1、Chapter 7 7-1.Reaction rate and its expressions 7-2.Reaction mechanism and elementary reaction 7-3.Reaction order and its characteristics 7-4.Collision and transition-state theory 7-5.Effect of temperature on reaction rate 7-6.Effect of catalyst on reaction rate1.Reaction Rate and Its ExpressionsChe
2、mical kinetics():(g)NH2(g)H3(g)N3221.Does this reaction proceed in a commercially feasible length of time?How is the rate of a reaction like this measured?2.What conditions affect the rate of a reaction?3.How do you express the relationship of rate of a reaction to the variables that affect rate?4.W
3、hat happens at the molecular level when a reaction occurs?1.Reaction Rate and Its ExpressionsI.Rate of reaction():Reactions proceed at a wide variety of rates.Some are very fast,like the explosions of dynamite.Others are very slow,as in the slow growth of corrosion on the prow of the Titanic on the
4、cold floor of the Atlantic Ocean.1.Reaction Rate and Its ExpressionsThe rate of reactions may be affected by the following factors:1.Concentrations of reactants:Orpiment,雌黄雌黄As2S3 鸡冠石Realgar,雄黄雄黄As4S4Arsenic trioxide，砒霜，砒霜As2O31.Reaction Rate and Its Expressions2.Concentrations of catalysts():(g)OO(
5、l)H2(aq)OH222HBr(aq)22 3.Temperature:Usually reactions speed up when the temperature increases.1.Reaction Rate and Its Expressions4.Surface area of a solid reactant or catalyst:The greater the surface area per unit volume,the faster is the reaction.1.Reaction Rate and Its Expressions is the increase
6、 in concentration of product of a reaction per unit time or the decrease in concentration of reactant per unit time.Unit:molL-1s-1.(g)O(g)NO4(g)ON22252The rate of this reaction has been extensively studied under various conditions.t)c(O)(O22Rate of formation of oxygen:1.Reaction Rate and Its Express
7、ionsTime(s)c(O2)0 0.0000 600 0.0021 1200 0.0036 1800 0.0048 2400 0.0057 3000 0.0063 3600 0.0068 4200 0.0072 4800 0.0075 5400 0.0077 6000 0.0078 1.Reaction Rate and Its ExpressionsThe negative sign occurs in a rate expression for a reactant in order to indicate a decrease in concentration and to give
8、 a positive value for the rate.116122sLmol102.5s600Lmol0.0015t)c(O)(ORate of consumption of N2O5:t)Oc(N)O(N52521:4:2)(O:)(NO:)O(N2252(g)O(g)NO4(g)ON222521.Reaction Rate and Its ExpressionsII.Average reaction rate:tc(R)tc(P)orAverage rate over the time interval tIII.Instantaneous reaction rate():dtdc
9、(R)tc(R)lim t0dtdc(P)tc(P)lim t0Whenever we speak of a reaction rate,we shall always mean an instantaneous rate.1.Reaction Rate and Its ExpressionsIV.Experimental determination of rate1.Withdraw samples at various times and analyze them.1.Reaction Rate and Its ExpressionsCH3CH2OCCH3OH2OH+CH3CH2OHHOC
10、CH3O+Ethyl acetateEthanolAcetic acid2.Continuously follow the progress of a reaction by observing the change in some physical property such as color.(aq)Cl(aq)IO(aq)I(aq)ClOThe hypoiodite ion,IO-,absorbs blue light near 400 nm.1.Reaction Rate and Its Expressions3.Continuously follow the progress of
11、a reaction by observing the change in pressure.(g)O(g)NO4(g)ON22252N2O5 crystals are sealed in a vessel equipped with a manometer.The vessel is then plunged into a water bath at 45 C,at which temperature the solid vaporizes and the gas decomposes.Chromatographypaper chromatography(纸色谱纸色谱)Chromatogra
12、phy(色谱色谱)is a group of similar separation techniques.Each depends on how fast a substance moves,in a stream of gas or liquid,past a stationary phase to which the substance may be slightly attracted.Column chromatography(柱色谱柱色谱)(A)A solution containing substances to be separated is poured into thetop
13、 of a column,which contains powdered chalk.(B)Pure liquid is added to the column,and the substances begin to separate into bands.(C)The substances separate further on the column.Each substance is collected in a separate flask as it comes off the column.Gas chromatographyThis is a chromatogram of a h
