廈大課件物化reaction-kinetics

1、1Reaction Kinetics (4) Xuan ChengXiamen UniversityPhysical Chemistry2Physical ChemistryTemperature Dependence of Rate ConstantsArrhenius Equation(17.66)*A & Ea: constants characteristic of the reaction (Arrhenius parameters)Ea: Arrhenius activation energyA: pre-exponential factor (Arrhenius A factor

2、) (frequency factor)(17.67)Some reactions are not Arrhenius-like, If Ea independent of temperature(17.68)(17.69)Reaction Kinetics3Physical ChemistryTemperature Dependence of Rate Constants(17.67)Slope = -Ea/RPlot lnk (log10k) vs. 1/TReaction Kinetics4Physical ChemistryTemperature Dependence of Rate

3、ConstantsArrhenius Equation(17.66)*Tolman 用統(tǒng)計(jì)平均的概念對(duì)基元反應(yīng)的活化能下了一個(gè)定義：活化分子的平均能量與反應(yīng)物分子平均能量之差值，稱為活化能。設(shè)基元反應(yīng)為 A P正、逆反應(yīng)的活化能可以用圖表示。Reaction Kinetics5復(fù)雜反應(yīng)的活化能 復(fù)雜反應(yīng)的活化能無(wú)法用簡(jiǎn)單的圖形表示，它只是組成復(fù)雜反應(yīng)的各基元反應(yīng)活化能的數(shù)學(xué)組合。這表觀活化能也稱為總包反應(yīng)活化能或?qū)嶒?yàn)活化能。 組合的方式?jīng)Q定于基元反應(yīng)的速率系數(shù)與表觀速率系數(shù)之間的關(guān)系，這個(gè)關(guān)系從反應(yīng)機(jī)理推導(dǎo)而得。例如：Physical ChemistryReaction Kinetics6溫度

4、對(duì)反應(yīng)速率影響的類型rTrTrTrTrT(1)(2)(3)(4)(5)通常有五種類型：（1）反應(yīng)速率隨溫度的升高而逐漸加快，它們之間呈指數(shù)關(guān)系，這類反應(yīng)最為常見。（2）開始時(shí)溫度影響不大，到達(dá)一定極限時(shí)，反應(yīng)以爆炸的形式極快的進(jìn)行。Physical ChemistryReaction Kinetics7溫度對(duì)反應(yīng)速率影響的類型（3）在溫度不太高時(shí)，速率隨溫度的升高而加快，到達(dá)一定的溫度，速率反而下降。如多相催化反應(yīng)和酶催化反應(yīng)。（4）速率在隨溫度升到某一高度時(shí)下降，再升高溫度，速率又迅速增加，可能發(fā)生了副反應(yīng)。（5） 溫度升高，速率反而下降。這種類型很少，如一氧化氮氧化成二氧化氮。rTrTrT

5、rTrT(1)(2)(3)(4)(5)Physical Chemistry8Chain Reactions & Free-Radical PolymerizationsPhysical ChemistryA series of steps: a reactive intermediate is consumedChain reactionReactants are converted to productsThe intermediate is regeneratedCycle is repeatedThe structure of chain reactionsChain carriers:

6、 the intermediates responsible for the propagation of a chain reaction Radical chain carriers: the chain carriers are radicals Radicals: Species with unpaired electronsIons, neutrons (in nuclear fission)Reaction Kinetics9Physical ChemistryChain Reactions & Free-Radical PolymerizationsA chain reactio

7、n leads to a complicated rate law.Derive the rate law for the formation of HBr according to the mechanism given below.Reaction Kinetics10Physical ChemistryChain Reactions & Free-Radical PolymerizationsThe following radical chain mechanism has been proposed: (a) Initiation:(b) Propagation:(c) Retarda

8、tion:(d) Termination:The third body M removes the energy of bination; the constant concentration of M has been absorbed into the rate constant. Reaction Kinetics11直鏈反應(yīng)的三個(gè)主要步驟（1）鏈引發(fā)（chain initiation） 處于穩(wěn)定態(tài)的分子吸收了外界的能量，如加熱、光照或加引發(fā)劑，使它分解成自由原子或自由基等活性傳遞物?；罨芟喈?dāng)于所斷鍵的鍵能。（2）鏈傳遞（chain propagation） 鏈引發(fā)所產(chǎn)生的活性傳遞物與

9、另一穩(wěn)定分子作用，在形成產(chǎn)物的同時(shí)又生成新的活性傳遞物，使反應(yīng)如鏈條一樣不斷發(fā)展下去。 兩個(gè)活性傳遞物相碰形成穩(wěn)定分子或發(fā)生岐化，失去傳遞活性；或與器壁相碰，形成穩(wěn)定分子，放出的能量被器壁吸收，造成反應(yīng)停止。（3）鏈終止（chain termination）Physical ChemistryReaction Kinetics12Physical ChemistryChain Reactions & Free-Radical PolymerizationsThe net rates of the formation of the two intermediates areReaction Ki

10、netics13Physical ChemistryChain Reactions & Free-Radical PolymerizationsThe steady-state concentrations of the intermediates areReaction Kinetics14Physical ChemistryChain Reactions & Free-Radical PolymerizationsBranching chain reactionThe chain produces more chain carriers than it consumesCombustion

