Transition Elements NEW - · PDF fileOCR Chemistry A H432 Transition Elements and ... manufacture sulphuric acid that oxidises sulphur ... for the reaction between the zinc and the

OCRChemistryAH432 TransitionElementsandQualitativeTests

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TransitionElementsElectronarrangementsofthed-blockelementsTheelementsinbetweenGroup2andGroup3arecorrectlyreferredtoasthed-blockelements.Thisisbecausethehighestenergysubshell(i.e.thesubshellbeingfilled)isad-subshell.Recallthat:

! the3dsubshellandthe4ssubshellareactuallyveryclosetogetherinenergy,but4sisslightlylowerinenergy,sothe4sfillsbefore3d

! ad-subshellcontainsfived-orbitals,socanholdupto10electrons! electronsgooneintoeachorbitalfirst,beforeanyorbitalgetsasecondelectron

Whenweexaminetheelectronarrangementsofthefirstrowofd-blockelementswefindtwoanomalies: Ca (20) 1s22s22p63s23p64s2 Sc (21) 1s22s22p63s23p63d14s2 Ti (22) 1s22s22p63s23p63d24s2 V (23) 1s22s22p63s23p63d34s2 Cr (24) 1s22s22p63s23p63d54s1 Mn (25) 1s22s22p63s23p63d54s2 Fe (26) 1s22s22p63s23p63d64s2 Co (27) 1s22s22p63s23p63d74s2 Ni (28) 1s22s22p63s23p63d84s2 Cu (29) 1s22s22p63s23p63d104s1 Zn (30) 1s22s22p63s23p63d104s2 Ga (31) 1s22s22p63s23p63d104s24p1Arathersimplisticandinadequateexplanationforthisistosaythathavingallthed-orbitalshalf-filledisanespeciallystablearrangement,soCrhas3dand4sallhalffilledratherthattheexpected3d44s2arrangement.MoshowsthesameeffectinPeriod5.InthesamewayitcanbesuggestedforCuthathavingthe3dorbitalsallfullismorestablethanhavingtheexpected3d94s2arrangement.Inotherperiods,AgandAushowthesameeffect.AmoresatisfactoryexplanationisbeyondthescopeoftheA-levelcourse.Electronarrangementsoftheionsofd-blockelements Puttingelectronsinto3dshieldsthe4sorbitalfromthenuclearcharge,andthe4selectronsnowbecomeeasiertoremovethanthe3di.e.3disnowthelowerenergysubshell.Asaresult,whenwestarttoremoveelectronsfromthed-blockatomstoformions,weremovethe4selectronsbeforeweremoveanyfrom3d.

d-blockelementsinPeriod4


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e.g.ironformsgreenFe2+andorangeFe3+ions Fe 1s22s22p63s23p63d64s2 Fe2+ 1s22s22p63s23p63d6 -2x4selectronsremoved Fe3+ 1s22s22p63s23p63d5 -2x4selectronsand1x3delectronremoved e.g.vanadiumformsvioletV2+ionsandgreenV3+ions V 1s22s22p63s23p63d34s2 V2+ 1s22s22p63s23p63d3 -2x4selectronsremoved V3+ 1s22s22p63s23p63d2 -2x4sand1x3delectronremovedBUTZinconlyformscolourless2+ions, Zn 1s22s22p63s23p63d104s2 Zn2+ 1s22s22p63s23p63d10 -2x4selectronsremovedScandiumonlyformscolourless3+ions, Sc 1s22s22p63s23p63d14s2 Sc3+ 1s22s22p63s23p6 -2x4sand1x3delectronremovedWemightproposethatcolourintheseionsisassociatedwithhavingapartially-filledd-subshell,andexperimentationwouldprovideaphysicalbasisforthissuggestion.Wealsohavethebasisforthedefinitionofatransitionelement–whichisNOTthesameasad-blockelement:Atransitionelementisad-blockelementthatformsatleastonestableionwithapartiallyfilleddsub-shell.ThereforeinPeriod4,ZnandScared-blockelementsbutNOTtransitionelements,whereasTi-Cuarebothd-blockelementsANDtransitionelements.TransitionMetalChemistryThepartially-filledd-subshellintransitionelements,andtheeasewithwhichelectronscanberemovedfrom,oraddedtotheseorbitals,givesthemtheircharacteristicproperties: -formingcolouredcompoundsandcomplexes -actingascatalysts -havingcompoundswithmultipleoxidationstatesAlltransitionmetalsformcompoundswithionswith+2oxidationnumber;inmostcasesthisisduetolosingthetwoelectronsfromthe4sorbital.However,the3delectronscanalsobelostallowingtransitionmetalstoformstableionswithhigheroxidationnumbers.Thishappensbecausethe3dand4senergylevelsaresocloseinenergy.Ionsoftransitionmetalsalsoreadilychangefromoneoxidationstatetoanother,byacceptingordonatingelectronsfromthe3dsubshell.


