大學化工專業(yè)英語

...wd......wd......wd...ChemicalEquilibriumandKinetics化學平衡和動力學Amajorobjectiveofchemististounderstandchemicalreactions,toknowwhetherunderagivensetofconditionstwosubstanceswillreactwhenmixed,todeterminewhetheragivenreactionwillbeexothermicorendothermic,andtopredicttheextenttowhichagivenreactionwillproceedbeforeequilibriumisestablished.化學家的一個主要目標是理解化學反響，知道在給定的一組條件下兩種物質混合時能否發(fā)生反響；確定一個給定的反響放熱還是吸熱；預測一個給定的反響在平衡前進展的程度。Anequilibriumstate,producedasaconsequenceoftwoopposingreactionsoccurringsimultaneously,isastateinwhichthereisnonetchangeaslongasthereisnochangeinconditions.兩個相反的反響同時進展時的狀態(tài)叫做平衡狀態(tài)，在平衡狀態(tài)下，只要條件不發(fā)生改變，這個狀態(tài)就不會有凈變化。Inthislessonitwillbeshownhowonepredicttheequilibriumstateofchemicalsystemsfromthermodynamicdata,andconverselyhowtheexperimentalmeasurementsonequilibriumstatesprovideusefulthermodynamicdata.在這節(jié)課中，將會展示假設何根據熱力學數據來預測化學體系的平衡狀態(tài)，以及相反地，平衡狀態(tài)的實驗測量假設何提供有用的熱力學數據。Thermodynamicsalonecannotexplaintherateatwhichequilibriumisestablished,nordoesitprovidedetailsofthemechanismbywhichequilibriumisestablished.單純的熱力學無法解釋建設平衡的速度，也不能提供平衡建設所遵循的機理信息。Suchexplanationscanbedevelopedfromconsiderationsofthequantumtheoryofmolecularstructureandfromstatisticalmechanics.從分子構造的量子理論和統(tǒng)計力學的角度考慮可以使這樣的解釋得到開展。Toappreciatefullythenatureofthechemicalequilibriumstate,itisnecessaryfirsttohavesomeacquaintancewiththefactorswhichinfluencereactionrates.要充分理解化學平衡狀態(tài)的性質，首先必須熟知影響反響速率的因素。Thefactorswhichinfluencetheratesofachemicalreactionaretemperature,concentrationsofreactants(orpartialpressuresofgaseousreactants),andpresenceofacatalyst.影響化學反響速率的因素有溫度、反響物濃度〔氣體反響物的分壓〕和催化劑。Ingeneral,foragivenreactionthehigherthetemperature,thefasterthereactionwilloccur.一般來說，對于一個給定的反響，溫度越高，反響速率越大。Theconcentrationsofreactantsorpartialpressureofgaseousreactantswillaffecttherateofreaction,anincreaseinconcentrationorpartialpressureincreasestherateofmostreactions.反響物濃度或氣體反響物的分壓影響反響的速率，對于大局部反響來說，增大濃度或分壓，反響速率增大。Substanceswhichaccelerateachemicalreactionbutwhichthemselvesarenotusedupinthereactionarecalledcatalysts.可以加快反響速率但本身不參加反響的物質稱為催化劑。DynamicEquilibrium動態(tài)平衡Inmanycases,directreactionsbetweentwosubstancesappeartoceasebeforeallofeitherstartingmaterialisexhausted.在許多情況下，兩種物質間的直接反響在所有原始物料耗盡之前就會停頓。Moreover,theproductsofchemicalreactionsthemselvesoftenreacttoproducethestartingmaterials.此外，化學反響自身的產物能夠發(fā)生反響生成原始物料。Forexample,nitrogenandhydrogencombineat500℃inthepresenceofacatalysttoproduceammonia:N2+3H2=2NH3例如，在催化劑存在下，氮和氫在500℃能夠反響生成氨：N2+3H2=2NH3Atthesametemperatureandinthepresenceofthesamecatalyst,pureammoniadecomposesintonitrogenandhydrogen:2NH3=3H2+N2在一樣溫度和一樣催化劑存在下，純潔的氨能夠分解生成氮和氫：2NH3=3H2+N2Forconvenience,thesetwoopposingreactionsaredenotedinoneequationbyuseofadoublearrow:為了方便起見，這兩個反響能夠用雙箭頭表示在一個方程式中：Thereactionproceedingtowardtherightiscalledtheforwardreaction;theotheriscalledthereversereaction.向右進展的反響稱為正反響，另外一個稱為逆反響。Ifeitherammoniaoramixtureofnitrogenandhydrogenissubjectedtotheabovecondition,amixtureofallthethreegaseswillresult.如果將氨或氮氣和氫氣的混合物放在上述條件下，將都會產生三種氣體的混合物。Therateofreactionbetweenthematerialswhichwereintroducedintothereactionvesselwilldecreaseafterthereactionstarts;becausetheirconcentrationsaredecreasing,conversely,afterthestartofthereactionthematerialbeingproducedwillreactfaster,sincetherewillbemoreofit.在反響開場后，放入反響器內的材料之間的反響速率將會減??；因為它們的濃度在減小，相反，在反響開場后，產物之間的反響將會加快，由于它們的濃度在增加。Thusthefasterforwardreactionbecomesslower,andtheslowerreversereactionspeedsup.