P Block Elements 12th PDF

Pre-Medical : Chemistry ALLEN NITROGEN FAMILY (GROUP 15)...

Pre-Medical : Chemistry ALLEN NITROGEN FAMILY (GROUP 15) Group 15 includes nitrogen phosphorus, arsenic, antimony and bismuth. PHYSICAL PROPERTIES: Dinitrogen is a diatomic gas while all others are solids. Except nitrogen, all the elements show allotropy. As we go down the group, there is a shift from non-metallic to metallic through metalloidic character. Nitrogen and phosphorus are non-metal, arsenic and antimony are metalloid and bismuth is a typical metal. CHEMICAL PROPERTIES Oxidation States and trends in a chemical reactivity : The common oxidation states of these elements are –3, +3 and +5. The tendency to exhibit –3 oxidation state decreases down the group , bismuth hardly forms any compound in –3 oxidation state. Nitrogen also exhibits +1, +2, +4 oxidation states when it reacts with oxygen. Phosphorus also shows +1 and +4 oxidation states in some oxyacids. EN In the case of nitrogen, all oxidation states from +1 to +4 tend to disproportionate in acid solution. For example, 3 HNO2 ¾¾® HNO3 + H2O + 2 NO 0 Similarly, in case of phosphorus nearly all intermediate oxidation states disproportionate into +5 and –3 both in alkali and acid. However +3 oxidation state in case of arsenic , antimony and bismuth become increasingly -2 stable with respect to disproportionation. Reactivity towards hydrogen : 19 All the elements of Group 15 form hydrides of the type EH3 where E=N, P, As, Sb or Bi. The stability of hydrides decreases from NH3 to BiH3. Consequently the reducing character of the hydrides increases. Ammonia is only a mild reducing agent while BiH3 is the strongest reducing agent amongst all the hydrides. Due to weaker Bi–H bond. 20 LL Reactivity towards oxygen : All these elements form two types of oxides : E2O3 and E2O5. The oxides of the type E2O3 of nitrogen and n phosphorus are purely acidic , that of arsenic and antimony amphoteric and those of bismuth is predominantly basic. io Reactivity towards halogens : ss These elements react to form two series of halides : EX3 and EX5. All the trihalides of these elements except those of nitrogen are stable. In case of nitrogen, only NF3 is known to be stable. Trihalides except BiF3 are predominantly covalent in nature. Se A COMPOUND OF NITROGEN Z:\NODE02\B0AI-B0\TARGET\CHEM\ENG\MODULE-5\1-P-BLOCK\1-THEORY.P65 1. NITROGEN GAS (N2) : Nitrogen is an important and essential constituent of proteins and amino acids. Nitrates and other nitrogen compounds are extensively used in fertilizers and explosive. (a) Preparation : (i) Laboratory method of preparation : NH4Cl(aq) + NaNO2(aq) ¾¾® N2(g) + H2O (l) + NaCl(aq) N2 is collected by the downward displacement of water. This reaction takes place in two steps as given below : D NH4Cl + NaNO2 ® NH4NO2 + NaCl ; NH4NO2 ¾¾® N2­ + 2H2O. E 29 Pre-Medical : Chemistry ALLEN (ii) By heating ammonium dichromate : D (NH4)2Cr2O7 ¾¾® N2 ­ + 4H2O + Cr2O3 (iii) Very pure nitrogen can be obtained by heating sodium or barium azide. D Ba(N3)2 ¾¾® Ba + 3N2 300ºC 2NaN3 ¾¾ ¾¾® 3N2 + 2Na (iv) Industrial methods of preparation : From liquified air by fractional distillation : The boiling point of N2 is –196oC and that of oxygen is – 183oC and hence they can be separated by fractional distillation of air. (b) Properties : (i) N2 is a colourless, odourless gas very less soluble in water. It is neither combustible nor a supporter of combustion. (ii) Reaction with oxygen: Dinitrogen combines with dioxygen only at very high temperature (at about 2000K) to form nitric oxide. (c) This reaction is endothermic. Uses : EN N2 + O2 ¾¾® 2NO 0 (i) For providing an inert atmosphere during many industrial processes where presence of air or O2 is to be -2 avoided. (ii) For manufacture of NH3 by the Haber’s process. 19 (iii) Liquid dinitrogen is used as a refrigerent to preserve biological materials, food items & cryosurgery. 2. AMMONIA (NH3) (a) Preparation : 20 LL (i) By the action of any base or alkali on any ammonium salt : n D NH4NO3 + NaOH ¾¾® NH3­ + NaNO3 + H2O io D (NH4)2SO4 + Ca(OH)2 ¾¾® 2NH3­ + CaSO4 + 2H2O ss This is a general method and is used as a test for ammonium salts. Ammonia is present in small quantities in air and soil where it is formed by the decay of nitrogenous Se organic matter e.g., urea. A NH2CONH2 + 2H2O ¾¾® (NH4)2CO3 2NH3 + H2O + CO2 Z:\NODE02\B0AI-B0\TARGET\CHEM\ENG\MODULE-5\1-P-BLOCK\1-THEORY.P65 (ii) Industrial methods of preparation : ˆˆˆˆˆˆˆˆˆˆˆˆ † 0 500 C,200atm Haber’s process : N2 + 3H2 ‡ˆˆˆˆˆˆˆˆˆˆˆ Iron oxide+K 2O & Al2 O3 ˆ 2NH3 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. (b) Physical properties : Ammonia is a colourless gas with a pungent odour. (c) Chemical