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AnillustrationofcolourandmultipleoxidationstatesAsolutioncontainingvanadate(V)ionsisyellow,butwhenareducingagentsuchaszincisadded,theoxidationstatecanbereducedfrom+5instepsdownto+2withacorrespondingchangeincolourateachstage: VO2

+(aq) Oxidationstate:+5 Colour:yellow

VO2+(aq) Oxidationstate:+4 Colour:blue*

*usuallyanintermediategreensolutionisseencontainingamixtureofVO2+andVO2+ions

beforetheblueappears. V3+

(aq) Oxidationstate:+3 Colour:green V2+

(aq) Oxidationstate:+2 Colour:lilacIneachstage,thevanadiumhasbeenprogressivelyreduced.Atthesametime,thezinchasbeenoxidized(Zn(s)"Zn2+(aq))sothisisaseriesofredoxreactions.CatalyticbehaviorintransitionmetalchemistryCatalystsprovideanalternativereactionpathwaywithloweractivationenergy.Weclassifycatalystsashomogeneous(inthesamephysicalstateasthereactants)orheterogeneous(inadifferentphysicalstatetothereactants).Examplesofheterogeneouscatalysis! Iron(solid)isusedasthecatalystfortheHaberprocess:N2(g)+3H2(g)⇌2NH3(g)! Solidvanadium(V)oxide,V2O5,isusedasthecatalystforthestepintheContactprocessto

manufacturesulphuricacidthatoxidisessulphurdioxidetosulphurtrioxide:SO2(g)+½O2(g)⇌SO3(g)! Nickelisusedinthecatalystinthehydrogenationofmargarines,saturating(someof)thealkene

functionalgroupsinpolyunsaturatedvegetableoils: -CH=CH-+H2"-CH2-CH2-! Manganese(IV)oxide,MnO2,isusedtocatalysethedecompositionofhydrogenperoxideinto

oxygenandwater: H2O2(aq)"H2O(l)+½O2(g)! Thereactionbetweensolidzincandacids(producingazincsaltandhydrogen)iscatalyzedbythe

presenceofcopper(II)ionsinthesolution.Thecopper(II)ionsreactwiththesurfaceofthesolidzincinadisplacementreaction,formingcoppermetal.Thecoppermetalthenactsasthecatalystforthereactionbetweenthezincandtheacid.

ExamplesofhomogeneouscatalysisInsolution,transitionmetalcompoundscanprovideanalternativereactionpathwayforredoxreactionsbyreactingwithonereactant,thenbeingregeneratedinthereactionwiththeotherreactant.e.g.ifAreactswithBinaredoxreaction,withAbeingoxidisedandBreduced,thealternativepathwaymightconsistofAbeingoxidizedbythecatalyst,whichisitselfreduced,thenthecatalystreducingB,andbeingoxidizedbacktoitsoriginalform.


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! Hydrogenperoxideoxidisestartrateionstocarbondioxidegasandmethanoateions.Thereactionproceedsveryslowlyifthereisnocatalyst,evenatelevatedtemperatures.

Thepresenceofcobalt(II)ionsinsolutioncatalysesthereaction. ThehydrogenperoxideinitiallyoxidisesthepinkCo2+,togreenCo3+,andisreducedtowater. Thecobalt(III)ioncausesoxidiationofthetartarateion,andasaresultofwhichtheCo3+is reducedbacktoCo2+andthepinkcolourreturns.

