化學(xué)原理Chemistry課件-post+7+kinetics+

1、Chemical Kinetics & Equilibrium Chapter 13 & 14 How fast or slow is the reaction? Which one is the rate-determining step? Nosaka et al. 2006. J. Phys. Chem. B.Kubo et al. 2006. J. Am. Chem. Soc.1. Reaction rate:rate = -DADtrate = DCDtaA + bB cC + dDrate = -DADt1a= -DBDt1b=DCDt1c=DDDt1dCH4 (g) + 2O2

2、(g) CO2 (g) + 2H2O (g)rate = -DCH4Dt= -DO2Dt12=DH2ODt12=DCO2DtBr2 (aq) + HCOOH (aq) 2Br- (aq) + 2H+ (aq) + CO2 (g)393 nmlightDetector393 nmBr2 (aq)I0ItUV-visible absorption spectroscopic spectrumA: absorbance e : molar coefficient (cm-1.M-1)l : light path length (cm) C: concentration (M)It = I0 e -

3、e l CLambert-Beer Lawaverage rate = -DBr2Dt= -Br2final Br2initialtfinal - tinitialslope oftangentslope oftangentslope oftangentinstantaneous rate = a rate for specific instance in timeInitial rate is the rate measured in the initial period of time, so as to avoid possible interference from reaction

4、intermediates or products. rate = k Br2k = 3.50 x 10-3 s-1 rate constantSlope = 3.50 x 10-3 s-1This reaction is called as first-order to Br2.This is called the Rate law.aA + bB cC + dDRate = k Ax ByIt means that the reaction is the (x + y)th order, the xth in A, and the yth in B.Note that: The rate

5、laws are always determined by experiment. X or y can be any value, but it does not necessarily equal to a or b (x a, y b) In general,F2 (g) + 2ClO2 (g) 2FClO2 (g)As F2 is constant, quadruples ClO2, the rate is quadrupled.y = 1Assume: rate = k F2x ClO2yAs ClO2 is constant, doubles F2, the rate is als

6、o doubled.y = 1Therefore: rate = k F2x ClO2yDetermine the rate law and calculate the rate constant for the following reaction from the following data:S2O82- (aq) + 3I- (aq) 2SO42- (aq) + I3- (aq)ExperimentS2O82-I-Initial Rate (M/s)10.080.0342.2 x 10-420.080.0171.1 x 10-430.160.0172.2 x 10-4Assume: r

7、ate = k S2O82-x I-yWe see that x= 1; y = 1k = rateS2O82-I-=2.2 x 10-4 M/s(0.08 M)(0.034 M)= 0.08/Msrate = k S2O82-I-The reaction is the 2rd order.First-Order ReactionsA productrate = -DADt= k ADADt= k A-A0 and A are the concentration of A at t = 0 and t = t, respectively.A = A0exp(-kt)The half-lifel

8、nA0A0/2k=tln2k=lnA = lnA0 - ktThe reaction 2A B is first order in A with a rate constant of 2.8 x 10-2 s-1 at 800C. How long will it take for A to decrease from 0.88 M to 0.14 M ?lnA = lnA0 - ktt =lnA0 lnAk= 66 sA0 = 0.88 MA = 0.14 MlnA0Ak=ln0.88 M0.14 M2.8 x 10-2 s-1=What is the half-life of the re

9、actant A in this reaction ?tln2k=Second-Order ReactionsA productrate = -DADtDADt= k A2-A0 and A is the concentration of A at time t =0, and t = t, respectively.1A=1A0+ ktt =1kA0IfZero-Order ReactionsA productrate = k A0 = kA = A0 - ktDADt= k-t =A02kSummary of Common Reaction KineticsOrderRate LawCon

10、centration-Time EquationHalf-Life012rate = krate = k Arate = k A2lnA = lnA0 - kt1A=1A0+ ktA = A0 - kttln2k=t =A02kt =1kA02. Effect of Temperature lnk = -1TB + CThe rate of most chemical reactions increases as the temperature rises. The experimental data of such effect can be well fitted to the follo

11、wing equation. The value of Ea varies from reaction to reaction (40 400 kJ/mol). Larger the value of Ea, stronger the temperature effect on the reaction rate. Ea is the minimum amount of energy required to react. It is now called as activation energy .A is the frequency factor.R is the gas constant

12、(8.314 J/Kmol).T is the absolute temperature (K).In 1888, Arrhenius proposed an empirical equation to describe the temperature effect.How to explain the concentration and temperature effect?(1) The collision modelThe reacting molecules must collide with one another. The colliding molecule should hav

