Ncert Solutions Chemistry Class 12th

7.57

The general electronic configuration of halogens is np⁵, where n = 2-6. Thus, halogens need only one more electron to complete their octet and to attain the stable noble gas configuration.

Also, halogens are highly electronegative with low dissociation energies and high negative electron gain enthalpies. Therefore, they have a high tendency to gain an electron.

Whenever an atom accepts an electron from another atom, the atom accepting the electron is getting reduced and the atom donating the electron is oxidized.

But the electron accepting atom acts as an oxidizing agent while the electron donating atom acts as reducing agent. Hence, they act as strong oxidizing agents.

7.56

SO₂ is a highly irritating gas and causes serious respiratory problems, and may cause a fit of coughing.

1. It reacts with water vapour present in the atmosphere to form sulphuric acid. This causes acid rain. Acid rain damages soil [soil become more acidic], plants, and buildings get corroded, especially those made of marble.

In the air, SO₂ is oxidized to SO₃ which is also an irritant.

2SO₂ + O₂  2SO₃ SO₃ + H₂O H₂SO₄

2. Even in very low concentrations, SO₂ causes irritation in the respiratory It causes throat and eye irritation and can also affect the larynx to cause breathlessness.

3. It is extremely harmful to plants. Plants exposed to sulphur dioxide for a prolong time lose colour from their This condition is known as chlorosis. This is because the formation of chlorophyll is retarded in presence of sulphur dioxide.

(i) Zone refining Is the method based on the principle that the impurities are more soluble in the melt than in the solid state of the metal.

(ii) Electrolytic refining works on the principle of refining impure metals by the use of electricity. In Electrolytic refining, the impure metal is made the anode and a strip of pure metal is made as the A solution of a soluble salt of the same metal is taken as the electrolyte. When an electric current is passed, metal ions from the electrolyte are deposited at the cathode as a pure metal and the impure metal from the anode dissolves into the electrolyte in the form of ions. The impurities present in the impure metal gets collected below the anode.

(iii) Vapour refining is the process of refining metal by converting it into its volatile compound and then, decomposing it to obtain a pure metal. To carry out this process,

1. The metal should form a volatile compound with an available reagent, and
2. The volatile compound should be easily decomposable so that the metal can be easily recovered
3. Nickel, Zirconium and titanium are refined using this method.