Metals and Nonmetals for (2009-2010)

Roasting. The process of converting a sulphide ore into its oxide by heating strongly in the presence of air is called roasting. SO₂ gas is released during roasting.

2ZnS + 3O₂ 2ZnO + 2SO₂

Calcinations. The process of converting a carbonate ore into its oxide by heating strongly in the absence of air is called calcination. CO₂ gas is released during calcination.

ZnCO₃ ZnO + CO₂

Conversion of sulphide or carbonate ores into corresponding oxides is necessary because it is very much easier to convert a metal oxide into free metal than metal sulphide or metal carbonate.

Extraction of metals low in the activity series. The oxides low in the activity series can be reduced to metals by heating alone. E.g.

1. When cinnabar (HgS mercuric sulphide- ore of mercury) is heated in the air, it is first converted into mercuric oxide. On further heating it reduces to mercury.

2HgS + 3O₂ 2HgO + 2SO2 ;

2HgO 2Hg + O₂

2. When cuprous sulphide is heated in air, it is first converted into cuprous oxide. On further heating it reduces to copper metal.

   2Cu₂S + 3O₂ 2Cu₂O + 2SO₂ ;

2Cu₂O + Cu2S 6Cu + SO₂

Extraction of metals in the middle of activity series The metals of medium reactivity are present as sulphide or carbonate ores. As it is easier to obtain metal from its oxide, they are first converted into their metal oxides by using roasting and calcination and then are reduced to free metal.

Extraction of metals of activity series The highly reactive metals are extracted by the electrolysis of their molten chlorides or oxides.

E.g. when molten sodium chloride is electrolysed, sodium metal is obtained on cathode and chlorine gas is obtained at anode.

2Na+ + 2e-    2Na ; 2Cl- - 2e -   Cl₂

Reducing agent The substances that reduce the oxides of metals into free metal are called reducing agents.

E.g. carbon (coke) is the reducing agent used in most metallurgical processes. The highly reactive metals like Na, Ca, Al, etc are used as reducing agents because they can displace metals of low reactivity from their compounds.

3MnO₂ + 4Al    3Mn + 2Al₂O₃

Thermit reaction The reaction between ferric oxide and aluminium is highly exothermic which is used to join railway tracks or cracked machine parts. This is known as thermit reaction.

Fe₂O₃ + 2Al   2Fe + 2Al₂O₃

Refining of metals The process of removing impurities from the extracted metal is called refining. The most widely method used for refining impure metals is electrolytic refining. The electrolyte used in the electrolytic refining is the electricity.