Higher the positive oxidation state of central atom, more will be its ploarising power which, in turn, increases the covalent character of bond formed between in central atom and the other atom.

The reducing character of the hydrides of Group 15 elements increases from NH₃  to BiH₃(Bismuthine) because the reducing character depends upon the stability of the hydride.The greater the unstability of anhydride,the greater is the its reducing character.Since,the BiH₃ is the least stable (because the size of central atom is greatest & therefore its tendency to form stable covalent bond with small hydrogen atom decreases,as a result the bond strength decreases) in this series,hence the reducing character increases. 

Dinitrogen (N₂) IS formed by sharing three electron pairs between two nitrogen atoms.The two nitrogen atoms are joined by triple bond(N≡N). The nitrogen atom is very small in size ,therefore the bond length is also quite small(109.8 pm) & as a result the bond dissociation energy is quite high(946Kj / mol).This reason leads N₂ to be very less reactive at room temperature.

Ammonia is prepared using the Haber’s process. The yield of ammonia can be maximized under the following conditions:
(i) Temperature ~ 700k.
(ii) High pressure of 200 x 10⁵ Pa (about 200 atm).
(iii) A catalyst such as iron oxide with small amounts of K₂,O and Al₂O₃.