Question 14 Marks
Those reactions in which two compounds react by an exchange of ions to form two new compounds are called double displacement reactions. A double displacement reaction usually occurs in solution and one of the products, being insoluble, precipitate out (separates as a solid). Any reaction in which an insoluble solid (called precipitate) is formed that separates from the solution is called a precipitation reaction. The reaction in which acid or acidic oxide reacts with base or basic oxide to form salt and water is called neutralisation reaction. For example, $2 \mathrm{NaOH}+\mathrm{H}_{2} \mathrm{SO}_{4} \longrightarrow \mathrm{Na}_{2} \mathrm{SO}_{4}+\mathrm{H}_{2} \mathrm{O}$
i. When hydrogen sulphide gas is passed through a blue solution of copper sulphate, a black precipitate of copper sulphide is obtained and the sulphuric acid so formed remains in the solution. Mention the type of reaction.
ii. Balance the following chemical reaction.
$\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2(\mathrm{aq})}+\mathrm{KI}_{(\mathrm{aq})} \longrightarrow \mathrm{PbI}_{2(\mathrm{~s})}+\mathrm{KNO}_{3(\mathrm{aq})}$
iii. Barium chloride in reaction with ammonium sulphate forms barium sulphate and ammonium chloride. Which type of chemical reaction represents in this reaction?
OR
Identify A in the following reaction.
$\mathrm{AlCl}_{3(\mathrm{aq})}+3 \mathrm{NH}_{4} \mathrm{OH}_{(\mathrm{aq})} \longrightarrow \mathrm{A}+3\mathrm{NH}_{4} \mathrm{Cl}_{(\mathrm{aq})}$
i. When hydrogen sulphide gas is passed through a blue solution of copper sulphate, a black precipitate of copper sulphide is obtained and the sulphuric acid so formed remains in the solution. Mention the type of reaction.
ii. Balance the following chemical reaction.
$\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2(\mathrm{aq})}+\mathrm{KI}_{(\mathrm{aq})} \longrightarrow \mathrm{PbI}_{2(\mathrm{~s})}+\mathrm{KNO}_{3(\mathrm{aq})}$
iii. Barium chloride in reaction with ammonium sulphate forms barium sulphate and ammonium chloride. Which type of chemical reaction represents in this reaction?
OR
Identify A in the following reaction.
$\mathrm{AlCl}_{3(\mathrm{aq})}+3 \mathrm{NH}_{4} \mathrm{OH}_{(\mathrm{aq})} \longrightarrow \mathrm{A}+3\mathrm{NH}_{4} \mathrm{Cl}_{(\mathrm{aq})}$
Answer
View full question & answer→i. Double displacement reaction
$\mathrm{CuSO}_{4}+\mathrm{H}_{2} \mathrm{~S} \longrightarrow 4 \mathrm{CuS}+\mathrm{H}_{2} \mathrm{SO}_{4}$
Both $\mathrm{CuSO}_{4}$ and $\mathrm{H}_{2} \mathrm{~S}$ exchange their ions to give new compounds- CuS and $\mathrm{H}_{2} \mathrm{SO}_{4}$. Hence, this is a double displacement reaction.
ii. $\mathrm{Zn}_{(\mathrm{s})}+\mathrm{H}_{2} \mathrm{SO}_{4(\mathrm{aq})} \longrightarrow \mathrm{ZnSO}_{4(\mathrm{aq})}+\mathrm{H}_{2(\mathrm{~g})}$
It is an example of single displacement reaction.
iii. $\mathrm{BaCl}_{2}+\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4} \rightarrow \underset{(\text { ppt. })}{\mathrm{BaSO}_{4}} \downarrow+2 \mathrm{NH}_{4} \mathrm{Cl}$
It is a precipitation reaction as well as double displacement reaction.
OR
$\mathrm{AlCl}_{3}+3 \mathrm{NH}_{4} \mathrm{OH} \longrightarrow \mathrm{Al}(\mathrm{OH})_{3}+3 \mathrm{NH}_{4} \mathrm{Cl}$
$\mathrm{CuSO}_{4}+\mathrm{H}_{2} \mathrm{~S} \longrightarrow 4 \mathrm{CuS}+\mathrm{H}_{2} \mathrm{SO}_{4}$
Both $\mathrm{CuSO}_{4}$ and $\mathrm{H}_{2} \mathrm{~S}$ exchange their ions to give new compounds- CuS and $\mathrm{H}_{2} \mathrm{SO}_{4}$. Hence, this is a double displacement reaction.
ii. $\mathrm{Zn}_{(\mathrm{s})}+\mathrm{H}_{2} \mathrm{SO}_{4(\mathrm{aq})} \longrightarrow \mathrm{ZnSO}_{4(\mathrm{aq})}+\mathrm{H}_{2(\mathrm{~g})}$
It is an example of single displacement reaction.
iii. $\mathrm{BaCl}_{2}+\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4} \rightarrow \underset{(\text { ppt. })}{\mathrm{BaSO}_{4}} \downarrow+2 \mathrm{NH}_{4} \mathrm{Cl}$
It is a precipitation reaction as well as double displacement reaction.
OR
$\mathrm{AlCl}_{3}+3 \mathrm{NH}_{4} \mathrm{OH} \longrightarrow \mathrm{Al}(\mathrm{OH})_{3}+3 \mathrm{NH}_{4} \mathrm{Cl}$