Question

Give an example of a Double displacement reaction, where a gas is evolved.

Answer 1

Double displacement reactions are those reactions in which two different compounds react with each other and exchange their ions to form two new compounds. There are two types of double displacement reactions, namely precipitation reaction and neutralization reaction.

Complete answer:
An example of a double displacement reaction where a gas is evolved is shown below.
$BaCl(s)+{{H}_{2}}S{{O}_{4}}(aq)\xrightarrow{{}}BaS{{O}_{4}}(aq)+HCl(g)$
The reaction shown above is a double displacement reaction, barium chloride ($BaCl$) reacts with sulphuric acid (${{H}_{2}}S{{O}_{4}}$) to give barium sulphate ($BaS{{O}_{4}}$) dissolved in the solution and evolved gas hydrochloric acid ($HCl$).
In this reaction, chlorine from barium chloride ($BaCl$) is replaced with sulphate ($S{{O}_{4}}$) and sulphate from sulphuric acid (${{H}_{2}}S{{O}_{4}}$) is replaced with chlorine.
A double displacement reaction is a reaction in which two different compounds exchange their ions. The above shown example can be accounted for by the precipitation reaction of barium sulphate ($BaS{{O}_{4}}$).
Another example of a double displacement reaction in which a gas is evolved is shown below.
$N{{a}_{2}}S+2HCl\xrightarrow{{}}2NaCl+{{H}_{2}}S\uparrow$
In above given reaction, sodium sulfate ($N{{a}_{2}}S$) reacts with hydrochloric acid ($HCl$) to form sodium chloride ($NaCl$) and hydrogen sulfide (${{H}_{2}}S$). Hydrogen sulfide is evolved as a gas in this reaction.
In this reaction, sodium from sodium sulfur ($N{{a}_{2}}S$) is replaced with chlorine, and chlorine from hydrochloric acid ($HCl$) is replaced with sulphur. Thus, two different compounds replace their ions with others to form two different compounds. And thus, this is called a double displacement reaction.

Note:
Many times, double displacement reaction occurs between ionic compounds in their aqueous solutions. The general reaction of double displacement reaction can be given as follows, $AB+CD\xrightarrow{{}}AD+BC$ where B from AB is replaced by D, and D from CD is replaced by B.