properties : (i) Its aqueous solution is weakly basic due to the formation of OH – ions. NH3 (g) + H2O (l) NH4+ (aq) + OH– (aq) 30 E  Pre-Medical : Chemistry ALLEN (ii) It forms ammonium salts with acids, e.g., NH4Cl, (NH4)2 SO4 etc. As a weak base, it precipitates the hydroxides of many metals from their salt solutions. For example , FeCl3 (aq) + NH4OH (aq) ¾¾® Fe2O3. xH2O (s) + NH4Cl (aq) (brown ppt) ZnSO4 (aq) + 2NH4OH (aq) ¾¾® Zn(OH)2 (s) + (NH4)2 SO4 (aq) (white ppt) (iii) The presence of lone pair of electrons on the nitrogen atoms of the ammonia molecule makes it a Lewis base. It donates the electrons pair and forms linkage with metal ions and the formation of such complex compounds finds applications in detection of metal ions such as Cu2+ , Ag+ Cu2+ (aq) + 4 NH3 (aq) [Cu(NH3)4]2+ (aq) (blue) (deep blue) Ag+ (aq) + Cl– (aq) AgCl (s) (colourless) (white ppt) AgCl (s) + 2 NH3 (aq) ¾¾® [Ag (NH3)2]Cl (aq) (white ppt) (colourless) 3. OXIDES OF NITROGEN : EN Nitrogen forms a number of oxides, N2O, NO, N2O3, NO2 or N2O4 and N2O5. All these oxides of nitrogen exhibit pp-pp multiple bonding between nitrogen and oxygen. 0 -2 19 20 LL n io ss Se A Z:\NODE02\B0AI-B0\TARGET\CHEM\ENG\MODULE-5\1-P-BLOCK\1-THEORY.P65 4. NITRIC ACID (HNO3) H m 102° 1p O 12 96 O 140.6 pm N 130° Structure of HNO3 pm O (a) Preparation : In the laboratory, nitric acid is prepared by heating KNO3 or NaNO3 and concentrated H2SO4 in a glass retort. NaNO3 + H2SO4 ¾¾® NaHSO4 + HNO3 E 31 Pre-Medical : Chemistry ALLEN On a large scale it is prepared mainly by Ostwald’s process. This method is based upon catalytic oxidation of NH3 by atmospheric oxygen. 4 NH3 (g) + 5O2 (g) 4 NO (g) + 6 H2O (g) (from air) Nitric oxide thus formed combines with oxygen giving NO2. 2 NO (g) + O2 (g) 2 NO2 (g) Nitrogen dioxide so formed, dissolves in water to give HNO 3. 3 NO2 (g) + H2O (l) ¾¾® 2 HNO3 (aq) + NO (g) NO thus formed is recycled and the aqueous HNO 3 can be concentrated by distillation upto ~ 68% by mass. Further concentration to 98% can be achieved by dehydration with concentrated H2SO4. (b) Physical properties : (i) It is a colourless liquid. Freezing point is 231.4 K and boiling point is 355.6 K. Laboratory grade nitric acid contains ~ 68% of the HNO3 by mass and has a specific gravity of 1.504. (ii) In the gaseous state, HNO3 exists as a planar molecule. (iii) EN In aqueous solution, nitric acid behaves as a strong acid giving hydronium and nitrate ions. HNO3 (aq) + H2O (l) ¾¾® 2H3O+ (aq) + NO3– (aq) 0 (iv) Concentrate nitric acid is a strong oxidising agent and attacks most metals except Au & Pt. The product -2 of oxidation depend upon the concentration of the acid, temperature and nature of metal. (v) Concentrate HNO3 also oxidises non-metals and their compounds. 19 conc. HNO3 Zn Zn(NO3)2 Cu Cu(NO3)2 Ccharcoal CO2 + H2O 20 LL Cdiamond × B H3BO3 n P4 H3PO4 ic acid H3AsO 4 io As S8 H2SO4 ss Sn H2SnO3 I2 HIO3 H2S S Se SO2 SO4—2 A Cgraphite C6(COOH)6 mellitic acid Z:\NODE02\B0AI-B0\TARGET\CHEM\ENG\MODULE-5\1-P-BLOCK\1-THEORY.P65 NO2 (Brown Gas) Types of Metal conc. HNO3 dil. HNO3 very dil. HNO3 negative SRP metal Metal nitrate Metal nitrate Metal nitrate + NO2 + N2 O + NH4NO3/(NH3) positive SRP metal Metal nitrate Metal nitrate × & Pb + NO2 + NO Metalloids, ic-acid × × Non-metals & Sn + NO2 EX. Zn(NO3 )2 + N2O ¬¾¾¾ dil.HNO3 conc.HNO3 ¾ Zn ¾¾¾¾¾ ® Zn(NO3 )2 + NO2 Cu(NO3 )2 + NO ¬¾¾¾ dil.HNO3 conc.HNO3 ¾ Cu ¾¾¾¾¾ ® Cu(NO3 )2 + NO2 32 E  Pre-Medical : Chemistry ALLEN SOME EXCEPTIONAL POINTS Mg and Mn form H2 gas on reaction with very dilute HNO3 Be, Al, Fe, Cr do not react with conc. HNO3 due to fromation of protective oxide layer. Au and Pt (Noble metals) do not react with conc. HNO3 Sn reacts with conc. HNO3 and forms H2SnO3 (metastannic acid) Au and Pt react only with aqua regia (3 part of conc. HCl and 1 part of conc. HNO3) 3HCl + HNO3 ®NOCl + 2H2O + Cl– Au + 3Cl - ® AuCl3 ¾¾¾ HCl ® HAuCl 4 tetrachloro auric acid Pt + 4Cl - ® PtCl 4 ¾¾¾ 2HCl ® H PtCl 2 6 hexachloro platinic acid conc. HNO3 + skin ® yellow spot (protein ® xanthoprotein) The major use of nitric acid is in the manufacture of ammonium nitrate for fertilizers and other nitrates for use in explosives and pyrotechnics. It is also used for the preparation of nitroglycerin, trinitrotoluene and other organic nitro compounds. Other major uses are in the pickling of EN stainless steel, etching of metals and as an oxidiser in rocket fuels. COMPOUNDS OF PHOSPHORUS : 0 1. Phosphine (PH3) : -2 (a) Preparation : (i) Phosphine is prepared by the reaction of calcium phosphide with water. 19 Ca3P2 + 6 H2O ¾¾® 3 Ca(OH)2 + 2 PH3 (ii) In the laboratory, it is prepared by heating white phosphorus with concentrated NaOH solution in an inert atmosphere of CO2. 