13、e a certain minimum energy (Ea).Ea As temperature increases, the fraction of active molecules increases. So the reaction rate increases with T. As concentration or pressure increases, the total number of active molecules increases, and so the reaction rate increases.The colliding molecules must form

14、 an activated complex in a preferable direction. In this transition state, the effective energy transfer and chemical bond rearrangement occur at one time. (2) Transition state modelCO2(g)+NO(g)CO(g)+NO2(g)OCONOProgress of reactionPotential energy kJEa = 134 kJEa = 360 kJDH = - 226 kJExothermic Reac

15、tionEndothermic ReactionA + B C + D3. Reaction MechanismsAt molecular level, the observed chemical reaction (called also as complicated reaction ) is actually composed of a series of simple elementary steps or reactions.Elementary step or reaction is the step or reaction that really occurs during th

16、e chemical reaction.2NO (g) + O2 (g) 2NO2 (g)N2O2 is detected during the reaction, called as the intermediateElementary step:NO + NO N2O2Elementary step:N2O2 + O2 2NO2Overall reaction:2NO + O2 2NO2+Complicated reaction:Based on this information, one proposed the following reaction mechanism:Unimolec

17、ular reactionA productsrate = k ABimolecular reactionA + B productsrate = k ABBimolecular reactionA + A productsrate = k A2For elementary steps, the rate raw can directly be written as. Writing plausible reaction mechanisms:The sum of the elementary steps must give the overall balanced equation for

18、the reaction.The rate-determining step should predict the same rate law that is determined experimentally.The rate-determining step is the slowest step in the sequence of steps leading to product formation.The experimental rate law for the reaction between NO2 and CO to produce NO and CO2 is rate =

19、kNO22. The reaction is believed to occur via two steps:Step 1:NO2 + NO2 NO + NO3Step 2:NO3 + CO NO2 + CO2What is the equation for the overall reaction?NO2+ CO NO + CO2What is the intermediate?NO3What can you say about the relative rates of steps 1 and 2?rate = kNO22 is the rate law for step 1 so ste

20、p 1 must be slower than step 24. Chemical equilibriumequilibriumN2O4 (g) 2NO2 (g)Rate(+)Rate(-) At the equilibrium, Rate(+) = Rate (-) Both N2O4 and NO2 remain constant. it is a dynamic equilibriumKc = NO22N2O4Equilibrium constantKp = NO2P2N2O4PKc KpHere P (pa), Po = 101 325 paHere C (mol/L), Co = 1

21、 mol/LaA + bB cC + dDIt is quite often written as If pressure is in a unit of atm,Kp = Kc(RT)DnFor a gas reaction, here Dn= (c + d) (a + b)The equilibrium concentrations for the reaction between carbon monoxide and molecular chlorine to form COCl2 (g) at 740C are CO = 0.012 M, Cl2 = 0.054 M, and COC

22、l2 = 0.14 M. Calculate the equilibrium constants Kc and Kp.CO (g) + Cl2 (g) COCl2 (g)Kc = COCl2COCl2=0.140.012 x 0.054= 220Kp = Kc(RT)DnDn = 1 2 = -1R = 0.0821T = 273 + 74 = 347 KKp = 220 x (0.0821 x 347)-1 = 7.714.2The concentration of solids and pure liquids are taken as a unit, and not included i

23、n the K expression.14.2CH3COOH (aq) + H2O (l) CH3COO- (aq) + H3O+ (aq)Kc =CH3COO-H3O+CH3COOHH2OSince H2O = constantCaCO3 (s) CaO (s) + CO2 (g)Kc =CaOCO2CaCO3Since CaCO3 = constant CaO = constantKc = CO2Kp = PCO2Kc = CH3COO-H3O+CH3COOH= Kc H2OConsider the following equilibrium at 295 K:The partial pr

24、essure of each gas is 0.265 atm. Calculate Kp and Kc for the reaction?NH4HS (s) NH3 (g) + H2S (g)Kp = PNH3H2SP= 0.265 x 0.265 = 0.0702Kp = Kc(RT)DnKc = Kp(RT)-DnDn = 2 0 = 2T = 295 KKc = 0.0702 x (0.0821 x 295)-2 = 1.20 x 10-414.2The factors influencing the chemical equilibrium: (1) Increasing A or B will shift the reaction to the right, until a new equilibrium is established to keep K.aA + bB cC + dD(2) Increasing system pressure will shift the reaction to the right if Dn 0, or to the left if Dn 0, or to the left if DH 0.If an external stress is applied