Give reasons for the following:

(a) Zinc and not copper is used for the recovery of metallic silver from complex [Ag(CN)₂] .

(b) Alumina is dissolved in cryolite for electrolysis instead of being electrolysed directly.

(c) Zinc oxide can be reduced to metal by heating with carbon but not Cr₂O₃.

(a) Though both Zinc and copper are more reactive than silver, but zinc is a stronger reducing agent than copper. (It can be noted that zinc is at much upper position than copper in the reactivity series of metal ) That’s why zinc is used for the recovery of silver. Another reason is affordability. Zinc is cheaper than copper so it is more cost effective.

(b) Alumina (Bauxite) , to give alumminium from its directly electrolysis need to be melted first. But it has a very high boiling point (nearly 2200 ° C). and a very large amount of energy is required to reach this temperature. So, we dissolve it in cryolite which decreases its boiling point. Cryolite acts as an impurity in alumina and hence decreases its boiling point and saves our energy and makes extraction easier.

(c) If we compare the affinity towards oxygen of these three metals, we will find out that chromium has the maximum affinity towards oxygen, the next is carbon and the least is zinc. As in zinc and carbon, carbon has more affinity so, it can reduce zinc by getting oxidisd. But, in chromium and carbon, chromium has higher affinity towards oxygen than carbon, so carbon is unable to get oxidized and reduce Cr₂O_3.