20 LL P4 + 3 NaOH + 3 H2O ¾¾® PH3 + 3 NaH2PO2 n (sodium hypophosphite) (iii) When pure, it is non inflammable but becomes inflammable owing to the presence of P2H4 or P4 io vapours. To purify it from the impurities , it is absorbed in HI to form phosphonium iodide (PH 4I) which on treating with KOH gives off phosphine. ss PH4I + KOH ¾¾® KI + H2O + PH3 Se (b) Properties : A (i) It is a colourless gas with rotten fish smell and is highly poisonous. It is slightly soluble in water. The solution of PH3 in water decomposes in presence of light giving red phosphorus and H2. Z:\NODE02\B0AI-B0\TARGET\CHEM\ENG\MODULE-5\1-P-BLOCK\1-THEORY.P65 (ii) When absorbed in copper sulphate or mercuric chloride, the corresponding phosphides are obtained. 3CuSO4 + 2PH3 ¾¾® Cu3P2 ¯ + 3H2SO4 3HgCl2 + 2 PH3 ¾¾® Hg3P2 ¯ (brownish black) + 6 HCl (iii) Phosphine is weakly basic and like ammonia, gives phosphonium compounds with acids e.g. PH3 + HBr ¾¾® PH4Br Uses : The spontaneous combustion of phosphine is technically used in Holme’s signals. Containers containing calcium carbide and calcium phosphide are pierced and thrown in the sea when the gases evolved burn and serve as a signal. It is also used in the production of smoke screens. Calcium phosphide reacts with water producing phosphine which burns in air to give clouds of phosphorus pentaoxide and that acts as smoke screens. E 33 Pre-Medical : Chemistry ALLEN 2. PHOSPHOROUS HALIDES Phosphorous forms two types of halides, PX3 (X = F, Cl, Br, I) and PX5 (X = F, Cl, Br). Phosphorous Trichloride (PCl3) (a) Method of preparation (i) It is obtained by passing dry chlorine over heated white phosphorus. P4 + 6Cl2 ® 4PCl3 (ii) It is also obtained by the action of thionyl chloride with white phosphorus. P4 + 8SOCl2 ® 4PCl3 + 4SO2 + 2S2Cl2 (b) Properties (i) It is a colourless oily liquid and hydrolyses in the presence of moisture. PCl3 + 3H2O ® H3PO3 + 3HCl (ii) It reacts with organic compounds containing –OH group such as CH3COOH, C2H5OH. 3CH3COOH + PCl3 ® 3CH3COCl + H3PO4 P 3C2H5OH + PCl3 ® 3C2H5Cl + H3PO3 Cl Cl Cl EN PHOSPHORUS PENTACHLORIDE (PCl5) (a) Method of preparation (i) Phosphorus pentachloride is prepared by the reaction of white phosphorus with excess of dry chlorine. 0 P4 + 10Cl2 ® 4PCl5 -2 (ii) It can also be prepared by the action of SO2Cl2 on phosphorus. P4 + 10SO2Cl2 ® 4PCl5 + 10SO2 (b) Properties 19 (i) PCl5 is a yellowish white powder and in moist air, it hydrolyses to POCl3 and finally gets converted to phosphoric acid. PCl5 + H2O ® POCl3 + 2HCl POCl3 + 3H2O ® H3PO4 + 3HCl 20 LL (ii) When heated, it sublimes but decomposes on stronger heating. n PCl 5 ¾¾¾ Heat ® PCl 3 + Cl 2 io (iii) It reacts with organic compounds containing –OH group converting them to chloro derivatives. C2H5OH + PCl5 ® C2H5Cl + POCl3 + HCl ss (iv) Finally divided metals on heating with PCl5 give corresponding chlorides. 2Ag + PCl5 ® 2AgCl + PCl3 Se Sn + 2PCl5 ® SnCl4 + 2PCl3 A (v) It is used in the synthesis of some organic compounds, e.g., C2H5Cl, CH3COCl. Z:\NODE02\B0AI-B0\TARGET\CHEM\ENG\MODULE-5\1-P-BLOCK\1-THEORY.P65 34 E  Pre-Medical : Chemistry ALLEN NCERT QUESTIONS (REASONING) Q.1 Why BiH3 is the strongest reducing agent amongst all the hydrides of Group 15 elements ? Ans. In hydrides of nitrogen family on moving down the group M–H bond length increases so bond strength decreases hence tendency to release hydrogen increases and reducing nature increases. Increasing order of reducing nature is NH 3< PH3 < AsH3 < SbH3 < BiH3 Q.2 Write the reaction of thermal decomposition of sodium azide. Ans. Thermal decomposition of sodium azide gives dinitrogen gas. 2NaN3 ® 2Na + 3N2 Q.3 Why N2 is less reactive at room temperature? Ans. N2 is less reactive at room temperature because of the high bond enthalpy of NºN bond. Q.4 Why does R3P = O exist but R3N = O does not (R = alkyl group)? Ans. Due to presence of vacant d orbital phosphorous can form five covalent bond, while nitrogen restricts its covalency to four due to absence of vacant d orbitals. Q.5 Ans. EN Why does nitrogen show catenation properties less than phosphorus? N–N bond is weaker than the single P–P bond due to high interelectronic respulsion of non-bonding electrons as a result the catenation tendency is weaker in nitrogen. 0 -2 Q.6 Mention the conditions required to maximise the yield of ammonia. Ans. 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 19 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. Q.7 How does ammonia react with a solution of Cu2+? 