Question

In which of the following, displacement reaction is possible:
(A) Solution of ${ AgNO }_{ 3 }$+Coin of copper
(B) Solution of ${ MgCl }_{ 2 }$+Coin of Aluminium
(C) Solution of ${ FeSO }_{ 4 }$+Coin of Silver
(D) Solution of ${NaCl}$+Coin of copper

Answer 1

Hint: Single displacement reaction products can be easily predicted in accordance with the electrochemical series i.e. using the redox potentials of the different species involved.

Complete step by step solution:
First let us understand displacement reactions. Displacement reaction is a type of reaction in which an ion in a compound is displaced by an element (single displacement reaction) or an ion belonging to another compound (double displacement reaction).
Below is an example of single displacement reaction:
$ A-B\quad +\quad C\quad \longrightarrow \quad C-B\quad +\quad A $
Where A and C are different metal species and B is an anion or
 A and C are different halogen species and B is a cation.
Below is an example of double displacement reaction:
$A-B\quad +\quad C-D\quad \longrightarrow \quad A-D\quad +\quad C-B\quad$
Where A and C are cations while B and D are anions.
In the above question, all the reactions mentioned are single displacement reactions which are basically oxidation-reduction reactions wherein one species is oxidised and another is reduced. Not all chemical species can oxidise or reduce each other. Whether it will be oxidised or reduced by another species depends upon their redox potential which is a measure of the tendency of a species to accept or donate electrons to an electrode (generally the Standard hydrogen electrode) and thus be reduced or oxidised respectively. For determining which species will displace the other in single displacement reactions, we can use the electrochemical series in which the species are arranged in the order of decreasing oxidation potential:

“Li > Cs > Rb > K >Ba > Sr >Na > Ca > Mg > Be > Al > H( water) > Mn > Zn > Cr(+3)
> Fe(+2) > Cd > Co > Ni > Sn > Pb > H(in acids) > Cu > Fe(+3) >Hg >Ag
> Pd > Ir > Pt(+2) > Au”

As ${Cu}$ metal has a greater oxidation potential than ${Ag}$ metal, this implies that ${ Ag }^{ + }$ ion has a greater reduction potential than ${ Cu }^{ 2+ }$ ion and therefore ${ Ag }^{ + }$ ion has a greater tendency to get reduced and oxidised the ${Cu}$ metal to ${ Cu }^{ 2+ }$
Oxidation reaction: ${ Cu }\longrightarrow { Cu }^{ 2+ }(aq)+2{ e }^{ - }$
Reduction reaction:${ 2Ag^{ + }(aq)+2{ e }^{ - }\longrightarrow 2Ag(s) }$
Al metal cannot displace Mg from its salt solution since the oxidation potential of Mg is more than that of Al due to which Mg metal has a greater tendency to get oxidised than Al metal.
Ag metal cannot displace Fe from its salt solution since according to the above mentioned electrochemical series, the oxidation potential of Fe metal is more than that of Ag metal due to which Fe metal has a greater tendency to get oxidised and form ${ Fe }^{ 2+ }$ ion.
Similarly Cu metal cannot displace sodium from its salt solution since the oxidation potential of Na metal is more than that of Cu due to which Na metal has a greater tendency to get oxidised and form ${ Na }^{ + }$ ion.

Therefore the correct answer is (A) ${ AgNO }_{ 3 }$+coin of copper.

Note: The redox potentials are actually the reduction potentials of the species and not the oxidation potentials so if the redox potential of a species is given, it is actually its reduction potential. Also if the redox potential of a species is more negative than the other, then the former has a less tendency to get reduced and the latter has a more tendency to get reduced.