PracticalRochellesaltreactionwithhydrogenperoxide,catalyzedbyCo2+(aq)–seeClassicChemistryDemos.! Thereactionbetweeniodideions,I-,andperoxydisulphateions,S2O8

2-formssulphateionsandiodine: 2I-(aq)+S2O8

2-(aq)"2SO4

2-(aq)+I2(aq)

Thisformationofiodinecanbehighlightedbytheadditionofstarch,whichformsablue-black complexwiththeiodine.ThereactioniscatalyzedbythepresenceofFe2+(aq),andwhenasmall amountofthisisaddedtheblue-blackcolourformsmuchmorequickly,withtheFe2+ionsleft unchangedattheendofthereaction. Step1: S2O8

2-(aq)+2Fe2+(aq)"2SO4

2-(aq)+2Fe3+(aq) reactioninvolvingcatalyst

Step2: 2I-(aq)+2Fe3+(aq)"I2(aq)+2Fe2+(aq) regenerationofcatalystComplexesandColourThesolidcompoundsandaqueousionsoftransitionmetalshavecharacteristiccolours: Cu2+(aq)ionsareblue Co2+(aq)ionsarepink Fe2+(aq)ionsarepalegreen Fe3+(aq)ionsareyellow-orange Mn2+(aq)ionsarepalepinkBycontrast,compoundsofZnandScarewhite(colourlessinsolution).ThisisbecauseZn2+ionshavea3d10electronarrangement,andSc3+haveno3delectrons,soneitherhastheincompletelyfilledd-orbitalsnecessaryforcolourintransitionmetalcompounds.However,considercopper(II)sulphate.Itisbluewhenhydratedcrystalsorinsolution,butwhitewhenanhydrous.Thecopperionsare[Ar]3d9ineverycase.Wemustconcludethattheunfilledd-subshellisonlyoneofthenecessaryrequirementsforcolour.Wheninsolutionandinthehydratedcrystallattice,thetransitionmetalionsarenotmerelydissolvedinwater,buttheyareactuallybondedtoanumberofwatermolecules,bydativecovalentbondsformedfromalonepaironwater'soxygenatomtothemetalion.


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e.g.theblueCu2+(aq)ionisactually[Cu(H2O)6]2+(aq) Thisisanexampleofacomplex.Becauseithasanoverallcharge,wecallitacomplexion.Becausetransitionmetalionsaresmalltheyhaveastrongelectricfieldaroundthemthatattractselectron-richspecies.Speciesthathavealonepairavailabletodonatecanformdativecovalentbondstothetransitionmetalion.WecallsuchspeciesLIGANDS.Aligandisamoleculeorionthatcandonatealonepairtoformadativecovalentbondtoatransitionmetalion.Itisthepresenceoftheseligandsaroundthecentraltransitionmetalionthatistheothernecessaryrequirementforcolour.Examplesofligands: H2O: :NH3 :Cl- :CN- :SCN- :OH- water ammonia chloride cyanide thiocyanate hydroxideThedativecovalentbondsaroundthetransitionmetalionrepeloneanother,andsoarearrangedaroundthetransitionmetalioningeometricarrangementspositioningthemasfarapartaspossible(likeinVSEPRTheory).Thenumberofdativecovalentbondsarrangedaroundatransitionmetaliscalledtheco-ordinationnumber.Complexionswithaco-ordinationnumberof6tendtobeoctahedralComplexionswithaco-ordinationnumberof4aretetrahedraloroccasionallysquareplanarDrawingcomplexes,andwritingformulaeFormulaeforcomplexionsarewritteninsquarebracketswiththeoverallchargeonthebrackets.Noindividualchargesonligandsorthemetalionareshown.Ligandsareinbracketsunlesssingleatoms,withthenumberofeachligandafterthebracketifmorethanone.Complexionsaredrawnin3Dusingshadedandwedgebondstorepresentbondsintoandoutoftheplaneofthepaper.Thebondsbetweenligandsandthetransitionmetalionaredativecovalentbonds,sowesometimesuseanarrowtoindicatethedirectioninwhichtheelectronpairhasbeendonated(towardsthetransitionmetalion),butthisisoptional.Aswiththeformula,squarebracketsareusedandtheoverallchargewrittenatthetopright.Examples:1) [Cu(H2O)6]2+ octahedral paleblue Name:hexaaquacopperIIion