20 LL Ans. The presence of a lone pair of electrons on the nitrogen atom of the ammonia molecule makes it a Lewis base. It donates the electron pair and forms linkage with metal ions and the formation of such complex compounds finds applications in detection of Cu2+. n ˆˆ† [Cu(NH3 ) 4 ]2 + (aq) Cu 2 + (aq) + 4NH 3 (aq) ‡ˆˆ io (blue) (deep blue) Q.8 Illustrate how copper metal can give different products on reaction with HNO3. ss Ans. 3Cu + 8 HNO3(dilute) ® 3Cu(NO3)2 + 2NO + 4H2O Cu + 4HNO3(conc.) ® Cu(NO3)2 + 2NO2 + 2H2O Se Q.9 Why is nitrogen di-oxide paramagnetic in gaseous state but the solid obtained on cooling it A is diamagnetic. Z:\NODE02\B0AI-B0\TARGET\CHEM\ENG\MODULE-5\1-P-BLOCK\1-THEORY.P65 Ans. NO2 contains odd number of electrons in its valence shell. On cooling it forms dimer and converted to stable N2O4 which is a colourless solid and diamagnetic in nature. Q.10 Why NH3 gas cannot be dried by passing over P2O5 , CaCl2 and H2SO4 ? Ans. CaCl2 + 8NH3 ¾¾® CaCl2.8NH3 P2O5 + 6NH3 + 3H2O ¾¾® 2(NH4)3PO4 H2SO4 +2NH3 ¾¾® (NH4)2SO4 So it is dried by passing over quick lime (CaO). CaO + H2O ¾¾® Ca(OH)2 Q.11 Why inert atmosphere of CO2 is taken in the formation of PH3 by the reaction of white phosphorous with conc. NaOH solution. Ans. To decrease the partial pressure of O2 in atmosphere. E 35 Pre-Medical : Chemistry ALLEN Q.12 Why does PCl3 fumes in moisture? Ans. PCl3 hydrolyses in the presence of moisture giving fumes of HCl PCl 3 + 3H2O ® H3 PO3 + 3HCl Q.13 What happens when PCl5 is heated? Ans. When heated, it sublimes but decomposes on stronger heating D PCl 5 ¾¾ ® PCl 3 + Cl2 Q.14 Can PCl5 act as an oxidising as well as reducing agent? Justify. Ans. It can act as an oxidising as well as reducing agent due to oxidising nature of P(V) and reducing nature of Cl– BEGINNER'S BOX-7 1. Which of the following halide of nitrogen is stable? (1) NF3 (2) NCl3 (3) NBr3 (4) NI3 2. The nitrogen oxide(s) that does not contain N–N bond(s) is (1) N2O (2) N2O3 (3) N2O4 (4) N2O5 3. What is false about N2O5 ? (1) It is anhydride of HNO3 EN (2) It is a powerful oxidizing agent 0 (3) Solid N2O5 is called nitronium nitrate (4) Structure of N2O5 contains no [N®O] bond -2 4. Conc. HNO3 is yellow coloured liquid due to – (1) Dissolution of NO in conc. HNO3 (2) Dissolution of NO2 in conc. HNO3 (3) Dissolution of N2O in conc. HNO3 (4) Dissolution of N2O3 in conc. HNO3 19 5. Which of the following process is not involved in Ostwald’s process for the manufacture of HNO3? (1) 4NH3 ( g ) + 5O 2 ( g ) ¾¾¾¾¾ Pt ® 4NO ( g ) + 6H 2 O ( g ) ˆˆˆ † (2) 2NO ( g ) + O2 ‡ˆˆ 500K,9 bar ˆ 2NO2 ( g ) 20 LL (3) 3NO2 ( g ) + H2 O ( l ) ® 2HNO3 ( aq ) + NO ( g ) n (4) None 6. Which of the following salts give NH3 in alkaline medium? io (1) (NH4)2CO3 (2) (NH4)2SO4 (3) NH4Cl (4) All of the above 7. Which of the following reaction is suitable for obtaining very pure nitrogen? ss (1) NH4Cl(aq) + NaNO2(aq) ¾¾® N2(g) + 2H2O(l) + NaCl(aq) Se (2) (NH4)2Cr2O7 ¾¾ D ® N2+4H2O + Cr2O3 A (3) Ba(N3)2 ¾¾ D ® Ba + 3N2 Z:\NODE02\B0AI-B0\TARGET\CHEM\ENG\MODULE-5\1-P-BLOCK\1-THEORY.P65 (4) All of the above 8. In warfare smoke screens are prepared from (1) CaC2 (2) PH3 (3) P2O5 (4) COCl2 9. White phosphorouus on reaction with NaOH gives PH3 as one of the products. This is a (1) dimerisation reaction (2) disproportionation reaction (3) condensation reaction (4) precipitation reaction 10. Which of the following is not correctly matched? (1) P4O10 + H2O ¾¾® reactants involved in formation of H3PO4 (2) CaC2 + H2O ¾¾® C2H2+Ca(OH)2; Ca3P2 + H2O ¾¾® PH3+Ca(OH)2 reactions involved in Holmes signal (3) PH3 + HI ¾¾® PH4I ¾¾¾ KOH ® KI + H2O + PH3; purification of PH3 (4) PH3 + HI ¾¾® PH4I ; shows Lewis acidic nature of PH3 36 E  Pre-Medical : Chemistry ALLEN OXYGEN FAMILY (GROUP 16 ) INTRODUCTION Oxygen, sulphur, selenium, tellurium and polonium constitute group 16 of the periodic table. This is sometimes known as group of chalcogens (ore forming elements) because a large number of metals ores are oxides or sulphides. Physical Properties : Oxygen and sulphur are non-metal, selenium and tellurium metalloids, whereas polonium is a metal. Polonium is radioactive and is short lived (Half-life 13.8 days). The melting and boiling points increase with an increase in atomic number down the group. The larger difference between the melting and boiling points of oxygen and sulphur may be explained on the basis of their atomicity; oxygen exist as diatomic molecules (O2) whereas sulphur exists as polyatomic molecule (S8). CATENATION Tendency for catenation decreases down the group. This property is prominently displayed by sulphur (S8). The S—S bond is important in biological system and is found in some proteins and enzymes such as cysteine. Chemical Properties Oxidation states and trends in chemical reactivity : EN The elements of group 16 exhibit a number of oxidation states. The stability of -2 oxidation state decreases down the group. Except oxygen other elements of the group exhibit + 2, + 4, + 6 oxidation states but + 4 and + 6 are more common. Sulphur, selenium and tellurium usually show + 4 oxidation in their compounds with oxygen and +6 oxidations state with fluorine. The stability of +6 oxidation state decreases down the 0 group and stability of + 4 oxidation state increases (inert pair effect). Bonding in + 4 and + 6 oxidation states -2 are primarily covalent. (i) Reactivity with hydrogen : All the elements of group 16 form hydrides of the type H2E (E = O, S, Se, Te, Po). 