因此，較快的正反響變慢，較慢的正反響加快。Ultimatelythetimecomeswhentheratesoftheforwardandreversereactionsbecomeequal,andtherewillbenofurthernetchange.最后，正反響和逆反響速率變得相等，且不再有進一步凈變化的時刻終于到來。Thissituationiscalledequilibrium.這種狀態(tài)稱為平衡狀態(tài)。Equilibriumisadynamicstatebecausebothreactionsarestillproceeding;butsincethetwoopposingreactionsareproceedingatequalrates,nonetchangeisobserved.平衡是動態(tài)的，因為反響仍然在進展；但是由于兩個相反方向的反響以一樣的速率進展，所以沒有凈變化被發(fā)現。Allchemicalreactionsultimatelyproceedtowardequilibrium.所有的化學反響最后都要向平衡狀態(tài)進展。Inapracticalsense,however,somereactionsgosofarinonedirectionthatthereversereactioncannotbedetected,andtheyaresaidtogotocompletion.然而，實際上，一些反響在一個方向上進展的很快以至于檢測不到逆反響，據此可說它們進展完全了。Theprinciplesofchemicalequilibriumapplyeventothese,anditwillbeseenthatformanyofthem,theextentofreactioncanbeexpressedquantitatively.化學平衡的原理甚至也適用于被認為是進展完全的反響，顯然許多這類反響其反響程度可以定量地表達。EquilibriumConstants平衡常數Equilibriumisastateofdynamicbalancebetweentwoopposingprocesses.Forageneralreactionatagiventemperature:A+B=C+D平衡是兩個相反過程的動態(tài)平衡的一種狀態(tài)。在給定溫度下，對于一個普通的反響：A+B=C+DAtthepointofequilibrium,thefollowingratiomustbeaconstant:在平衡時，以下比值肯定是一個常數：Theconstant,K,iscalledtheequilibriumconstantofthereaction.常數K稱為反響的平衡常數。Ithasaspecificvalueatagiventemperature.在一個給定溫度下，它有一個比值。Iftheconcentrationofanyofthecomponentsinthesystematequilibriumischanged,theconcentrationsoftheothercomponentswillchangeinsuchamannerthatthedefinedratioremainsequaltoKaslongasthetemperaturedoesnotchange.如果平衡時體系中任意組分的濃度改變，那么只要溫度不變，其他組分的濃度將以保持相等K的固定比例的方式變化。Theequilibriumconstantexpressionquantitativelydefinestheequilibriumstate.平衡常數表達式定量地定義了平衡狀態(tài)。Moregenerally,forthereversiblereaction更一般地說，對于一個可逆反響theequilibriumconstantexpressioniswrittenasfollows:平衡常數表達式可以寫成：Byconvention,theconcentrationtermsofthereactionproductsarealwaysplacedinthenumeratoroftheequilibriumconstantexpression.按照慣例，反響產物濃度項通常是放在平衡常數表達式的分子上。Itshouldbenotedthattheexponentsoftheconcentrationtermsintheequilibriumconstantexpressionarethecoefficientsoftherespectivespeciesinthebalancedchemicalequation.應該注意的是，平衡常數表達式中濃度項的指數是平衡化學方程式中各個物種的系數。ChemicalKinetics化學動力學Whenasystemisintheequilibriumstate,therateoftheforwardreactionisidenticaltotherateofthereversereaction.當一個體系到達平衡狀態(tài)時，正反響的反響速率等于逆反響的反響速率。Itisimportanttoknowjusthowfastareactantisbeingusedupinaprocess,orthespeedwithwhichaproductisbeingformed.重要的是，要知道在反響過程中，反響物消耗完的速率，或者產物生成的速率。Itisalsoimportanttohavedetailedinformationaboutratesofreactantsinordertotesttheoriesandmechanismsforvariouskindsofchemicalprocesses.同樣重要的是，要有關于反響物反響速率的詳細信息，以便測試各種化學過程的理論和機制。Experimentsshowthatanumberofreactionvariablesaffectreactionrates:實驗說明，許多反響變量都影響反響速率：Temperature.Theratesofchemicalreactionsaretemperature-dependent.Therefore,itiscommonpracticewhenstudyingtherateinthelaboratorytocarryoutreactionsatconstanttemperature(isothermally),thuseliminatingonevariable.溫度?；瘜W反響速率隨溫度變化。因此，常見的做法是在實驗室、在恒溫下研究反響的反響速率〔等溫地〕，這樣可以消除一個變量。PressureandVolume.Pressureisimportantinakineticconsiderationofgasphasereactions.Usually,volumeisfixedbyrunningthereactionincontaineroffixeddimensions.Forsolidandliquidstatereactions,pressureisusuallyatmospheric,andthevolumeofthereactingsystemisrelativelyunimportantbecausethereislittlechangeinvolume.壓力與體積。壓力是氣相反響動力學的一個重要的影響因素。通常在固定尺寸的容器中反響時，反響體積是固定的。對于固體和液體反響，壓力通常是大氣壓，反響體系的體積是相對不重要的，由于反響前后它的體積變化很小。Concentration.Atanyparticulartemperature,theratesofmostchemicalreactionsarefunctionsoftheconcentrationsofoneormoreofthecomponentsof