下载专业英语.docx

下载专业英语.docx

《下载专业英语.docx》由会员分享，可在线阅读，更多相关《下载专业英语.docx（26页珍藏版）》请在冰点文库上搜索。

下载专业英语

ToothDecayandFluoridation

Toothenamelconsistsmainlyofamineralcalledhydroxyapatite,Ca10（PO4）6（OH）2.Itisthehardestsubstanceinthebody.Toothcavitiesarecausedbythedissolvingactionpfacidsontoothenamel:

Ca10（PO4）6（OH）2（s）+8H-（aq）→lOCa2+（aq）+6HPO42-（aq）+2H20（l）

    TheresultantCa2+andHPO42-ionsdiffuseoutofthetoothenamelandarewashedawaybysaliva.Theacidsthatattackthehydroxyapatiteareformedbytheactionofspecificbacteriaonsugarsandothercarbohydratespresentintheplaqueadheringtotheteeth.

    Fluorideion,presentindrinkingwater,toothpaste,orothersources,canreactwithhydroxyapatitetoformfluoroapatite,Ca10（PO4）6F2.Thismineral,inwhichF-hasreplacedOH-,ismuchmoreresistanttoattackbyacidsbecausethefluorideionisamuchweakerBrensted-Lowrybasethanthehydroxideion.

    Becausethefluorideionissoeffectiveinpreventingcavities,itisaddedtothepublicwatersupplyinmanyplacestogiveaconcentrationof1mg/L（1ppm）.ThecompoundaddedmaybeNaForNa2SiF6.Thelattercompoundreactswithwatertoreleasefluorideionsbythefollowingreaction:

SiF62-aq）+2H20（l）-→6F-（aq）+4H+（aq）+SiO2（s）

    About80percentofalltoothpastesnowsoldintheUnitedStatescontainfluoridecompounds,usuallyatthelevelof0.1percentfluoridebymass.Themostcommoncompoundsintoothpastesarestannousfluoride,SnF2,sodiummonofluorophosphate,Na2PO3F,andsodiumfluoride,NaF.

DiamondCoatings

Besidesbeingoneofthehardestknownsubstances,diamondisalsohighlyresistanttocorrosion,andhasexceptionalhigh-temperaturestability.Atpresent,commercialdiamondsarewidelyusedtostrengthencuttingandgrindingtools.Thesediamondsareembeddedinthetoolsandarenotintimatelyanduniformlypartofthematerial.

    Scientistshaverecentlydevelopedproceduresforapplyinganultrathinlayerofpolycrystallinediamondonmanymaterials.Thesethinfilmsimpartthehardnessanddurabilityofdiamondtoavarietyofmaterials─glass,paper,plastics,metals,andsemiconductordevices.Imaginescratchproofglass;cuttingtoolsthatvirtuallyneverneedsharpening;surfacesthatarechemical-resistant;temperaturesensorsthatoperateathightemperaturesinharshenvironments.Becausediamondiscompatiblewithbiologicaltissue,itcanbeusedtocoatprostheticmaterialsandbiosensors.Whendepositedonsilicon,polycrystallinediamondthinfilmscanserveashigh-temperaturesensorsandbeusedinelectronicdevicessuchasflat-screendisplays.Insomeoftheseapplicationsthefilmmustbedopedduringdepositionwithanotherelement,suchasboron,tocreateasemiconductor.

    Oneprocedureforgeneratingdiamondfilmsinvolvesexposingamixtureofmethanegas,CH4,andhydrogengas,H2,tointensemicrowaveradiationinthepresenceoftheobjecttobecoated.Typically,totalgaspressureisabout50torr,andH2ispresentinlargeexcess.Underappropriateconditions,theCH4decomposes,depositingathinfilmofdiamond.TheH2dissociatesintoatomichydrogen,whichimpedesformationofgraphite.Atomichydrogenreactsfasterwithgraphitethanwithdiamond,effectivelyremovinggraphitefromthegrowingfilm.Althoughtheprocessisnotfundamentallyexpensiveorsophisticated,widespreadcommercial-applicationswilldependonproducingfilmsofuniformthicknessandminimalsurfaceroughness.Inpartthisdependsonhavingahighdensityofsitesonthesurfaceatwhichdiamondfilmformationcaninitiate.Thesesitesarecallednucleationsites.Researchersarepresentlyworkingtoreducethedepositiontemperatures,increasedepositionrates,anddeveloptechniquesforforminguniformfilmsonobjectsofvarying  shapeandsize.

WaterSoftening

WatercontainingarelativelyhighconcentrationofCa2+,Mg2+andotherdivalentcationsiscalledhardwater.Althoughthepresenceoftheseionsisgenerallynotahealththreat,theycanmakewaterunsuitableforsomehouseholdandindustrialuses.Forexample,theseionsreactwithsoapstoformaninsolublesoapscum,thestuffofbathtubrings.Inaddition,mineraldepositsmayformwhenwatercontainingtheseionsisheated.Whenwatercontainingcalciumionsandbicarbonateionsisheated,somecarbondioxideisdrivenoff.Asaresult,thesolutionbecomeslessacidic,andinsolublecalciumcarbonateforms:

Ca2+（aq）+2HCO3-（aq）→CaCO3（s）+CO2（g）+H2O（l）

    ThesolidCaCO3coatsthesurfaceofhot-watersystemsandteakettles,therebyreducingheatingefficiency,Thesedeposits,calledscale,canbeespeciallyseriousinboilerswherewaterisheatedunderpressureinpipesrunningthroughafurnace.Formationofscalereducestheefficiencyofheattransferandreducestheflowofwaterthroughpipes.

