Question

Use atomic properties to explain the reduction of a less active metal by a more active one:

(a) in aqueous solution;

(b) in the molten state. Give a specific example of each process.

Short answer

a. Reduction by a more active metal is also used in situations that do not involve an oxide. In the extraction of gold, after the pretreatment by leaching, the gold(I) complex ion is reduced with zinc to form the metal and a zinc complex ion:

$2\mathrm{Au}{\left(\mathrm{CN}\right)}_2{{}^{-}}_{\text{(aq)}}{\text{+\hspace{0.17em}Zn}}_{\text{(s)}}\to 2{\mathrm{Au}}_{\text{(s)}}\text{+Zn}{\left(\mathrm{CN}\right)}_4{{}^{2-}}_{\text{(aq)}}$

b. It forms ions of the more active element and removes ions of the less active element from solution. For example molten calcium is used to reduce molten RbCl.

data-custom-editor="chemistry" ${\text{Ca}}_{\text{(l)}}{\text{+2RbCl}}_{\text{(l)}}\stackrel{\text{Δ}}{\to }{\text{CaCl}}_{\text{2(l)}}{\text{+2Rb}}_{\text{(g)}}$

Step-by-step solution

Reduction

Sometimes in the extraction of less reactive metals, a few active metals are used as reducing agents. For example to reduce chromium oxide, the metal aluminium is used.

Subpart (a)

Reduction by a more active metal is also used in situations that do not involve an oxide. In the extraction of gold, after the pretreatment by leaching, the gold(I) complex ion is reduced with zinc to form the metal and a zinc complex ion:

$2\mathrm{Au}{\left(\mathrm{CN}\right)}_2{{}^{-}}_{\text{(aq)}}{\text{+\hspace{0.17em}Zn}}_{\text{(s)}}\to 2{\mathrm{Au}}_{\text{(s)}}\text{+Zn}{\left(\mathrm{CN}\right)}_4{{}^{2-}}_{\text{(aq)}}$

Subpart (b)

The ionisation energy of the most active elements in molten state is lower. When the more active element loses electrons and the less active one gains them, a greater amount of energy is released.

- It forms ions of the more active element and removes ions of the less active element from solution. For example, molten calcium is used to reduce molten RbCl.

${\text{Ca}}_{\text{(l)}}{\text{+2RbCl}}_{\text{(i)}}\stackrel{\text{Δ}}{\to }{\text{CaCl}}_{\text{2(l)}}{\text{+2Rb}}_{\text{(ag)}}$