14、exane mixture,showing its separation into four different substances.Such hexane mixtures occur in gasoline;hexane is also used as a solvent to extract the oil from certain vegetable seeds.Infrared SpectroscopyVibration of the HCl moleculeThis vibration of molecules is revealed in their absorption of
15、 infrared radiation.A chemical bond acts like a stiff spring connecting nuclei.As a result,the nuclei in a molecule vibrate,rather than maintaining fixed positions relative to each other.A Fourier transform infrared(FTIR)spectrometer傅立叶变换红外（傅立叶变换红外（FTIR FTIR）光谱仪）光谱仪the IR spectrum can act as a compo
16、unds“fingerprint(指纹指纹).”nuclear magnetic resonance(NMR)spectroscopyHigh-resolution NMR spectrum of ethanol,CH3CH2OHEthanol,whose molecular structure is has protons in three different chemical environments:a proton bonded to an oxygen atom(H-O-),the protons in a-CH2-group,and the protons in a-CH3 gro
17、up.2.Reaction Mechanism and Elementary reactionI.Elementary and complex reaction:Elementary reaction():reactions that occur by one step to give products.Complex reaction():reactions that take place by a sequence of steps,to finally give the products.(1)I2(g)2I(g)(fast,equilibrium)H2(g)+I2(g)=2HI(g)C
18、O(g)+H2O(g)=CO2(g)+H2(g)(2)H2(g)+2I(g)=2HI(g)(slow,rate-determining step)The set of elementary reactions whose overall effect is given by the net chemical equation is called the.(g)CONO(g)CO(g)(g)NO222.Reaction Mechanism and Elementary reactionreaction)y(elementarNONONONO322reaction)y(elementarCONOC
19、ONO223(1)I2(g)2I(g)(fast,equilibrium)(2)H2(g)+2I(g)=2HI(g)(slow,)H2(g)+I2(g)=2HI(g)(overall reaction)II.Molecularity of reaction():Unimolecular reaction:An elementary reaction that involves one reactant molecule.Bimolecular reaction:two reactant molecules Termolecular reaction:three reactant molecul
20、es 223CHCHCHnecyclopropaNO(g)(g)NO(g)NO(g)NO322HI(g)2I(g)2(g)H22.Reaction Mechanism and Elementary reactionThe molecularity is the number of molecules on the reactant side of an.2.Reaction Mechanism and Elementary reactionExample 7-1:The mechanism for the gas-phase chlorination of CHCl3 with Cl2 to
21、produce CCl4 is:)CClHClClCHCl(423Cl2Cl233CClHClCHClCl4CClCClCl3What is the molecularity of each step in the mechanism?Solution:The molecularity of any elementary reaction equals the number of reactant molecules.Thus,the forward part of the first step is;the reverse of the first step,the second step,
22、and the third step are each.III.Law of mass action and rate equationFor an elementary reaction at a specified temperature,the rate is proportional to the product of the concentration of reactant molecule.bBaAbBaAckccc1.Law of mass action():Elementary reaction:aA+bB=productsRate equation():An equatio
23、n that relates the rate of a reaction to the concentrations of reactants raised to various powers.2.Reaction Mechanism and Elementary reaction“Active mass”was an early term for concentration;hence the term mass action.k:rate constant.Depending on the nature of reaction and the temperature.Physical m
24、eaning?bBaAbBaAckcccThe unit of k depends on the form of the rate equation.223CHCHCHnecyclopropaNO(g)(g)NO(g)NO(g)NO32263HCkc1111sLmolsLmol:kofunit2NO2kc112111sLmol)L(molsLmol:kofunit2.Reaction Mechanism and Elementary reaction2.Notes:1.Law of mass action is permissible only for one-step elementary
25、reactions.bBaAckcaA+bB=dD+eEElementary reaction:Complex reaction:nBmAckcThe exponents m and n(frequently integers)must be determined experimentally.(g)O(g)NO4(g)ON2225252ONkc2.Reaction Mechanism and Elementary reaction2.Concentration of a pure solid or liquid is not included in the rate equation.(g)
