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Types of Chemical Reactions

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Types of Chemical Reactions - Lesson Summary

In general chemical reactions are classified into:

  •    Combination reactions
  •    Decomposition reactions
  •    Single displacement reactions
  •    Double displacement reactions


Combination reactions:
A combination reaction is one in which two or more reactants combine to form a single product. Combination reactions are again of three types.

They are:
Combination reactions between elements.
Combination reactions between compounds.
Combination reactions between elements and compounds.

Combination reactions between elements:
In these reactions elements were combined to form a product.

Examples :
Formation of calcium oxide by the combination elements calcium and oxygen.
2Ca + O→ 2CaO
Formation of ammonia by the combination of elements nitrogen and hydrogen.
N2 + 3H2 → 2NH3

Combination reactions between compounds:
In these reactions compounds were combined to form product.

Example:
Reaction of calcium oxide in water to form calcium hydroxide
CaO + H2O  → Ca(OH)2

Combination reactions between elements and compounds:
In these reactions elements and compounds combined to form product.

Example:
Formation of sulphur trioxide by the combination of sulphur dioxide and oxygen.
2SO2 + O2   2SO3

Decomposition reaction:
Decomposition reactions are those in which a substance splits into two or more simpler substances. Decomposition reactions are the opposite of combination reactions.  Decomposition reactions are of three types:

  •    Thermal decomposition
  •    Electrolysis
  •    Photolysis


Decomposition by the application of heat on a substance is called thermal decomposition. 
Example: 
Thermal decomposition of calcium carbonate.
CaCO3 + Δ (Heating) → CaO + CO2

Decomposition of any substance by passing current through it is called electrolysis. 
Example: 
Electrolysis of water 

2H2O → 2H2 + O2

The decomposition reaction resulting from action of light energy is called photolysis. 
Example: 
Photolysis of silver chloride
2AgCl  → 2Ag + Cl2

Single displacement reaction: 
Single displacement reaction is the one in which one element substitutes or displaces another element in a compound to give new compound. Generally in a single displacement reaction,  less active metal is displaced by a more active metal.

It is important to note that in a single displacement reaction, one of the reactants is always an element. 
Example: 
The reaction of magnesium with copper chloride
Mg + CuCl2 →  MgCl2 + Cu

Double displacement reaction:
The reactions which involves exchange of ions (cations and anions) between the reactants are called double displacement reactions.
AB + CD → AC + BD
Example:


Double displacement reactions classified into different types. They are: 

  • Neutralization reactions
  • Precipitation reactions
  • Gas forming reactions


Neutralization reaction:
Reaction in which hydrogen ions of an acid react with hydroxide ions of a base to form water is an neutralization reaction. 
Example :

The reaction of hydrochloric acid with sodium hydroxide.
H+Cl  + Na+OH → Na+Cl + H2O

Precipitation reaction:
Reactions which proceeds through the formation of precipitation are called precipitation reactions.
Example:
In the reaction of barium chloride with sodium sulphate produces precipitate of barium sulphate along with formation of sodium chloride.
BaCl2 (aq) + Na2SO4 (aq) → BaSO(s) + NaCl (aq)

Gas forming reaction:
In these reactions gas is produces as one of the product during reaction.



Example:
Na2CO3 (s) + 2HCl (aq) → 2NaCl (aq) + H2O (l) + CO2 (g)


Based on energy differeneces between reactants and products the chemical reactions are classified into two types. They are exothermic reaction and endothermic reaction.

Exothermic reaction:
Chemical reactions in which heat energy released are known as exothermic reactions.
Burning of methane gas in air produces heat energy.
CH4 (g) + 2O(g) → CO(g) +  2H2O (l)+ heat
All combustion reactions are exothermic reactions.

Endothermic reaction:
Reactions which involves absorption of energy are known as endothermic reactions.
Example:
Formation of nitric oxide from nitrogen and oxygen.
N2 (g)+ O2 (g) + Heat → 2NO 


Combustion reaction:
Burning of a substance in the presence of oxygen which resulting release of energy is called combustion.
Example:
Combustion of ethylene gas.
C2H4 + 3O2 → 2CO2 + 2H2O

Oxidation reaction: 
Oxidation reaction involves the addition of oxygen or the removal of hydrogen from a substance.
Example: 
Rusting of iron

Rusting of iron:
Iron when reacted with both water and oxygen are present (moist air), corrodes. Its silvery colour changes to a reddish-brown, because hydrated oxides are formed which is commonly called as rust.
Reaction showing the formation of rust when reacted with water in presence of oxygen (atmosphere).                        
4Fe + 3O₂+ XH₂O → 2Fe₂O₃.XH₂O

Reduction reaction:
The reaction which involves addition of hydrogen or removal of oxygen from a substance is called reduction reaction.
Example: 
Photosynthesis is a reduction reaction.

Redox reaction:
The reactions in which both oxidation and reduction occurring together are known as redox reactions.
Example:
CuO + H2 → Cu + H2O


The above is a redox reaction as copper is reduced and hydrogen is oxidised.

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