    Theremovaloftheionsthatcausehardwateriscalledwatersoftening.Notallmunicipalwatersuppliesrequirewatersoftening.Inthosethatdo,thewaterisgenerallytakenfromundergroundsourcesinwhichithashadconsiderablecontactwithlimestone,CaCO3,andothermineralscontainingCa2+,Mg2+,andFe2+Thelime-sodaprocessisusedforlarge-scalemunicipalwater-softeningoperations.Thewateristreatedwithlime,CaO[orslakedlime,Ca（OH）2],andsodaash,Na2CO3.

    ThesechemicalscauseprecipitationofCa2+asCaCO3（Ksp=2.8X10-9）andMg2+asMg（OH）2;,（Ksp=1.8X10-11）:

Ca2+（aq）+CO32-（aq）→CaCO3（s）

Mg2+（aq）+2OH（aq）→Mg（OH）2（s）

    Ionexchangeisatypicalhouseholdmethodforsoftening.Inthisprocedurethehardwaterispassedthroughabedofanionexchangeresin:

plasticbeadswithcovalentlyboundaniongroupssuchas―COO-or―SO3-.

    ThesenegativelychargedgroupshaveNa+ionsattachedtobalancetheircharges.TheCa2+ionsandothercationsinthehardwaterareattractedtotheanionicgroupsanddisplacethelower-chargedNa+ionsintothewater.Thus,onetypeofionisexchangedforanother.Tomaintainchargebalance,2Na+ionsenterthewaterforeachCa2+removed.IfwerepresenttheresinwithitsanionicsiteasR―COO-wecanwritetheequationfortheprocessasfollows:

2Na（R―COO）（s）+Ca2+（aq）==Ca（R―COO）;（s）+2Na+（aq）

    WatersoftenedinthiswaycontainsanincreasedconcentrationofNa+ions.AlthoughNa+ionsdonotformprecipitatesorcauseotherproblemsassociatedwithhard-watercations,individualsconcernedwiththeirsodiumintakeshouldavoiddrinkingwatersoftenedinthisway.Thisincludespeoplewithhighbloodpressure（hypertension）orcertainotherhealthproblems.WhenalltheavailableNa+ionshavebeendisplacedfromtheion-exchangeresin,theresinisregeneratedbyflushingitwithaconcentratedsolutionofNaCl.HomeownersaccomplishthistaskbychargingtheirunitswithlargeamountsofNaCl（s）,whichcanbepurchasedinlargebagsatmostgrocerystores.ThehighconcentrationofNa+forcestheequilibriumshownintheaboveequationtoshifttotheleft,causingtheNa+ionstodisplacethehard-watercations,whichareflusheddownthedrain.

OtherEnergySources

Ofthevarioussourcesofenergythatcouldserveasalternativestofossilfuels,onlynuclearandsolarenergiesarepotentiallycapableoffurnishingsufficientquantitiestosatisfytheworld'sneeds.Nuclearpowercurrentlysuppliesabout8percentofthegrossenergyconsumedintheUnitedStates.

    Solarenergyistheworld'slargestenergysource.Onaclearday,about1kJofsolarenergyreacheseachsquaremeterofEarth'ssurfaceeverysecond.Thesolarenergythatfallsononly0.1percentofU.S.landareaisequivalenttoalltheenergythatthisnationcurrentlyuses.Harnessingthisenergyisdifficultbecauseitisdilute（itisdistributedoverawidearea）anditfluctuateswithtimeandweatherconditions.Theeffectiveuseofsolarenergywilldependonthedevelopmentofsomemeansofstoringthecollectedenergyforuseatalatertime.Anypracticalmeansfordoingthiswillalmostcertainlyinvolveuseofanendothermicchemicalprocessthatcanbelaterreversedtoreleaseheat.Onesuchreactionisthefollowing:

CH4（g）+H2O（g）+heat==CO（g）+3H2（g）

    Thisreactionproceedsintheforwarddirectionathightemperatures,whichcanbeobtainedinasolarfurnace.TheCOandH2,formedinthereactioncouldthenbereactedlater,withtheheatreleasedbeingputtousefulwork.

    Solarenergycanbeconverteddirectlyintoelectricitybyuseofphotovoltaicdevices,sometimescalledsolarcells.Theefficienciesofsolarenergyconversionbyuseofsuchdeviceshaveincreaseddramaticallyduringthepastfewyearsasaresultofintensiveresearchefforts.Photovoltaicsarevitaltothegenerationofpowerforsatellites.However,forlarge-scalegenerationofusefulenergyatEarth'ssurface,theyarenotyetpracticalbecauseofhighunitcost.Evenifthecostsarereduced,somemeansmustbefoundtostoretheenergyproducedbythesolarcells,becausethesunshinesonlyintermittentlyandduringonlypartofthedayatanyplace.Onceagain,thesolutiontothisproblemwillalmostcertainlybetousetheenergytorunachemicalreactioninthedirectioninwhichitisendothermic.

What's"Free"AboutFreeEnergy?

TheGibbsfreeenergyisaremarkablethermodynamicquantity.Becausesomanychemicalreactionsarecarriedoutunderconditionsofnear-constantpressureandtemperature,chemists,biochemists,andengineersusethesignandmagnitudeof△Gasexceptionallyusefultoolsinthedesignandimplementationofchemicalandbiochemicalreactions.Wewillseeexamplesoftheusefulnessof△Gthroughouttheremainderofthischapterandthistext.

      TherearetwocommonquestionsthatoftenarisewhenonefirstlearnsabouttheGibbsfreeenergy:

Whydoest