19 (ii) Reactivity with oxygen : All these elements form oxides of the EO2 and EO3 types where E = S, Se, Te or 20 Po. Ozone (O3) and sulphur dioxide (SO2) are gases while selenium dioxide (SeO2) is solid. Reducing property of dioxide decreases from SO2 to TeO2 ; SO2 is reducing while TeO2 is an oxidising agent. Besides EO2 type LL sulphur, selenium and tellurium also form EO 3 type oxides (SO3, SeO3, TeO3). Both types of oxides are acidic in nature. n (iii) Reactivity toward the halogens : Elements of group 16 form a larger number of halides of the type EX6, EX4 and EX2 where E is an element of the group –16 and X is an halogen. The stabilities of the halides io decrease in the order F > Cl > Br > l. Amongst hexahalides, hexafluorides are the only stable halides. All hexafluorides are gaseous in nature. They have octahedral structure. Sulphur hexafluoride SF6 is exceptionally ss stable for steric reasons. Amongst tetrafluorides, SF4 is a gas , SeF4 liquid and TeF4 a solid Se All elements except selenium form dichlorides and dibromides. The well known monohalides are dimeric in nature, Examples are S2F2, S2Cl2, S2Br2, Se2Cl2 and Se2Br2. These dimeric halides undergo disproportionation A as given 2Se2Cl2 ¾¾® SeCl4 + 3Se. Z:\NODE02\B0AI-B0\TARGET\CHEM\ENG\MODULE-5\1-P-BLOCK\1-THEORY.P65 COMPOUNDS OF OXYGEN 1. DIOXYGEN (O2) (a) Preparation of DiOxygen (O2) : By thermal decomposition of oxides of metals. 0 0 350 C ¾® 2 Hg + O2 ; 2 Ag2O ¾¾¾ 450 C 2 HgO ¾¾ ¾ ¾® 4 Ag + O2 D D 3 MnO2 ¾¾® Mn3O4 + O2 ; 2 Pb3O4 ¾¾® 6 PbO + O2 D KClO3 ¾¾® 2 KCl + 3O2 (laboratory method) D D 4 K2Cr2O7 ¾¾® 4 K2CrO4 + 2 Cr2O3 + 3O2 ; 2 KMnO4 ¾¾® K2MnO4 + MnO2 + O2 D 2PbO2(s) ¾¾® 2PbO(s) + O2(g) E 37 Pre-Medical : Chemistry ALLEN Properties: Dioxygen is a colourless and odourless gas. Oxygen atom has three stable isotopes: 16O, 17O and 18O. Molecular oxygen. O2 is paramagnetic (by MOT) Dioxygen directly reacts with nearly all metals and non-metals except some metals ( e.g., Au, Pt) and some noble gases. 2. OZONE (O3) : Preparation : It is prepared by passing silent electric discharge through pure and dry oxygen. 2O3 DH (298K) = +142 kJ mol - -1 3O2 + + + O O O 117° O O O O -½ O O-½ Mixture obtained contains 5-10% ozone by volume and this mixture is called ozonised oxygen. EN Since the formation of ozone from oxygen is an endothermic process, it is necessary to use a silent electrical discharge in its preparation to prevent its decomposition. If concentrations of ozone greater than 10 per cent are required, a battery of ozonisers can be used, and pure ozone (b.p. 385 K) can be condensed in a vessel surrounded by liquid oxygen. 0 Properties : -2 Pure ozone is a pale blue gas, dark blue liquid and violet-black solid. Ozone is thermodynamically unstable with respect to oxygen since its decomposition into oxygen results in 19 the liberation of heat (DH is negative) and an increase in entropy (DS is positive). These two effects reinforce each other, resulting in large negative Gibbs energy change (DG) for its conversion into oxygen. Therefore, high concentrations of ozone can be dangerously explosive. Oxidising behaviour of ozone : 20 LL Due to the ease with which it liberates atoms of nascent oxygen (O3 ® O2 + O), it acts as a powerful oxidising agent. For example, it oxidises lead sulphide to lead sulphate and iodide ions to iodine. n io O3 ss ite — —2 ate — —2 (NO2 , SO3 ) (NO3 , SO4 ) NO NO2 Se SO2 SO3 A HX X2 (except HF) I2 PbS PbSO4 Z:\NODE02\B0AI-B0\TARGET\CHEM\ENG\MODULE-5\1-P-BLOCK\1-THEORY.P65 Hg Hg2O dry I2 I4O9 moist I2 HIO3 O2 Tests for Ozone Tailing of mercury : Pure mercury is a mobile liquid but when brought in contact with O3 its mobility decreases and it starts sticking to glass surface forming a type of tail due to the dissolution of Hg 2O (mercury sub-oxide) in Hg. 2 Hg + O3 ¾¾® Hg2O + O2 38 E  Pre-Medical : Chemistry ALLEN Estimation of Ozone : When ozone reacts with an excess of potassium iodide solution buffered with a borate buffer (pH 9.2), iodine is liberated which can be titrated against a standard solution of sodium thiosulphate. This is a quantitative method for estimating O 3 gas. 2KI + H2O + O3 ® O2 + I2 + 