Question

An example of double-displacement reaction is:
A. \[BC{l_3} + C{l^ - } \to BCl_4^ - \\ \]
B .\[HCN + {H_2}O \to {H_3}{O^ + } + C{N^ - }\\ \]
C. \[{H_2}{O_2} + 2B{r^ - } + 2{H^ + } \to 2{H_2}O + B{r_2}\\ \]
D. \[2NaCl + CaS{O_4} \to N{a_2}S{O_4} + CaC{l_2}\\ \]

Answer 1

Hint:
We know that, the chemical reaction in which two separate atoms are displaced by other atoms or groups of atoms or in which two compounds react by exchanging or displacing ions to form new compounds are called double displacement reactions.

Complete step by step solution
Double displacement reaction is classified as Neutralization reaction and Precipitation reaction.
- Precipitation reaction
This type of reaction is formed when two soluble salts are combined in aqueous solution. And in one of them the product is insoluble salt which is known as precipitate. When two or more compounds react in solution one of the products is not soluble in water, it forms a solid precipitate which can be filtered and collected. For example,
\[AgN{O_3} + NaCl \to AgCl\left( {ppt} \right) + NaN{O_3}\]
- Neutralization reaction
The reaction between an acid and a base producing water and salt is called a neutralization reaction. The acid and base neutralize the acidity and basicity of each other and therefore it is termed as neutralization. For example,
\[HCl + NaOH \to NaCl + {H_2}O\]

Precipitation is not mandatory for all double displacement reactions, as there are many such reactions in which one reactant is acid and another is base. The acid reacts with base and forms salt and water which is found dissolved in water that is formed.
\[{H_2}S{O_4} + 2KOH \to {K_2}S{O_4} + 2{H_2}O\]

Now, we consider an example here to discuss the double displacement reaction. If \[NaCl\] react with \[CaS{O_4}\], \[Na\] combine with \[S{O_4}\] then it gives \[N{a_2}S{o_4}\]. And when \[Ca\] molecule combines with \[Cl\] then it gives \[CaC{l_2}\]. Here, we see that chlorine molecule is replaced by sulphate molecule and replacement of two molecules is also known as double displacement reaction. And this condition is fulfilled in the given reaction. The reactions in other options do not fulfill this criterion. So we can say that it is a type of double displacement reaction.
\[2NaCl + CaS{O_4} \to N{a_2}S{O_4} + CaC{l_2}\]

Therefore, the correct answer is D.

Note:
Displacement reactions can also be a redox reaction, as it often includes simultaneous oxidation and reduction .