(i) As we move down the group 17, the size of the atom increases from fluorine to chlorine. The larger difference in the size of N and Cl results in the weakness of strength of N-Cl bond. 
On the other hand, the difference in size of N and F is small; consequently, the N-F bond is quite strong. As a result, NF₃ is an exothermic compound. 

 (ii)

1. F-F bond has low enthalpy because the fluorine atom has a small size and due to their small size, there is repulsion between two atoms making its bond enthalpy lower, hence more reactivity is more.

2. It has a small size and high charge density due to which it is the most electronegative element.

(i) The acidity of a molecule depends on the polarity of the bond between central atom and the hydrogen atom. Greater the polarity higher will be the acidity.

And the polarity of the bond depends on the electronegativity of the central atom.  In a period, the electronegativity decreases in the order Cl > S > P. As a result, the loss of H⁺ ions decreases. 

 Thus, the acidic strength of the hydrides decreases in the 

 Following order: 

 HCl > H₂S > PH₃

(ii) Nitrogen does not form pentahalide because it does not have d-orbital. P, As, Sb form pentahalide. Bi does not form pentahalide. The tendency to form pentahalide decrease down the group. This because of inert pair effect.
Due to the inert pair effect, ns² electron remains inert in a chemical reaction and element shows -2 oxidation state. Inert pair effect increases down the group. Thus the tendency to form pentahalides decrease down the group 15.    

(b) 

(i) P₄ + 10SO₂Cl₂--->  4PCl₅ + 10SO₂

(ii) 2XeF₂ + 2H₂O---->  2Xe + 4HF + O₂

(iii) I₂ + 10HNO₃--> 2HIO₃+10NO₂+4H₂O

i) 



ii)

(b)

(i) Due to the small size of oxygen, it has less tendency for catenation and the high tendency of pp-pp multiple bonds, hence forms stable O₂ molecules whereas sulphur because of its higher tendency for catenation and lesser tendency to form pp-pp multiple bonds forms S₈ molecules having 8-membered puckered ring.

(ii)Inter -halogen bonds are weaker (it is between two different halogen like ICl) because of its partly ionic character due to the difference in electronegativity.

While when the same halogen forms X₂ molecules like I₂. They form covalent bonds which are stronger than interhalogen compound and weak bond obviously is more reactive than the stronger bond and that’s why ICl is more reactive than I₂.

(iii) Fluorine is a much stronger oxidising agent than chlorine. The oxidising power depends on three factors.

1. Bond dissociation energy

2. Electron gains 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 oxidising agent than chlorine.

i) White phosphorus is most reactive of all the allotropes of phosphorus because it is unstable due to the angular strain on P₄ molecule with the bond angle of 60°.

(ii) Nitrogen oxide emitted from the exhausts of supersonic jet aeroplanes readily combine with ozone to form nitrogen dioxide and diatomic oxygen.

NO(g) + O3(g) →NO2(g) +O2(g)

Since supersonic jets fly in the stratosphere near the ozone layer, they are responsible for the depletion of ozone layer.

(iii) The size of a fluorine atom is very small as compared to a chlorine atom. Therefore, the repulsion between electrons in the outer most shell of the two atoms in a fluorine molecule is much greater than that in a chlorine molecule. Hence, it requires less energy to break up the fluorine molecule, making its bond dissociation energy lesser than that of chlorine molecule.

(iv) Helium, being light, non-inflammable and unreactive, is used for filling of balloons for metrological observations.

(v) XeF2 + PF5→  [XF]⁺+[PF6]^-