion reactions is still a significant challenge due to the high temperatures and pressures required. Research and development in this field aim to find ways to replicate the fusion reactions occurring in stars, such as the Sun, to generate clean and sustainable energy.

Combination Chemical Reaction:

A combination chemical reaction is a type of reaction where two or more substances combine to form a new compound. In this reaction, the reactants combine to create a single product. Combination reactions are typically characterized by the absence of any decomposition or displacement of elements or compounds.

Example: The reaction between iron and sulfur to form iron sulfide is an example of a combination chemical reaction:

Fe (s) + S (s) → FeS (s)

In this reaction, iron (Fe) reacts with sulfur (S) to form iron sulfide (FeS).

Solution: Combination reactions can be useful in the synthesis of various compounds. They are often employed in the laboratory to produce specific substances, such as pharmaceuticals, polymers, or dyes. Understanding the conditions and reactants required for a successful combination reaction allows chemists to design and control the synthesis of desired compounds.

Combination Reaction Examples:

Combination reactions occur in various chemical and biological processes. Here are a few examples:

a. Formation of Water:
2H₂ (g) + O₂ (g) → 2H₂O (l)

In this reaction, hydrogen gas (H₂) combines with oxygen gas (O₂) to produce water (H₂O).

b. Formation of Rust:
Fe (s) + O₂ (g) + H₂O (l) → Fe₂O₃·xH₂O (s)

When iron (Fe) reacts with oxygen (O₂) and water (H₂O), it forms hydrated iron(III) oxide, commonly known as rust.

c. Formation of Ammonium Nitrate:
NH₃ (g) + HNO₃ (l) → NH₄NO₃ (aq)

Ammonia gas (NH₃) reacts with nitric acid (HNO₃) to produce ammonium nitrate (NH₄NO₃), which is commonly used as a fertilizer.

10 Examples of Combination Reaction:

Here are ten additional examples of combination reactions:

a. Magnesium reacting with oxygen:
2Mg (s) + O₂ (g) → 2MgO (s)

b. Sodium reacting with chlorine:
2Na (s) + Cl₂ (g) → 2NaCl (s)

c. Carbon reacting with oxygen:
C (s) + O₂ (g) → CO₂ (g)

d. Hydrogen reacting with chlorine:
H₂ (g) + Cl₂ (g) → 2HCl (g)

e. Calcium reacting with nitrogen:
3Ca (s) + N₂ (g) → Ca₃N₂ (s)

f. Lithium reacting with nitrogen:

6Li (s) + N₂ (g) → 2Li₃N (s)

g. Phosphorus reacting with chlorine:
P₄ (s) + 6Cl₂ (g) → 4PCl₃ (l)

h. Aluminum reacting with oxygen:
4Al (s) + 3O₂ (g) → 2Al₂O₃ (s)

i. Hydrogen reacting with sulfur:
H₂ (g) + S (s) → H₂S (g)

j. Carbon monoxide reacting with oxygen:
2CO (g) + O₂ (g) → 2CO₂ (g)

Combination Reaction Notes:

In combination reactions, two or more reactants come together to form a single product. Here are some important notes about combination reactions:

– Combination reactions are characterized by the absence of any displacement or decomposition of elements or compounds.
– They often involve the combination of elements to form compounds or the combination of compounds to form more complex compounds.
– Combination reactions can be exothermic, meaning they release energy, or endothermic, requiring an input of energy to proceed.
– The reactants in a combination reaction may be either elements or compounds.
– Combination reactions are represented by the general chemical equation: A + B → AB, where A and B are the reactants, and AB is the product.

Combination Reaction Formula:

The general formula for a combination reaction can be expressed as follows:

A + B → AB

In this formula, A and B represent the reactants, and AB represents the product formed by their combination.

For example, in the reaction between hydrogen gas (H₂) and oxygen gas (O₂) to form water (H₂O):

H₂ + O₂ → 2H₂O

In this reaction, hydrogen and oxygen combine to produce water molecules. The coefficient “2” in front of H₂O indicates that two molecules of water are formed for every two molecules of hydrogen and one molecule of oxygen.

Note: It is important to balance the coefficients in a chemical equation to ensure the conservation of atoms and mass.


  • Students achieve knowledge about combination reactions.
  • Students achieve skills to perform a combination reaction using quick lime and water.
  • Students will be able to distinguish a combination reaction from a given set of chemical reactions.

Frequently Asked Questions About Combination Reaction

Why Is A Decomposition Reaction Hailed The Opposite Of A Combination Reaction?