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Aqueoussolutionsofallthetransitionmetalions(withnootherligandspresent)haveaco-ordinationnumberof6andareoctahedral.2) [CuCl4]2- tetrahedral yellow Name:tetrachlorocuprateIIionItisthesizeoftheligandsthathasthemostsignificanteffectonthegeometryofthecomplexions–chlorideionsaremuchlargerthanwatermolecules,soonlyfourchlorideionswillpackaroundthesmalltransitionmetalion,wheresixwatermoleculeswillfit.3) [Fe(H2O)5SCN]2+ octahedral bloodred Name:pentaaquathiocyanatoironIIIionTheligandsdon’tallhavetobethesame,herefivewaterligandsandathiocyanateligandformdativebondstotheiron(III)ion.Noticealsothattheoverallchargeonthecomplexionis2+eventhoughtheironisin+3oxidationstate,becauseofthe1-chargeontheSCN-ion.CrystallisingcomplexesComplexionsarenotfoundontheirown–thereisalwaysanoppositely-chargediontobalancethechargesalthoughwemayignoreitmuchofthetime.Wemayfocusonthe[Cu(H2O)6]2+ionwhenwedissolvecopper(II)sulphateinwater,forgettingtheSO4

2-ionisalsopresentinequalconcentrations.Similarlywhenweform[CuCl4]2-bytheadditionofhydrochloricacidtocoppersulphatesolution,wehavetwoH+ionspresentforevery[CuCl4]2-ion.Thisisofnoconsequencewhenweareconsideringsolutions–the"other"ionsarespectators,butwhenweevaporatethewaterandformcrystals,theseotherionsbecomepartofthegiantioniclatticealongwiththecomplexions.e.g.K3Fe(CN)6containsthecomplexion[Fe(CN)6]3-andthreeK+ionstobalancethecharges.Name:potassiumhexacyanoferrate(III)TypicalionswemayfindinthisroleincludeNa+,K+orNH4

+.TheseotherionsareNOTligandsandareNOTbondedtothecentralmetalion.LigandSubstitutionWhenanewligandisaddedtoasolutioncontainingacomplex,thenewligandcanreplacetheoriginalligandtoformanewcomplex.Thisisknownasligandsubstitution.


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Examples:1)AdditionofexcessconcentratedammoniasolutiontoCu2+(aq)resultsintheformationasolutioncontainingadarkbluecomplexion. [Cu(H2O)6]2++4NH3"[Cu(NH3)4(H2O)2]2++2H2O bluedarkblue octahedral octahedral2)IfconcentratedhydrochloricacidisaddedtoasolutioncontainingCu2+(aq),thesolutionturnsgreenandprogressivelymoreyellowaschlorideligandsreplacethewatermolecules.Thenewcomplexisyellow,butthesolutioncontainingbothcomplexesappearsgreen. [Cu(H2O)6]2++4Cl-⇌[CuCl4]2-+6H2O blue yellow octahedral tetrahedralPractical: Nuffield19.2aPart1experiments1-4 ClassicChemistryDemonstration#8p.16MultidentateligandsAlltheligandswehaveseensofarhaveformedasingledativecovalentbondtothetransitionmetalion.Werefertotheseasmonodentateligands.Largermoleculesmayhavemorethanonesitethatisabletodonatealonepair,andsomaybeabletoformmorethanonedativecovalentbondtothetransitionmetalion.Examples:salicylate(2-hydroxybenzoate)ions Salicylicacid(abenzeneringwithanadjacent–OHand–COOHgroup)doesnotactasaligand,butthesalicylateiondoes(presumablybecauseitismoreelectron-rich).TheOofthe–OHgroupandthe–O-ofthe–COO-groupeachhavealonepairwhichcanformabondtothetransitionmetalion,sotwodativecovalentbondsareformed.Thisasabidentateligand.Whenthisisaddedtoasolutioncontainingyellowaqueousiron(III)ions,thesixwaterligandsarereplacedwiththreesalicylateligandstoformadeeppurplecomplexofiron(III)salicylate.(Notethatthiscomplexhasnocharge–itisnotacomplexion,justacomplex).


