Buffer solutions PDF

Preview

Summary

This page describes simple acidic and alkaline buffer solutionsand explains how they work...

Description

2014.12.26.

buffersolutions

BUFFERSOLUTIONS Thispagedescribessimpleacidicandalkalinebuffersolutions andexplainshowtheywork.

Whatisabuffersolution? Definition AbuffersolutionisonewhichresistschangesinpHwhensmall quantitiesofanacidoranalkaliareaddedtoit. Acidicbuffersolutions AnacidicbuffersolutionissimplyonewhichhasapHlessthan 7.Acidicbuffersolutionsarecommonlymadefromaweakacid andoneofitssaltsoftenasodiumsalt. Acommonexamplewouldbeamixtureofethanoicacidand sodiumethanoateinsolution.Inthiscase,ifthesolution containedequalmolarconcentrationsofboththeacidandthe salt,itwouldhaveapHof4.76.Itwouldn'tmatterwhatthe concentrationswere,aslongastheywerethesame. YoucanchangethepHofthebuffersolutionbychangingthe ratioofacidtosalt,orbychoosingadifferentacidandoneofits salts. Note:Ifyouneedtoknowaboutcalculationsinvolvingbuffer solutions,youmaybeinterestinmychemistrycalculations book.

Alkalinebuffersolutions AnalkalinebuffersolutionhasapHgreaterthan7.Alkaline buffersolutionsarecommonlymadefromaweakbaseandone ofitssalts. Afrequentlyusedexampleisamixtureofammoniasolutionand ammoniumchloridesolution.Iftheseweremixedinequalmolar proportions,thesolutionwouldhaveapHof9.25.Again,it doesn'tmatterwhatconcentrationsyouchooseaslongasthey arethesame.

http://www.chemguide.co.uk/physical/acidbaseeqia/buffers.html

1/10

2014.12.26.

buffersolutions

Howdobuffersolutionswork? Abuffersolutionhastocontainthingswhichwillremoveany hydrogenionsorhydroxideionsthatyoumightaddtoit otherwisethepHwillchange.Acidicandalkalinebuffer solutionsachievethisindifferentways. Acidicbuffersolutions We'lltakeamixtureofethanoicacidandsodiumethanoateas typical. Ethanoicacidisaweakacid,andthepositionofthisequilibrium willbewelltotheleft:

Addingsodiumethanoatetothisaddslotsofextraethanoate ions.AccordingtoLeChatelier'sPrinciple,thatwilltipthe positionoftheequilibriumevenfurthertotheleft. Note:Ifyoudon'tunderstandLeChatelier'sPrinciple,follow thislinkbeforeyougoanyfurther,andmakesurethatyou understandabouttheeffectofchangesofconcentrationon thepositionofequilibrium. UsetheBACKbuttononyourbrowsertoreturntothispage.

Thesolutionwillthereforecontaintheseimportantthings: lotsofunionisedethanoicacid; lotsofethanoateionsfromthesodiumethanoate; enoughhydrogenionstomakethesolutionacidic. Otherthings(likewaterandsodiumions)whicharepresent aren'timportanttotheargument. Addinganacidtothisbuffersolution Thebuffersolutionmustremovemostofthenewhydrogenions otherwisethepHwoulddropmarkedly. Hydrogenionscombinewiththeethanoateionstomake ethanoicacid.Althoughthereactionisreversible,sincethe ethanoicacidisaweakacid,mostofthenewhydrogenionsare removedinthisway.

Sincemostofthenewhydrogenionsareremoved,thepHwon't http://www.chemguide.co.uk/physical/acidbaseeqia/buffers.html

2/10

2014.12.26.

buffersolutions

changeverymuchbutbecauseoftheequilibriainvolved,itwill fallalittlebit. Addinganalkalitothisbuffersolution Alkalinesolutionscontainhydroxideionsandthebuffersolution removesmostofthese. Thistimethesituationisabitmorecomplicatedbecausethere aretwoprocesseswhichcanremovehydroxideions. Removalbyreactingwithethanoicacid Themostlikelyacidicsubstancewhichahydroxideionisgoing tocollidewithisanethanoicacidmolecule.Theywillreactto formethanoateionsandwater.

Note:Youmightbesurprisedtofindthiswrittenasaslightly reversiblereaction.Becauseethanoicacidisaweakacid,its conjugatebase(theethanoateion)isfairlygoodatpickingup hydrogenionsagaintoreformtheacid.Itcangetthesefrom thewatermolecules.Youmaywellfindthisreactionwritten asoneway,buttobefussyaboutit,itisactuallyreversible!

Becausemostofthenewhydroxideionsareremoved,thepH doesn'tincreaseverymuch. Removalofthehydroxideionsbyreactingwithhydrogenions Rememberthattherearesomehydrogenionspresentfromthe ionisationoftheethanoicacid.

