Chemistry - an illustrated guide to science (794128), страница 16
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The symbol ‘X’ isoften used to denote a halogen atomand ‘X- ’ a halogen ion.outer shell1 Electron structure● Allhalogen atoms have sevenelectrons in their outer shell.A halogen atom needs one moreelectron to fill the outer shell, and itcan obtain this either by forming asingle covalen t bon d or by forming anion, X- . Halogens form both covalen tcom pou n ds and ion ic com pou n ds.2 Halogen atom and moleculeX2 Halogen atom andmoleculeHalogen atomX● Halogensexist as diatomic molecules.Each atom in the molecule provides asingle electron to form a covalentbond.
The result is that each atom hascontrol over eight electrons.X3 Physical propertiesHalogen molecule3 Physical propertiesFluorineElementChlorineBromineIodineAtomic number9173553Relative atomic mass19.035.579.9126.9State at 20 °CgasgasliquidsolidColorpale yellowpale greenred-brownblackm.p./ °C–220–101–7113b.p./ °C–188–3559183Solubility/ g per 10 0 gof water at 20 °Creacts readilywith water0.59 (reactsslightly3.60.018is a gradation of physicalproperties going down group 7.1.
State changes from solid to liquid togas. Bromine is one of only twoelements that exist as liquids at roomtemperature.2. The color darkens from pale yellowto black.3. Melting point and boiling pointincrease.● There is a gradual decrease inchemical reactivity going downgroup 7.● Fluorine oxidizes water to give oxygen:2F2 + 2H2O ➞ 4HF + O2● Chlorinereacts less vigorously withwater, forming an acidic solution:C2 + H2O● BromineHCl + HOCland iodine form solutions inwater, although the latter is not verysoluble.© Diagram Visual Information Ltd.● There92PATTERNS—NON-METALSKey wordsbrominechlorinedistillationiodineoxidationoxidizing agentLaboratory preparationof the halogens1 Laboratory preparation of chlorinea1 Laboratory preparationof chlorine● Chlorin eis made in the laboratory bythe oxidation of concentratedhydrochloric acid using a suitableoxidizin g agen t such as manganesedioxide (manganese(IV) oxide):MnO2(s) + 4HCl(aq) ➞MnCl 2(aq) + 2H2O(l) + Cl 2(g)gas is first passed through waterto remove any hydrogen chloride gas,and then through concentratedsulfuric acid to dry the gas.
Chlorine ismore dense than air and is collectedby downward delivery.● Chlorine can also be convenientlymade in the laboratory from bleachingpowder, using dilute hydrochloric acid:edb● Thec2 Laboratory preparation of brominekCa(OCl) 2(s) + 4HCl(aq) ➞CaCl 2(aq) + 2H2O(l) + 2Cl 2(g)2 Laboratory preparationof bromine● Brom in eois made in a similar way tonchlorine:lMnO2(s) + 2NaBr(aq) + 2H2SO4 (aq) ➞MnSO4 (aq) + Na2SO4 (aq)+ 2H2O(l) + Br 2(g)it boils at 59°C, bromine isremoved from the reaction mixture bydistillation .fpm● Because3 Laboratory preparation of iodine3 Laboratory preparationof iodineis made in a similar way tobromine.
