一级反应动力学、二级反应动力学基本原理

1、编辑pptKinetics动力学Thermodynamics determines whether and in which direction a reaction will happen, and what the concentrations of constituents are involved in the reaction热力学决定了反应是否发生、反应的方向以及参与到反应之中的物质浓度问题。Kinetics say how fast a reaction happens, sequence of steps in the reaction, and some of the fac

2、tors that control the rates of reactions动力学则说明反应发生的速率有多快、反应步骤的顺序以及控制反应速率的一些因素Further Reading (Chapter 3 in Hobbs)编辑ppt Rate of reaction is typically measured as the change in concentration (moles/L) with time反应速率通常通过浓度反应速率通常通过浓度(mol/L)随时间的变化来测量随时间的变化来测量 This change may be a decrease or an increase改变

3、可能是增加的也可能是减小的改变可能是增加的也可能是减小的 Likewise the concentration change may be of reactants or products同样，改变浓度的物质可能是反应物也可能是生成物同样，改变浓度的物质可能是反应物也可能是生成物Rate = _ = _change in timechange in timein productsin reactantsRate =concentration changetime change反应速率=浓度/时间编辑ppt Rate has units of moles per liter per unit t

4、ime- M s-1, M h-1速率的单位是摩尔每升每单位时间，如速率的单位是摩尔每升每单位时间，如M s-1、M h-1 Consider the hypothetical reaction 对于这个假设的反应对于这个假设的反应aA + bB cC + dD We can write t ARate of reaction=1a t B1b t C1c t D1d=-编辑ppt Note the use of the negative sign注意负号的使用注意负号的使用- rate is defined as a positive quantity反应速率被定义为正量反应速率被定义为正量

5、- rate of disappearance of a reactant is negative那么，反应物的消耗速率就是负的那么，反应物的消耗速率就是负的2N2O5(g) 4NO2(g) + O2(g) t N2O5Rate of reaction=12 t NO214 t O2=-编辑ppt Rate may be expressed in three main ways:反应速率主要有以下三个表达方式：反应速率主要有以下三个表达方式： Average reaction rate: a measure of the change in concentration with time 1、

6、平均反应速率：浓度随时间变化的度量、平均反应速率：浓度随时间变化的度量2. Instantaneous rate: rate of change of concentration at any particular instant during theReaction 2、瞬时速率：反应过程中某一特定瞬间的浓度变化速率、瞬时速率：反应过程中某一特定瞬间的浓度变化速率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时即反应物最初混合时

7、的瞬时速率时即反应物最初混合时的瞬时速率，编辑pptRate 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：用来描述相关的化合物的转化速率的数学函数，即各种参与反应的物质的浓度的函数May or may NOT have a theoretical bas

8、is可能有或没有理论基础“Many reactions occur at a decreasing rate with increasing time because the reaction rate diminishes as the concentrations of the reactants diminish”“很多反应随着时间的增加反应速率逐渐减小？，因为反应物浓度减小会降低反应速率”Hobbs (2000)编辑pptRate Laws-dAdt=kAm“m” indicates the order of the reaction with respect to species “

9、A”. Determined experimentally.“m”指关于物质A的反应级数，由实验确定。“k” = rate coefficient (or specific reaction rate coeff.); determined for a specific reaction at a specific temp“K”=速率常数（或特定的速率系数），由特定温度下的具体反应确定A = productsReaction rate编辑pptRate LawsdAdt=-kAmBnCo“m, n, o” indicate the order of the reaction with res

10、pect to each species; determine experimentallyoverall reaction order = m+n+o“m, n, o”分别是关于每种物质的反应级数，实验确定的总反应级数=m+n+oReaction orders can be: first, second, third反应级数可以是一级、二级、三级A + B + C = products编辑pptFirst order reactions一级动力反应-dAdt=kA1=kAktteAA-=0Take integral of equation above, and get equation be

11、low:将以上等式积分，得到下面的等式：编辑pptt = 0.693 / kPlot of first order reaction一级动力反应的图片半衰期的长度固定编辑pptExample of 1st order reaction1级动力反应范例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. 如果反应物只有一种，而且指数为1，那么称这种反应为一级动力反应Write rate law for above

12、 equation:H2O2 (l) H2O(l) + 1/2O2(g)Wikipedia编辑pptSecond Order Reaction二级动力反应 If integrated如果积分: 得到1/At = k”t + 1/Ao Inverse concentration linear in time 浓度的倒数和时间程线性关系-dAdt=kA2编辑pptt = 1 / k AoPlot of second order reaction 2级动力反应图AtimeHalf lives are not constant with time for second order reactions二

13、级反应动力学反应物质的半衰期随时间推移变化编辑pptExample of 2nd order reaction二级动力反应范例A second-order reaction depends on the concentrations of one second-order reactant, or two first-order reactants.二级动力反应取决是否有一个二级反应物或者两个一级反应物？Write rate law for above equations:2NO2 (g) 2NO(g) + O2(g)WikipediaCO2 + H2O H2CO3编辑pptPseudo fi

