Q. 295.0( 2 Votes )

(a) Give reasons

Answer :


(i) Bond enthalpy of F2 is lower than that of Cl2 because F atom is small in size and due to this the electron- electron repulsions between the lone pairs of F-F electrons are very large due to which F-F bond is weakened. Less energy would be required to break this bond. Thus, the bond dissociation energy of F2 is lower than that of Cl2.

(ii) Electronegative elements like N,F,O can form intermolecular hydrogen bonding with water to stabilise the molecule. PH3 has lower boiling point than NH3 because NH3 molecule possess N atom due to which there are intermolecular hydrogen bondings which binds them strongly, whereas, PH3 has weaker van der Waals forces.



Structure of BF3


Structure of (HPO3)3


Structure of XeF4



(i) 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, small air bubbled are formed as solubility of nitrogen decreases and it separates from the blood. This leads to a dangerous medical condition called bends.

So, helium mixed with oxygen under pressure is given to sea- divers for artificial respiration because of its very low solubility in blood.

(ii) Fluorine being the most electronegative atom does not exhibit positive oxidation state. Also it does not have d-orbitals for octet expansion and therefore, it shows only a negative oxidation state of -1.

(iii) S-S bond is stronger than O-O bond and sulphur has an extra d orbital present. Oxygen atom is smaller in size as compared to sulphur due to lone pair of electrons in O-O bonds in oxygen experiences more repulsions as compared to the S-S bonds and thus, S-S forms a strong bond.



Structure of XeF2


Structure of H2S2O8

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