(a) Name the method of refining which is

(i) used to obtain semiconductor of high purity,

(ii) used to obtain low boiling metal.

(b) Write chemical reactions taking place in the extraction of copper from Cu₂S.

(a)

The process of purifying the crude metals is called refining.

The refining method chosen depends on the physical and chemical properties of a particular metal.

i. The refining method used to obtain semi-conductors such as germanium, silicon and gallium of high purity is Zone Refining. It was invented by William Pfann, and this method is based on the fact that impurities are more soluble than the pure metal in the melt.

ii. Distillation process is used to obtain metals which have low boiling point like zinc and mercury. In this method, the impure metal is heated to its boiling point in a vessel. The vapours of metal thus formed are collected and cooled in a separate vessel to get pure metal. The impurities being non-volatile remain behind.

(b) The steps involved in extraction of Copper from Cu₂S are:

•Roasting of copper (I) sulphide

2Cu₂S(s) + 3O₂(g) → 2Cu₂O(s) + 2SO₂(g)

•Reduction of copper (I) oxide with copper (I) sulphide

2Cu₂O(s) + Cu₂S(s) → 6CuS(s) + SO₂(g)

The copper obtained here is very impure. Hence it undergoes electrolytic refining. The diagram below shows the process.

•At the cathode, copper (II) ions are deposited as copper.

Cu²⁺(aq) + 2e^-→ Cu(s)

•At the anode, copper goes into solution as copper (II) ions.

Cu(s) → Cu²⁺(aq) + 2e^-