Question

The following four solutions are kept in separate beakers and copper metal is put in each of them. Which solution will become blue after some time
(A) $AgN{O_3}$solution
(B) $Zn{(N{O_3})_2}$ solution
(C) $Ba{(N{O_3})_2}$ solution
(D) $NaN{O_3}$ solution

Answer 1

Hint: The feasibility of a reaction is described according to the chemical reactivity series. Solution will turn blue only which reacts with copper to form copper nitrate that is blue in colour.

Complete Step by Step Solution:
In the chemical reactivity series, the elements are arranged in the decreasing order of chemical reactivity. Thus, the element placed higher in the chemical reactivity series is more reactive and displaces the elements lower in the chemical reactivity series.

Thus, it is correct to say that copper will displace the metal from its solution which is placed below copper in the chemical reactivity series. $Zn,Ba,Na$are placed above copper hence, have greater reactivity so copper cannot displace them in their solution. But copper can displace silver from the $AgN{O_3}$ solution.
$Zn{(N{O_3})_2} + C{u^{ + 2}} \to Cu{(N{O_3})_2} + Z{n^{ + 2}}$
$Ba{(N{O_3})_2} + C{u^{ + 2}} \to Cu{(N{O_3})_2} + B{a^{ + 2}}$
\[NaN{O_3} + C{u^{ + 2}} \to Cu{(N{O_3})_2} + N{a^ + }\]

The above written reactions are not feasible hence, adding copper metal to these solutions will not turn solution blue. Thus, the blue colour of the solution is because of the reaction
\[AgN{O_3} + C{u^{ + 2}} \to Cu{(N{O_3})_2} + A{g^ + }\]
Hence, the correct answer is A.

Note: A feasible reaction is one which happens spontaneously in a practical time period without supply of extra energy. The spontaneity criteria for a reaction is given by Gibbs free energy which is negative for a spontaneous reaction. Kinetics play a crucial role in defining feasibility of a reaction. The conversion of graphite to diamond is a feasible reaction as it has a negative value of Gibbs free energy but it takes an infinite time hence the reaction is not possible.