一級反應(yīng)動力學、二級反應(yīng)動力學基本原理

1、Kinetics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(yīng)是否發(fā)生、反應(yīng)的方向以及參與到反應(yīng)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 facto

2、rs that control the rates of reactions 動力學則說明反應(yīng)發(fā)生的速率有多快、反應(yīng)步驟的順序以及控制反應(yīng)速率的一些因素 Further Reading (Chapter 3 in Hobbs),Rate of reaction is typically measured as the change in concentration (moles/L) with time 反應(yīng)速率通常通過濃度(mol/L)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能是減小的 Likewise

3、 the concentration change may be of reactants or products 同樣，改變濃度的物質(zhì)可能是反應(yīng)物也可能是生成物,反應(yīng)速率=濃度/時間,Rate has units of moles per liter per unit time - M s-1, M h-1 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(shè)的反應(yīng) aA + bB cC + dD We can write,Note the use of the negative sign 注意負號的使用

4、 - rate is defined as a positive quantity 反應(yīng)速率被定義為正量 - rate of disappearance of a reactant is negative 那么，反應(yīng)物的消耗速率就是負的 2N2O5(g) 4NO2(g) + O2(g),Rate may be expressed in three main ways: 反應(yīng)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(yīng)速率：濃度隨時間變化的度量 2.

5、Instantaneous rate: rate of change of concentration at any particular instant during the Reaction 2、瞬時速率：反應(yīng)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：t=0時即反應(yīng)物最初混合時的瞬時速率，,Rate Laws 速率定律,Rate law = a mathematical function describi

6、ng 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(guān)的化合物的轉(zhuǎn)化速率的數(shù)學函數(shù)，即各種參與反應(yīng)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(chǔ) “Many reactions occur at a decreasing rate with increasing time becau

7、se the reaction rate diminishes as the concentrations of the reactants diminish” “很多反應(yīng)隨著時間的增加反應(yīng)速率逐漸減?。?，因為反應(yīng)物濃度減小會降低反應(yīng)速率”,Hobbs (2000),Rate Laws,“m” indicates the order of the reaction with respect to species “A”. Determined experimentally. “m”指關(guān)于物質(zhì)A的反應(yīng)級數(shù)，由實驗確定。 “k” = rate coefficient (or specific re

8、action 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(yīng)確定,A = products,Reaction rate,Rate Laws,“m, n, o” indicate the order of the reaction with respect to each species; determine experimentally overall reaction order = m+n+o “m, n, o”分別是關(guān)于每種物質(zhì)的反應(yīng)級數(shù)，

9、實驗確定的總反應(yīng)級數(shù)=m+n+o Reaction orders can be: first, second, third 反應(yīng)級數(shù)可以是一級、二級、三級,A + B + C = products,First order reactions一級動力反應(yīng),Take integral of equation above, and get equation below: 將以上等式積分，得到下面的等式：,t = 0.693 / k,Plot of first order reaction 一級動力反應(yīng)的圖片,半衰期的長度固定,Example of 1st order reaction1級動力反應(yīng)范例

10、,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(yīng)物只有一種，而且指數(shù)為1，那么稱這種反應(yīng)為一級動力反應(yīng) Write rate law for above equation:,H2O2 (l) H2O(l) + 1/2O2(g),Wikipedia,Second Order Reaction二級動力反應(yīng),If integrated如果積分: 得到1/At = k”t + 1/Ao Inverse

11、concentration linear in time 濃度的倒數(shù)和時間程線性關(guān)系,t = 1 / k Ao,Plot of second order reaction 2級動力反應(yīng)圖,A,time,Half lives are not constant with time for second order reactions 二級反應(yīng)動力學反應(yīng)物質(zhì)的半衰期隨時間推移變化,Example of 2nd order reaction二級動力反應(yīng)范例,A second-order reaction depends on the concentrations of one second-order

12、 reactant, or two first-order reactants. 二級動力反應(yīng)取決是否有一個二級反應(yīng)物或者兩個一級反應(yīng)物？ Write rate law for above equations:,2NO2 (g) 2NO(g) + O2(g),Wikipedia,CO2 + H2O H2CO3,Pseudo first order reactions偽一級反應(yīng),pseudo-first order: concentration of one reactant remains essentially constant over time (often because it is

