(a) What is meant by corrosion? Name any two methods used for the prevention of corrosion.
(b) Suppose you have to extract metal M from its enriched sulphide ore. If M is in the middle of the reactivity series, write various steps used in extracting this metal.
(a) Corrosion is a process in which metal reacts with substances present in the environment to form surface compounds.
Prevention:
(i) Galvanisation is a process to prevent corrosion of iron.
(ii) Electroplating is also used to prevent corrosion.
(b)(i) Concentration of ores: Sulphide ore will be concentrated by froth- floatation process. Sulphide ore will be collected in froth whereas gangue will be left behind.
(ii) Roasting: Sulphide ore is heated strongly in the presence of O2 to form metal oxide and sulphur dioxide.
2MS + 3O2 → 2MO + 2SO2
(iii) Reduction: MO reacts with carbon (acts as reducing agent) to form metal and CO.
MO + C → M + CO
(iv) Electrolytic refining: Impure metal ‘M’ is purified by electrolytic refining. Impure metal is taken as anode, pure metal is taken as cathode, soluble salt of metal is taken as electrolyte. Impure metal forms metal ions which gain electrons and form pure metal at cathode.