一級反應動力學、二級反應動力學基本原理（課堂PPT）

1、1Kinetics動力學Thermodynamics determines whether and in which direction a reaction will happen, and what the concentrations of constituents are involved in the reaction熱力學決定了反應是否發(fā)生、反應的方向以及參與到反應之中的物質(zhì)濃度問題。Kinetics say how fast a reaction happens, sequence of steps in the reaction, and some of the factors

2、 that control the rates of reactions動力學則說明反應發(fā)生的速率有多快、反應步驟的順序以及控制反應速率的一些因素Further Reading (Chapter 3 in Hobbs)2 Rate of reaction is typically measured as the change in concentration (moles/L) with time反應速率通常通過濃度反應速率通常通過濃度(mol/L)隨時間的變化來測量隨時間的變化來測量 This change may be a decrease or an increase改變可能是增加的也可

3、能是減小的改變可能是增加的也可能是減小的 Likewise the concentration change may be of reactants or products同樣，改變濃度的物質(zhì)可能是反應物也可能是生成物同樣，改變濃度的物質(zhì)可能是反應物也可能是生成物Rate = _ = _change in timechange in timein productsin reactantsRate =concentration changetime change反應速率=濃度/時間3 Rate has units of moles per liter per unit time- M s-1,

4、M h-1速率的單位是摩爾每升每單位時間，如速率的單位是摩爾每升每單位時間，如M s-1、M h-1 Consider the hypothetical reaction 對于這個假設的反應對于這個假設的反應aA + bB cC + dD We can write t ARate of reaction=1a t B1b t C1c t D1d=-4 Note the use of the negative sign注意負號的使用注意負號的使用- rate is defined as a positive quantity反應速率被定義為正量反應速率被定義為正量- rate of disapp

5、earance of a reactant is negative那么，反應物的消耗速率就是負的那么，反應物的消耗速率就是負的2N2O5(g) 4NO2(g) + O2(g) t N2O5Rate of reaction=12 t NO214 t O2=-5 Rate may be expressed in three main ways:反應速率主要有以下三個表達方式：反應速率主要有以下三個表達方式： Average reaction rate: a measure of the change in concentration with time 1、平均反應速率：濃度隨時間變化的度量、平均

6、反應速率：濃度隨時間變化的度量2. Instantaneous rate: rate of change of concentration at any particular instant during theReaction 2、瞬時速率：反應過程中某一特定瞬間的濃度變化速率、瞬時速率：反應過程中某一特定瞬間的濃度變化速率3. Initial rate: instantaneous rate at t = 0- that is, when the reactants are first mixed 3、初始速率：、初始速率：t=0時即反應物最初混合時的瞬時速率時即反應物最初混合時的瞬時速率

7、，6Rate Laws 速率定律Rate law = a mathematical function describing the turnover rate of the compound of interest as a function of the concentrations of the various species participating in the reaction 速率定律：用來描述相關的化合物的轉(zhuǎn)化速率的數(shù)學函數(shù)，即各種參與反應的物質(zhì)的濃度的函數(shù)May or may NOT have a theoretical basis可能有或沒有理論基礎“Many reacti

8、ons occur at a decreasing rate with increasing time because the reaction rate diminishes as the concentrations of the reactants diminish”“很多反應隨著時間的增加反應速率逐漸減?。?，因為反應物濃度減小會降低反應速率”Hobbs (2000)7Rate Laws-dAdt=kAm“m” indicates the order of the reaction with respect to species “A”. Determined experimentall

9、y.“m”指關于物質(zhì)A的反應級數(shù)，由實驗確定?！発” = rate coefficient (or specific reaction rate coeff.); determined for a specific reaction at a specific temp“K”=速率常數(shù)（或特定的速率系數(shù)），由特定溫度下的具體反應確定A = productsReaction rate8Rate LawsdAdt=-kAmBnCo“m, n, o” indicate the order of the reaction with respect to each species; determine

10、experimentallyoverall reaction order = m+n+o“m, n, o”分別是關于每種物質(zhì)的反應級數(shù)，實驗確定的總反應級數(shù)=m+n+oReaction orders can be: first, second, third反應級數(shù)可以是一級、二級、三級A + B + C = products9First order reactions一級動力反應-dAdt=kA1=kAktteAA-=0Take integral of equation above, and get equation below:將以上等式積分，得到下面的等式：10t = 0.693 / kP

11、lot of first order reaction一級動力反應的圖片半衰期的長度固定11Example of 1st order reaction1級動力反應范例If a reaction has a single reactant and the value of the exponent is one, then it is said to be a first-order reaction. 如果反應物只有一種，而且指數(shù)為1，那么稱這種反應為一級動力反應Write rate law for above equation:H2O2 (l) H2O(l) + 1/2O2(g)Wikipe

12、dia12Second Order Reaction二級動力反應 If integrated如果積分: 得到1/At = k”t + 1/Ao Inverse concentration linear in time 濃度的倒數(shù)和時間程線性關系-dAdt=kA213t = 1 / k AoPlot of second order reaction 2級動力反應圖AtimeHalf lives are not constant with time for second order reactions二級反應動力學反應物質(zhì)的半衰期隨時間推移變化14Example of 2nd order reac

13、tion二級動力反應范例A second-order reaction depends on the concentrations of one second-order reactant, or two first-order reactants.二級動力反應取決是否有一個二級反應物或者兩個一級反應物？Write rate law for above equations:2NO2 (g) 2NO(g) + O2(g)WikipediaCO2 + H2O H2CO315Pseudo first order reactions偽一級反應 pseudo-first order: concentra

