...wd......wd......wd...ChemicalEquilibriumandKinetics化學(xué)平衡和動(dòng)力學(xué)Amajorobjectiveofchemististounderstandchemicalreactions,toknowwhetherunderagivensetofconditionstwosubstanceswillreactwhenmixed,todeterminewhetheragivenreactionwillbeexothermicorendothermic,andtopredicttheextenttowhichagivenreactionwillproceedbeforeequilibriumisestablished.化學(xué)家的一個(gè)主要目標(biāo)是理解化學(xué)反響，知道在給定的一組條件下兩種物質(zhì)混合時(shí)能否發(fā)生反響；確定一個(gè)給定的反響放熱還是吸熱；預(yù)測一個(gè)給定的反響在平衡前進(jìn)展的程度。Anequilibriumstate,producedasaconsequenceoftwoopposingreactionsoccurringsimultaneously,isastateinwhichthereisnonetchangeaslongasthereisnochangeinconditions.兩個(gè)相反的反響同時(shí)進(jìn)展時(shí)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(gè)狀態(tài)就不會(huì)有凈變化。Inthislessonitwillbeshownhowonepredicttheequilibriumstateofchemicalsystemsfromthermodynamicdata,andconverselyhowtheexperimentalmeasurementsonequilibriumstatesprovideusefulthermodynamicdata.在這節(jié)課中，將會(huì)展示假設(shè)何根據(jù)熱力學(xué)數(shù)據(jù)來預(yù)測化學(xué)體系的平衡狀態(tài)，以及相反地，平衡狀態(tài)的實(shí)驗(yàn)測量假設(shè)何提供有用的熱力學(xué)數(shù)據(jù)。Thermodynamicsalonecannotexplaintherateatwhichequilibriumisestablished,nordoesitprovidedetailsofthemechanismbywhichequilibriumisestablished.單純的熱力學(xué)無法解釋建設(shè)平衡的速度，也不能提供平衡建設(shè)所遵循的機(jī)理信息。Suchexplanationscanbedevelopedfromconsiderationsofthequantumtheoryofmolecularstructureandfromstatisticalmechanics.從分子構(gòu)造的量子理論和統(tǒng)計(jì)力學(xué)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(xué)平衡狀態(tài)的性質(zhì)，首先必須熟知影響反響速率的因素。Thefactorswhichinfluencetheratesofachemicalreactionaretemperature,concentrationsofreactants(orpartialpressuresofgaseousreactants),andpresenceofacatalyst.影響化學(xué)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(duì)于一個(gè)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(duì)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(zhì)稱為催化劑。DynamicEquilibrium動(dòng)態(tài)平衡Inmanycases,directreactionsbetweentwosubstancesappeartoceasebeforeallofeitherstartingmaterialisexhausted.在許多情況下，兩種物質(zhì)間的直接反響在所有原始物料耗盡之前就會(huì)停頓。Moreover,theproductsofchemicalreactionsthemselvesoftenreacttoproducethestartingmaterials.此外，化學(xué)反響自身的產(chǎn)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(gè)反響能夠用雙箭頭表示在一個(gè)方程式中：Thereactionproceedingtowardtherightiscalledtheforwardreaction;theotheriscalledthereversereaction.向右進(jìn)展的反響稱為正反響，另外一個(gè)稱為逆反響。Ifeitherammoniaoramixtureofnitrogenandhydrogenissubjectedtotheabovecondition,amixtureofallthethreegaseswillresult.如果將氨或氮?dú)夂蜌錃獾幕旌衔锓旁谏鲜鰲l件下，將都會(huì)產(chǎn)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(nèi)的材料之間的反響速率將會(huì)減??；因?yàn)樗鼈兊臐舛仍跍p小，相反，在反響開場后，產(chǎn)物之間的反響將會(huì)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(jìn)一步凈變化的時(shí)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(dòng)態(tài)的，因?yàn)榉错懭匀辉谶M(jìn)展；但是由于兩個(gè)相反方向的反響以一樣的速率進(jìn)展，所以沒有凈變化被發(fā)現(xiàn)。Allchemicalreactionsultimatelyproceedtowardequilibrium.所有的化學(xué)反響最后都要向平衡狀態(tài)進(jìn)展。Inapracticalsense,however,somereactionsgosofarinonedirectionthatthereversereactioncannotbedetected,andtheyaresaidtogotocompletion.