Hydrogen iodide is made insitu by reacting sodium iodide withconcentrated sulfuric acid:© Diagram Visual Information Ltd.● Iodin etMnO2(s) + 2KI(aq) + 2H2SO4 (aq) ➞MnSO4 (aq) + K2SO4 (aq) +2H2O(l) + I 2(g)● Iodineis removed from the reactionmixture by sublimation. On heating, itchanges directly from solid to vaporand then back to solid on cooling.rqsa concentrated hydrochloric acidb manganese dioxidec warm gentlyd water to remove HCl fumese concentrated H2SO4 to dry Cl 2f chlorine gasg concentrated brineh chlorine gasi hydrogen gasj waterk concentrated sulfuric acidl manganese oxide + sodium bromidem warm gentlyp cold watero fumes of HBrp bromineq manganese oxide + potassium iodine+ concentrated H2SO4s warm gentlyt cold water93Compounds of chlorinePATTERNS—NON-METALSKey words1 Chlorine and metalsCalcium burnsin chlorinechloridechlorinecovalentcompoundhalideAluminum reacts whenwarmed in a stream ofchloridealuminumwhite smokechlorinehydrogenchloridesodium chloridesulfuric acid1 Chlorine and metalschlorine● Metals,calciumto fumecupboardsuch as calcium, burn inchlorin e to produce thecorresponding metal chloride:Ca(s) + Cl 2(g) ➞ CaCl 2(s)● Aluminumreacts with chlorine to formaluminum chloride:heat2Al(s) + 3Cl 2(g) ➞ 2AlCl 3(s)● Unlikemany metal chlorides,aluminum chloride is hydrolyzed bywater, giving off hydrogen chloridegas:2 Laboratory preparation of hydrogen chlorideconcentrated sulfuric acidAlCl 3(s) + 3H2O(l) ➞Al(OH) 3(s) + 3HCl(g)It is for this reason that aluminumhalides fume when they come intocontact with moist air.hydrogen chloridesodium chloride2 Laboratory preparationof hydrogen chloride● Hydrogenchloride is made by thereaction of sodiu m chloride withconcentrated su lfu ric acid:NaCl(s) + H2SO4 (aq) ➞NaHSO4 (s) + HCl(g)3 Compounds with non-metalsClClPCClCarbon tetrachloride (CCl4 )ClCClCHCCClCHCClClHClBenzene hexachloride (BHC)ClHCl C H CCCCCl 2 CH2CCCl C H CClHAldrinCHforms covalen t com pou n dswith non-metals such as carbon,phosphorus, and sulfur.4 Pesticides● Chlorinatedcompounds provide arange of pesticides.● DDT, BHC, Aldrin, and Dieldrin havebeen the source of environmentalconcern, and their use is nowprohibited or severely restricted.ClCCClClCC CHHHC● ChlorineSulfur monochloride (S2 Cl2 )HHC CClCCC CHDichlorophenyl- HHtrichloroethaneHH(DDT)C CClCH3 Compounds withnon-metalsSClPhosphorus trichloride (PCl3)4 The structure of somechlorinated pesticidesHSClClHClClClCl CCCCl 2CCl CClHH CCCH2CH CHDieldrinHCCHO© Diagram Visual Information Ltd.Cl94PATTERNS—NON-METALSKey wordscarbonatecovalent bondcovalentcompoundhydrogenchlorideionionic compoundHydrogen chloridein solution1 In organic solventsHydrogen chloride● Hydrogenchloride gas is a covalen tcom pou n d.
In solutions in organicsolvents, it remains a covalentcompound. It becomes an ion iccom pou n d in aqueous solutions.1 In organic solvents● Insolution in organic solvents such asmethylbenzene, hydrogen chlorideremains a covalent compound. Thehydrogen atom and the chlorine atomeach donate one electron to form thecovalen t bon d.● The solution contains no ions anddoes not conduct electricity.● The solution has no effect on bluelitmus paper or on carbon ates, thusshowing that it is not an acid.HClH–ClH–ClH–ClH–ClH–Cl2 In aqueous solution● Inaqueous solution, hydrogenchloride becomes an ionic compound.The hydrogen atom loses an electronto become a hydrogen ion , H+ , andthe chlorine atom gains an electron tobecome a chloride ion, Cl - .● The solution contains ions andconducts electricity.
The ions are ableto carry a charge through the solution.● The solution turns blue litmus paperred and reacts with carbonates,showing that it is an acid:2 In aqueous solutionNa2CO3(aq) + 2HCl(aq) ➞2NaCl(aq) + CO2(g) + H2O(l)HH+Cl –© Diagram Visual Information Ltd.Cl –H+Cl –H+H+Cl –Cl95Acid/ base chemistryof the halogensPATTERNS—NON-METALSKey wordshalidehydrochloric acidhydrogenchloridenitric acid1 Laboratory preparation of hydrochloric acidaaoxidizing agentreducing agentsilver nitrate1 Laboratory preparation ofhydrochloric acida● Hydrochloricacid is made in thelaboratory by dissolving hydrogenchloride gas in water.● Hydrogen chloride is very soluble inwater.