11、 of hydrogenChain-branching stepsInitiation A highly exothermic reaction that is not a chain reaction may lead to an explosion!Reaction Kinetics15Physical ChemistryChain Reactions & Free-Radical PolymerizationsExplosion A chain-branching explosion may occur when there are chain-branching steps in a

12、reaction, for then the number of chain carriers grows exponentially and the rate of reaction may cascade into an explosion.The reaction between hydrogen and oxygenAlthough the net reaction is very simple, the mechanism is very complex and has not yet been fully elucidated.Involving a chain reactionT

13、he chain carriers: H, O, OH, and O2HReaction Kinetics16Physical ChemistryChain Reactions & Free-Radical PolymerizationsInitiation: Propagation: A chain-branching explosionThe occurrence of an explosion depends on the temperature and pressure of the systemReaction Kinetics17Physical ChemistryAt very

14、low pressure, the system is outside the explosion region and the mixture reacts smoothly.Chain Reactions & Free-Radical PolymerizationsIncreasing the pressure (along the vertical line) takes the system through the first explosion limit (if T 730K). The mixture then explodes.The reaction is smooth wh

15、en the pressure is above the second explosion limit.The explosion limits of the H2+O2 reactionThermal explosionSteady reactionexplosion 600 700 800 90043256lg (P/Pa)T/KUpperlimitLowerlimitThe pressure is increased to above the third explosion limit, a thermal explosion occurs.Reaction Kinetics18擬定反應(yīng)

16、歷程的一般方法1.寫出反應(yīng)的計(jì)量方程。2.實(shí)驗(yàn)測(cè)定速率方程，確定反應(yīng)級(jí)數(shù)。3.測(cè)定反應(yīng)的活化能。4.用順磁共振(EPR)、核磁共振(NMR)和質(zhì)譜等手段測(cè)定中間產(chǎn)物的化學(xué)組成。Physical ChemistryReaction Kinetics19擬定反應(yīng)歷程的一般方法5.擬定反應(yīng)歷程。7.從動(dòng)力學(xué)方程計(jì)算活化能，是否與實(shí)驗(yàn)值相等。6.從反應(yīng)歷程用穩(wěn)態(tài)近似、平衡假設(shè)等近似方法推導(dǎo)動(dòng)力學(xué)方程，是否與實(shí)驗(yàn)測(cè)定的一致。8.如果(6)(7)的結(jié)果與實(shí)驗(yàn)一致，則所擬的反應(yīng)歷程基本準(zhǔn)確，如果不一致則應(yīng)作相應(yīng)的修正。Physical ChemistryReaction Kinetics20擬定反應(yīng)歷程的例

17、子1.反應(yīng)計(jì)量方程C2H6C2H4+H22.實(shí)驗(yàn)測(cè)定速率方程為一級(jí)，r =kC2H63.實(shí)驗(yàn)活化能Ea=284.5 kJmol-14.發(fā)現(xiàn)有CH3,C2H5等自由基。Physical ChemistryReaction Kinetics21擬定反應(yīng)歷程的例子5.擬定反應(yīng)歷程。Physical ChemistryReaction Kinetics22擬定反應(yīng)歷程的例子8.動(dòng)力學(xué)方程、活化能與實(shí)驗(yàn)值基本相符，所以擬定的反應(yīng)歷程是合理的。6.根據(jù)歷程，用穩(wěn)態(tài)近似作合理的近似得動(dòng)力學(xué)方程為：7.Physical Chemistry23速率決定步驟 在連續(xù)反應(yīng)中，如果有某步很慢，該步的速率基本上等于整個(gè)

18、反應(yīng)的速率，則該慢步驟稱為速率決定步驟，簡(jiǎn)稱速?zèng)Q步或速控步。利用速?zèng)Q步近似，可以使復(fù)雜反應(yīng)的動(dòng)力學(xué)方程推導(dǎo)步驟簡(jiǎn)化。Physical ChemistryReaction Kinetics24速率決定步驟慢步驟后面的快步驟可以不考慮。只需用平衡態(tài)近似法求出第1，2步的速率。雖然第二步是速?zèng)Q步，但中間產(chǎn)物C的濃度要從第一步快平衡求。例1.慢快快Physical Chemistry例2. 快 慢 快 快Reaction Kinetics25穩(wěn)態(tài)近似 從反應(yīng)機(jī)理導(dǎo)出速率方程必須作適當(dāng)近似，穩(wěn)態(tài)近似是方法之一。 假定反應(yīng)進(jìn)行一段時(shí)間后，體系基本上處于穩(wěn)態(tài)，這時(shí)，各中間產(chǎn)物的濃度可認(rèn)為保持不變，這種近似處理的方法稱為穩(wěn)態(tài)近似，一般活潑的中間產(chǎn)物可以采用穩(wěn)態(tài)近似。Physical ChemistryReaction Kinetics26氫與碘的反應(yīng)分別用穩(wěn)態(tài)近似和平衡假設(shè)來(lái)求中間產(chǎn)物I的表達(dá)式，并比較兩種方法的適用范圍。Physical ChemistryReaction Kinetics27用穩(wěn)態(tài)近似法求碘原子濃度因?yàn)?1)是快平衡，k-1很大；(2)是慢反應(yīng)，k2很小，分母中略去2k2H2項(xiàng)，得：與實(shí)驗(yàn)測(cè)定的速率方程一致。Physical ChemistryReac