然而，實(shí)際上，一些反響在一個(gè)方向上進(jìn)展的很快以至于檢測不到逆反響，據(jù)此可說它們進(jìn)展完全了。Theprinciplesofchemicalequilibriumapplyeventothese,anditwillbeseenthatformanyofthem,theextentofreactioncanbeexpressedquantitatively.化學(xué)平衡的原理甚至也適用于被認(rèn)為是進(jìn)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(dá)。EquilibriumConstants平衡常數(shù)Equilibriumisastateofdynamicbalancebetweentwoopposingprocesses.Forageneralreactionatagiventemperature:A+B=C+D平衡是兩個(gè)相反過程的動(dòng)態(tài)平衡的一種狀態(tài)。在給定溫度下，對(duì)于一個(gè)普通的反響：A+B=C+DAtthepointofequilibrium,thefollowingratiomustbeaconstant:在平衡時(shí)，以下比值肯定是一個(gè)常數(shù)：Theconstant,K,iscalledtheequilibriumconstantofthereaction.常數(shù)K稱為反響的平衡常數(shù)。Ithasaspecificvalueatagiventemperature.在一個(gè)給定溫度下，它有一個(gè)比值。Iftheconcentrationofanyofthecomponentsinthesystematequilibriumischanged,theconcentrationsoftheothercomponentswillchangeinsuchamannerthatthedefinedratioremainsequaltoKaslongasthetemperaturedoesnotchange.如果平衡時(shí)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(shù)表達(dá)式定量地定義了平衡狀態(tài)。Moregenerally,forthereversiblereaction更一般地說，對(duì)于一個(gè)可逆反響theequilibriumconstantexpressioniswrittenasfollows:平衡常數(shù)表達(dá)式可以寫成：Byconvention,theconcentrationtermsofthereactionproductsarealwaysplacedinthenumeratoroftheequilibriumconstantexpression.按照慣例，反響產(chǎn)物濃度項(xiàng)通常是放在平衡常數(shù)表達(dá)式的分子上。Itshouldbenotedthattheexponentsoftheconcentrationtermsintheequilibriumconstantexpressionarethecoefficientsoftherespectivespeciesinthebalancedchemicalequation.應(yīng)該注意的是，平衡常數(shù)表達(dá)式中濃度項(xiàng)的指數(shù)是平衡化學(xué)方程式中各個(gè)物種的系數(shù)。ChemicalKinetics化學(xué)動(dòng)力學(xué)Whenasystemisintheequilibriumstate,therateoftheforwardreactionisidenticaltotherateofthereversereaction.當(dāng)一個(gè)體系到達(dá)平衡狀態(tài)時(shí)，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(chǎn)物生成的速率。Itisalsoimportanttohavedetailedinformationaboutratesofreactantsinordertotesttheoriesandmechanismsforvariouskindsofchemicalprocesses.同樣重要的是，要有關(guān)于反響物反響速率的詳細(xì)信息，以便測試各種化學(xué)過程的理論和機(jī)制。Experimentsshowthatanumberofreactionvariablesaffectreactionrates:實(shí)驗(yàn)說明，許多反響變量都影響反響速率：Temperature.Theratesofchemicalreactionsaretemperature-dependent.Therefore,itiscommonpracticewhenstudyingtherateinthelaboratorytocarryoutreactionsatconstanttemperature(isothermally),thuseliminatingonevariable.溫度。化學(xué)反響速率隨溫度變化。因此，常見的做法是在實(shí)驗(yàn)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(gè)變量。PressureandVolume.Pressureisimportantinakineticconsiderationofgasphasereactions.Usually,volumeisfixedbyrunningthereactionincontaineroffixeddimensions.Forsolidandliquidstatereactions,pressureisusuallyatmospheric,andthevolumeofthereactingsystemisrelativelyunimportantbecausethereislittlechangeinvolume.壓力與體積。壓力是氣相反響動(dòng)力學(xué)的一個(gè)重要的影響因素。通常在固定尺寸的容器中反響時(shí)，反響體積是固定的。對(duì)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(duì)不重要的，由于反響前后它的體積變化很小。Concentration.Atanyparticulartemperature,theratesofmostchemicalreactionsarefunctionsoftheconcentrationsofoneormoreofthecomponentsof