2KOH ( Na 2 S2O3.5H2 O ) I2 ¾¾¾¾¾¾® 2NaI + Na2S4O6 (sodium tetra thionate) Depletion of ozone layer : Experiments have shown that nitrogen oxides (particularly nitric oxide) combine very rapidly with ozone and there is, thus, the possibility that nitrogen oxides emitted from the exhaust systems of supersonic jet aeroplanes might be slowly depleting the concentration of the ozone layer in the upper atmosphere. NO(g) + O3(g) ® NO2(g) + O2(g) Another threat to this ozone layer is probably posed by the use of freons which are used in aerosol sprays and as refrigerants. COMPOUNDS OF SULPHUR : 1. Hydrogen Sulphide (H2S) Preparation : FeS + H2SO4 ¾¾® FeSO4 + H2S It is prepared in kipp’s apparatus Properties : (i) (ii) (iii) EN Colourless gas with rotten egg smell. Moderately soluble in water but solubility decreases with increasing temperature. It gives black ppt with lead acetate 0 (CH3 COO)2 Pb+ H2S ¾¾® PbS + 2CH 3COOH salt black ppt -2 acid Reducing behaviour : Acts as a strong reducing agent as it decomposes evolving hydrogen. 19 H2S + SO2 ¾¾ ¾¾® H2O + S; moisture redox 2FeCl 3 + H2 S ¾¾¾ ® 2FeCl 2 + S + 2HCl 2. yellow SO2 (Sulphur Dioxide) green 20 LL Preparation : n Burn (i) S + O2 or air ¾¾ ¾® SO2 (ii) By reaction of metal sulphites with dilute HCl (Laboratory method) io Na2SO3 + 2HCl ¾¾® 2NaCl + SO2 + H2O Similarly bisulphites also give SO2 with dilute HCl ss NaHSO3 + HCl ¾¾® NaCl + SO2 + H2O (iii) By heating sulphides (metal sulphide ores) in excess of air. Se 2 ZnS + 3O2 ¾¾® 2ZnO + 2SO2 By this method SO2 is obtained in large scale A Properties : Z:\NODE02\B0AI-B0\TARGET\CHEM\ENG\MODULE-5\1-P-BLOCK\1-THEORY.P65 (i) Colourless gas with pungent smell. (ii) It is heavier than air and is highly soluble in water. (iii) Acidic Nature : Acidic oxide and thus dissolve in water forming sulphurous acid. SO2 + H2O ¾¾® H2SO3 Reducing nature : SO2 (Orange) Cr2O7 2– Cr+3 (Green) – +2 (Purple) MnO4 Mn +3 +2 (Yellow)Fe Fe (Green) 2– SO4 The above reactions are not given by CO2, so they are used to distinguish between SO2 and CO2 E 39 Pre-Medical : Chemistry ALLEN Oxidising nature : Acts as oxidising agent with strong reducing agent SO2 (Br–, I–)X– X2 H2S S (Milky sulphur) S 3. SULPHURIC ACID (H2SO4) : Manufacture by contacts process : Water Conc. H2SO4 spray spray Conc. H 2SO 4 Impure SO3 + O 2 Dry SO2 + O2 SO3 Sulphur EN V2O5 Preheater Quartz 0 -2 Air Sulphur Waste Waste Catalytic Oleum converter 19 burner water Acid (H2S2O7) Dust Arsenic purifier Washing and Drying containing gelatinous precipitator cooling tower tower hydrated ferric oxide Flow diagram for the manufacture of sulphuric acid 20 LL Sulphuric acid is one of the most important industrial chemicals worldwxide. n Sulphuric acid is manufactured by the contact process which involves three steps : io (i) burning of sulphur or sulphide ores in air to generate SO2 ss (ii) Conversion of SO2 to SO3 by the reaction with oxygen in the presence of a catalyst (V2O5), and (iii) Absorption of SO3 in H2SO4 to give Oleum (H2S2O7) Se The SO2 produced is purified by removing dust and other impurities such as arsenic compounds. A The key step in the manufacture of H2SO4 is the catalytic oxidation of SO2 with O2 to give SO3 in the presence of V2O5 (catalyst). Z:\NODE02\B0AI-B0\TARGET\CHEM\ENG\MODULE-5\1-P-BLOCK\1-THEORY.P65 V2 O5 2SO2(g) + O2(g) ¾¾ ¾® 2SO3(g) DrH– = – 196.6 kJ mol–1. The reaction is exothermic reversible and the forward reaction leads to a decrease in volume. Therefore, low temperature and high pressure are the favourable conditions for maximum yield. But the temperature should not be very low other wise rate of reaction will become slow. Dilution of oleum with water gives H2SO4 of the desired concentration. In the industry two steps are carried out simultaneously to make the process a continuous one and also to reduce the cost. SO3 + H2SO4 ¾¾® H2S2O7 ; H2S2O7 + H2O ¾® 2H2SO4 (Oleum) The sulphuric acid obtained by Contact process is 96-98% pure. 40 E  Pre-Medical : Chemistry ALLEN Properties : Sulphuric acid is a colourless, dense, oily liquid with a specific gravity of 1.84 at 298 K. The acid freezes at 283 K and boils at 611 K. It dissolves in water with the evolution of a larger quantity of heat. The chemical reaction of sulphuric acid are as a result of the following characteristics : (a) low volatility (b) strong acidic character (c) strong affinity for water and (d) ability to act as an oxidising agent in aqueous solution, (i) Sulphuric acid ionises in two steps. H2SO4(aq) + H2O(l) ® H3O+ (aq) + HSO4– (aq) ; Ka1 = very larger (Ka > 10) 1 HSO4– (aq) + H2O(l) ® H3O+ (aq) + SO42– (aq) ; Ka2 = 1.2 × 10–2 The larger value of Ka (Ka > 10) means that H2SO4 is largely dissociated into H+ and HSO4–. Greater