Question

When zinc is added to \[CuS{O_4}\] solution, copper is precipitated because:
A.\[C{u^{2 + }}\] is reduced
 B.\[C{u^{2 + }}\] is oxidised
 C.\[CuS{O_4}\] is ionised
 D.\[CuS{O_4}\] is hydrolysed

Answer 1

Hint: It is a displacement reaction when zinc reacts with copper sulphate and displaces copper from copper sulphate and forms zinc sulphate. It is a redox reaction as well because the oxidation number of copper changes from +2 to 0 and the oxidation number of zinc changes from 0 to +2.

Complete step-by-step answer:
If we are given the reactivity series or even if we are aware of it, we will know that zinc is more reactive than copper. So, when zinc is dipped in copper sulphate solution, being more reactive than copper, zinc easily displaces copper from its copper sulphate solution and make its own salt i.e. zinc sulphate which can be seen as precipitate at the bottom of the beaker.
Such reactions are called displacement reactions. In these reactions, more reactive metal replaces less reactive metal from its salt and makes its own salt solution. The colour of solution changes from blue of copper sulphate to green of zinc sulphate and we can see a layer of zinc sulphate near the piece of metal.
We can understand the mechanism of this reaction by the movement of electrons. Electrons from the surface of the zinc are transferred to copper (II) ions in solution. For a very small amount of zinc, we get partial reduction of Cu (II) to Cu (I), but when a large amount of zinc is added, copper gets deposited on the surface of the electrode.
Let us understand it now in terms of redox reaction. In a redox reaction, one compound is reduced while the other is oxidised. Here, we can write the chemical reaction as
 \[Zn(s) + CuS{O_4}(aq) \to ZnS{O_4}(aq) + Cu(s)\]
We can see from the equation that when zinc forms zinc sulphate, its oxidation state changes from zero to +2 and when copper sulphate changes from +2 to zero. The substance which is reduced in a reaction is called the oxidizing agent as it gains electrons. The substance which is oxidized in a reaction is called a reducing agent as it loses electrons. Here, copper gains electron, so we can say that \[C{u^{2 + }}\] is reduced whereas zinc loses two electrons to become \[Z{n^{2 + }}\] , then we say it is oxidized.
 \[C{u^{2 + }} + 2{e^ - } \to Cu\]
 \[Zn \to Z{n^{2 + }} + 2{e^ - }\]

Hence, the correct option is (A).

Note: The electrolysis of an aqueous solution of copper sulphate results in transfer of copper metal from anode to cathode and we say that copper sulphate is ionised in aqueous solution and the ions remain as it is in the solution. Hydrolysis of copper sulphate is not possible because it is already acidic in nature