Hydroxideionscancombinewiththesetomakewater.Assoon asthishappens,theequilibriumtipstoreplacethem.Thiskeeps onhappeninguntilmostofthehydroxideionsareremoved.

Again,becauseyouhaveequilibriainvolved,notallofthe hydroxideionsareremovedjustmostofthem.Thewater http://www.chemguide.co.uk/physical/acidbaseeqia/buffers.html

3/10

2014.12.26.

buffersolutions

formedreionisestoaverysmallextenttogiveafewhydrogen ionsandhydroxideions.

Alkalinebuffersolutions We'lltakeamixtureofammoniaandammoniumchloride solutionsastypical. Ammoniaisaweakbase,andthepositionofthisequilibriumwill bewelltotheleft:

Addingammoniumchloridetothisaddslotsofextraammonium ions.AccordingtoLeChatelier'sPrinciple,thatwilltipthe positionoftheequilibriumevenfurthertotheleft. Thesolutionwillthereforecontaintheseimportantthings: lotsofunreactedammonia; lotsofammoniumionsfromtheammoniumchloride; enoughhydroxideionstomakethesolutionalkaline. Otherthings(likewaterandchlorideions)whicharepresent aren'timportanttotheargument. Addinganacidtothisbuffersolution Therearetwoprocesseswhichcanremovethehydrogenions thatyouareadding. Removalbyreactingwithammonia Themostlikelybasicsubstancewhichahydrogenionisgoing tocollidewithisanammoniamolecule.Theywillreacttoform ammoniumions.

Most,butnotall,ofthehydrogenionswillberemoved.The ammoniumionisweaklyacidic,andsosomeofthehydrogen ionswillbereleasedagain. Removalofthehydrogenionsbyreactingwithhydroxideions Rememberthattherearesomehydroxideionspresentfromthe reactionbetweentheammoniaandthewater.

Hydrogenionscancombinewiththesehydroxideionstomake water.Assoonasthishappens,theequilibriumtipstoreplace http://www.chemguide.co.uk/physical/acidbaseeqia/buffers.html

4/10

2014.12.26.

buffersolutions

thehydroxideions.Thiskeepsonhappeninguntilmostofthe hydrogenionsareremoved.

Again,becauseyouhaveequilibriainvolved,notallofthe hydrogenionsareremovedjustmostofthem. Addinganalkalitothisbuffersolution Thehydroxideionsfromthealkaliareremovedbyasimple reactionwithammoniumions.

Becausetheammoniaformedisaweakbase,itcanreactwith thewaterandsothereactionisslightlyreversible.Thatmeans that,again,most(butnotall)ofthethehydroxideionsare removedfromthesolution.

Calculationsinvolvingbuffersolutions Thisisonlyabriefintroduction.Therearemoreexamples, includingseveralvariations,over10pagesinmychemistry calculationsbook. Acidicbuffersolutions Thisiseasiertoseewithaspecificexample.Rememberthatan acidbuffercanbemadefromaweakacidandoneofitssalts. Let'ssupposethatyouhadabuffersolutioncontaining0.10mol dm3ofethanoicacidand0.20moldm3ofsodiumethanoate. HowdoyoucalculateitspH? Inanysolutioncontainingaweakacid,thereisanequilibrium betweentheunionisedacidanditsions.Soforethanoicacid, youhavetheequilibrium:

Thepresenceoftheethanoateionsfromthesodiumethanoate willhavemovedtheequilibriumtotheleft,buttheequilibrium stillexists. http://www.chemguide.co.uk/physical/acidbaseeqia/buffers.html

5/10

2014.12.26.

buffersolutions

Thatmeansthatyoucanwritetheequilibriumconstant,Ka,for it:

Whereyouhavedonecalculationsusingthisequation previouslywithaweakacid,youwillhaveassumedthatthe concentrationsofthehydrogenionsandethanoateionswere thesame.Everymoleculeofethanoicacidthatsplitsupgives oneofeachsortofion. That'snolongertrueforabuffersolution:

Iftheequilibriumhasbeenpushedevenfurthertotheleft,the numberofethanoateionscomingfromtheethanoicacidwillbe completelynegligiblecomparedtothosefromthesodium ethanoate. Wethereforeassumethattheethanoateionconcentrationisthe sameastheconcentrationofthesodiumethanoateinthis case,0.20moldm3. Inaweakacidcalculation,wenormallyassumethatsolittleof theacidhasionisedthattheconcentrationoftheacidat equilibriumisthesameastheconcentrationoftheacidwe used.Thatisevenmoretruenowthattheequilibriumhasbeen movedevenfurthertotheleft. Sotheassumptionswemakeforabuffersolutionare:

Now,ifweknowthevalueforKa,wecancalculatethehydrogen ionconcentrationandthereforethepH. Kaforethanoicacidis1.74x105moldm3. http://www.chemguide.co.uk/physical/acidbaseeqia/buffers.html

6/10

2014.12.26.

buffersolutions

RememberthatwewanttocalculatethepHofabuffersolution containing0.10moldm3ofethanoicacidand0.20moldm3of sodiumethanoate.

