Question

During the process of electrolytic refining of copper, some metals present as impurity settle as ‘anode mud’. These are:
(a). Sn and Ag
(b). Pb and Zn
(c). Ag and Au
(d). Fe and Ni

Answer 1

- Hint: In the electro-refining process of copper the impurity settles down as ‘anode mud’ which contains all precious metals that are less electropositive or insoluble in nature.

Complete step-by-step solution -

To know what ‘anode mud’ is at first we should know how electro-refining of copper takes place.
The electrolyte we use is a mixture of dilute $H_2SO_4$ (5-10%) and Copper Sulphate solution (15%). The electrolytic cell should be lead coated.
For anode we use a wide rod of blister copper and for cathode we use thin rod of pure copper.
After passing the electric current copper gets ionised as Cu+2 from anode rod and comes to the electrolyte solution then reduced as copper metal and stored on cathode rod. Thus the anode rod gets thinner whereas the cathode rod gets thicker with pure copper metal (99.99%).
The blister copper used as anode contains some impurity. In the impurity the more electropositive metals (like: Fe, Ni, Zn etc.) get soluble but the less electropositive precious metals (like: Ag, Au, Pt etc.) get mix with other impurities and settle down as ‘mud’ or ‘sludge’ form at the bottom of anode. This is known as ‘anode mud’.
Electrode reaction:
\[CuS{{O}_{4}}\rightleftharpoons C{{u}^{+2}}+S{{O}_{4}}^{2-}\]
Cathode reaction: \[Cu\rightleftharpoons C{{u}^{+2}}+2e\]
Anode reaction: \[C{{u}^{+2}}+2e\rightleftharpoons Cu\]
‘Anode mud’ is used to recover precious metals like: gold, silver etc.
Therefore, ‘anode mud’ contains Ag and Au and thus option C is the correct one.

Note: We should know what blister copper is-
In the self-reduction process of copper when molten copper is getting cool, soluble sulphur dioxide comes out as a bubble. This causes the surface of copper blistery. It contains 97-98% copper.