13、in large excess compared to the other reactant) 偽一級動力反應(yīng)：一種反應(yīng)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）,B濃度不變,Reaction Mechanism反應(yīng)機理,Often we write a chemical reaction 我們經(jīng)常寫化學反應(yīng)式 How we 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(yīng)的機理,Single Step (Elementary) React

14、ion Process單步（基本）反應(yīng)過程,Unimolecular 單分子反應(yīng) CaCO3 Ca2+ + CO32- Bimolecular 雙分子反應(yīng) CO2 + H2O H2CO3 Termolecular 三分子反應(yīng) 2 FeS2 + 3 O2 + 2 H2O 2 Fe3+ + 4H2SO4 No four or more molecule processes have been identified because chances of 4 molecules colliding is very, very small (making reaction rate super smal

15、l) 沒有4或者4以上的分子反應(yīng)被檢測到，因為4分子同時碰撞的機會非常非常小（使得反應(yīng)速率非常?。?Rate Laws For Elementary基本反應(yīng)速率定律,Simple to write 書寫簡單 Unimolecular H2CO3 H+ + HCO3- dH2CO3/dt = kH2CO3 Bimolecular CO2 + H2O H2CO3 dCO2/dt = kCO2H2O,Reaction order is not necessarily related to the stoichiometry of the reaction, unless the reaction i

16、s elementary. 反應(yīng)級數(shù)不一定和反應(yīng)的化學計量數(shù)有關(guān)，除非反應(yīng)是基本反應(yīng) In single step process, reaction order = coefficient of reactant in chemical equation 在單步反應(yīng)過程中，反應(yīng)級數(shù)=化學式中反應(yīng)物的系數(shù) Complex reactions may or may not have reaction orders equal to their stoichiometric coefficients 復雜反應(yīng)的反應(yīng)級數(shù)可能和化學計量系數(shù)相等 Not all first order reaction

17、s, are single step, monomolecular reactions 不是所有的一級反應(yīng)都是單步反應(yīng)、單分子反應(yīng),Important Note重要提示,Hobbs (2000),Overview of Elementary Reactions基本反應(yīng)回顧,Note: for Zero-Order reactions rate is independent of the concentration of reactant. Doubling concentration has no effect on rate. 提示：零級反應(yīng)速率不依賴反應(yīng)物的濃度，雙倍的反應(yīng)物濃度不影響反應(yīng)

18、速率。,Arrhenius 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(yīng)會發(fā)生一系列的基本步驟. usually one of these steps is much slower than the others Rate Determining Step 通常其中的一個步驟會比其他的步驟反應(yīng)慢很多，這個就是速率決定步驟 empirically, the effect of T on the rate of this reaction

19、 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(yīng)速率（也就是整體反應(yīng)速率）的影響可以用阿倫紐斯方程描述,pre-exponential factor or “frequency factor”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

20、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?,Increased Temperature = Faster Reaction Rates 升溫=更高的反應(yīng)速率,Determination of Activation Energy (Ea)活化能測定,ln k2 - ln k1 = -Ea/R(1/T2 - 1/T1),K = reaction rate 反應(yīng)速率 Ea = activation energy 活化能 R = gas constant (8.314 J/mol*K

21、, or 0.008314 KJ/mol*K) 氣體常數(shù) T = temperature in Kelvin 開爾文溫度,Example: 例如 k is determined at two different temperatures for the following reaction: 在如下反應(yīng)中，兩種不同的溫度確定了兩個k值 2 HI(g) - H2(g) + I2(g) k1 = 2.15 x 10-8Ms-1 at 650K k2 = 2.39 x 10-7Ms-1 at 700K Determine the activation energy.,Fig. 13.20a,b,Th

22、ree processes can explain slowness to reach equilibrium conditions Transport Diffusion Surface Chemical Kinetics 三個過程可以解釋達到反應(yīng)平衡緩慢的原因 輸運 擴散 表面化學動力學,Drever (1997),Example of diffusion limitations 擴散作用邊界,Example of surface kinetic limitations,Examples of Kinetics in Nature自然中的動力學范例,Kinetic limitations

23、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,Dolomite Problem關(guān)于白云石的問題,Ubiquitous 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

24、，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)2,Dolomite 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),Comparison between Lab and Field dissolution rates 實驗室和野外的溶解速率比較,Dissolution rate depends on mineral surface area (difficult to