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[Fe(H2O)6]3+(aq)+3HOC6H4COO-(aq)"[Fe(HOC6H4COO)3](s)+6H2O(l)

yellow purpleEDTA(ethylenediaminetetraaceticacid) ThisligandexistsincomplexesastheEDTA4-ion.Ithassixlonepairswhichcanformdativecovalentbonds,andishenceahexadentateligand.EDTAisusedtobindmetalions,removingthemfromsolution,andisreferredtoasachelatingagent.Usesincludebindingcalciumandmagnesiumionstoreducewaterhardness,orbeingaddedtobloodtotreatpatientssufferingfromleadormercurypoisoning.Practical:NuffieldExpt19.2aPart2(omitting1,2-dihydroxybenzene)ethane-1,2-diamine NH2CH2CH2NH2

ThishastwoNatomseachwithalonepairthatcanformabondtothetransitionmetalion.Thisisthereforeabidentateligand.Forexample,nickelformsanoctahedralcomplexwithco-ordinationnumber6whenitbondswiththreeoftheseligands: [Ni(NH2CH2CH2NH2)3]2+ ethanedioateions C2O4

2- Theethanedioateionisabidentateligand.EachO-canformadativecovalentbondtothetransitionmetalion.

Cr3+formsanoctahedralcomplexwithtwoethanedioateionsandtwowatermolecules. [Cr(C2O4)2(H2O)2]-


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PtO

OOH

C OOC

C

O

O

OH

C OO OC

O C

O

OPt

OH

C OOCC

O

O O

O

O

H

C O O

CC

O

O O

O

Wemightbegiventhestructureofacomplexandaskedtoworkoutanddrawtheligandspresent,indicatinghowtheyactasligands.Todothisweneedtobreakthedativecovalentbondstothetransitionmetalionandrestorethelonepairtotheatomsintheligandthatformedthebond:e.g.[Pt(C2O4)(HOC6H4OCOO)]-1)Takethecomplexionapart:2)Lookforatomswhosevalencyisnotsatisfiedwithbonds–onetoofewbondsindicatesanegativeion.3)Addinthelonepairswhichformedthedativecovalentbondstothemetalions

-


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BiologicalroleofligandsubstitutionHaemaglobininbloodisresponsibleforthetransportofoxygentocellsforrespiration,andthetransportawayofcarbondioxide.Haemaglobinisacomplexprotein,containingfournon-proteincomponentscalledhaemgroups,eachofwhichhasanFe2+ionatitscentre,andfourdativecovalentbondsbetweentheironandfourN-atomsinthehaemstructure,andafurtherdativecovalentbondtotheproteinglobin.

Oxygencanbindtotheironinthehaemgroupasaligand,givingaco-ordinationnumberof6,inordertobetransported.Thecolourofthecomplexwhenoxygenisboundasaligandistherichredofoxygenatedblood.Carbonmonoxidecanalsobindtotheironatthesamebindingsiteastheoxygenwould,thestabilityconstantforthiscomplexisgreater(seelater)sothebondbetweentheironandtheCOisstrongerandmuchlesslikely

tobreak.COwillthereforebindpreferentiallyifbothcarbonmonoxideandoxygenarepresentinthelungs;thebindingisirreversible,sothathaemaglobinbecomesuseless.Sincetobaccosmokingproducescarbonmonoxide,andthisisoneofthereasonswhysmokersbecomeshortofbreath.IsomerismincomplexionsStereoisomerismariseswhentheligandscanbearrangedindifferentspatialarrangements.Incomplexes,aswithorganicsubstances,wewillseebothcis-transisomerismandopticalisomerism.Cis–TransisomerismForexamplewhenacomplexcontainsfourofoneligandandtwoofanother,wecanhavecisandtransisomers.e.g.cobaltformsanoctahedralcomplexofformula[Co(NH3)4Cl2]+.Thecobaltisinoxidationstate+3here.Itwasfoundthatagreensaltandapurplesaltbothwiththesameformulacouldbecrystallized.Thiswasclearevidencefortwodifferentisomers:

cis-isomer trans-isomerpurple green

Clat90°tooneanother Clat180°tooneanother (adjacent=cis) (opposite/across=trans)


