《基礎化學》英文教學課件：chapter-7

1、Chapter 7 Rate of Reaction 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 Rat

2、e and Its ExpressionsChemical kinetics (化學動力學):Does this reaction proceed in a commercially feasible length of time? How is the rate of a reaction like this measured?What conditions affect the rate of a reaction?How do you express the relationship of rate of a reaction to the variables that affect r

3、ate?What happens at the molecular level when a reaction occurs?1. Reaction Rate and Its ExpressionsI. Rate of reaction (化學反應速率):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 t

4、he 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:Na2S2O3 + Na3AsO3 As2S3Orpiment, 雌黃As2S3 雞冠石Realgar, 雄黃As4S4Arsenic trioxide ，砒霜As2O31. Reaction Rate and Its Expressio

5、ns2. Concentrations of catalysts (催化劑):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 Expres

6、sionsReaction rate is the increase in concentration of product of a reaction per unit time or the decrease in concentration of reactant per unit time.Unit: molL-1s-1.The rate of this reaction has been extensively studied under various conditions.Rate of formation of oxygen:1. Reaction Rate and Its E

7、xpressions1. 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 a positive value for the rate.Rate of consumption of N2O5:1. Reaction Rate and Its ExpressionsII. Average reaction rate:orAverage ra

8、te over the time interval tIII. Instantaneous reaction rate (瞬時反應速率):t0t0Whenever we speak of a reaction rate, we shall always mean an instantaneous rate.1. Reaction Rate and Its ExpressionsIV. Experimental determination of rateWithdraw samples at various times and analyze them.1. Reaction Rate and

9、Its Expressions+Ethyl acetateEthanolAcetic acid2. Continuously follow the progress of a reaction by observing the change in some physical property such as color.The hypoiodite ion, IO-, absorbs blue light near 400 nm.1. Reaction Rate and Its Expressions3. Continuously follow the progress of a reacti

10、on by observing the change in pressure.N2O5 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(紙色譜)Chromatography(色譜) is a group of similar

11、 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(柱色譜)(A) A solution containing substances to be separated is poured into thetop of a column, which contains

12、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 hexane mixture, showing

13、 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 infrared radiation

14、. 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傅立葉變換紅外（FTIR ）光譜儀the IR spectrum can act as a compounds “fingerprint(指紋).”nuclea

15、r 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 group.2. Reaction Mech

16、anism and Elementary reactionI. Elementary and complex reaction: Elementary reaction (基元反應): reactions that occur by one step to give products.Complex reaction (復合反應): 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) = 2

17、HI(g)CO(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 reaction mechanism (反應機理).2. Reaction Mechanism and Elementary reaction(1) I2(g) 2I(g) (fast, equilibri

18、um )(2) H2(g) + 2I(g) = 2HI(g) (slow, rate-determining step)H2(g) + I2(g) = 2HI(g)(控速步驟)(overall reaction )II. Molecularity of reaction (反應分子數(shù)):Unimolecular reaction: An elementary reaction that involves one reactant molecule.Bimolecular reaction: two reactant molecules Termolecular reaction: three

19、reactant molecules 2. Reaction Mechanism and Elementary reactionThe molecularity is the number of molecules on the reactant side of an elementary reaction.2. Reaction Mechanism and Elementary reactionExample 7-1: The mechanism for the gas-phase chlorination of CHCl3 with Cl2 to produce CCl4 is:What

20、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 unimolecular; the reverse of the first step, the second step, and the third step are each bimolecular.III. Law o

21、f mass action and rate equationFor an elementary reaction at a specified temperature, the rate is proportional to the product of the concentration of each reactant molecule. 1. Law of mass action (質(zhì)量作用定律): Elementary reaction:aA + bB = productsRate equation (速率方程): An equation that relates the rate

22、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 meaning?The unit of

23、 k depends on the form of the rate equation.2. Reaction Mechanism and Elementary reaction2. Notes:1. Law of mass action is permissible only for one-step elementary reactions.aA + bB = dD + eEElementary reaction:Complex reaction:The exponents m and n (frequently integers) must be determined experimen