14、tion of one reactant remains essentially constant over time (often because it is in large excess compared to the other reactant) 偽一級動力反應：一種反應物剩余的濃度基本不變（經(jīng)常由于它與其他物質(zhì)相比，濃度過量）A+BC+DdAdt=-kABdAdt=-kA where k=kBlnA= lnA0-kt B濃度不變16Reaction Mechanism反應機理 Often we write a chemical reaction 我們經(jīng)常寫化學反應式 How we

15、write it is often not how it happens 但是我們所寫的通常不是它如何發(fā)生的 Many kinds of reaction mechanisms 有很多反應的機理17Single Step (Elementary) Reaction Process單步（基本）反應過程 Unimolecular 單分子反應 CaCO3 Ca2+ + CO32- Bimolecular 雙分子反應CO2 + H2O H2CO3 Termolecular 三分子反應2 FeS2 + 3 O2 + 2 H2O 2 Fe3+ + 4H2SO4 No four or more molecu

16、le processes have been identified because chances of 4 molecules colliding is very, very small (making reaction rate super small)沒有4或者4以上的分子反應被檢測到，因為4分子同時碰撞的機會非常非常?。ㄊ沟梅磻俾史浅Ｐ。?8Rate Laws For Elementary基本反應速率定律 Simple to write 書寫簡單 Unimolecular H2CO3 H+ + HCO3-dH2CO3/dt = kH2CO3 Bimolecular CO2 + H2O

17、 H2CO3dCO2/dt = kCO2H2O19Reaction order is not necessarily related to the stoichiometry of the reaction, unless the reaction is elementary.反應級數(shù)不一定和反應的化學計量數(shù)有關，除非反應是基本反應In single step process, reaction order = coefficient of reactant in chemical equation在單步反應過程中，反應級數(shù)=化學式中反應物的系數(shù)Complex reactions may or

18、 may not have reaction orders equal to their stoichiometric coefficients復雜反應的反應級數(shù)可能和化學計量系數(shù)相等Not all first order reactions, are single step, monomolecular reactions不是所有的一級反應都是單步反應、單分子反應Important Note重要提示Hobbs (2000)20Overview of Elementary Reactions基本反應回顧Note: for Zero-Order reactions rate is indepen

19、dent of the concentration of reactant. Doubling concentration has no effect on rate.提示：零級反應速率不依賴反應物的濃度，雙倍的反應物濃度不影響反應速率。21Arrhenius Equation and Transition State Theory阿倫紐斯方程和過渡態(tài)理論 reactions occur as a sequence of elementary steps 化學反應會發(fā)生一系列的基本步驟. usually one of these steps is much slower than the ot

20、hers Rate Determining Step 通常其中的一個步驟會比其他的步驟反應慢很多，這個就是速率決定步驟 empirically, the effect of T on the rate of this reaction step (and therefore on the overall reaction rate) is described by the Arrhenius equation: 依照經(jīng)驗，時間對這個步驟反應速率（也就是整體反應速率）的影響可以用阿倫紐斯方程描述kA eERTa=-/pre-exponential factor or “frequency fac

21、tor”A：指前因子或振動因子describes collision frequency and the orientation probability描述碰撞頻率和定向概率Activation energy活化能（Ea是物質(zhì)分裂形成新物質(zhì)的最低能量） describes the fraction of species with energy greater than Ea 描述了物質(zhì)能量大于Ea的部分?22Increased Temperature = Faster Reaction Rates升溫=更高的反應速率23Determination of Activation Energy (E

22、a)活化能測定ln k2 - ln k1 = -Ea/R(1/T2 - 1/T1)K = reaction rate 反應速率Ea = activation energy 活化能R = gas constant (8.314 J/mol*K, or 0.008314 KJ/mol*K) 氣體常數(shù)T = temperature in Kelvin 開爾文溫度24Example: 例如k is determined at two different temperatures for the following reaction: 在如下反應中，兩種不同的溫度確定了兩個k值 2 HI(g) - H2

23、(g) + I2(g)k1 = 2.15 x 10-8Ms-1 at 650Kk2 = 2.39 x 10-7Ms-1 at 700KDetermine the activation energy.25Fig. 13.20a,b26 Three processes can explain slowness to reach equilibrium conditionsTransportDiffusionSurface Chemical Kinetics三個過程可以解釋達到反應平衡緩慢的原因輸運擴散表面化學動力學 27Drever (1997)Example of diffusion limit

24、ations 擴散作用邊界Example of surface kinetic limitations28Examples of Kinetics in Nature自然中的動力學范例 Kinetic limitations for dolomite precipitation 白云石沉淀的動力學邊界 Why silicate mineral weathering rates are so much slower in nature than in the lab 為什么硅酸鹽礦物在自然環(huán)境中的風化速率中比在試驗中慢得多29Dolomite Problem關于白云石的問題Ubiquitous

25、in nature.在環(huán)境中普遍存在Yet, scientists havent been able to participate it at earths surface conditions in the lab目前，科學家們還不能在實驗室中模擬地球表面環(huán)境形成它Is there a kinetic limitation?是否存在動力學的邊界？CaMg(CO3)230Dolomite Problem白云石的問題One hypothesis is that microbes drive dolomite precipitation by reducing sulfate (an inhibitor), and bringing together ingredients for dolomite (Ca, Mg, CO3) on their cell walls:一種假說就是微生物會減少硫酸鹽（阻聚劑）的含量，進而促進白云石的沉淀，Work by Judy Mackenzie (ETH)31Comparison between Lab and Field dissolution rates實驗室和野外的溶解速率比較 Dissolution rate depe