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Chapter Five Rates of Chemical Reaction

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  1. Chapter Five Rates of Chemical Reaction Weather can occur ? ---chemical thermodynamics How fast (reaction rate)? ---chemical kinetics Chemical Reaction

  2. 化学反应速度的快慢——主要决定于反应的内在机理。化学反应速度的快慢——主要决定于反应的内在机理。 反应机理:化学反应所经历的途径或具体步骤, 又称为反应历程。 化学动力学的基本任务就是研究反应的机理。

  3. 反应机理可以告诉我们: • 某一反应究竟是经过哪些步骤完成的, • 了解各个步骤的特征和相互联系, • 揭示化学反应速度的本质,使人们能够自觉控制反应速度。

  4. 问题 1. 为什么有些口服药物的服用方法是2次/天? 3次/天? 2. 为什么静脉滴注某些青霉素类药物要加快滴注的速度?

  5. 二、酶作为生物催化剂的特性:

  6. 2. )

  7. Chapter FiveRates of Chemical Reaction 5-1 Rates and Mechanisms of Chemical Reactions 5-2 Theories of Reaction Rate 5-3 Reaction Rates and Concentrations 5-4 Effect of Temperature on Reaction Rates 5-5 Effect of Catalyst on Reaction Rates

  8. 5-1 Rates and Mechanisms of Chemical Reactions Reaction rate: changes in a concentration of a product or areactant per unit time. The rate is defined to be a positive number. The unit of reaction rate is mol·L-1·S-1, mol·L-1·min-1 or mol·L-1·h-1 et al

  9. Define reaction rate : 1. Average reaction rate 2N2O5 = 4NO2 + O2 average reaction rate is obtained by dividing the change in concentration of a reactant or product by the time interval over which the change occurs

  10. SAMPLE EXERCISE :H2O2 = H2O + 1/2 O2 0 〃: c1(H2O2) = 15.88×10-3 mol/L 5〃 : c2(H2O2) = 12.8×10-3 mol/L So:

  11. Reaction Rates and Stoichiometry H2O2 = H2O + 1/2 O2 用不同物质浓度变化所表示的反应速率之间存在着一定的关系是:它们之间的速率比正好等于反应式中各物质分子式的系数之比,

  12. aA + bB cC + dD • To generalize, for the reaction This equation can be used to establish the relationship between rate of change of one reactant or product to another reactant or product. • You have to be able to do this on the test, too!

  13. - Δc dc V = lim —————— = -———— Δt dt Δt0 Define reaction rate : 2. Instantaneous reaction rate 2N2O5 = 4NO2 + O2 The instantaneous rate of change at a point is the same as the slope of the tangent line. That is, it's the slope of a curve. c c t v t

  14. 5-1.2 The Mechanisms of Chemical Reactions • A reaction mechanism is a description of the path that a reaction takes. • elementary reaction? • overall reaction? • types of elementary reactions

  15. 例如: 5-1.2 The Mechanisms of Chemical Reactions elementary reaction: is one that the reactants can convert directly into the products in a single step when they act each other. (one-step reaction) 基元反应:反应物间相互作用直接转化为生成物的反应。 (简称元反应)。

  16. overall reaction:Many reactions are actually made up ofseveral elementary steps,which are combined to yield the overall reaction. NO2 + CO → NO + CO2 NO2 + NO2 → NO3 + NO (slow)NO3 + CO → NO2 + CO2 (fast) This means that the rate of the overall reaction is dominated(控制)by the rate of the first reaction, this is the rate-determining step.

  17. H2(g) + I2(g)  2 HI(g) I2(g) → 2I (fast) H2+ 2I → 2HI (slow) ——rate-determining step.

  18. Types of Elementary Reactions • molecularity --for Elementary Reactions: • unimolecular reaction: an elementary reaction in which the rearrangement of a single molecule produces one or more molecules of product. I2(g)─→ 2I(g) • bimolecular reaction: the collision and combination of two reactants to give an activated complex in an elementary reaction. NO(g) + O3(g) ─→ NO2(g) + O2(g)

  19. _termolecular reaction: an elementary reaction involving the simultaneous collision of any combination of three molecules, ions, or atoms. 2 NO + H2 ─→ N2 + H2O2

  20. 5-2 Theories of Reaction Rate • One is the collision theory : The collision theory is based on the kinetic theory and assumes a collision between reactants before a reaction can take place. 1918年 Lewis以气体分子运动论为基础提出 • Another is the transition state theory: The transition state theory suggests that as reactant molecules approach each other closely they are momentarily in a less stable state than either the reactants or the products.

  21. 5-2.1 Collision Theory and Activation Energy ●Contents of Collision Theory: ⑴ reacting molecules must come so close that they collide. ⑵"effective" collisions : not every collision between molecules creates products, only few collisions between reactant molecules will react.

  22. ●Contents of Collision Theory: • According to this theory, product formation can only take place when there are "effective" collisions between reactant molecules involved in the rate determining step of the process.