24、tally.2. Reaction Mechanism and Elementary reaction2. Concentration of a pure solid or liquid is not included in the rate equation.3. Concentration of a reactive solvent in diluted solutions is not included in the rate equation.sucrosefructoseglucose2. Reaction Mechanism and Elementary reactionDeter

25、mining the rate equation:2. Reaction Mechanism and Elementary reaction3. Reaction order:The reaction order (反應級數(shù)) with respect to a given reactant species equals the exponent (指數(shù)) of the concentration of that species in the rate equation, as determined experimentally.m is the reaction order with res

26、pect to the reactant A;n is the reaction order with respect to the reactant B.aA + bB = dD + eE2. 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: ze

27、ro-order reaction;m+n = 1: first-order reaction;m+n = 2: second-order reaction.aA + bB = dD + eE2. Reaction Mechanism and Elementary reactionThe reaction is second-order in A, first-order in B, and third-order overall.3. Reaction Order and its CharacteristicsA rate equation tells you how the rate of

28、 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.Change of concentration with time:Using calculus (微積分), we can transform a rate equation into a mathe

29、matical relationship between concentration and time.3. Reaction Order and its CharacteristicsI. First-order reaction (一級反應)c: concentration at time t;c0: initial concentration.3. Reaction Order and its Characteristicsln cA t: the slope is k (time-1).3. Reaction Order and its CharacteristicsHalf-life

30、 (半衰期, t1/2): The time it takes for the reactant concentration to decrease to one-half of its initial value.The half-life of a first-order reaction is only related to the rate constant, but independent of initial concentration.3. Reaction Order and its CharacteristicsExample 7-2: Soil was found to b

31、e 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

32、 20% of its initial value?Assume the initial radioactivity of Co-60 is R, then:3. Reaction Order and its CharacteristicsSolution:II. Second-order reaction1/cA t: the slope is k (concentration-1 time-1).3. Reaction Order and its Characteristics3. Reaction Order and its Characteristics3. Reaction Orde

33、r and its Characteristics3. Reaction Order and its CharacteristicsHalf-life (t1/2) of a second-order reaction:The half-life of a second-order reaction depends on initial concentration and becomes larger as time goes on.Example 7-3: Saponification of ethyl acetate at 298K is a second-order reaction:C

34、H3COOC2H5 + NaOH CH3COONa + C2H5OHThe initial 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 t1/2.3. Reaction Order and its CharacteristicsSolution:III. Zero-order reactioncA t: the sl

35、ope is -k (concentration time-1).3. Reaction Order and its CharacteristicsThe half-life of a zero-order reaction depends on initial concentration and becomes smaller as time goes on.3. Reaction Order and its CharacteristicsRelationships for zero-, first-, and second-order reactions4. Collision and T

36、ransition-State TheoryThe rate of reaction depends on temperature:25 C:35 C:Nitrosyl chloride(亞硝酰氯)4. Collision and Transition-State TheoryI. Collision theoryCollision theory (碰撞理論) of reaction rates is a theory assumes that, for a reaction to occur, reactant molecules must collide with an energy gr

37、eater than some minimum value and with the proper orientation.4. Collision and Transition-State TheoryThe minimum energy required for a collision to react is called the activation energy (活化能, Ea). Unit: kJ mol-1.The value of Ea depends on the particular reaction.II. Activation energyActivated molec

38、ules(活化分子)Fraction of moleculesKinetic energy4. Collision and Transition-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 w

39、ith the reactant molecules properly oriented.1. Z (collision frequency, 碰撞頻率): As 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%.4. Collision and Transition-State Theory2. f

40、 (fraction of activated molecules): f changes rapidly in most reactions with even small temperature changes.Fraction of moleculesKinetic energyEaLow temperatureHigh temperature4. Collision and Transition-State Theory25 Cf = 1.2 10-15;35 Cf = 3.8 10-15;For the reaction:3. p (fraction of molecules pro

41、perly oriented): p is independent of temperature changes.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 moleculesKinetic energyReactions with large Ea occur slo

42、wly; Reactions with small Ea go fast.4. Collision and Transition-State TheoryIII. Transition-state theory (過渡態(tài)理論)1. Transition-state theory explains the reaction resulting from the collision of two molecules in terms of an activated complex. An activated complex (活化絡合物) (transition state) is an unst

