Chapter 15.14, Problem 15.31P

Interpretation Introduction

Interpretation:

The separation of ${\text{Cd}}^{2+}$ from ${\text{Zn}}^{2+}$ by bubbling ${\text{H}}_{\text{2}}\text{S}$ through an acidic solution of ${\text{Cd}}^{2+}$ and ${\text{Zn}}^{2+}$ mixture should be determined.

Concept introduction:

Solubility product in acid:

In the separation of metal ions by precipitation of metal sulphides, the soluble metal sulphides can be precipitated by changing concentration of ${\text{H}}_{\text{3}}{\text{O}}^{\text{+}}$ ion and the reaction is,

${\text{MS}}_{\text{(s)}}{\text{ + 2 H}}_{\text{3}}{\text{O}}^{\text{+}}{}_{\text{(aq)}}\stackrel{}{\rightleftharpoons }{\text{M}}^{\text{2+}}{}_{\text{(aq)}}{\text{ + H}}_{\text{2}}{\text{S}}_{\text{(aq)}}{\text{ + 2H}}_{\text{2}}{\text{O}}_{\text{(l)}}$

The equilibrium constant of the above reaction is,

${\text{K}}_{\text{spa}}\text{=}\frac{{\text{[M}}^{\text{2+}}{\text{][H}}_{\text{2}}\text{S]}}{{{\text{[H}}_{\text{3}}{\text{O}}^{\text{+}}\text{]}}^2}$

In the above equation, the reaction quotient ${\text{Q}}_{\text{c}}$ exceeds ${\text{K}}_{\text{spa}}$ results the formation of very insoluble sulphides.