14、rst order reactions伪一级反应 pseudo-first order: concentration of one reactant remains essentially constant over time (often because it is in large excess compared to the other reactant) 伪一级动力反应：一种反应物剩余的浓度基本不变（经常由于它与其他物质相比，浓度过量）A+BC+DdAdt=-kABdAdt=-kA where k=kBlnA= lnA0-kt B浓度不变编辑pptReaction Mechanism反

15、应机理 Often we write a chemical reaction 我们经常写化学反应式 How we write it is often not how it happens 但是我们所写的通常不是它如何发生的 Many kinds of reaction mechanisms 有很多反应的机理编辑pptSingle Step (Elementary) Reaction Process单步（基本）反应过程 Unimolecular 单分子反应 CaCO3 Ca2+ + CO32- Bimolecular 双分子反应CO2 + H2O H2CO3 Termolecular 三分子反应

16、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 small)没有4或者4以上的分子反应被检测到，因为4分子同时碰撞的机会非常非常小（使得反应速率非常小）编辑pptRate Laws For Elementary基本反应速率定律 Simple to write 书写简单 Unimol

17、ecular H2CO3 H+ + HCO3-dH2CO3/dt = kH2CO3 Bimolecular CO2 + H2O H2CO3dCO2/dt = kCO2H2O编辑pptReaction order is not necessarily related to the stoichiometry of the reaction, unless the reaction is elementary.反应级数不一定和反应的化学计量数有关，除非反应是基本反应In single step process, reaction order = coefficient of reactant in

18、 chemical equation在单步反应过程中，反应级数=化学式中反应物的系数Complex reactions may or may not have reaction orders equal to their stoichiometric coefficients复杂反应的反应级数可能和化学计量系数相等Not all first order reactions, are single step, monomolecular reactions不是所有的一级反应都是单步反应、单分子反应Important Note重要提示Hobbs (2000)编辑pptOverview of Ele

19、mentary Reactions基本反应回顾Note: for Zero-Order reactions rate is independent of the concentration of reactant. Doubling concentration has no effect on rate.提示：零级反应速率不依赖反应物的浓度，双倍的反应物浓度不影响反应速率。编辑pptArrhenius Equation and Transition State Theory阿伦纽斯方程和过渡态理论 reactions occur as a sequence of elementary step

20、s 化学反应会发生一系列的基本步骤. usually one of these steps is much slower than the others Rate Determining Step 通常其中的一个步骤会比其他的步骤反应慢很多，这个就是速率决定步骤 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: 依照经验，时间对这个步骤反应速率（也就是

21、整体反应速率）的影响可以用阿伦纽斯方程描述kA eERTa=-/pre-exponential factor or “frequency factor”A：指前因子或振动因子describes collision frequency and the orientation probability描述碰撞频率和定向概率Activation energy活化能（Ea是物质分裂形成新物质的最低能量） describes the fraction of species with energy greater than Ea 描述了物质能量大于Ea的部分?编辑pptIncreased Temperatu

22、re = Faster Reaction Rates升温=更高的反应速率编辑pptDetermination of Activation Energy (Ea)活化能测定ln k2 - ln k1 = -Ea/R(1/T2 - 1/T1)K = reaction rate 反应速率Ea = activation energy 活化能R = gas constant (8.314 J/mol*K, or 0.008314 KJ/mol*K) 气体常数T = temperature in Kelvin 开尔文温度编辑pptExample: 例如k is determined at two diff

23、erent temperatures for the following reaction: 在如下反应中，两种不同的温度确定了两个k值 2 HI(g) - H2(g) + I2(g)k1 = 2.15 x 10-8Ms-1 at 650Kk2 = 2.39 x 10-7Ms-1 at 700KDetermine the activation energy.编辑pptFig. 13.20a,b编辑ppt Three processes can explain slowness to reach equilibrium conditionsTransportDiffusionSurface Ch

24、emical Kinetics三个过程可以解释达到反应平衡缓慢的原因输运扩散表面化学动力学 编辑pptDrever (1997)Example of diffusion limitations 扩散作用边界Example of surface kinetic limitations编辑pptExamples of Kinetics in Nature自然中的动力学范例 Kinetic limitations for dolomite precipitation 白云石沉淀的动力学边界 Why silicate mineral weathering rates are so much slowe

25、r in nature than in the lab 为什么硅酸盐矿物在自然环境中的风化速率中比在试验中慢得多编辑pptDolomite Problem关于白云石的问题Ubiquitous in nature.在环境中普遍存在Yet, scientists havent been able to participate it at earths surface conditions in the lab目前，科学家们还不能在实验室中模拟地球表面环境形成它Is there a kinetic limitation?是否存在动力学的边界？CaMg(CO3)2编辑pptDolomite Probl

26、em白云石的问题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:一种假说就是微生物会减少硫酸盐（阻聚剂）的含量，进而促进白云石的沉淀，Work by Judy Mackenzie (ETH)编辑pptComparison between Lab and Field dissolution rates实验室和野外的溶解速率比较 Dissolution rate depends on mineral surface a