Difference between Combination and Decomposition Reactions. - Future Study Point

Difference between Combination and Decomposition Reactions.

Difference;Combination and decomposition reaction

Difference between Combination and Decomposition Reactions.

Difference;Combination and decomposition reaction

During a chemical reaction when two or more reactants form a single product then such a reaction is known as a combination reaction while a decomposition reaction is the type of reaction in which a single reactant breaks down into two or more products. An example of a combination reaction is, that calcium oxide reacts with water to form calcium hydroxide and an example of a decomposition reaction is, that calcium carbonate when heated break down into calcium oxide and carbon dioxide.

Table of content

  • Combination reaction
  • Examples of Combination reaction
  • Exothermic reaction
  • Decomposition reaction
  • Endothermic reaction
  • Types of decomposition reactions
  • Example of decomposition reaction

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Displacement and Double displacement Reactions

Combination reaction:

A combination reaction is the type of reaction in which two or more reactants forms a single product. In simple language, we can say that when two or more substances(elements or compounds) combine to form a single product, the reaction is called a combination reaction.

Examples of Combination reactions:

1-Calcium oxide reacts vigorously with water to produce slaked lime(calcium hydroxide) releasing a large amount of heat.

CaO(s) + H2O(l) → Ca(OH)2(aq) + Heat

In this reaction calcium oxide and water combine to form a single product calcium hydroxide.

2-Burning of coal

C(s) + O2 (g)→ CO2(g)

3- Hydrogen and oxygen combine to form water

2H2(g) + O2 (g)→ 2H2O(l)

Exothermic reaction:

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We observe that in most of the combination reactions heat is also evolved with the formation of the product. Reactions in which heat is also released along with the product formation are called exothermic chemical reactions.

Examples of exothermic chemical reactions:

1-All combustion reactions are exothermic chemical reactions because the coal, petrol, CNG, and LPG get fired at their ignition temperature and release a lot of heat.

CH4+ 2O2→ CO2+ 2H2O +Heat

2- Respiration reaction is an exothermic reaction because glucose combines with oxygen forms carbon dioxide, and water and releases heat(energy in the form of ATP).

C6H12O6 +6O2→ 6CO2+ 6H2O +ATP(Heat)

3- Vegetable oil, and compost of the remainder of animals and plants are decomposed into simpler molecules and release heat therefore these reactions are also examples of exothermic reactions.

Decomposition reaction:

A decomposition reaction is a chemical reaction in which one substance is breakdown into 2 or more substances known as a decomposition reaction.

Endothermic reaction:

It can be observed that most of the decomposed reactions are needed a certain amount of energy to break down the reactant. The type of reactions in which heat is absorbed are known as endothermic chemical reactions.

Types of decomposition reactions:

1-Thermal decomposition reaction: The type of reaction in which reactants absorb heat during the formation of the products is known as the thermal decomposition reaction.


(i) Crystal of ferrous sulfate when heated, its green colour turns to white and then to brown, it occurs because after heating ferrous sulphate crystal(FeSO4.7H2O) loses water and anhydrous FeSO4 is formed then on further heating it is decomposed into ferric oxide with the release of sulphur dioxide and sulphur trioxide gases..

(ii)On heating calcium carbonate it decomposes into calcium oxide and carbon dioxide.

(iii)On heating lead nitrate it decomposes into lead oxide, nitrogen dioxide and oxygen.

Electrolytic decomposition reaction: The type of decomposition reaction which is needed electrical energy for its decomposition is known as an electrolytic decomposition reaction. As an example, if water is passed through the process of electrolysis, it is decomposed into hydrogen and oxygen as below, hydrogen gas is separated at the cathode and oxygen is at the anode.

electrolysis of water

Photo decomposition reaction:

White silver chloride turns grey in presence of the sunlight, this is due to the decomposition of silver chloride into silver and chlorine by the light.

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