Double Displacement Reaction
These notes are based on the chapter Chemical Reactions and Equations from NCERT book based on CBSE syllabus.
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.
Activity to show double displacement reaction
- Take about 3 mL of sodium sulphate solution in a test tube.
- Take about 3 mL of barium chloride solution in another tes tube.
- Pour the content of second test tube in the first test tube and mix both solutions.
Observation:
You will notice that the content of the test tube shows white precipitate. The white substance formed in this chemical reaction is barium sulphate.
Explanation: 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
More examples of double displacement reaction:
When sodium hydroxide (a base) reacts with hydrochloric acid, sodium chloride and water are formed.
NaOH + HCl → NaCl + H2O
When sodium hydroxide reacts with sulphuric acid then sodium sulphate and water are formed.
2NaOH + H2SO4 → Na2SO4 + 2H2O
Double displacement reaction, in which precipitate is formed, is also known as precipitation.
As the examples of reaction between sodium hydroxide and acids mentione above show, neutralisation reactions are good examples of double displacement reaction. Double displacement is always involved in neutralisation reaction.
Exothermic and Endothermic Reaction
A chemical reaction always involves exchange of heat between the reaction mixture and the environment. Heat is either absorbed or released during a chemical reaction. Based on this fact, there are two types of chemical 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.
Combustion is an example of exothermic reaction. Energy is released whenever something is burnt. We know that burning is another name for combustion.
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.
CaO + H2O → Ca(OH)2
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.