Ncert Solutions Chemistry Class 12th

The steps involved in the extraction of zinc from zinc blende (ZnS) are as followed:-

• Concentration: the ore is crushed and then concentrated by froth floatation
• Roasting: the concentrated ore is heated in the presence of an excess of air at about 1200K to form zinc
• ZnS + 3 O₂ → 2 ZnO + 2 SO₂

• Reduction: ZnO obtained above is mixed with powdered coke and heated to 1673K in a fireclay
• ZnO + C → Zn + CO
• Electrolytic Refining: Zinc is refined by the process of electrolytic refining. In this process, impure zinc is made the anode and a pure copper strip is made the cathode. The electrode used in an acidified solution of zinc sulphate (ZnSO₄). Electrolysis results in the transfer of zinc in pure form from the anode to the cathode.
• Anode: Zn → Zn²⁺ +2e^-
• Cathode: Zn²⁺ +2e^-+ → Zn

During the extraction of iron, the reduction of iron oxides takes place in the blast furnace. In the process, hot air is blown from the bottom of the furnace and coke is burnt to raise the temperature up to 2200K in the lower portion itself. The temperature is lower in the upper part. Thus, it is the lower part where the reduction of iron oxides (Fe₂O₃ and Fe₃O₄) takes place.

The reactions taking place in the lower temperature range (500K – 800K ) in the blast furnace are:-

3 Fe₂O₃ + CO → 2 Fe₃O₄ + CO₂

Fe₃O₄ + 4 CO → 3 Fe + 4 CO₂

Fe₂O₃ + CO → 2 Fe₃ + CO₂

The reactions taking place in the higher temperature range ( 900 – 1500K ) in the blast furnace are:-

C + CO₂→ 2 CO

FeO + CO → Fe + CO₂

The silicate impurity of the ore is removed as a slag by calcium oxide ( CaO), which is formed by the decomposition of limestone (CaCO₃).

CaCO₃→ CaO + CO₂

CaO + SiO₂→ CaSiO₃ (Calcium silicate/ slag)

7.48

Catenation is the bonding of atoms of the same element into a series to form a chain. Catenation is much more common in phosphorous compounds than in nitrogen compounds.

This is because of the relative weakness of the NN single bond as compared to the PP single bond. Since nitrogen atom is smaller, there is greater repulsion of electron density of two nitrogen atoms, thereby weakening the NN single bond. T

his is the reason why Nitrogen shows catenation properties less than phosphorous.

Phosphorous has larger atomic size because of which it has less repulsion of electron density of two phosphorous atoms and hence P-P single bond is stronger.

Zone refining method is based on the principle that impurities are more soluble in the molten state of metal (the melt) than in the solid state. The impure metal is heated with the help of a circular mobile heater at one end. This results in the formation of the molten zone or melt. As the heater is removed along with the length of the rod, the pure metal crystallizes out of the melt and impurities pass into the adjacent molten zone. This process is repeated several times till the impurities are completely driven to the end of the rod which is then cut off and discarded. The method is very useful for semiconductor and other metals of very high purity, e.g., silicon, germanium, boron, and gallium.

Column chromatography It is based on the principle that different components of a mixture are differently adsorbed on an adsorbent. The mixture to be separated is put in a liquid or gaseous medium which is moved through the adsorbent. Different components are adsorbed at different levels on the column. Later, the adsorbed components are removed (eluted) by using suitable solvents (eluents).

There are several chromatographic techniques such as paper chromatography, column chromatography, gas chromatography, etc. chromatography is used for the purification of elements which are available in minute quantities and the impurities are not very different in chemical properties from the element to be purified.