The p-Block Elements Group 15: Occurrence and Atomic Properties The differentiating electron in group 15 elements tends to enter the p orbital, resulting in a valence shell electronic configuration of ns 2np 3. The atomic and ionic radii of group 15 elements increase in size down the group due to the addition of a new principal energy level in each successive element. There is a considerable increase in the covalent radius from nitrogen to phosphorus, which is due to the effective shielding of the s and p electrons present in the penultimate shell of phosphorus. Group 15 elements show a higher value of ionisation enthalpy as compared to group 14 elements. As we move down the group, the ionisation energy gradually decreases. As we move down the group, the electronegativity gradually decreases. Group 15: Physical Properties and Oxidation States Group 15 elements are less metallic as compared to group 14 elements, but their metallic character increases down the group. The melting point of the elements increases from nitrogen to arsenic and decreases from arsenic to bismuth. The boiling point gradually increases from nitrogen to antimony. The density of the elements increases regularly from nitrogen to bismuth. All the elements, except for bismuth, show allotropy. Nitrogen shows an oxidation state of -3 in nitrides by gaining electrons. The elements of this group form covalent bonds and show a formal oxidation state of -3. The elements show positive oxidation states of +3 and +5. In group 15 elements, the covalent character decreases from nitrogen to bismuth. Nitrogen exhibits various oxidation states from -3 to +5. Group 15: Anomalous Properties The unique properties of nitrogen are attributed to its • Small atomic size • High electro-negativity or high ionisation enthalpy • Non availability of d-orbitals • Propensity to form multiple bonds The tendency of nitrogen to form pp-pp bonds is one of the major features that distinguishes it from phosphorous and other group 15 elements. The other elements do not form pp-bonds because of their relatively larger size. The catenation tendency is less for nitrogen when compared to the other elements of the group. Nitrogen doesn’t form dp-pp or dp-dp bonds due to the absence of d-orbitals. Group 15: Chemical Reactivity The stability of hydrides decreases from NH 3 to BiH 3. This is due to an increase in the size of the central atom down the group. Hydrides of group 15 elements are good reducing agents. The reducing character of hydrides increases from NH 3 to BiH 3. NH 3 is the strongest base among all the hydrides. Nitrogen forms five oxides with oxidation state ranging from +1 to +5, while the other elements form oxides only in +3 and +5 oxidation states. The lower oxides of nitrogen are neutral, while the higher oxides are acidic. All the elements of group +15 form trihalides and pentahalides. All the elements of group 15 react with metals to form their binary compound showing -3 oxidation state. Group 15: Nitrogen – Dinitrogen Commercially, dinitrogen is mainly obtained by the fractional distillation of liquid air. In the laboratory, dinitrogen is generally prepared by gently heating equimolar aqueous solutions of ammonium chloride and sodium nitrite. NH 4Cl(aq) + NaNO 2(aq) → NaCl(aq) + NH 4 NO 2(aq) Ammonium Chloride Sodium Nitrite Sodium Chloride Ammonium Nitrite NH 4NO 2(aq) → 2H 2O(vap) + N 2(g) Sodium azide or barium azide, when heated carefully to about 573K, undergoes thermal decomposition to produce dinitrogen. 2NaN 3 → 2Na + 3N2 Sodium Azide Sodium Dinitrogen Gas Ba(N 3)2 → Ba + 3N 2 Barium Azide Barium Dinitrogen Gas Dinitrogen is a colourless, odourless and tasteless non-toxic gas. A molecule of dinitrogen consists of a triple bond that has a very high bond dissociation energy of 945.4 kJmol -1. Hence, dinitrogen is inert at room temperature. At high temperatures, dinitrogen reacts directly with metals such as magnesium, calcium and aluminium to form the respective nitrides. N2 + 3Mg → Mg 3 N2 Dinitrogen Magnesium Magnesium Nitride N2 + 3Ca → Ca 3 N2 Dinitrogen Calcium Calcium Nitride 3N 2 + 6Al → 6AlN Dinitrogen Aluminium Aluminium Nitride It also reacts with hydrogen at high temperature and pressure in the presence of a catalyst. N2 + 3H 2 2NH 3 Dinitrogen Hydrogen Ammonia It is widely used as an inert atmosphere in iron, steel and other metallurgical industries. A large amount of dinitrogen is used to prepare ammonia and nitrolim. Liquid dinitrogen is used as a refrigerant to preserve biological material. Lsn 6: Group 15: Nitrogen - Ammonia Ammonium chloride, when heated with lime or caustic soda, evolves ammonia gas. 2NH 4Cl + Ca(OH) 2 2NH 3 + 2H 2O + CaCl 2 Ammonium Chloride