JEE Chemistry The P Block Elements Notes Set 14

Group 15: Nitrogen Family

N and P are non-metals, As is a metalloid, Sb and Bi are metals. Configuration is ns2 np3.

1. Covalent radius N < P < As < Sb < Bi. First ionisation enthalpy N > P > As > Sb > Bi. Electronegativity N > P > As > Sb = Bi.
2. Catenation is weak, strongest for P. The N-N bond is very weak (about 160 kJ per mole) compared with P-P (about 200) because of lone pair repulsion in the small N atom.
3. All except N and Bi show allotropy. Phosphorus has white, red and black forms.
4. Nitrogen is made by heating \( \ce{NH4NO2 -> N2 + 2H2O} \) or ammonium dichromate; the purest N2 comes from heating sodium azide, \( \ce{2NaN3 -> 2Na + 3N2} \).
5. N2 is inert because of its triple bond. Hot Ca, Mg and Al form nitrides. Heated CaC2 absorbs N2 to give calcium cyanamide, a fertiliser.
6. Nitrides are ionic (Li3N, Mg3N2), covalent (BN, AlN, Si3N4) or interstitial (MN of Sc, Ti, Zr, Hf). Hexagonal BN is called inorganic graphite and its cubic form borazon is harder than diamond.

Ammonia

1. Lab preparation: ammonium salt heated with a base. Haber process: \( \ce{N2 + 3H2 <=> 2NH3} \) at about 450 C, 200 atm, Fe catalyst with Mo promoter. Hydrolysis of nitrides also gives NH3.
2. NH3 cannot be dried with conc. H2SO4, P2O5 or anhydrous CaCl2 because it reacts with them; quicklime (CaO) is used.
3. Alkali metals dissolve in liquid ammonia to give a blue, conducting, strongly reducing solution. Acetic acid is a strong acid in liquid ammonia because NH3 accepts protons more readily than water.

Oxides and Oxoacids of Nitrogen

1. Nitrous acid is both an oxidising agent (liberates I2 from KI) and a reducing agent (decolourises acidic KMnO4).
2. Ostwald process: \( \ce{4NH3 + 5O2 -> 4NO + 6H2O} \) over Pt gauze at 800 C; NO + O2 gives NO2; \( \ce{3NO2 + H2O -> 2HNO3 + NO} \).
3. Conc. nitric acid oxidises non-metals to their highest oxoacids: S gives H2SO4, P gives H3PO4, I2 gives HIO3, C gives H2CO3 (CO2), As gives H3AsO4.
4. Products with metals depend on acid strength and the metal. Very dilute acid with Mg or Mn gives H2. Dilute acid with Cu, Pb, Ag gives NO. Conc. acid gives NO2. Zn with cold dilute acid gives N2O. Zn, Fe, Sn with very dilute acid can give NH4NO3.
5. Au and Pt are not attacked. Fe and Al become passive in conc. HNO3. Nitric acid turns proteins yellow (xanthoproteic test).

Phosphorus and Phosphine

1. White phosphorus is made by heating phosphate rock with sand and coke at about 1500 C. It turns into red phosphorus on heating at 240 to 250 C in an inert atmosphere with iodine catalyst.
2. Hot conc. H2SO4 oxidises P to H3PO4. Metals give phosphides, which hydrolyse to PH3: \( \ce{Ca3P2 + 6H2O -> 3Ca(OH)2 + 2PH3} \).
3. Phosphine has a rotten fish smell, is insoluble in water and is a weak base. The purest PH3 is obtained from PH4I with KOH. It turns CuSO4 black (Cu3P2) and AgNO3 black.
4. P2O5 is a very powerful dehydrating agent: it dehydrates H2SO4 and HNO3 to give SO3 and N2O5.