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36沸点和蒸馏BoilingPointsofPureLiquids沸点的纯液体Anygivenliquid,whenadmittedintoaclosedevacuatedspace,evaporatesuntilthevaporattains acertaindefinitepressure,whichdependsonlyuponthetemperature.Thispressure,whichisthepressureexertedbythevaporinequilibriumwiththeliquid,isthevaporpressureoftheliquidatthattemperature.Asthetemperatureincreases.thevaporpressureofatypicalliquidxincreasesregularlyasshownbythegeneralizedvaporpressure-temperaturecurveBC,inFig.5.任何给定的液体,当进入一个封闭的疏散空间,蒸发到蒸汽形成某种明确的压力,仅仅依赖于温度。这种压力,这是由蒸汽压力与液体平衡的,是液体的蒸气压在温度。随着温度的升高。这个压力，是通过在与液体平衡的蒸汽施加的压力，是液体在该温度下的蒸汽压，其随着温度的增加而增加。一个典型液体x的蒸汽压的增加经常如图五所示，广义蒸汽压-温度曲线BC。Atthetemperature,Tp,wherethevaporpressurereaches101.3kPa.xbeginstoboilandTpiscalledtheNORMALBOILINGPOINTofx.Everyliquidwhichdoesnotdecomposebeforeitsvaporpressurereaches101.3kPa.hasitsowncharacteristicboilingpoint.Ingeneral,theboilingpointofasubstancedependsuponthemassofitsmoleculesandthestrengthoftheattractiveforcesbetweenthem.Foragivenhomologousseries,theboilingpointsofthemembercompoundsrisefairlyregularlywithincreasingmolecularweight.在温度、Tp,那里的蒸汽压力达到101.3kPa。开始煮和xTp被称为正常沸点液体的x。在这个温度，当蒸汽压达到101.3kPa时，液体x开始沸腾，这个蒸汽压力被叫做x的正常沸点。任何一种液体在它的蒸汽压达到101.3kPa之前都不分解，每种液体的沸点都有其自身的特点。一般来说,一种物质的沸点取决于它的分子质量和它们之间的引力的强度。对于一个给定的同系物，其成员化合物的沸点随分子量的增加而增加。Polarliquidstendtoboilhigherthannonpolarliquidsofthesamemolecularweight,andassociatedpolarliquidsusuallyboilconsiderablyhigherthannonassociatedpolarcompounds.极性液体的沸点往往高于相同分子量的非极性液体，而相关的极性液体的沸点远远高于一般的极性化合物。Theboilingpointisacharacteristicconstantthatiswidelyusedintheidentificationofliquids.Becauseofitsmarkeddependenceuponpressureanditsrathererraticresponsetoimpurities,however,itisgenerallylessreliableandusefulincharacterizationandasacriterionofpuritythanisthemeltingpointforsolids1.沸点是一个常数,广泛用于识别液体。然而，由于对压力的显著依赖性和对杂质的相当无规律的反应，沸点用于鉴别和作为纯度的判据方面，较之固体的熔点一般不太可信和较少使用。BoilingPointsofSolutions沸点的解决方案Thenormalboilingpointofanysolutionisthetemperatureatwhichthetotalvaporpressureofthesolutionisequalto101.3kPa.Theeffectofanysolute,Y,ontheboilingpointofXwilldepend,then,uponthenatureofY.IfYislessvolatilethanX.thenthetotalvaporpressureofthesolutionislower,atanygiventemperature,thanthevaporpressureofpureX.正常沸点的任何解决方案都是在其溶液总蒸汽压力等于101.3kPa时的温度。任何溶质，Y在X上的沸点的效果将取决于Y的性质。如果Y比X难挥发。那么其总蒸汽压都比在任何给定的温度下纯X的蒸汽压低。SuchacaseisrepresentedbycurveBC,inwhichtheexperimentallydeterminedvaluestorthevaporpressuresofasolutionareplottedagainsttemperature.Thevaporpressureofthesolutiondoesnotreach101.3kPa.untilatemperatureTpisattained.Inotherwords,thepresenceofthelessvolatilesoluteraisestheboilingpointofXfromTptoTp.Asolutionofsugarorsaltinwaterisafamiliarexampleofthistypeofsolution.