26、CO(g)OC(s)222Okc3.Concentration of a reactive solvent in diluted solutions is not included in the rate equation.112212OHCkc612661262112212OHCOHCOHOHCsucrosefructoseglucose2.Reaction Mechanism and Elementary reactionDetermining the rate equation:2.Reaction Mechanism and Elementary reaction?B?Ackc B?A
27、ckc B2Ackc 3.Reaction order:The with respect to a given reactant species equals the exponent()of the concentration of that species in the rate equation,as determined experimentally.m is the reaction order with respect to the reactant A;n is the reaction order with respect to the reactant B.aA+bB=dD+
28、eEnBmAckc2.Reaction Mechanism and Elementary reactionThe overall order of a reaction equals the sum of the orders of the reactant species in the rate equation.In this example,the overall order is m+n.m+n=0:zero-order reaction;m+n=1:first-order reaction;m+n=2:second-order reaction.aA+bB=dD+eEnBmAckc2
29、.Reaction Mechanism and Elementary reactionThe reaction is second-order in A,first-order in B,and third-order overall.B2Ackc 3.Reaction Order and its CharacteristicsA rate equation tells you how the rate of a reaction depends on reactant concentrations at a particular moment.But often you would like
30、 to have a mathematical relationship showing how a reactant concentration changes over a period of time.Using calculus(),we can transform a rate equation into a mathematical relationship between concentration and time.nBmAckctcA3.Reaction Order and its CharacteristicsI.First-order reaction()AAkcdtdc
31、t0ccAAdtkcdc0productsaA dtkcdcAAc:concentration at time t;c0:initial concentration.3.Reaction Order and its Characteristicsln cA t:the slope is k(time-1).ktcclnAA0ktlnclnc0AAktAAecc00AAlnctklncbxay(g)O(g)NO4(g)ON2225252ONkc3.Reaction Order and its Characteristics:The time it takes for the reactant c
32、oncentration to decrease to one-half of its initial value.2ln2/cclnkt00AA2/1k0.693t2/1ktcclnAA03.Reaction Order and its Characteristics2252ONO4ON252ONkck0.693t2/114s104.800.693s101.443Example 7-2:Soil was found to be contaminated with radioactive Co-60,which has a half-life of 5.26 years(radioactive
33、 substances decay by first order processes and their rate of decay is normally reported by stating their half-lives.).The soil was collected for storage.How much years must pass before the radioactivity drops to 20%of its initial value?k0.693t2/112/1a0.132a5.260.693t0.693kAssume the initial radioact
34、ivity of Co-60 is R,then:ta0.132R20%Rln13.Reaction Order and its Characteristicsa12.2t Solution:II.Second-order reaction2AAkcdtdc1/cA t:the slope is k(concentration-1 time-1).ktc1c10AA3.Reaction Order and its CharacteristicsproductsaA 0AAc1ktc1bxay3.Reaction Order and its Characteristics(g)ONO(g)2(g
35、)NO2222NO2kc0AAc1ktc13.Reaction Order and its Characteristics(g)ONO(g)2(g)NO2222NO2kc0AAlnctklnc3.Reaction Order and its Characteristics0AAc21cktc1c10AA2/1tt 2/1AA21ktc1c1000A2/1kc1tExample 7-3:Saponification of ethyl acetate at 298K is a second-order reaction:CH3COOC2H5+NaOH CH3COONa+C2H5OHThe init
36、ial concentrations of all reactants are 0.0100 mol L-1,NaOH takes to use up 0.00566 mol L-1 after 20 minutes of reaction.Calculate the rate constant k and half-life t1/2.)c1c1(t1k0AAmin15.3Lmol0.0100minLmol6.521kc1t111A2/103.Reaction Order and its CharacteristicsSolution:)Lmol0.01001Lmol0.00566Lmol0
37、.01001(min20111111minLmol6.52ktc1c10AAIII.Zero-order reactionkkcdtdc0AAcA t:the slope is-k(concentration time-1).k2ct0A2/1ktccAA03.Reaction Order and its Characteristics0AAcktcbxay3.Reaction Order and its Characteristics4.Collision and Transition-State TheoryThe rate of reaction depends on temperatu
38、re:25 C:116sLmol104.9k35 C:115sLmol101.5kCl(g)NOCl(g)ClNO(g)2Nitrosyl chloride()2ClNOcck 4.Collision and Transition-State TheoryI.Collision theoryof reaction rates is a theory assumes that,for a reaction to occur,reactant molecules must collide with an than some minimum value and with the.Cl(g)NOCl(
