Types of Chemical Reaction
On this page, you will read about two types of chemical reaction, viz. combination reaction and decomposition reaction. You will also read various types of decomposition reactions such as photolytic decomposition and electrolytic decomposition.
These notes are based on the chapter Chemical Reactions and Equations from NCERT book based on CBSE syllabus.
Chemical reactions can be classified as following types:
- Combination Reaction
- Decomposition Reaction
- Displacement Reaction
- Double Displacement Reaction
- Oxidation and Reduction Reaction
Combination Reaction
Reactions in which two or more reactants combine to form one product are called COMBINATION REACTION.
A general combination reaction can be represented by the chemical equation given here.
A + B → AB
In this imaginary reaction, A and B are reactants. They undergo combination reaction to form the product AB.
Example 1: When magnesium is burnt in air (oxygen), magnesium oxide is formed. In this reaction, magnesium combines with oxygen to give magnesium oxide.
Mg + O2 → 2MgO
Magnesium + Oxygen → Magnesium oxide
Example 2: When carbon is burnt in oxygen (air), carbon dioxide is formed. In this reaction, carbon combines with oxygen to give carbon dioxide.
C + O2 → CO2
Carbon + Oxygen → Carbon dioxide
Example 3: When hydrogen reacts with chlorine, hydrogen chloride is formed. Int this reaction, hydrogen combines with chlorine to give hydrogen chloride.
H2 + Cl2 → 2HCl
Hydrogen + Chlorine → Hydrogen chloride
Example 4: When calcium oxide reacts with water, calcium hydroxide is formed. In this reaction, calcium combines with hydroxide ion to give calcium hydroxide.
CaO + H2O → Ca(OH)2
Calcium oxide + Water → Calcium hydroxide
Example 5: When carbon monoxide reacts with oxygen, carbon dioxide is formed. In this reaction, carbon monoxide combines with oxygen to give carbon dioxide.
2CO + O2 → 2CO2
Carbon monoxide + Oxygen → Carbon dioxide
Decomposition Reaction
Reactions in which one compound decomposes in two or more compounds or element are known as DECOMPOSITION REACTION. Decomposition reaction is just opposite of combination reaction.
A general decomposition reaction can be represented as follows:
AB → A + B
In this imaginary reaction, AB is a reactant. It undergoes decomposition to give two products, i.e. A and B.
Example 1: When calcium carbonate is heated, it decomposes into calcium oxide and carbon dioxide
CaCO3 → CaO + CO2
Calcium carbonate → Calcium oxide + Carbon dioxide
Example 2: When ferric hydroxide is heated, it decomposes into ferric oxide and water
2Fe(OH)3 → Fe2O3 + 3H2O
Ferric hydroxide → Ferric oxide + Water
Example 3: When lead nitrate is heated, it decomposes into lead oxide, nitrogen dioxide and oxygen.
2Pb(NO3)2 → 2PbO + 4NO2 + O2
Lead nitrate → Lead oxide + Nitrogen oxide + Oxygen
In above examples, compound is decomposed because of heating, so, these reactions are called THERMAL DECOMPOSITION REACTION.
Electrolytic Decomposition
Reactions in which compounds decompose into simpler compounds because of passage of electricity, are known as ELECTROLYTIC DECOMPOSITION. In other words, decomposition because of electric current is called electrolytic decomposition. This is also known as ELECTROLYSIS.
Example 1: When electricity is passed in water, it decomposes into hydrogen and oxygen.
2H2O → 2H2 + O2
Photolysis or Photo Decomposition Reaction:
Reactions in which a compound decomposes because of sunlight are known as PHOTOLYSIS or PHOTO DECOMPOSITION REACTION. In other words, decomposition due to light is called photo-decomposition.
Example 2: When silver chloride is put in sunlight, it decomposes into silver metal and chlorine gas.
2AgCl → 2Ag + Cl2
Similarly, when silver bromide is put under sunlight, it decomposes into silver metal and bromine gas.
2AgBr → 2Ag + Br2
Photographic paper has coating of silver chloride, which turns into grey when exposed to sunlight. It happens because silver chloride is colourless while silver is a grey metal. In this era of digital cameras, many of you may not be aware about photographic film. Now-a-days, such films are used for taking X-ray of internal organs.