(a) Due to relatively stable half - filled p-orbitals of group-15 elements.
(b) \((i)\ CaF_{ 2 }+H_{ 2 }SO_{ 4 }\longrightarrow CaSO_{ 4 }+2HF\)
\(\\ (ii)\ SO_{ 2 }(g)+C1_{ 2 }(g)\longrightarrow SO_{ 2 }C1_{ 2 }(l)\)
\(\\ (iii)\ 2NH_{ 4 }C1+Ca(OH)_{ 2 }\longrightarrow 2NH_{ 3 }+2H_{ 2 }O+CaC1_{ 2 }\)

Nitrogen has a strong tendency to form multiple bonds because of its small size and high electronegativity. Therefore, it exists as a diatomic molecule, \(N\equiv N\) . On the other' hand, phosphorus because of its large size and small electronegativity does not show any tendency to form multiple bonds and therefore, 'diatomic molecule like \(N\equiv N\)  is not formed. Instead, it prefers to form stable tetraatomic, \({ P }_{ 4 }\)  molecules in which each P is linked to three other P atoms by three single covalent bonds. The four atoms in \({ P }_{ 4 }\) molecule lie at the corners of a regular tetrahedron.

(i) \({ NH }_{ 4 }{ NO }_{ 2 }(s)\underrightarrow { heat } { N }_{ 2 }+{ 2H }_{ 2 }O\) 
                'A'                 'B'
(ii) \({ N }_{ 2 }+{ 3H }_{ 2 }\longrightarrow 2{ NH }_{ 3 }(g)\) 
                              'C' basic
(iii) \({ 4NH }_{ 3 }+{ 5O }_{ 2 }\longrightarrow 4NO+{ 6H }_{ 2 }O\) 
\(\\ 2NO+{ O }_{ 2 }\longrightarrow { 2NO }_{ 2 }\) 
                  'D' (part of acid rain)
\({ 3NO }_{ 2 }+{ H }_{ 2 }O\longrightarrow 2{ HNO }_{ 3 }+NO\)      

(a) (i) Haber's process:
\(N_{ 2 }(g)+{ 3H }_{ 2 }(g)\overset { 200\ atm }{ \underset { 673\ k,\ Fe/Mo }{ \rightleftharpoons } } { 2NH }_{ 3 }(g)+heat\)
Since the process is exothermic, therefore, the optimum temperature of 673 K is needed. There is decrease in number of moles from reactants to products, therefore, high pressure \(200\times { 10 }^{ 5 }\) pa (200 atm) is needed. Iron or molybdenum is used as catalyst with small amount of \({ K }_{ 2 }O\)  and  \({ AI }_{ 2 }{ O }_{ 2 }\) to increase the rate of reaction.
(ii) Contact process:
\({ 2SO }_{ 2 }(g)+{ O }_{ 2 }(g)\longrightarrow { 2SO }_{ 3 }(g)+heat\)
The process is exothermic, therefore, the optimum temperature of 720 K is suitable. The forward reaction leads to decrease in volume, therefore, high-pressure of 2 bar is needed. \({ V }_{ 2 }{ O }_{ 5 }\)  should be used as catalyst.
(b) (i)

(ii)

(iii)
​​​​

(a) \((i)\ { 2F }_{ 2 }+2{ H }_{ 2 }O\longrightarrow { 4H }^{ + }(aq)+{ 4F }^{ - }(aq)+{ O }_{ 2 }(g)\)
\(\\ (ii)\ { Ca }_{ 3 }{ P }_{ 2 }+6{ H }_{ 2 }O\longrightarrow 3Ca(O{ H) }_{ 2 }+{ 2PH }_{ 3 }\)
\(\\ (iii)\ { 6XeF }_{ 4 }+12{ H }_{ 2 }O\longrightarrow 4Xe+{ 2XeO }_{ 3 }+24HF+{ 3O }_{ 2 }\)
(b) (i) 

(a) (i)

(b) (i) It is because P-P single bond is stronger than the single N-N bond.
(ii) It is because there is more interelectronic repulsion between valence electrons in 'F' atoms as compared to 'CI' atoms.
(iii) It is because HF molecules are associated with intermolecular H-bonding while HCI is not that is why HF is liquid and has higher boiling point than HCI which is a gas.

(a) (i)

(ii)

(b) (i) It is because there is more interelectronic repulsion in oxygen due to smaller size than sulphur, less energy is released on addition of electrons, therefore, its electron gain enthalpy is less than sulphur.
(ii) It is due to absence of d-orbitals in nitrogen atom, it cannot show pentavalently.
(iii) HI has lower bond dissociation energy than HCI due to longer bond length it is stronger acid than HCI.

(b) \((i)\quad { 3HgCI }_{ 2 }+{ 2PH }_{ 3 }\longrightarrow { Hg }_{ 3 }{ p }_{ 2 }+6HCI\)
\(\\ (ii)\quad { SO }_{ 2 }+{ H }_{ 2 }{ SO }_{ 4 }\longrightarrow { H }_{ 2 }{ S }_{ 2 }{ O }_{ 7 }\)
\(\\ (iii)\quad { 6XeF }_{ 4 }+{ 12H }_{ 2 }O\longrightarrow 4Xe+{ 2XeO }_{ 3 }+24HF+{ 3O }_{ 2 }\)

(a) (i) \(3Cu+{ 8HNO }_{ 3 }(dil.)\longrightarrow { { 3 }Cu(NO_{ 3 }) }_{ 2 }+2NO+{ 4H }_{ 2 }O\)
(ii) \({ XeF }_{ 4 }+{ O }_{ 2 }{ F }_{ 2 }\overset { { -130°C } }{ \longrightarrow } { XeF }_{ 6 }+{ O }_{ 2 }\)
(b) (i) The property of catenation depends upon the strength of the element-element bond. Since P-P bond strength (213 kJ \({ mol }^{ -1 }\) ) is much more than N-N bond strength (159 kJ \({ mol }^{ -1 }\)), phosphorus has marked catenation properties than nitrogen.
(ii) Due to small size and high electronegativity, oxygen atom forms \(p\pi -p\pi \)  double bond, 0= 0. The intermolecular forces in oxygen are weak van der Waal's forces and therefore, oxygen exists as a gas. On the other hand, sulphur does not form stable \(p\pi -p\pi \)  bonds and do not exists as \({ S }_{2}\) It is linked by single bonds and form polyatomic complex molecules having eight atoms per molecule \({ S }_{ 8 }\) and have puckered ring structure. Therefore, S atoms are strongly held together and it exists as a solid.
(iii) All the halogens are coloured. This is due to absorption of radiations in the visible region which results in the excitation of outer electrons to higher energy levels. By absorbing different quanta of radiations, they display different colours. The fluorine atom is the smallest and the force of attraction between the nucleus and the outer electrons is very large. As a result, it requires large excitation energy and absorbs violet light (high energy) and therefore, appears pale yellow. On the other hand, iodine needs very less excitation energy and absorbs yellow light of low energy. Thus, it appears dark violet. Similarly, we can explain the greenish yellow colour of chlorine and reddish brown colour of iodine.