Question

On the basis of reactivity metals are grouped into three categories: 

1. Metals of low reactivity
2. Metals of medium reactivity
3. Metals of high reactivity

Therefore metals are extracted in pure form from their ores on the basis of their chemical properties.

Metals of high reactivity are extracted from their ores by electrolysis of the molten ore.

Metals of low reactivity are extracted from their sulphide ores, which are converted into their oxides. The oxides of these metals are reduced to metals by simple heating.

(a) Name the process of reduction used for a metal that gives vigorous reaction with air and water both.

(b) Carbon cannot be used as a reducing agent to obtain aluminium from its oxide? Why?

(c) Describe briefly the method to obtain mercury from cinnabar. Write the chemical equation for the reactions involved in the process.

                                     OR

(c) Differentiate between roasting and calcination giving chemical equation for each. 

Answer 1

(a) Metals towards the top of the reactivity series have a high level of reactivity. By heating with carbon, they cannot be extracted from their compounds. This is due to the fact that certain metals are more apt to bind oxygen than carbon. By using electrolytic reduction, these metals are produced. For instance, electrolysis of their molten chlorides yields sodium, magnesium, and calcium. While chlorine is released at the anode (the positively charged electrode), where the metals are deposited at the cathode (the negatively charged electrode).

(b) Alumina cannot be converted from carbon to aluminium because oxygen has a stronger affinity for aluminium than carbon does.

(c) Cinnabar (HgS) is an ore of mercury. When it is heated in air, it is first converted into mercuric oxide (HgO). Mercuric oxide is then reduced to mercury on further heating.

\[\ce{2HgS(s) + 3O_2(g)->[Heat] 2HgO(s) + 2SO_2(g)}\]

\[\ce{2HgO(s)->[Heat]2Hg(l) + O_2(g)}\]

                                      OR

(c) Difference between roasting and calcination are:

Calcination: Calcination is the process of converting a concentrated ore into its oxide by heating it below its melting point without the presence of air. It often applies to ores of hydroxide and carbonate. 

Example:    \[\ce{MgCO_3->[\Delta] MgO+CO_2}\]

In this process, magnesium carbonate is heated without the presence of air to produce magnesium oxide.

Roasting: Roasting is the name for the metallurgical procedure in which an ore is heated over its melting point in the presence of extra air to produce its oxide. Sulphur is typically emitted as sulphur dioxide, a gas, after being transformed from a sulphide to an oxide.

Example: \[\ce{2ZnS + 3O_2->[\Delta]2ZnO + 2SO_2}\]

In this process, zinc sulphide is changed into zinc oxide and sulphur is liberated as the gas sulphur dioxide.