根据溶质所确定实验值来绘制温度-蒸汽压力图，如曲线BC所示。直到温度达到Tp时，其溶液的蒸汽压才达到101.3kPa。换句话说,较低挥发性溶质的存在使X的沸点从Tp提高到Tp。这种类型的一个广为人知的例子就是糖或盐溶解在水中。Figure5.Generalizedvaporpressurediagramsforapureliquid(BO,forasolutioninwhichthesoluteislessvolatilethanthesolvent(BC),andforasolutioninwhichthesoluteismorevolatilethanthesolvent(BC).图5。广义水汽压图表示纯液体(BC,溶液中溶质比溶剂(BC)难挥发，以及另一溶液中含有更不稳定的溶剂(B“C”)。Ontheotherhand,ifYismorevolatilethanX.thenthetotalvaporpressureofthesolutionishigherthanthatofpureX,asshownbycurveBC.Thevaporpressureofsuchasolutionreaches101.3kPa.attempratureTp;hencetheeffectofthemorevolatilesoluteistolowertheboilingpointofXfromTptoTp.Asolutionofacetoneinwaterisanexampleofthistype.另一方面,如果Y比X更易挥发。那么其溶液的总蒸汽压比纯X的高。,如图所示曲线BC所示。当一溶液的蒸汽压力达到101.3kPa、温度达到Tp,因此更多的挥发性溶质的作用是使X的沸点从TP降低到TP。丙酮在水中的溶液就是这种类型的一个例子。InanysolutionoftwoliquidsXandY,themoleculesofXaredilutedbymoleculesofY,and,conversely,themoleculesofYaredilutedbymoleculesofX.YouwouldthereforeexpectthevaporpressureduetoXtobelessthanthatofpureX;infact,youmightpredictthatthePARTIALPRESSUREduetoXwouldbeproportionaltothemolecularconcentrationofX.在任何溶液中的两种液体X和Y，X的分子被Y的分子稀释，相反,Y的分子被X的分子稀释。你因此会认为由此X的蒸汽压就小于纯的X。事实上,你可以预测,X的局部压力,与X的组分的浓度成正比。Similarly,thepartialpressureofYmightbeexpectedtobeproportionaltothemolecularconcentrationofY.Thisis,infact,therelationshipwhichholdsforso-calledidealsolutions.ItisexpressedinRaoultsLaw-thepartialpressureofacomponentinasolutionatagiventemperatureisequaltothevaporpressureofthepuresubstancemultipliedbyitsmolefractioninsolution.Insymbols,forasolutionofcomponentsXandY.同样地,局部压力的Y也许被希望是与Y的分子浓度成正比。.这是,事实上,这种关系可以使用与理想溶液。这由拉乌尔定律表述为：溶液中成分的局部压力等于在给定温度下的纯物质的蒸汽压乘以该物质在溶液中的摩尔分数。用符号X和Y表示溶液的组分。Px=Px0NxwherePx=thepartialpressureofXinsolution,Px=X在溶液中的分压Px0=thevaporpressureofpureXatthattemperature,Px0=纯X在该温度下的蒸汽压Nx=themolefractionofXinthesolution.Nx=X在溶液中的摩尔分数Similarly.相同的，Py=Py0NywherePy=thepartialpressureofYinsolution.Py=Y在溶液中的分压Py0=thevaporpressureofpureYatthattemperature,Py0=纯Y在该温度下的蒸汽压Ny=themolefractionofyinsolution.Ny=Y在溶液中的摩尔分数Thetotalpressure,PTofthesolutionwouldbethesumofthepartialpressuresofXandY.溶液的总压，PT，是X和Y的分压的加和。PT=Px+PyTemperature-CompositionDiagramforSolutionswhichFollowRaoultsLaw.溶液的温度组成图遵循拉乌尔定律ThesefactsarerepresentedgraphicallyinFig.6whichshowsatypicaltemperature-compositiondiagram.Thisdiagramisatemperature-compositionplotoftheexperimentallydeterminedvaluesforthesystembenzene-toluene,butitisrepresentativeoftheplotforallsolutionswhicharedescribedbyRaoultsLaw.根据事实，用图6来表示，图6是一个典型的温度-组成图。这个图是用实验方法对苯-甲苯系统测定值而得到，但对所有可以用拉乌尔定律来表示的溶液，这是一个典型的图。