ThenallyouhavetodoistofindthepHusingtheexpression pH=log10[H+] Youwillstillhavethevalueforthehydrogenionconcentration onyourcalculator,sopressthelogbuttonandignorethe negativesign(toallowfortheminussigninthepHexpression). Youshouldgetananswerof5.1totwosignificantfigures.You can'tbemoreaccuratethanthis,becauseyourconcentrations wereonlygiventotwofigures.

Youcould,ofcourse,beaskedtoreversethisandcalculatein whatproportionsyouwouldhavetomixethanoicacidand sodiumethanoatetogetabuffersolutionofsomedesiredpH.It isnomoredifficultthanthecalculationwehavejustlookedat. SupposeyouwantedabufferwithapHof4.46.Ifyouunlog thistofindthehydrogenionconcentrationyouneed,youwill finditis3.47x105moldm3. FeedthatintotheKaexpression.

AllthismeansisthattogetasolutionofpH4.46,the concentrationoftheethanoateions(fromthesodiumethanoate) inthesolutionhastobe0.5timesthatoftheconcentrationof theacid.Allthatmattersisthatratio. Inotherwords,theconcentrationoftheethanoatehastobehalf http://www.chemguide.co.uk/physical/acidbaseeqia/buffers.html

7/10

2014.12.26.

buffersolutions

thatoftheethanoicacid. Onewayofgettingthis,forexample,wouldbetomixtogether 10cm3of1.0moldm3sodiumethanoatesolutionwith20cm3 of1.0moldm3ethanoicacid.Or10cm3of1.0moldm3sodium ethanoatesolutionwith10cm3of2.0moldm3ethanoicacid. Andthereareallsortsofotherpossibilities.

Note:Ifyourmathsisn'tverygood,theseexamplescan lookabitscary,butinfacttheyaren't.Gothroughthe calculationslinebyline,andmakesurethatyoucansee exactlywhatishappeningineachlinewherethenumbers arecomingfrom,andwhytheyarewheretheyare.Thengo awayandpractisesimilarquestions. Ifyouaregoodatmathsandcan'tseewhyanyoneshould thinkthisisdifficult,thenfeelveryfortunate.Mostpeople aren'tsolucky!

Alkalinebuffersolutions Wearetalkinghereaboutamixtureofaweakbaseandoneof itssaltsforexample,asolutioncontainingammoniaand ammoniumchloride. Themodern,andeasy,wayofdoingthesecalculationsistore thinkthemfromthepointofviewoftheammoniumionrather thanoftheammoniasolution.Onceyouhavetakenthisslightly differentviewpoint,everythingbecomesmuchthesameas before. SohowwouldyoufindthepHofasolutioncontaining0.100mol dm3ofammoniaand0.0500moldm3ofammoniumchloride? Themixturewillcontainlotsofunreactedammoniamolecules andlotsofammoniumionsastheessentialingredients. Theammoniumionsareweaklyacidic,andthisequilibriumis setupwhenevertheyareinsolutioninwater:

YoucanwriteaKaexpressionfortheammoniumion,andmake thesamesortofassumptionsaswedidinthepreviouscase:

http://www.chemguide.co.uk/physical/acidbaseeqia/buffers.html

8/10

2014.12.26.

buffersolutions

Thepresenceoftheammoniainthemixtureforcesthe equilibriumfartotheleft.Thatmeansthatyoucanassumethat theammoniumionconcentrationiswhatyoustartedoffwithin theammoniumchloride,andthattheammoniaconcentrationis allduetotheaddedammoniasolution. ThevalueforKafortheammoniumionis5.62x1010moldm3. RememberthatwewanttocalculatethepHofabuffersolution containing0.100moldm3ofammoniaand0.0500moldm3of ammoniumchloride. JustputallthesenumbersintheKaexpression,anddothesum:

Questionstotestyourunderstanding Ifthisisthefirstsetofquestionsyouhavedone,pleasereadthe introductorypagebeforeyoustart.YouwillneedtousetheBACKBUTTON onyourbrowsertocomebackhereafterwards. questionsonbuffersolutions answers

Wherewouldyouliketogonow? Totheacidbaseequilibriamenu... http://www.chemguide.co.uk/physical/acidbaseeqia/buffers.html

9/10...