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Date Created: 09/09/15
Chemistry 165 221 Chapter 22 The Halogens and the Noble Gases The halogens group VII are the most reactive of the nonmetallic elements They are all oxidizing agents easily reduced and are found in nature only in combination with other elements The noble gases are very unreactive and are found in elemental form only Element Ionization Electron x1 Ionic Common enthalpy affinity Radius oxidation kl marl k1 mo11 A numbers Hulagens F 1687 334 398 117 1 C1 1257 355 316 167 1 1 3 5 7 Br 1146 325 296 182 1 1 3 5 I 1015 295 266 206 1 1 3 5 7 At 270 Noble Gases He 2378 48 0 Ne 2087 120 0 Ar 1527 96 0 Kr 1357 96 0 2 Xe 1177 77 26 0 2 4 6 8 Rn 1043 Recall C Si N P O S differences in properties Group VII shows less striking anomalies Major oddity is that F has lower electron affinity than others This is surprising since F is very electronegative This anomaly is due to e e repulsion in the compact ion F But lattice enthalpies for metal uorides are generally much greater than corresponding chlorides due to the small ionic radius of F e g NaCl 787 kImol vs NaF 1506 kImol We can make HCl from NaCl and H2804 2 NaCl 5 H2504 aq gt NazSO4 aq 2 HCl g LeBlanc process for sodium carbonate NaZSO4 s 2 C s gt Nags s 2 C02 g Nags s CaCO3 gt Na2C03 s CaS s The soluble Na2C03 is dissolved and purified Byproduct CaS and HCl were serious pollution problems The Alkali Act Britain 1863 was the first serious attempt to control industrial emissions Later versions of this process made C12 from HCl and recycled S from CaS to sulfuric acid Chemistry 165 222 We can make HCl into chlorine This is an oxidation from 1 to 0 oxidation state 4 HCl uq Mn02 s gt C12 g MnC12 aq 2 H20 1 Scheele noted a green gas from this reaction in 1774 but Davy 1811 was the rst to show that it was an element C12 can be used as a bleach but a more convenient form is made by reaction with CaOH2 CaOH2 s C12 3 gt CaClOC1 s H20 l 00 is the hypochlorite ion with C1 in the 1 oxidation state more on this later The other C1 ends up as chloride anion so this is a disproportionation Chlorine can be produced from HCl more efficiently by the Deacon Process 1870 s 2HC1 g 02 g gt C12 g H20 g Modern processes use electrolysis of brines NaCl mostly Review section 127 Myanmar Cl K 1 I that was M cum 1 Anode 2C1 aq gt C12 g2e Cathode 2H2Ol Ze gt H2 g 20H m1 Overall 2H2Ol 2Cl aq gt ZOH aq H2 g C12 g AG 422 k Chemistry 165 223 Note that the oxidation of water at the anode is suppressed by employing an electrode material that has a higher over potential for 02 evolution than for C12 evolution R1102 works well Other halogens Br2 and 12 are made by oxidation of Br and I using C12 E V C12 g 2e gt 2C1 1358 Br2 l 2e39 2B1 1065 12 s Ze gt 21 0535 Solution Chemistry C12 aq 2 H2O 2 HOCl aq Cl aq H3O aq At equilibrium mostly C12 the presence of base shifts equilibrium to the right Overall C12 aq 2 OH39 aq 2 OCl aq Cl aq H20 1 OCl is hypochlorite ion Chlorine is in the 1 oxidation state This ion will disproportionate 3 00 1111 gt 2 C1 C103 1 1 5 hypochlorite chloride chlorate Chlorate will also disproportionate on heating eg 4 KC103S gt 3 KClO4 s KCls perchlorate echem alternative 003 0111 3H2Ol gt 004 aq 2H30 aq 2e Chemistry 165 224 Oxidation 02 or Cl g Chemistry 165 225 Halogen Oxides eg Dichlorine monoxide 2 C12 8 HgO s gt C120 g HgClz39HgO 5 forms 0039 in aqueous base C120 g 2 OH uq gt 2 OCl aq H20 1 Chlorine dioxide can be made from C12 C12 g 2 AgC103 s gt 2 C102 g 2 AgCl s 02 g oxidation state of Cl 0 4 1 C102 is used in bleaching It reacts with aqueous base 2 C102 g 2 OH39 1117 gt C102 uq 003 aq H20 1 C1207 be prepared by dehydration of perchloric acid with P205 12 HClO4 l P4010 S gt 6 C1207 l 4 H3PO4 l Fluorine Cannot be made by electrolytic oxidation of F in aqueous solution because water is oxidized rst Think about this in terms of adding F2 to water E V 2 F2 g 4 e gt 4 F uq 287 6 H20 1 gt 02 g 4 H30 123 Overall 2 F2 g 6 H20 1 gt 02 g 4 F aq 4 H30 164 Moisson 1886 managed to make elemental F2 by electrolysis of KF in HF solvent Note that HF has