A Decomposition Reaction Is One When A Substance Decays Into Two Or More Products. This Reaction Is The Opposite Of What A Combination Does.

Can A Combination Reaction Be A Redox Reaction?

In Case, Most Common Oxidation-reduction (Redox) Reactions Are Combination Reactions.

Can A Combination Reaction Be An Oxidation Reaction?”]A Combination Reaction Can Be An Oxidation Reaction If One Of The Reactants Is Oxygen.

Can The Combination Reaction Be Endothermic?

While All Combination Reactions Are Exothermic, There Can Be An Exemption. The Production Of Nitric Oxide (No) From Nitrogen And Oxygen Is An Endothermic Reaction.

What Is A Dehydration Synthesis Reaction?

Dehydration Synthesis Is The Formation Of Larger Molecules From Smaller Reactants, Followed By The Loss Of A Water Molecule.

How Do Hydrolysis Reactions Compare To Dehydration Synthesis Reactions?

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.

Which Combination of Factors Will Affect the Rate of the Following Reaction?

Several factors can influence the rate of a combination reaction:

a. Concentration: Increasing the concentration of reactants generally leads to a higher reaction rate. This is because a higher concentration provides more reactant particles, increasing the chances of successful collisions.

b. Temperature: Higher temperatures generally increase the rate of a combination reaction. As temperature rises, the kinetic energy of the reactant particles increases, leading to more frequent and energetic collisions.

c. Surface area: If a solid reactant is involved, increasing its surface area can enhance the reaction rate. A larger surface area provides more contact points for reactant particles, allowing for more frequent collisions.

d. Catalysts: Catalysts are substances that can speed up a reaction without being consumed in the process. They lower the activation energy required for the reaction to occur, thereby increasing the rate. Catalysts provide an alternative reaction pathway with lower energy barriers, facilitating the formation of products.

e. Pressure (for gaseous reactions): In the case of gaseous reactants, increasing the pressure can influence the reaction rate. Higher pressure results in a greater number of particles in a given volume, leading to more frequent collisions.

Which of the Following General Reactions Appropriately Models a Combination Reaction?

The appropriate general reaction that models a combination reaction is:

A + B → AB

In this reaction, the reactants A and B combine to form a single product, AB. This represents a combination reaction where two or more substances unite to create a new compound.

What Is the Product S in This Combination Reaction?

To provide the product S in a combination reaction, the specific reaction needs to be provided. Without the reaction, it is not possible to determine the product S.

What Is the Reverse of a Combination Reaction?

The reverse of a combination reaction is a decomposition reaction. In a decomposition reaction, a compound breaks down into two or more simpler substances. It is the opposite of a combination reaction, where multiple substances combine to form a compound.

For example, the reverse of the combination reaction:

A + B → AB

Would be:

AB → A + B

In this decomposition reaction, the compound AB breaks down into its constituent elements A and B.

What Is a Combination Reaction and Example?

A combination reaction refers to a chemical reaction in which two or more substances combine to form a new compound. It is characterized by the absence of any displacement or decomposition reactions.

Example: The reaction between hydrogen gas (H₂) and oxygen gas (O₂) to form water (H₂O) is an example of a combination reaction:

2H₂ + O₂ → 2H₂O

In this reaction, two molecules of hydrogen gas react with one molecule of oxygen gas to produce two molecules of water.

What Is the Combination Reaction?

A combination reaction is a type of chemical reaction where two or more substances combine to form a single product. It can involve the combination of elements to form compounds or the combination of compounds to form more complex compounds. The general form of a combination reaction can be represented as:

A + B → AB

Where A and B are the reactants, and AB is the product formed by their combination.

What Is a Combination Reaction with 10 Examples?

Here are ten examples of combination reactions:

1. 2H₂ + O₂ → 2H₂O (formation of water)
2. 2Na + Cl₂ → 2NaCl (formation of sodium chloride)
3. CaO + CO₂ → CaCO₃ (formation of calcium carbonate)
4. 2Mg + O₂ → 2MgO (formation of magnesium oxide)
5. N₂ + 3H₂ → 2NH₃ (formation of ammonia)
6. C + O₂ → CO₂ (formation of carbon dioxide)
7. 4Fe + 3O₂ → 2Fe₂O₃ (formation of iron(III) oxide)
8. 2SO₂ + O₂ → 2SO₃ (formation of sulfur trioxide)
9. C₃H₈ + 5O₂ → 3CO₂ + 4H₂O (combustion of propane)
10. 2H₂ + S → 2H₂S (formation of hydrogen sulfide)

In each of these examples, two or more substances combine to form a new compound.