43、able grouping of atoms that can break up to form products.reactantsactivated complexproducts4. Collision and Transition-State Theory2. Potential-energy diagram (勢能圖) for reactionsThe difference in energy between the activated complex and the reactant molecules is the activation energy for the forwar

44、d reaction.4. Collision and Transition-State TheoryThe difference in energy between the products and the reactants equals the heat of reaction.4. Collision and Transition-State Theory2cyclobutane(環(huán)丁烷)ethylene(乙烯)Ahmed H. ZewailNobel prizein Chemistry 19994. Collision and Transition-State TheoryThe t

45、otal time of the reaction was 700 fs (10-15 s).Femtochemistry (飛秒化學)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

46、 the same temperature and have the same orientation requirements 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 Ra

47、teI. Effect of temperature on reaction rateThe change in rate 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 equationThis

48、 equation expresses the dependence of the rate constant on temperature, and is called the Arrhenius equation.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 freque

49、ncy constant or preexponential factor. It is related to the frequency of collision with proper orientation (pZ).5. Effect of Temperature on Reaction RateAssume:Rate constant k1 for T1; Rate constant k2 for T25. Effect of Temperature on Reaction RateExample 7-5: The rate constants of hydrolysis of CO

50、(CH2COOH)2 in aqueous solution are k283 = 1.08 10-4 s-1 at 283K, and k333 = 5.48 10-2 s-1 at 333K. Calculate the activation energy of the reaction Ea and the rate constant k303 at 303 K.5. Effect of Temperature on Reaction RateSolution:5. Effect of Temperature on Reaction RateArrhenius published thi

51、s equation in 1889 and suggested that molecules must be given enough energy to become “activated” before they could react. Collision and transition-state theories, which enlarged on this concept, were developed later (1920s and 1930s, respectively).Ea: Experimental activation energy (實驗活化能).5. Effec

52、t of Temperature on Reaction RateActivation energy: What is it ?6. Effect of Catalyst on Reaction RateThe synthesis of ammonia is exothermic, and although the reaction is faster at high temperatures, the equilibrium concentration of ammonia is much lower. In industry, high temperature also means hig

53、h energy costs, so accelerating reactions without resorting to high temperatures is a more economical procedure.I. Catalyst (催化劑)6. Effect of Catalyst on Reaction RateA catalyst is a substance that increases the rate of reaction without being consumed in the overall reaction.6. Effect of Catalyst on

54、 Reaction RateII. Characteristics of a catalyst1. A catalyst must participate in at least one step of a reaction and be generated in a later step.2. Catalysts are often quite specific (專一)-they increase the rate of certain reactions, but not others.6. Effect of Catalyst on Reaction Rate3. A catalyst

55、 has no effect on the equilibrium composition of a reaction mixture. A catalyst merely speeds up the attainment of equilibrium.A catalyst is useful for a reaction that is normally slow but has a large equilibrium constant. However, if the reaction has an exceedingly small equilibrium constant, a cat

56、alyst is of little help.6. Effect of Catalyst on Reaction Rate4. Catalysts provide an alternative reaction pathway with a lower activation energy.6. Effect of Catalyst on Reaction Rate Almost all enzymes are protein molecules; An enzyme has enormous catalytic activity; Enzymes are highly specific.II

57、I. Enzymatic catalysisEnzymes(酶) are living catalysts of biological organisms.The substance whose reaction the enzyme catalyzes is called the substrate (底物).6. Effect of Catalyst on Reaction RateEnzyme action (lock-and-key model). The enzyme (E) has an active site to which the substrate (S) binds to

58、 form an enzyme-substrate complex (ES). The active site of the enzyme acts like a lock into which the substrate (key) fits. While bound to the enzyme, the substrate may have bonds weakened or new bonds formed to yield the products (P), which leave the enzyme.E + S ES E + PSummaryThe reaction rate is defined as the increase of product concentration per unit time (or the decrease of reactant concentration per unit time). It is found that reaction rates are proportional to concentrations of reactants raised to various power (usually 1 or 2, but