the 1 1 value of dissociation constant (Ka) the stronger is the acid. (ii) The acid forms two series of salts : Normal sulphates (such as sodium sulphate and copper sulphate and acid sulphate (e.g., sodium hydrogen sulphate) (iii) Sulphuric acid, because of its low volatility can be used to manufacture more volatile acid from their corresponding salts. EN 2MX + H2SO4 ® 2HX + M2SO4 (X = F, Cl, NO3) NaCl + H2SO4 ¾¾® NaHSO4 + HCl (M = Metal) 0 KNO3 + H2SO4 ¾¾® KHSO4 + HNO3 -2 (iv) Concentrated sulphuric acid is a strong dehydrating agent. Many wet gases can be dried by passing them through sulphuric acid, provided the gases do not react with the acid. Sulphuric acid removes water from 19 organic compound; it is evident by its charring action on carbohydrates. H 2SO 4 C12H22O11 ¾¾ ¾ ¾® 12C + 11H2O (charring action of sugar) 20 LL H 2SO 4 H2C2O4 ¾¾ ¾ ¾® CO + CO – H 2O 2 n (v) Hot concentrated sulphuric acid is moderately strong oxidising agent. In this respect it is intermediate between phosphoric and nitric acids. Both metals and non-metals are oxidised by concentrated sulphuric acid, which io is reduced to SO2. Cu + conc. 2H2SO4 ¾¾® CuSO4 + 2H2O ss 3S + conc. 2H2SO4 ¾¾® 3SO2 + 2H2O Se C + conc. 2H2SO4 ¾¾® CO2 + 2SO2 + 2H2O A KBr + conc.H2SO4 ¾¾® KHSO4 + 2H2O + Br2 + SO2 Z:\NODE02\B0AI-B0\TARGET\CHEM\ENG\MODULE-5\1-P-BLOCK\1-THEORY.P65 E 41 Pre-Medical : Chemistry ALLEN NCERT QUESTIONS (REASONING) Q.1 H2S is less acidic than H2Te. Why? Ans. Due to the decrease in bond (E–H) dissociation enthalpy down the group, acidic character increases. Q.2 Why is H2O a liquid and H2S a gas? Ans. Hydrogen bonds are present between H2O molecules while between H2S molecules, vander Waal’s forces are present. Q.3 Why is dioxygen a gas but sulphur a solid? Ans. Oxygen exist as a O2 molecule while sulphur exist as a S8 molecule due to more molecular mass sulphur is solid. Q.4 Knowing the electron gain enthalpy values for O ® O– and O ® O2– as –141 and 702 kJ mol–1 respectively, how can you account for the formation of a large number of oxides having O2– species and not O–? Ans. Consider lattice energy factor in the formation of compounds. Q.5 Which of the following does not react with oxygen directly? Zn, Ti, Pt, Fe EN Ans. Pt is a noble metal which does not react directly with oxygen. Q.6 Complete the following reactions: (i) C2H4 + O2 ® (ii) 4Al + 3O2 ® 0 Ans. (i) C2H4 + 3O2 ® 2CO2 + 2H2O (ii) 4Al + 3O2 ® 2Al2O3 -2 Q.7 How is O3 estimated quantitatively? Ans. When ozone reacts with an excess of potassium iodide solution buffered with a borate buffer (pH 9.2), iodine 19 is liberated which can be titrated against a standard solution of sodium thiosulphate. This is a quantitative method for estimating O3 gas. Q.8 What happens when sulphur dioxide is passed through an aqueous solution of Fe(III) salt? Ans. 2Fe+3 + SO2 + 2H2O ® 2Fe+2 + SO4–2 + 4H+ 20 LL Q.9 How is the presence of SO2 detected? Ans. It has colourless gas with pungent smell and decolourise acidified KMnO4 solution. n Q.10 Write the conditions to maximise the yield of H2SO4 by Contact process. io Ans. The reaction is exothermic, reversible and the forward reaction leads to a decrease in volume. Therefore, low temperature and high pressure are the favourable conditions for maximum yield. But the temperature ss should not be very low otherwise rate of reaction will become slow. Q.11 Why is K a2 = K a1 for H2SO4 in water? Se Ans. It is difficult to remove H+ ion from HSO4– ion. A BEGINNER'S BOX-8 Z:\NODE02\B0AI-B0\TARGET\CHEM\ENG\MODULE-5\1-P-BLOCK\1-THEORY.P65 1. Which of the following is not oxidised by O3? (1) KI (2) FeSO4 (3) KMnO4 (4) K2MnO4 2. In which of the following reaction conc. H2SO4 is not used as on oxidising agent? (1) Cu + 2H2SO4 ¾¾® CuSO4 + SO2 + 2H2O (2) 2HI + H2SO4 ¾¾® I2+SO2+2H2O (3) CaF2 + H2SO4 ¾¾® CaSO4 + 2HF (4) None 3. Hot conc. H2SO4 acts as strong oxidising agent which of the following element is oxidised by conc. H2SO4 into two gaseous products? (1) Cu (2) S (3) C (4) Zn 4. HCOOH reacts with conc.H2SO4 to produce (1) CO (2) CO2 (3) NO (4) NO2 42 E  Pre-Medical : Chemistry ALLEN HALOGEN FAMILY (GROUP-17) (A) PHYSICAL PROPERTIES (i) Fluorine and chlorine are gases, bromine is a liquid whereas iodine is a solid. (ii) Their melting and boiling points steadily increase with atomic number. (iii) All halogens are coloured. This is due to absorption of radiations in visible region which results in the excitation of outer electrons to higher energy level. By absorbing different quanta of radiation, they display different colours. For example, F2, has yellow, Cl2, greenish yellow, Br2, red and I2, violet colour. (iv) Fluorine and chlorine react with water. Bromine and iodine are only sparingly