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Cis-transisomerismisalsopossibleinsomecomplexeswithco-ordinationnumber4iftheyhaveasquare-planarconfiguration.Nickelformsasquareplanarcomplex[NiCl2(NH3)2].Thishasnooverallcharge,asthenickelisinthe+2oxidationstate. cis-isomer trans-isomerAsimilarcomplexofplatinum[PtCl2(NH3)2]isoneofthemosteffectivedrugsinchemotherapyforcancer.Itisthecis-formofthecomplexwhichisbiologicallyactive.Thedrugisaliquid,usuallyadministeredintravenouslyasadrip,andgoesunderthenameofcis-platin.Theimportanceoftheexactshapeandstructureofthemoleculeisemphasizedbythefactthatthetrans-formisineffective.Thebenefitofusingcis-platintotreatcanceristhatitworksbybindingtotheDNAoffast-growingcancercells,changingtheDNAstructureandpreventingthemfromdividingtoreproduce.Chemotherapyhasunpleasantsideeffects,however,andanewgenerationofcancer-treatmentdrugsrequiringlowerdosesandwithfewersideeffectsthancis-platinhavebeendeveloped,e.g.carboplatin.OpticalisomerismOpticalisomerismariseswithoctahedralcomplexescontainingmultidentateligands,wherethesecanbearrangedaroundthecentraltransitionmetalatomasnon-superimposablemirrorimages.Recallthatopticalisomersrotateplane-polarisedlightdifferently–oneisomerwillrotateittotheright,andtheothertotheleft.Anequalmixtureofisomerswillhavenoeffectonplanepolarizedlightbecausetherotationscancelout.Examples: [Ni(en)3]2+-acomplexwiththreebidentateligands


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[Co(en)2(H2O)2]2+-acomplexwithtwobidentateligandsandtwomonodentateligands.Complexeslikethiswillhavecisandtransisomers,buttherewillbetwocisisomerareopticalisomersofeachother.TwocisisomersThetrans-isomerCasestudiesinTransitionMetalchemistryReactionsofcoppercompoundsi)Cu2+(aq)withsodiumhydroxideBlueaqueouscopper(II)ionsreactwithhydroxideionstoformablueprecipitateofcopper(II)hydroxide.Theprecipitateisinsolubleinexcesssodiumhydroxide. Cu2+

(aq)+2OH-(aq)" Cu(OH)2(s)

ii)Cu2+(aq)withammoniasolutionRememberthatammoniaformsanequilibriumreactingwithwaterwhenitdissolves,NH3(aq)+H2O(aq)⇌NH4

+(aq)+OH-

(aq)soammoniasolutionisbothasourceofNH3andOH-ligands.InitiallyitistheOH-ionsthatreactwiththeaqueouscopper(II)ions,formingabluecopperhydroxideprecipitateasabove.Whenexcessammoniasolutionisadded,thecopperhydroxideprecipitateredissolvesundergoingaligandsubstitutionreactiontoformadarkbluesolutioncontainingtheoctahedraltrans-complexof[Cu(NH3)4(H2O)2]2+Overall: [Cu(H2O)6]2+(aq)+4NH3(aq)" [Cu(NH3)4(H2O)2]2+(aq)+4H2O(l)


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iii)Cu2+(aq)withconcentratedhydrochloricacidWhenconcentratedhydrochloricacidisaddedtoblueaqueouscopper(II)ions,ayellowsolutioncontainingtetrahedral[CuCl4]2-(aq)isformedinaligandsubstitutionreaction,althoughagreencolourisoftenseenwhenboththeaqueousionsandthenewcomplexionarepresentinsolution.Asthisisanequilibrium,thecolourofthesolutionchangesastherelativeconcentrationsofchlorideionsandwaterpresentchange. [Cu(H2O)6]2+(aq)+4Cl-(aq)⇌[CuCl4]2-(aq)+6H2O(l)iv)Cu2+(aq)withiodideionsAqueouscopper(II)ionsreactwithiodideionsinaredoxreactionthatformsbrownaqueousiodineandawhiteprecipitateofcopper(I)iodide.Notethatthecopper(I)ionhasa3d10electronarrangement,soitisnotsurprisingwithnopartiallyfilledd-subshellthatitiswhite. 2Cu2+