Slaked Lime Ammonia Water Calcium Chloride In accordance with Le Chatelier’s principle, high pressure and low temperature favours the better yield of ammonia. A high pressure of 200-300 atmospheres, an optimum temperature about 700K and the catalyst iron oxide along with promoter alumina are used to obtain better yields of ammonia. Fe 3O4, Al 2O3 200-300 atm -1 N2 + 3H 2 2NH 3 ∆H = -92.2 KJ mol Nitrogen Hydrogen 700K Ammonia On account of the presence of one lone pair in ammonia, the geometry of the molecule is trigonal pyramid. An aqueous solution of ammonia precipitates hydroxides of metal from their aqueous salt solutions. An aqueous solution of ammonia combines with copper sulphate solution to form a deep blue coppertetraammine complex. CuSO 4 + 2NH 4OH Cu(OH) 2 + (NH4)2SO 4 Blue ppt. Cu(OH) 2 + (NH 4)2SO 4 + 2NH 4 OH [ Cu(NH 3)4 ] SO 4 + 4H2O Deep Blue Ammonia is used in the manufacture of certain industrially important chemicals such as nitric acid, baking soda and washing soda. Group 15: Nitrogen – Oxides N2O is prepared by heating NH 4NO 3 gently upto 240 °C. NH NO + N O + 2H O 4 3 240 ° 2 2 Ammonium nitrate Nitrous oxide NO is prepared by the reduction of nitrite salt with FeSO 4. 2NaNO 3 + 2FeSO 3 + 3H 2 SO 4 Fe(SO 4)3 + 2NaHSO 4 + 2H 2O + 2NO Sodium nitrite Ferrous sulphate Nitric oxide N2O and NO are both neutral in nature. Dinitrogen trioxide (N 2O3) is obtained as a blue liquid when a mixture of NO and NO 2 or N 2O4 is cooled to below -20 °C. NO 2 is obtained by heating Pb(NO 3)2 at about 673 K. 2Pb(NO ) 4N O + 2PbO + O 3 2 673 2 2 Lead nitrate Nitrogen dioxide At low temperatures, NO 2 associates to a solid dimer called N 2O4. N2O5 is prepared by the dehydration of HNO 3 with P 4O10. 4HNO 3 + P4O10 4HPO 3 + 2N 2O5 Nitric acid Phosphorus Metaphosphoric Dinitrogen pentoxide acid pentoxide N2O3, NO 2 or N 2O4 and N2O5 are acidic in nature. Group 15: Nitrogen – Nitric Acid In the laboratory, HNO 3 is prepared by heating NaNO 3 with concentrated H 2SO 4. On a commercial scale, HNO 3 is prepared by Ostwald’s process. The steps are: • Catalytic oxidation of NH 3 into NO • Oxidation of NO to NO 2 • Formation of HNO3 from NO 2 + − An aqueous solution of HNO 3 undergoes ionisation to produce H 3O and NO 3. Hot and concentrated HNO 3 acts as a powerful oxidising agent. Concentrated HNO 3 reacts with “Cu” and “Zn” to give NO 2. Dilute HNO 3 reacts with “Cu” to give NO, and with “Zn” to give N 2O. Noble metals dissolve in aqua regia, of which HNO 3 is a constituent. Concentrated HNO 3 oxidises non-metals to their corresponding higher oxoacids. The brown ring test confirms the presence of nitrate ions in a given salt. Group 15: Nitrogen – Allotropes The existence of an element in more than one physical form is called allotropy. The different physical forms of the same element are called allotropes. White phosphorus is a soft, waxy and translucent solid. The angular strain in the molecule makes white phosphorus unstable and highly reactive. Red phosphorus is an iron-grey lustrous crystalline solid that is odourless, non-toxic and insoluble in water. Black phosphorus is highly polymerised and thermally the most stable allotrope of phosphorus. Group 15: Phosphorus – Preparation of Phosphine Phosphine is a hydride of phosphorus analogous to ammonia. The hydrolysis of metallic phosphides with water or dilute mineral acids like hydrochloric acid or sulphuric acid produces phosphine. On a laboratory scale, phosphine is prepared by hydrolyzing white phosphorus with a concentrated solution of sodium hydroxide. Pure phosphine can be obtained by using alcoholic potassium hydroxide instead of an aqueous sodium hydroxide solution. Phosphine has pyramidal geometry with a bond angle of 93.5 °. Group 15: Phosphorus – Properties and Uses of Phosphine Phosphine is a colourless and extremely poisonous gas that smells of rotten fish. Phosphine is slightly soluble in water, and more soluble in carbon disulphide and other organic solvents. The bond angle in the phosphonium ion is greater than in phosphine because the extent of repulsive interaction in it is less than in phosphine. An aqueous solution of phosphine in the presence of light decomposes to red phosphorus and hydrogen. It burns with an explosion when it comes in contact with even small amounts of oxidising agents. It precipitates metal phosphides from their aqueous metal salt solutions. It finds use in the manufacture of Holme’s signals and smoke screens. Group 15: Phosphorus – Halides In the laboratory, PCl 3 is prepared by heating white phosphorus in a current of dry chlorine. PCl3 has pyramidal geometry. It acts as a Lewis base due to its ability to denote its lone pair of electrons.