Chemistry Class 12 NCERT Solutions: Chapter 7 The p Block Elements Part 3

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Q: 16. Can act as an oxidising as well as a reducing agent? Justify.


Can only act as an oxidizing agent. The highest oxidation state that can show is . In PCl5, phosphorus is in its highest oxidation state . However, it can decrease its oxidation state and act as an oxidizing agent.

Q: 17. Justify the placement of and in the same group of the periodic table in terms of electronic configuration, oxidation state and hydride formation.


The elements of group 16 are collectively called Chalcogens.

(i) Elements of group have six valence electrons each. The general electronic configuration of these elements is , where n varies from to .

(ii) Oxidation state:

As these elements have six valence electrons , they should display an oxidation state of . However, only oxygen predominantly shows the oxidation state of owing to its high electronegativity. It also exhibits the oxidation state of , zero , and . However, the stability of the oxidation state decreases on moving down a group due to a decrease in the electronegativity of the elements. The heavier elements of the group show an oxidation state of , and due to the availability of .

(iii) Formation of hydrides:

These elements form hydrides of formula , where . Oxygen and sulphur also form hydrides of type . These hydrides are quite volatile in nature.

Q: 18. Why is dioxygen a gas but sulphur a solid?


Oxygen is smaller in size as compared to sulphur. Due to its smaller size, it can effectively form bonds and form molecule. Also, the intermolecular forces in oxygen are weak van der Wall’s, which cause it to exist as gas. On the other hand, sulphur does not form molecule but exists as a puckered structure held together by strong covalent bonds. Hence, it is a solid.

Q: 19. Knowing the electron gain enthalpy values for and as and respectively, how can you account for the formation of a large number of oxides having species and not ?

(Hint: Consider lattice energy factor in the formation of compounds).


Stability of an ionic compound depends on its lattice energy. More the lattice energy of a compound, more stable it will be.

Lattice energy is directly proportional to the charge carried by an ion. When a metal combines with oxygen, the lattice energy of the oxide-involving ion is much more than the oxide involving . Hence, the oxide having ions are more stable than oxides having . Hence, we can say that formation of is energetically more favourable than formation of .

Q: 20. Which aerosols deplete ozone?


Freons or chlorofluorocarbons are aerosols that accelerate the depletion of ozone. In the presence of ultraviolet radiations, molecules of break down to form chlorine: free radicals that combine with ozone to form oxygen.

Q: 21. Describe the manufacture of by contact process?


Sulphuric acid is manufactured by the contact process. It involves the following steps:

Step(i): Sulphur or sulphide ores are burnt in air to form .

Step(ii): By a reaction with oxygen, is converted into in the presence of as a catalyst.

Step (iii): produced is absorbed on to give

This oleum is then diluted to obtain of the desired concentration. In practice, the plant is operated at (pressure) and (temperature). The sulphuric acid thus obtained is pure.

Q: 22. How is an air pollutant?


Sulphur dioxide causes harm to the environment in many ways:

1. It combines with water vapour present in the atmosphere to form sulphuric acid. This causes acid rain. Acid rain damages soil, plants, and buildings, especially those made of marble.

2. Even in very low concentrations, causes irritation in the respiratory tract. 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 long time lose colour from their leaves. This condition is known as chlorosis. This happens because the formation of chlorophyll is affected by the presence of sulphur dioxide.

Q: 23. Why are halogens strong oxidising agents?


The general electronic configuration of halogens is , where . 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. Hence, they act as strong oxidizing agents.

Q: 24. Explain why fluorine forms only one Oxoacid, .


Fluorine forms only one Oxoacid i.e., because of its high electronegativity and small size.