Chemistry Class 12 NCERT Solutions: Chapter 7 The p Block Elements Part 9

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Q: 26. Considering the parameters such as bond dissociation enthalpy, electron gain enthalpy and hydration enthalpy, compare the oxidising power of and .


Fluorine is a much stronger oxidizing agent than chlorine. The oxidizing power depends on three factors.

1. Bond dissociation energy

2. Electron gain enthalpy

3. Hydration enthalpy

The electron gain enthalpy of chlorine is more negative than that of fluorine. However, the bond dissociation energy of fluorine is much lesser than that of chlorine. Also, because of its small size, the hydration energy of fluorine is much higher than that of chlorine. Therefore, the latter two factors more than compensate for the less negative electron gain enthalpy of fluorine. Thus, fluorine is a much stronger oxidizing agent than chlorine.

Q: 27. Give two examples to show the anomalous behaviour fluorine.


Anomalous behaviour of fluorine

(i) It forms only one Oxoacid as compared to other halogens that form a number of oxoacids.

(ii) Ionisation enthalpy, electronegativity, and electrode potential of fluorine are much higher than expected.

Q: 28. Sea is the greatest source of some halogens. Comment.


Sea water contains chlorides, bromides, and iodides of , and . However, it primarily contains NaCl. The deposits of dried up sea beds contain sodium chloride and carnallite, . Marine life also contains iodine in their systems. For example, sea weeds contain upto iodine as sodium iodide. Thus, sea is the greatest source of halogens.

Q: 29. Give the reason for bleaching action of .


When chlorine reacts with water, it produces nascent oxygen. This nascent oxygen then combines with the coloured substances present in the organic matter to oxide them into colourless substances.

Coloured substances Oxidized colourless substance.

Q: 30. Name two poisonous gases which can be prepared from chlorine gas.


Two poisonous gases that can be prepared from chlorine gas are

(i) Phosgene (COCl2)

(ii) Mustard gas (ClCH2CH2SCH2CH2Cl)

Q: 31. Why is more reactive than ?


is more reactive than because bond in is weaker than bond in .

Q: 32. Why is helium used in diving apparatus?


Air contains a large amount of nitrogen and the solubility of gases in liquids increases with increase in pressure. When sea divers dive deep into the sea, large amount of nitrogen dissolves in their blood. When they come back to the surface, solubility of nitrogen decreases and it separates from the blood and forms small air bubbles. This leads to a dangerous medical condition called bends. Therefore, air in oxygen cylinders used for diving is diluted with helium gas. This is done as He is sparingly less soluble in blood.

Q: 33. Balance the following equation:


Balanced equation

Q: 34. Why has it been difficult to study the chemistry of radon?


It is difficult to study the chemistry of radon because it is a radioactive substance having a half-life of only days. Also, compounds of radon such as have not been isolated. They have only been identified.