(aq)+4I-(aq)" 2CuI(s)+I2(aq)Oxidationstates: +2-1+1-10Copper(I)compoundsareunstableinsolution.Whensolidcopper(I)oxidereactswithhotdilutesulphuricacid,abrownprecipitateofcopperandabluesolutionofcopper(II)sulphateareformedinadisproportionationreaction: Cu2O(s)+H2SO4(aq)" CuSO4(aq)+Cu(s)+H2O(l)Oxidationnumbers: +1 +2 0 +1 Reactionsofironcompoundsi)Fe2+(aq)andFe3+(aq)withsodiumhydroxidePalegreeniron(II)ionsinsolutionreactwithsodiumhydroxidetoformadirtygreenprecipitateofiron(II)hydroxide.Onstandingtheprecipitateturnsbrownatitssurfaceasiron(II)hydroxideisoxidizedtoiron(III)hydroxide. Fe2+(aq)+2OH-

(aq)" Fe(OH)2(s)Paleyellowiron(III)ionsinsolutionreactwithsodiumhydroxidetoformanorange-brownprecipitateofiron(III)hydroxide.Fe3+(aq)+3OH-

(aq)" Fe(OH)3(s)Neitherprecipitateredissolvesonadditionofexcesssodiumhydroxide.ii)Fe2+(aq)andFe3+(aq)withammoniasolutionItisonlythehydroxideionsintheammoniasolutionthatreactwithiron(II)andiron(III)ionstoformiron(II)hydroxideandiron(III)hydroxideexactlyasabove.Thereisnofurtherreactionwiththeammoniapresent,andtheprecipitatesdonotredissolve.


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iii)oxidationofiron(II)withacidifiedmanaganate(VII)Palegreenaqueousiron(II)ionswilldecolourisepurplemanagante(VII)ionsinacidicsolutioninaredoxreaction.Theiron(II)isoxidizedtopaleyellowaqueousiron(III)ions,whilethemanganate(VII)isreducedtopalepinkaqueousmanganese(II)ions. MnO4

-(aq)+8H+

(aq)+5Fe2+(aq)" 5Fe3+(aq)+Mn2+(aq)+4H2O(l)iv)reductionofiron(III)withiodideionsPaleyellowaqueousiron(III)ionscanbereducedtopalegreeniron(II)ionsbyredoxreactionwithcolourlessaqueousiodideions.Inthisreactioniodineisformedinaqueoussolution,whichisbrown,andmasksthecolourchangeoftheironions. 2Fe3+(aq)+2I-(aq)" I2(aq)+2Fe2+(aq) Reactionsofmanganesecompoundsi)Mn2+(aq)withsodiumhydroxidePalepinkaqueousmanganese(II)ionsreactwithsodiumhydroxidetoformapalebrownprecipitateofmanganese(II)hydroxide,whichdarkensonstandinginair.Theprecipitateisdoesnotredissolveorreactfurtheronfurtheradditionofsodiumhydroxide. Mn2+

(aq)+2OH-(aq)" Mn(OH)2(s)

ii)Mn2+(aq)withammoniasolutionItisonlythehydroxideionsintheammoniasolutionthatreactwithmanganese(II)ionstoformmanganese(II)hydroxideexactlyasabove.Thereisnofurtherreactionwiththeammoniapresent,andtheprecipitatesdonotredissolve.ReactionsofchromiumcompoundsThecolourofthecomplex[Cr(H2O)6]3+(aq)ispalepurple.Thismaybeasurprisesincewehaveconsistentlyreferredtothecolourchangewhenacidifieddichromate(VI)ionsareusedasanoxidizingagentasorangefordichromate(VI)andgreenforchromium(III).Thisisbecausethechromium(III)ionformedinthepresenceofsulphuricacidisactuallynot[Cr(H2O)6]3+but[Cr(H2O)5(SO4)]+andthisisthegreenionwehavebeenreferringtoasCr3+(aq).i)Cr3+(aq)withsodiumhydroxidePalepurpleaqueouschromium(III)ionsreactwithsodiumhydroxidetoformagreen-greyprecipitateofchromium(III)hydroxide. Cr3+(aq)+3OH-

(aq)" Cr(OH)3(s)Onadditionofexcesssodiumhydroxide,thisprecipitateredissolvestoformadarkgreensolutioncontainingthecomplexion[Cr(OH)6]3-(aq). Cr(OH)3(s)+3OH-

(aq)" [Cr(OH)6]3-(aq)ii)Cr3+(aq)withammoniasolutionInitiallyitistheOH-ionsthatreactwiththeaqueouschromium(III)ions,formingagreen-greychromiumhydroxideprecipitateasabove.