soluble in water. But are soluble in organic solvents such as chloroform, carbon tetrachloride, carbon disulphide and hydrocarbons to give coloured solutions. (v) Except the smaller enthalpy of dissociation of F2 compared to that of Cl2. The X-X bond disassociation enthalpies from chlorine onwards show the expected trend : Cl – Cl > Br – Br > F – F > I – I. The reason for the smaller enthalpy of dissociation of F2 is the relatively larger electrons- electrons repulsion among the lone pairs in F2 molecule where they are much closer to each other than in case of Cl2. (B) CHEMICAL PROPERTIES (i) EN Oxidation states and trends in chemical reactivity All the halogens exhibit –1 oxidation state. However, chlorine, bromine and iodine exhibit + 1, + 3, + 5 0 and + 7 oxidation states also. The higher oxidation states of chlorine, bromine and iodine are realised -2 mainly when the halogens are in combination with the small and highly electronegative fluorine and oxygen atoms e.g., in interhalogens, oxides and oxoacids. 19 The fluorine atom has no d orbitals in its valence shell and therefore cannot expand its octet. Being the most electronegative, it exhibits only – 1 oxidation state. All the halogens are highly reactive. They react with metals and non-metals to form halides. The reactivity of the halogens decreases down the group. Standard Reduction Potential (SRP) 20 LL X2 + 2e– ¾¾® 2X– n F2 + 2e– ¾¾® 2F– E–= + 2.87 V ; Cl2 + 2e– ¾¾® 2Cl– E– = + 1.36 V io Br2 + 2e – ¾¾® 2Br – E– = + 1.09 V ; I2 + 2e– ¾¾® 2I– E–= + 0.54 V More the value of the SRP, more powerful is the oxidising agent. ss Hence the order of oxidising power is F2 > Cl2 > Br2 > I2 Order of reducing behaviour is I– > Br– > Cl– > F– Se (ii) Halogen oxides : A Fluorine forms two oxides OF2 and O2F2. However, only OF2 is the thermally stable at 298 K. These oxide are essentially oxygen fluorides because of the higher electronegativity of flurorine than oxygen. Z:\NODE02\B0AI-B0\TARGET\CHEM\ENG\MODULE-5\1-P-BLOCK\1-THEORY.P65 Both are strong fluorinating agents. O2F2 oxidises plutonium to PuF6 and the reaction is used in removing plutonium as PuF6 from spent nuclear fuel. Chlorine, bromine and iodine form oxides in which the oxidation states of these halogen vary from + 1 to + 7. A combination of kinetic and thermodynamic factors lead to the generally decreasing order of stability of oxides formed by halogens, I > Cl > Br. The higher oxides of halogens tend to be more stable than the lower ones. This is called middle row anamoly. Chlorine oxides, Cl2O, ClO2, Cl2O6 and Cl2O7 are highly reactive oxidising agents and tend to explode. ClO2 is used as a bleaching agent for paper pulp and textiles and in water treatment. The bromine oxides, Br2O, BrO2, BrO3 are the least stable halogen oxides and exist only at low temperature. They are very powerful oxidising agents. The iodine oxides, I2O4, I2O5, I2O7 are insoluble solids and decompose on heating. I2O5 is very good oxidising agent and is used in the estimation of carbon monoxide. E 43 Pre-Medical : Chemistry ALLEN COMPOUNDS OF HALOGEN 1. CHLORINE GAS (Cl2) (a) Preparation : (i) By heating chloride with concentrated H2SO4 in presence of MnO2. 4H+ + MnO2 + 2X– ¾¾® X2 + Mn+2 + 2H2O Bromides and iodides also liberate Br2 and I2 respectively with concentrated H2SO4 and MnO2. (ii) CaOCl2 + 2HCl ¾¾® CaCl2 + Cl2 + H2O 2KMnO4 + 16 HCl ® 2 KCl + 2 MnCl2 + 5 Cl2 + 8 H2O PbO2 + 4 HCl ® PbCl2, + Cl2 + 2 H2O (iii) Manufacture of chlorine : l Deacon’s process : By oxidation of hydrogen chloride gas by atmospheric oxygen in the presence of CuCl2 (catalyst) at 723 K. 4 HCl + O2 ¾¾ ¾® 2 Cl2 + 2 H2O CuCl2 l Electrolytic process : Chlorine is obtained by the electrolysis of brine (concentrated NaCl solution). EN Chlorine is liberated at anode. It is obtained as a by–product in many chemical industries e.g.; in manufacturing of sodium hydroxide. NaX (aq) ¾¾® Na+ (aq) + X– (aq) 0 Anode : 2X– ¾¾® X2 + 2e– -2 (b) Properties : (i) It is a greenish–yellow gas with pungent and suffocating odour. It is about 2–5 times heavier than air. It 19 can be liquefied into greenish–yellow liquid which boils at 239 K. It is soluble in water. (ii) Oxidising & bleaching properties : Chlorine dissolves in water giving HCl and HOCl. Hypochlorous acid (HOCl) so formed, gives nascent oxygen which is responsible for oxidising and bleaching properties of chlorine. 20 It is a powerful bleaching agent ; bleaching action is due to oxidation. LL Cl2 + H2O ¾¾® 2 HCl + O n Coloured substance + O ® Colourless substance It bleaches vegetable or organic matter in the presence of moisture. Bleaching effect of chlorine is permanent. io Cl2 ss 2+ 3+ Fe Fe SO32— SO4 2— SO2 H2SO4 Se I2 HIO3 A Br /I— — Br 2/I2