Boilingpoints(ordinates)areplottedagainstcompositionexpressedasmolefractions(abscissae).Purebenzene(100percentx)boilsat80.1(pointA)andpuretoluene(100percenty)at110.6(pointB).Allmixturesofthetwoboilatintermediatetemperatures,asshownbytheliquid(lower)curve.Thiscurveshowsthetemperatureatwhichamixtureofbenzeneandtolueneofanygivencompositionbeginstoboil.Thevapor(upper)curverepresentsthecompositionofthevaporinequilibriumwiththeliquidatanygiventemperature.沸点（纵坐标）绘制对组成表示为摩尔分数（横坐标）。纯的苯(100%x)沸点为80.1(A点)、纯甲苯(100%y)的沸点为110.6(B点)。所有混合物的沸点在两个纯组分的沸点之间,如图所示液体(下图)曲线。该曲线显示了在任何给定的苯-甲苯的组成开始沸腾的温度。蒸汽(上面)曲线表示了在任何给定温度下气-液平衡时的组成。Forexample.considerthechangeswhichoccurwhena20molepercentbenzene80molepercenttoluenesolution(representedbypointP)isheated.At101.6,correspondingtopointL1theliquidbeginstoboil.Thefirsttraceofvaporwhichisformedis,ofcourse,inequilibriumwiththeliquidat101.6.Ithasthecomposition38molepercentbenzene62molepercenttoluene,asrepresentedbypointV1,andisthereforeconsiderablyricherthantheliquidinbenzene.以20%的苯和80%的甲苯（P点）溶液被加热时出现的变化为例。在101.6,对应点L1液体开始沸腾。首次出现微量的蒸汽，它的组成在101.6是与液体平衡的。它由38%的苯和62%的甲苯组成，V1为代表点，因此比苯含量更大。Figure6.Temperature-Compositiondiagramofthesystembenzene-toluene.图6.温度组成图 苯-甲苯系统。Asthedistillationproceeds,theconcentrationoftolueneintheliquidphaseandtheboilingpointincreasecontinuously,followingthevaluesrepresentedbyL1B.Finally,attheendofthedistillation,theliquidphaseispuretolueneboilingat110.6.Similarly,thevaporbecomesprogressivelyricherintoluenealso,followingV1B.Always,however,itisricherinbenzenethanistheliquidwithwhichitisinequilibrium,asshownbythepointsofintersectionofanyhorizontallinewiththevaporandliquidcurves2.在精馏过程中，随着L1B所表示的值，甲苯在液相中的浓度和沸点逐步增加。最后，在精馏结束时，液相是纯的甲苯，沸点为110.6。同样地，根据V1B曲线，甲苯蒸汽也随之变浓。然而，蒸汽较之其处于平衡的液体总是富含苯的，正如任一水平线与蒸汽和液体曲线的交点所示。Obviously,asinglesimpledistillationcouldneverseparates20:80molarmixtureofbenzeneandtolueneintothepurecomponents.Butnowconsiderwhatwouldbeaccomplishedifthefirsttraceofvaporformedbydistillationofthemixturewerecooledandcondensed.Itwould,ofcourse,formliquidcorrespondingtopointL2ofcomposition38molepercentbenzene62molepercenttolueneat94.5.很明显,单一简单的精馏不能分离苯-甲苯20:80摩尔的纯组分混合物。但是现在考虑如果是通过精馏混合物使首先出现的微量蒸汽被冷却和浓缩的话可以实现。当然，这将会形成液体相应的点L2，在94.5的组成成分是38%摩尔的苯和62%摩尔的甲苯。Now,ifliquidL2weredistilled,thefirsttraceofvaporformedwouldhavethecomposition59molepercentbenzene41molepercenttoluene(pointV2)andwhencooledwouldcondensetoliquidat88(pointL3).This,inturn,maybefurtherenrichedinbenzeneasindicatedonthegraph.现在，如果液体L2被蒸馏，首先形成的微量蒸汽将会由59%摩尔的苯和41%摩尔的甲苯（点V2)组成，冷却到88时将得到冷凝液（点L3）。反过来，这可能进一步增加了苯在图上的显示。
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