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Whenexcessammoniasolutionisadded,thechromiumhydroxideprecipitateredissolvesundergoingaligandsubstitutionreactiontoformapurplesolutioncontainingtheoctahedralcomplex[Cr(NH3)6]3+Overall: [Cr(H2O)6]3+(aq)+6NH3(aq)" [Cr(NH3)6]3+(aq)+6H2O(l)iii)oxidationofchromium(III)withalkalinehydrogenperoxideHotalkalinehydrogenperoxideisapowerfuloxidizingagent,andreactsinaredoxreactiontooxidiseaqueouschromium(III)ions(greenorpalepurple,dependingontheligandsbondedtothechromium)toyellowchromate(VI)ions. 3H2O2(aq)+2Cr3+(aq)+10OH-

(aq)" 2CrO42-(aq)+8H2O(l)

iv)reductionofdichromate(VI)withzincunderacidicconditionsThedichromate(VI)ionisorangeinaqueoussolution.Itcanbereducedinaredoxreactionwithareducingagentsuchaszincmetalunderacidicconditions,formingchromium(III)ionsinsolution.Thechromium(III)ionformedislikelytobegreen,asaligandfromtheacidwillformpartoftheCr3+(aq)complex. Cr2O7

2-(aq)+14H+

(aq)+3Zn(s)" 2Cr3+(aq)+3Zn2+(aq)+7H2O(l)Ifanexcessofzincisused,thechromium(III)ionsinsolutioncanbereducedinafurtherredoxreactiontopalebluechromium(II)ions. Zn(s)+2Cr3+(aq)" Zn2+(aq)+2Cr2+(aq)QualitativeTestsforIonsTestsforanionsWehavealreadymetthetestsforcarbonate,sulphate,chloride,bromideandiodideions.Therearespecificprecautionstobetakeninpreventinginterferingionsfrominvalidatingtheresultsofthesetests,andanorderinwhichthetestsshouldbedoneifthesamesampleistobeusedforallthetests. Carbonate: addH+

(aq) observefizzing Sulphate: addBa2+(aq) observewhiteprecipitate Chloride: addAg+(aq) observewhiteprecipitate,solubleindiluteammoniasolution Bromide: addAg+(aq) observecreamprecipitate,solubleinconc.ammoniasolution Iodide: addAg+(aq) observeyellowprecipitate,insolubleinammoniasolutionTestsforcationsInthistopicwehaveseenthattheprecipitationreactionsoftransitionmetalswithsodiumhydroxidesolutionwillallowustoidentifycopper(II),iron(II),iron(III),chromium(III)andmanganese(II)ionsinsolutionbasedonthecolouroftheprecipitateandtheactionofexcesssodiumhydroxideonit. Copper(II) addNaOH(aq) blueprecipitatethatdoesnotredissolveinexcessNaOH(aq) Iron(II) addNaOH(aq) dirtygreenprecipitatethatdoesnotredissolveinexcessNaOH(aq) Iron(III) addNaOH(aq) orange-brownprecipitatethatdoesnotredissolveinexcessNaOH(aq) Manganese(II) addNaOH(aq) palebrownprecipitatethatdoesnotredissolveinexcessNaOH(aq)

Chromium(III) addNaOH(aq) green-greyprecipitate,redissolvesinexcesstogiveagreensolution


p. 16

Ifaprecipitatedoesnotformwhensodiumhydroxideisadded,thecationpresentmightbetheammoniumion,NH4

+.Thepresenceofammoniumionsistestedbywarmingthesolutionwithsodiumhydroxide.Thiscausesanyammoniumionspresenttoreactwiththehydroxideionstoproduceammoniagas. NH4

+(aq)+OH-

(aq)" NH3(g)+H2O(l)Theevolutionofammoniagaswillcausedampredlitmuspaperheldatthemouthofthetesttubetoturnblue.

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