1、Kinetics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 factors
2、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) Rate of reaction is typically measured as the change in concentration (moles/L) with time反反应应速率通常通速率通常通过浓过浓度度(mol/L)随随时间时间的的变变化来化来测测量量 This change may be a decrease or an increase改改变变可能是增加的也可
3、能是减小的可能是增加的也可能是减小的 Likewise the concentration change may be of reactants or products同同样样，改，改变浓变浓度的物度的物质质可能是反可能是反应应物也可能是生成物物也可能是生成物Rate = _ = _change in timechange in timein productsin reactantsRate =concentration changetime change反应速率=浓度/时间 Rate has units of moles per liter per unit time- M s-1, M h
4、-1速率的速率的单单位是摩位是摩尔尔每升每每升每单单位位时间时间，如，如M s-1、 、M h-1 Consider the hypothetical reaction 对对于于这这个假个假设设的反的反应应aA + bB cC + dD We can write t ARate of reaction=1a t B1b t C1c t D1d=- Note the use of the negative sign注意注意负负号的使用号的使用- rate is defined as a positive quantity反反应应速率被定速率被定义为义为正量正量- rate of disappea
5、rance of a reactant is negative那么，反那么，反应应物的消耗速率就是物的消耗速率就是负负的的2N2O5(g) 4NO2(g) + O2(g) t N2O5Rate of reaction=12 t NO214 t O2=- Rate may be expressed in three main ways:反反应应速率主要有以下三个表达方式：速率主要有以下三个表达方式：1. Average reaction rate: a measure of the change in concentration with time 1、平均反、平均反应应速率：速率：浓浓度随度随
6、时间变时间变化的度量化的度量2. Instantaneous rate: rate of change of concentration at any particular instant during theReaction 2、瞬、瞬时时速率：反速率：反应过应过程中某一特定瞬程中某一特定瞬间间的的浓浓度度变变化速率化速率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时时即反即反应应物最初混合物最初混合时时的瞬的瞬时时速率速率，
7、 ，Rate 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 basis可能有或没有理论基础“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)Rate Laws-dAdt=kAm“m” indicates the order of the reaction with respect to species “A”. Determined experimentally
9、.“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 rateRate LawsdAdt=-kAmBnCo“m, n, o” indicate the order of the reaction with respect to each species; determine ex
10、perimentallyoverall 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 = productsFirst order reactions一级动力反应-dAdt=kA1=kAktteAA-=0Take integral of equation above, and get equation below:将以上等式积分，得到下面的等式：t = 0.693 / kPlot o
11、f first order reaction一级动力反应的图片半衰期的长度固定Example 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 equation:H2O2 (l) H2O(l) + 1/2O2(g)WikipediaSeco
12、nd Order Reaction二级动力反应If integrated如果积分:得到1/At = k”t + 1/AoInverse concentration linear in time浓度的倒数和时间程线性关系-dAdt=kA2t = 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二级反应动力学反应物质的半衰期随时间推移变化Example of 2nd order reaction二级动力反应范例A sec
13、ond-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 H2CO3Pseudo first order reactions伪一级反应pseudo-first order: concentration of one reactant
14、 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Reaction Mechanism反应机理Often we write a chemical reaction 我们经常写化学反应式How we write it is often not how
15、 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Single 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 H2CO3Termolecular 三分子反应2 FeS2 + 3 O2 + 2 H2O 2 Fe3+ + 4H2SO4No four or more molecule processes have been identified b
16、ecause 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）Rate Laws For Elementary基本反应速率定律Simple to write 书写简单Unimolecular H2CO3 H+ + HCO3-dH2CO3/dt = kH2CO3Bimolecular CO2 + H2O H2CO3dCO2/dt = kCO2H2OReaction order is
17、 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 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 the
18、ir 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)Overview of Elementary Reactions基本反应回顾Note: for Zero-Order reactions rate is independent of the concentration of reactant. Doubl
19、ing 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。Arrhenius Equation and Transition State Theory阿伦纽斯方程和过渡态理论 reactions occur as a sequence of elementary steps 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
20、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 a
21、nd 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?Increased Temperature = Faster Reaction Rates升温=更高的反应速率Determination of Activation Energy (Ea)活化能测定ln k2 - ln k1 = -Ea/R(1/T2 - 1/T1)K = react
22、ion 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Example: 例如k is determined at two different 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 1
23、0-7Ms-1 at 700KDetermine the activation energy.Fig. 13.20a,bThree processes can explain slowness to reach equilibrium conditionsTransportDiffusionSurface Chemical 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 Drever (1997)Example of diffusion limitations 扩散作用边界Example of surface kinetic limitationsExamples
24、 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 slower 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Dolomite 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 i
25、t 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)2Dolomite Problem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
26、 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)Comparison 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 area (difficult to measure or estimate in the field) 溶解速率取决于矿物表面积（很难测量或者预测野外矿物的表面积） In order to compare lab rat
27、es to field rates, need to know surface area of all minerals involved in reaction from pedon to catchment scale 为了比较实验室溶解速率和溶解野外速率，需要得到从单个土体到流域内所有矿物的表面积 Most studies find slower weathering rates in field compared to lab 大多研究表明野外风化速率比实验室的要慢 Hypotheses: 假说： Reactive surfaces weathered away over long t
28、ime periods in field (vs. fresh surfaces from ground up minerals used in lab experiments) 在野外的矿物风化需要的周期更长（相对于实验室未风化的矿物表面） Precipitation of secondary minerals on grain surfaces may isolate fresh mineral surfaces from weathering 次生矿物在颗粒表面的沉淀，可以使未风化的矿物隔离，免于风化 Hydrology: preferential flow of water through macro pores may bypass mineral surfaces, decreasing weathering rates in field 水文学：通过大孔隙的优势水流可能会绕开矿物表面，减少野外的风化Drever (1997)
