Question

Copper turnings are added to \[{\text{MgC}}{{\text{1}}_{\text{2}}}\] solution. Then the solution will:
A) have no effect
B) precipitate \[{\text{Mg}}\] Metal
C)precipitate \[{\text{MgO}}\]
D)none of the above

Answer 1

Hint:First,we will write the overall cell reaction. Then estimate the sign of the standard cell potential. From the sign of the standard cell potential, we can judge if the reaction is spontaneous or not.

 Complete step by step answer:
The chemical formula \[{\text{MgC}}{{\text{1}}_{\text{2}}}\] represents magnesium chloride. When we add copper turning to a solution containing magnesium chloride, there are chances that no reaction occurs. There are also chances that copper will displace magnesium from magnesium chloride to form cupric chloride and magnesium metal.
\[{\text{Cu + MgC}}{{\text{1}}_{\text{2}}} \to {\text{CuC}}{{\text{1}}_{\text{2}}} + {\text{Mg}}\]
Out of these two possibilities, only one is correct. It means that either no reaction will occur or copper will displace magnesium from magnesium chloride. We can say, out of the above two possibilities, which one is actually happening, based on the cell potential of the overall cell reaction.
For the reaction between copper and magnesium chloride, the cell potential will be given by the following expression
\[{\text{E}}_{cell}^o = {\text{E}}_{{\text{M}}{{\text{g}}^{2 + }}{\text{|Mg}}}^o - {\text{E}}_{Cu{\text{|Cu}}}^o\]
But since,
\[{\text{E}}_{{\text{M}}{{\text{g}}^{2 + }}{\text{|Mg}}}^o < {\text{E}}_{Cu{\text{|Cu}}}^o\]
Hence,
\[{\text{E}}_{cell}^o{\text{E}}_{{\text{M}}{{\text{g}}^{2 + }}{\text{|Mg}}}^o - {\text{E}}_{Cu{\text{|Cu}}}^o < 0\]
and
\[{\text{E}}_{cell}^o < 0\]
For a non-spontaneous cell reaction, the standard cell potential is negative. Hence, copper cannot displace magnesium from magnesium chloride solution.
Hence, no change will occur and the option A is the correct answer.

Note:For a spontaneous cell reaction, the standard cell potential is positive. For such reactions, \[{\text{E}}_{cell}^o < 0\] and \[\Delta {G^o}0\] Thus, for a spontaneous cell reaction, the standard Gibbs free energy change is negative.