JEE Chemistry The P Block Elements Notes Set 02

Group 15: The Nitrogen Family

Nitrogen is a diatomic gas with a triple bond, while phosphorus shows allotropes: white (P4, reactive, soluble in CS2, glows), red (polymeric, less reactive) and black (most stable, layered, conducting). Hydrides fall in basicity, NH3 to BiH3, and the reducing power rises. Ammonia is prepared in the Haber process (Fe catalyst with K2O and Al2O3 promoters) and dried with CaO. PH3 comes from white P with NaOH, or from Ca3P2 and water (Holme's signal). The oxyacids of phosphorus are H3PO2 (monobasic, P(+1)), H3PO3 (dibasic) and H3PO4 (tribasic).

Oxides and Oxyacids of Nitrogen

N2O and NO are neutral, N2O3 is an acid anhydride of HNO2, NO2 and N2O4 are mixed anhydrides, and N2O5 is the anhydride of HNO3. The acid character increases with oxidation state. Nitric acid is made by the Ostwald process: 4NH3 + 5O2 to 4NO (over Pt-Rh), 2NO + O2 to 2NO2 and 3NO2 + H2O to 2HNO3 + NO. Metals react with dilute HNO3 to give NO, with conc. HNO3 to give NO2, and Zn with very dilute acid gives N2O. The brown ring test detects nitrate.

Group 16: The Oxygen Family

Oxygen has no d orbitals and shows -2 mostly, while sulphur forms many oxidation states. Ozone is a diamagnetic, bent, strongly oxidising molecule that is endothermic in formation; it is estimated using KI in a borate buffer followed by titration with hypo. Sulphur dioxide is both an oxidising and a reducing agent. Sulphuric acid is made by the contact process, using V2O5; SO3 is absorbed in H2SO4 to give oleum, which is then diluted. Thiosulphate has two non-equivalent sulphur atoms. Hydrides H2O to H2Te decrease in thermal stability and bond angle, and increase in acidity and reducing power.