It is dissolved by passingthrough an inverted filter funnel, therim of which sits just below the waterlevel. When water is sucked into thefunnel, the water level drops, and thefunnel rim is no longer submerged.This prevents water being sucked backinto the apparatus.bHydrogen chloride enterswater via a filter funnelThe level in thebeaker dropsThe cyclestarts again● Alla hydrochloric acid (HCl)b plug of liquid in funnel2 Solubility of the halogens2F2 + 2H2O4HF + O2Flouorine is so reactive it decomposes water producing hydrofluoric acid and oxygenCl 2 + H2O2 Solubility of the halogensHCl + HOClChlorine is the next most reactive halogen after fluorinehalogens are oxidizin g agen ts.However, oxidizing power decreasesdown the group:fluorine > chlorine > bromine >iodine● Halide ions are redu cin g agen ts. Thereducing power increases down thegroup:fluorine < chlorine < bromine <iodine3 Chloride test● Thepresence of halide ions in solutioncan be detected by adding a few dropsof dilute n itric acid followed byseveral drops of silver n itrate solution.1.
Chloride ions form a whiteprecipitate of insoluble silver chloride:3 Chloride testAg+ (aq) + Cl - (aq) ➞ AgCl(s)2. Bromide ions form a creamprecipitate of insoluble silver bromide:solutionAg+ (aq) + Br - (aq) ➞ AgBr(s)Ag+ (aq) + I - (aq) ➞ AgI(s)silver nitratesolutionDissolve unknown substance,adding dilute nitric acid tothe solution. Add a few dropsof silver nitrate solutionColored precipitationproves presence ofchloride, bromide,or iodine© Diagram Visual Information Ltd.3. Iodide ions form a yellowprecipitate of insoluble silver iodide:96PATTERNS—NON-METALSKey wordschloridechlorinenoble gasesoxidizing agentreactivity seriesredox reactionsulfurRedox reactions of thehalogens1 Calcium burns in chlorinechlorine gasplate1 Calcium and chlorine● Whenhydrogen sulfide and chlorineare mixed, elemental su lfu r is formed.Chlorin e acts as an oxidizin g agen tand oxidizes the hydrogen sulfide byremoving hydrogen.
In turn, thechlorine gains hydrogen and isreduced to hydrogen chloride:platesulfur coatinginside gas jarshydrogen sulfidegas8H2S(g) + 8Cl 2(g) ➞ S8 (s) + 16HCl(g)2 Chlorine and ferrouschloride● Chlorinecan also be used to oxidizeiron(II) to iron(III). When chlorine isbubbled into iron(II) chloride solution,the color changes from green toyellow-brown, showing the formationof iron(III). The chlorine atoms arereduced to chloride ions:Chlorine gas and hydrogensulfide gas are separatedby platePlate is removed2 Reaction of chlorine with ferrous chloridechlorine gas2FeCl 2(aq) + Cl 2(g) ➞ 2FeCl 3(aq)3 Halogens and metals● Halogensreadily oxidize metals.Fluorine oxidizes all metals, includinggold and silver, easily.● Chlorine oxidizes all but the leastreactive metals. When iron is heated ina stream of dry chlorine, iron(III)chloride is produced:2Fe(s) + 3Cl 2(g) ➞ 2FeCl 3(s)● Theease with which halogens oxidizemetals decreases down the group, buteven iodine will slowly oxidize metalslow in the reactivity series.green iron chloridesolutionChlorine gas is passed intoferrous chloride solution3 Halogens and metalsYellow-brown ironchloride solutioniron(III) chloridedry chlorine4 Halogens and non-metals© Diagram Visual Information Ltd.● Fluorineoxidizes most non-metalsexcept nitrogen and most of the n oblegases.● Chlorine reacts directly withphosphorus and sulfur, but carbon,nitrogen, and oxygen do not reactdirectly with chlorine, bromine, oriodine.● The relative reactivities of the halogensin redox reaction s with non-metals isillustrated at right by their reactionwith hydrogen.heat4 Halogens and non-metalsReactionObservationsH2(g) + F2(g) → 2HF(g)explosiveH2(g) + Cl 2(g) → 2HCl(g)explosive in sunlight but slow in the darkH2(g) + Br 2(g) → 2HBr(g)needs heat and a catalystH2(g) + I 2(g) → 2Hl(g)slow even when heated97Reactivity of the halogens1 Chemical reactivity of halogens with each otherChlorineBrominePATTERNS—NON-METALSKey wordsIodineChloridedisplacementreactionhalidehalogensimmiscible1 Reactivity of halogens● Thechemical reactivity of thehalogen s decreases down group 7:fluorine > chlorine > bromine >iodine● A more reactive halogen will displacethe ions of a less reactive halogenfrom a metal halide solution.
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