39、g)ClNO(g)24.Collision and Transition-State TheoryThe minimum energy required for a collision to react is called the activation energy(,Ea).Unit:kJ mol-1.The value of Ea depends on the particular reaction.II.Activation energyActivated molecules()Fraction of molecules aEKinetic energy4.Collision and T
40、ransition-State TheoryIn collision theory,the rate constant for a reaction is given as a product of three factors:(1)Z,the collision frequency;(2)f,the fraction of activated molecules;(3)p,the fraction of collision that occur with the reactant molecules properly oriented.1.Z(collision frequency,):As
41、 the temperature rises,the gas molecules move faster and therefore collide more frequently.At 25 C,a 10 C rise in temperature increases the collision frequency by about 2%.p fZk4.Collision and Transition-State Theory2.f(fraction of activated molecules):f changes rapidly in most reactions with even s
42、mall temperature changes./RTEaef Fraction of moleculesKinetic energyEaLow temperatureHigh temperature4.Collision and Transition-State Theory25 Cf=1.2 10-15;35 Cf=3.8 10-15;Cl(g)NOCl(g)ClNO(g)21amolkJ85EFor the reaction:3.p(fraction of molecules properly oriented):p is independent of temperature chan
43、ges.Steric effect in collisionsa.No reaction.b.No reaction.c.Reaction takes place.4.Collision and Transition-State TheoryActivation energy and reaction rateactivated moleculesFraction of molecules aEKinetic energy?aE/RTEaefReactions with large Ea occur slowly;Reactions with small Ea go fast.1amolkJ2
44、5060:E4.Collision and Transition-State TheoryIII.Transition-state theory()explains the reaction resulting from the collision of two molecules in terms of an activated complex.An activated complex()(transition state)is an unstable grouping of atoms that can break up to form products.Cl(g)NOCl(g)ClNO(
45、g)2ClClNOClClNOClClNOClClNOClNO2reactantsactivated complexproducts4.Collision and Transition-State TheoryClNOClNOClClNO22The difference in energy between the activated complex and the reactant molecules is the activation energy for the forward reaction.4.Collision and Transition-State TheoryThe diff
46、erence in energy between the products and the reactants equals the heat of reaction.4.Collision and Transition-State TheoryCC CCHHHHHHHHC CHHHH2cyclobutane()ethylene()Ahmed H.ZewailNobel prizein Chemistry 19994.Collision and Transition-State TheoryThe total time of the reaction was 700 fs(10-15 s).F
47、emtochemistry()Two possible potential-energy curves for the decomposition of cyclobutane to ethylene.4.Collision and Transition-State Theorya.Which reaction has a higher activation energy for the forward reaction?b.If both reactions were run at the same temperature and have the same orientation requ
48、irements to react,which one would have the higher rate constant?c.Are these reactions exothermic or endothermic?Example 7-4:Considering the following potential energy curves for two different reactions:5.Effect of Temperature on Reaction RateI.Effect of temperature on reaction rateThe change in rate
49、 constant with temperature varies considerably from one reaction to another.In many cases,the rate of reaction approximately doubles for a 10 C rise,and this is often given as an approximate rule(vant Hoff).III.Arrhenius equationRTEaAekThis equation expresses the dependence of the rate constant on t
50、emperature,and is called the.5.Effect of Temperature on Reaction RateS.Arrhenius(1859-1927)SwedenNobel prize(Chemistry,1903)Ea is the(experimental)activation energy;A(assumed to be a constant)is called the frequency constant or preexponential factor.It is related to the frequency of collision with p
