: 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.
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.
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. 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.
Oxidation: Addition of oxygen or non-metallic element or removal of hydrogen or metallic element from a compound is known as oxidation. Elements or compounds in which oxygen or non-metallic element is added or hydrogen or metallic element is removed are called to be oxidized. So, oxidation can involve any of the following:
Oxidizing agent: Compounds which can add oxygen or a non-metallic compound or remove hydrogen or metallic element are known as oxidizing agents.
Reduction: Addition of hydrogen or metallic element or removal of oxygen or non-metallic element from a compound is called reduction. The compound or element which goes under reduction is called to be reduced. So, reduction can involve any of the following:
To make it easier to understand, here is table to show comparision between oxidation and reduction.
|Addition of oxygen||Addition of hydrogen|
|Addition of non-metallic element||Addition of metal|
|Removal of hydrogen||Removal of oxygen|
|Removal of metallic element||Removal of non-metallic element|
Reducing agent: Compounds or elements which can cause reduction are called reducing agents.
In a chemical reaction, both oxidation and reduction take place simultaneously and such reactions are also known as REDOX REACTIONS. In the word REDOX, Red stands for reduction and Ox stands for oxidation.
Example: When iron reacts with air, it forms iron oxide (rust)
4Fe + 3O2 ⇨ 2Fe2O3
In this reaction, oxygen is added to iron, thus, iron is oxidized. Here oxygen is oxidizing agent.
Example: When cupric oxide reacts with hydrogen, it gives copper and water.
CuO + H2 ⇨ Cu + H2O
In this reaction, oxygen is removed from copper and oxygen is added to hydrogen. So, cupric oxide is reduced to copper and hydrogen is oxidized to water. Cupric oxide is oxidizing agent and hydrogen is reducing agent.
Example: When sodium hydroxide reacts with hydrochloric acid, it gives sodium chloride and water.
NaOH + HCl ⇨ NaCl + H2O
In this reaction, sodium hydroxide is reduced to sodium chloride since hydrogen is removed from sodium hydroxide. Hydrochloric acid is oxidized to water, since oxygen is added to hydrogen chloride and non-metallic element chloride is removed. Sodium hydroxide is oxidising agent and hydrochloric acid is reducing agent.
In this reaction oxidation and reduction both takes place simultaneously, thus it is an example of redox reaction.
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