a very high boiling point 195 C due to H bonding HF is a quite weak acid due to the high strength of the H F bond Recall that HCl HBr and HI are strong acids Fluorine has a unique chemistry due to very high oxidizing ability Recall that C12 prepared by electrolysis was used to make 12 and Br2 from I39 and Br Similarly F2 will oxidize C139 to C12 eg 2NaCl s F2 g gt 2NaF s C12 g Chemistry 165 226 F also stabilizes high oxidation states e g PbF4 is stable while PbCl4 spontaneously loses C12 to form PbClz Another example is PtF6 PtCl6 is unknown Several examples of this arise when F2 reacts with halide ions eg KCl 5 2172 g gt KClF4 C1F4 ion contains C1 in 3 oxidation state KIs 4F2g gt IF7g Note that water vapor burns with F2 at 100 C 2F2 g H20 g gt 4HF g 02 g An important use of uoride not fluorine is in preventing tooth decay Tooth enamel is mostly hydroxyapitite Ca5PO43OH which can be converted to Ca5PO43F Note that F39 and OH39 are about the same size The uoride is less soluble in mildly acidic aqueous solution Covalent Fluorides Electron deficient uorides such as BF3 PbF5 and SbF5 are powerful Lewis acids With less electronegative elements bonds to F become more ionic Eg CF4 and SiF4 are gases GeF4 is a molecular liquid but SnF4 and PbF4 are solids with some ionic character In contrast San and PbFz are ionic solids In general metal uorides in higher oxidation states are more covalent in character Reaction of aqueous alkali with F2 gives OF2 2172 g 20H3911q gt OFZ g 2F39 uq H200 In contrast to C120 see 225 which reacts with water to give HOCl OF2 exhibits reactivity consistent with O in the unusual 1 2 oxidation state Reaction with water affords Oz OFZ g H20 1 02 g 21Faq Fluorinated Organic Compounds Tetra uoroethylene C2F4 can be polymerized to polyTFE known as Te on Valuable due to extremely low chemical reactivity Repels water Chemistry 165 227 Organic compounds containing chlorine and uorine are known as chloro uorocarbons CFC s Typical of these compounds is CF2C12 which was once widely employed as the working uid in refrigerators and air conditioners This compound and related molecules have now been banned by international treaties due to their effect on stratosphere ozone see Oxtoby pages 621624 Ozone is continuously created in the upper atmosphere by high energy photolysis of 02 hi gt 02 g 200 nm 20 g 0g02g gt 038 Ozone efficiently absorbs light with wavelengths of 200350 nm Depletion of the ozone concentration in the stratosphere leads to an increase in high energy UV light reaching the surface of the earth CFC s are very unreactive Released CFC s eventually reach the stratosphere where photolysis can cleave CCl bonds hi CF2C12 g gt CFzCl g Cl g Atomic chlorine is very reactive with O and 03 C1 O gt C10 C1 03 gt ClO 02 C10 catalytically destroys ozone M is N2 or Oz 2C10 M gt CIOOCI M CIOOCI h1 gt 000 C1 CIOOM gtClOzM 2C1 203 gt 200 202 Overall 203 gt 302 One Cl atom can convert several thousand molecules of ozone to oxygen Noble Gases Very unreactive elements High ionization potentials Until 1962 these elements were called the inert gases In March 1962 Neil Bartlett in Vancouver BC noted that reaction of PtF6 with Oz was known to give a novel salt Chemistry 165 228 02 g PtF6S gt 021PtF6139 Since the ionization potential of 02 is similar to that of Xenon he decided to try the reaction of Xe with PtF6 Reaction occurred to give a complex mixture of Xe containing compounds Leaving out the Pt Bartlett soon discovered that F2 g would react with Xe g Xe g F2 g M XeFZ s with excess Xe 1 atm Higher uorides can be prepared by using more F2 and higher temp Xe g 212 g 29 XeF4 s 6 atm Xe g 3F2 g M XeF6 s 60 atm XeF6 must be handled with care It reacts with water XeF6 s 3H20 l gt XeO3 aq 6HF g Xe03 is explosively unstable Other Reactions of Xenon Fluorides XeF2s SbF5l gt XeFSbF639 XeF4 NMe4F gt NMe4XeF539 Xer oxidizes water to 02 2XeF2s 2H20l gt ZXe g 4HF g Oz 5 XeF4 uorinates normally unreactive Pt metal XeF4 s Pt 5 gt Xe g PtF4 s Krypton can also be induced to react with F2 but resulting compounds are very unstable Kr 5 F2 5 77K gt K1132 h D