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3 appreciate general trends in the chemistry of elements of Group15,16,17 and 18. learn the preparation, properties and uses of dinitrogen and phosphorus and some of their important compounds. describe the preparation,properties and uses of dioxygen and ozone and chemistry of somesimple oxides.

4 know allotropic forms of sulphur,chemistry of its important compounds and the structures of its oxoacids describe the preparation, properties and uses of and Cl hydrochloric acid; know the chemistry of interhalogens and structures of oxoacids of halogens; enumerate the uses of noblegases; appreciate the importance of these elements and their compounds in our day to day life.

5 In Class XI, you have learnt that the p-block elements are placed in groups 13 to 18 of the periodic table. Their valence shell electronic configuration is ns2np1–6 (except He which has 1s2 configuration). The properties of p-block elements like that of others are greatly influenced by atomic sizes, ionisation enthalpy, electron gain enthalpy and electronegativity. The absence of dorbitals in second period and presence of d or d and f orbitals in heavier elements (starting from third period onwards) have significant effects on the properties of elements. In addition, the presence of all the three types of elements; metals, metalloids and non-metals bring diversification in chemistry of these elements. Having learnt the chemistry of elements of Groups 13 and 14 of the p-block of periodic table in Class XI, you will learn the chemistry of the elements of subsequent groups in this Unit

6 Atomic and Physical Properties of Group 15 Elements

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11 Preparation Dinitrogen is produced commercially by the liquefaction and fractional distillation of air. Liquid dinitrogen (b.p. 77.2 K) distils out first leaving behind liquid oxygen (b.p. 90 K). In the laboratory, dinitrogen is prepared by treating an aqueous solution of ammonium chloride with sodium nitrite. NH4CI(aq) + NaNO2(aq) → N2(g) + 2H2O(l) + NaCl (aq) Small amounts of NO and HNO3 are also formed in this reaction; these impurities can be removed by passing the gas through aqueous sulphuric acid containing potassium dichromate. It can also be obtained by the thermal decomposition of ammonium dichromate. (NH4)2Cr2O7 → N2 + 4H2O + Cr2O3 Very pure nitrogen can be obtained by the thermal decomposition of sodium or barium azide. Ba(N3)2 → Ba + 3N2

12 Properties Dinitrogen is a colourless, odourless, tasteless and non-toxic gas. Nitrogen atom has two stable isotopes: 14N and 15N. It has a very low solubility in water (23.2 cm3 per litre of water at 273 K and 1 bar pressure) and low freezing and boiling points (Table 7.1). Dinitrogen is rather inert at room temperature because of the high bond enthalpy of N ≡ N bond. Reactivity, however, increases rapidly with rise in temperature. At higher temperatures, it directly combines with some metals to form predominantly ionic nitrides and with non-metals, covalent nitrides. A few typical reactions are: 6Li + N2 Heat... → 2Li3N 3Mg + N2 Heat... → Mg3N2

13 Ammonia

14 On a small scale ammonia is obtained from ammonium salts which decompose when treated with caustic soda or calcium hydroxide. 2NH4Cl + Ca(OH)2 → 2NH3 + 2H2O + CaCl2 (NH4)2 SO4 + 2NaOH → 2NH3 + 2H2O + Na2SO4 On a large scale, ammonia is manufactured by Haber’s process. N2(g) + 3H2(g) Ö 2NH3(g); Df H0 = – 46.1 kJ mol–1 In accordance with Le Chatelier’s principle, high pressure would favour the formation of ammonia. The optimum conditions for the production of ammonia are a pressure of 200 × 105 Pa (about 200 atm), a temperature of ~ 700 K and the use of a catalyst such as iron oxide with small amounts of K2O and Al2O3 to increase the rate of attainment of equilibrium. The flow chart for the production of ammonia is shown in Fig. 7.1. Earlier, iron was used as a catalyst with molybdenum as a promoter. © NCERT not to be republished PREPARATIONPREPARATION

15 PhosphinePhosphine

16 Preparation Phosphine is prepared by the reaction of calcium phosphide with water or dilute HCl. Ca3P2 + 6H2O → 3Ca(OH)2 + 2PH3 Ca3P2 + 6HCl → 3CaCl2 + 2PH3 In the laboratory, it is prepared by heating white phosphorus with concentrated NaOH solution in an inert atmosphere of CO2. 2 P 3NaOH 3H O PH 3NaH PO sodium hypophosphite When pure, it is non inflammable but becomes inflammable owing to the presence of P2H4 or P4 vapours. To purify it from the impurities, it is absorbed in HI to form phosphonium iodide (PH4I) which on treating with KOH gives off phosphine. 4 2 3 PH I KOH KI H O PH Properties It is a colourless gas with rotten fish smell and is highly poisonous. It explodes in contact with traces of oxidising agents like HNO3, Cl2 and

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21 Groups 13 to 18 of the periodic table consist of p-block elements with their valence shell electronic configuration ns2np1–6. Groups 13 and 14 were dealt with in Class XI. In this Unit remaining groups of the p-block have been discussed. Group 15 consists of five elements namely, N, P, As, Sb and Bi which have general electronic configuration ns2np3. Nitrogen differs from other elements of this group due to small size, formation of p ð–p ð multiple bonds with itself and with highly electronegative atom like O or C and non-availability of d orbitals to expand its valence shell. Elements of group 15 show gradation in properties. They react with oxygen, hydrogen and halogens. They exhibit two important oxidation states, + 3 and + 5 but +3 oxidation is favoured by heavier elements due to ‘inert pair effect’. Dinitrogen can be prepared in laboratory as well as on industrial scale. It forms oxides in various oxidation states as N2O, NO, N2O3, NO2, N2O4 and N2O5. These oxides have resonating structures and have multiple bonds. Ammonia can be prepared on large scale by Haber’s process

22 PX3 and PX5. PCl3 is prepared by the reaction of white phosphorus with dry chlorine while PCl5 is prepared by the reaction of phosphorus with SO2Cl2. Phosphorus forms a number of oxoacids. Depending upon the number of P–OH groups, their basicity varies. The oxoacids which have P–H bonds are good reducing agents. The Group 16 elements have general electronic configuration ns2np4. They show maximum oxidation state, +6. Gradation in physical and chemical properties is observed in the group 16 elements. In laboratory, dioxygen is prepared by heating KClO3 in presence of MnO2. It forms a number of oxides with metals. Allotropic form of oxygen is O3 which is a highly oxidising agent. Sulphur forms a number of allotropes. Of these, á– and â– forms of sulphur are the most important. Sulphur combines with oxygen to give oxides such as SO2 and SO3

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