基础化学-chapter_7_1

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 rate At the end of the session, you

2、are expected to 1. define reaction rate and its expression;2. distinguish between elementary reaction and complex reaction;3. write the rate equation for a reaction;4. define the half-life of a reaction;5. formulate concentration-time relationship for different reaction orders.Learning Objectives1.

3、Reaction Rate and Its ExpressionsChemical 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 react

4、ion to the variables that affect rate?4. What 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 growt

5、h of corrosion on the prow of the Titanic on the 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:0s20s40s1. Reaction Rate and Its Expressions2. Concentrations of catalysts ():(g)OO(l)H2

6、(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 increas

7、e 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 Expr

8、essionsTime (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

9、to give 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

10、 rate ():dtdc(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 ExpressionsCH3CH2OCCH

11、3OH2OH+CH3CH2OHHOCCH3O+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

12、follow the progress of 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.2. Reaction Mechanism and El

13、ementary 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)CO(g) + H2O(g)

14、= 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(elementarCONO

15、CONO223(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

16、: three reactant molecules 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 chlori

17、nation of CHCl3 with Cl2 to 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

18、 the first step, the second step, 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

19、:aA + bB = productsRate equation (): An equation 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 t

20、he nature of reaction and the temperature. Physical meaning ? 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 o

21、f mass action is permissible only for one-step elementary 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. Concentratio

22、n of a pure solid or liquid is not included in the rate equation.(g)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

23、 rate equation:2. Reaction Mechanism and Elementary reaction?B?Ackc B?Ackc 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

24、reactant A;n is the reaction order with respect to the reactant B.aA + bB = dD + 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-or

25、der reaction;m+n = 1: first-order reaction;m+n = 2: second-order reaction.aA + bB = dD + eEnBmAckc2. 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 th

26、e rate of a reaction depends on reactant concentrations at a particular moment. But often you would like 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

27、 concentration and time.nBmAckctcA3. Reaction Order and its CharacteristicsI. First-order reaction ()AAkcdtdct0ccAAdtkcdc0productsaA dtkcdcAAc: concentration at time t;c0: initial concentration.3. Reaction Order and its Characteristicsln cA t: the slope is k (time-1).ktcclnAA0ktlnclnc0AAktAAecc00AAl

28、nctklncbxay(g)O(g)NO4(g)ON2225252ONkc3. Reaction Order and its Characteristics: The time it takes for the reactant concentration to decrease to one-half of its initial value.2ln2/cclnkt00AA2/1k0.693t2/1ktcclnAA03. Reaction Order and its Characteristics2252ONO4ON252ONkck0.693t2/ 114s104.800.693s101.4

29、43Example 7-2: Soil was found to be contaminated with radioactive Co-60, which has a half-life of 5.26 years (radioactive 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 radioactivity of Co-60 is R, then:ta0.132R20%Rln13. Reaction Order and its Characteristicsa12.2t Solution:II. Second-order reaction2AAkcdtdc1/cA t: the slop