Double Displacement Reaction
Reactions in which ions are exchanged between two reactants are called double displacement reactions.
In other words, in case of double displacement reaction, two compounds displace each other's ions. You may recall that in case of single displacement reaction, a more reactive metal displaced a less reactive metal from a compound. But in double displacement reaction displacement takes place from both the reactants.
AB + CD → AC + BD
In this imaginary example, AB and CD are reactants. Both the compounds displace ions from each other to form new compounds. In this case, products are AC and BD.
More Examples
Example 1: When solution of barium chloride reacts with the solution of sodium sulphate, white precipitate of barium sulphate is formed along with sodium chloride.
BaCl2 + Na2SO4 → BaSO4 + 2NaCl
Example 2: When sodium hydroxide (a base) reacts with hydrochloric acid, sodium chloride and water are formed.
NaOH + HCl → NaCl + H2O
Double displacement reaction, in which precipitate is formed, is also known as precipitation reaction.
Neutralisation reactions are also examples of double displacement reaction.
Exothermic and Endothermic Reaction
Reactions which produce energy are called exothermic reaction. On the other hand, reactions which absorb energy are called endothermic reaction.
Most of the combination reactions are exothermic. Most of the decomposition reactions are endothermic. Respiration is a combination reaction in which energy is released.
Example of Combination Reaction: When quick lime (calcium oxide) is added to water, it combines to form calcium hydroxide and releases energy. Cooking involves chemical reactions which are endothermic as cooking is possible because of heating.
Summary
Double Displacement: Reaction in which ions are exchanged between two reactants.
Exothermic Reaction: Reaction in which heat is evolved.
Endothermic Reaction: Reaction in which heat is absorbed.