  23. Br(g) + HI(g) ─→ HBr(g) + I(g) • Straight on collision, hydrogen facing incoming bromine. Reaction occurs. • Straight on collision, hydrogen facing away from incoming bromine. Reaction does not occur. • Straight on collision, bromine at 90 degrees. Reaction does not occur.

  24. What constitutes an effective collision? 发生有效碰撞的两个基本前提: ● enough energy

  25. ● proper orientation 对 HCl 和 NH3的气相反应

  26. Activation molecule: is the molecule have enough energy and can produce effective collision • the average molecules must absorb some energyto become activation molecules

  27. 活化分子一般只占极少数,它具有的最低能量为Ec 。通常把活化分子具有的平均能量与反应物分子的平均能量之差称为反应的活化能,用符号 Ea 表示。

  28. 在一定温度下,反应的活化能越大,活化分子的分子分数越小,活化分子越少,有效碰撞次数就越少,因此化学反应速率越慢;在一定温度下,反应的活化能越大,活化分子的分子分数越小,活化分子越少,有效碰撞次数就越少,因此化学反应速率越慢; 反应的活化能越小,活化分子的分子分数越大,活化分子越多,有效碰撞次数就越多,化学反应速率越快。

  29. Figure:As the activation energy of a reaction decreases, the number of molecules with at least this much energy increases, as shown by the yellow shaded areas.

  30. 每一反应的活化能数值各异,可以通过实验和计算得 到 。活化能越小, 反应速率越快。Ea是动力学参数。 ▲ 一些反应的Ea 2SO2(g) + O2(g) = 2SO3(g), Ea=251 kJ·mol-1 N2(g) + 3H2(g) = 2NH3(g), Ea=175.5 kJ·mol-1 HCl + NaOH → NaCl+H2O, Ea≈20 kJ·mol-1 ▲ 离子反应和沉淀反应的Ea都很小 一般认为Ea小于63 kJ·mol-1的为快速反应 小于40 kJ·mol-1和大于400 kJ·mol-1的都很难测定出

  31. ★ 反应物的能量必 须爬过一个能垒 才能转化为产物 ★ 即使是放热反应 (△rH为负值), 外界仍必须提供 最低限度的能量, 这个能量就是 反应的活化能

  32. 5-2.2 The Transition State Theory Transition state theory (TST) is also called activated complex theory. reactants pass through high-energy transition states before forming products, they are associated in an unstable entity called an activated complex, then change into products. 要点: 化学键重排、 活化络合物形成

  33. Example :NO2(g) + CO(g) ─→ NO(g) + CO2(g) ΔH = Ea,f - Ea,r =358 kJ·mol-1 -132 kJ·mol-1 ΔH = –226kJ·mol-1.

  34. 理论受限 活化络合物的结构无法在实验中加以确定 计算过于复杂 酶作用的机制——中间产物学说 • 酶与底物形成酶-底物中间复合物, • 中间复合物再分解成产物和酶。 • E + S = E-S  P + E • 许多实验事实证明了E-S复合物的存在。

  35. Factors That Affect Reaction Rates • Concentration of Reactants • Temperature • Catalysts ___Speed by changing mechanism

  36. 5-3 Reaction Rates and Concentrations Chemical reactions are faster when the concentrations of the reactants are increased . Because more molecules will exist in a given volume. More collisions will occur and the rate ofa reaction will increase.

  37. Concentration and Rate Each reaction has its own equation that gives its rate as a function of reactant concentrations. this is called its Rate Law A rate law shows the relationship between the reaction rate and the concentrations of reactants. 说明在一定温度下,反应速度与反应物浓度之间的定量关系 ——质量作用定律

  38. 5-3.1 The Rate Law The rate of a reaction is proportional to the product of the concentrations of the reactants raised to somepower. aA + bB → cC + dD v∝[A]m[B]n v = k[A]m[B]n 反应速率方程或称质量作用定律

  39. 参见中文P101 v = k[A]m[B]n • kis a rate constant that has a specific value • for each reaction.Thevalue of k is • determined experimentally. • “Constant” is relative here- • k changes with T, • the unit(量纲) depend on m + n • ① when [A]=[B]=1mol·L-1, v =k • ② the greater the k , the faster the rate

  40. 质量作用定律(law of mass action) 对于基元反应,反应速率与反应物浓度的幂乘积成正比。幂指数就是基元反应方程中各反应物的系数,这就是质量作用定律。

  41. 质量作用定律仅适用于基元反应。 2N2O5(g)→4NO2(g)+ O2 (g) 对于复杂反应, 它适用于每一步的基元反应,它的反应速度取决于定速步骤。 N2O5 → NO3+NO2(慢) NO2+NO3 → NO2+O2+NO (快) NO+NO3 → 2NO2 (快) 实验证明 速率方程应为

  42. v = k[A]m[B]n aA + bB → cC + dD [A], [B] are the concentration of A and B; mand nare themselves constants for a given reaction, it must be determined experimentally in general, m and n are not equal to the stoichiometric coefficientsa and b

  43. 5-3.2 Order of A Reaction The order of a reaction with respect toone of the reactantsis equal to the power to which the concentration of that reactant is raised in the rate equation. The sum of the powers to which all reactant concentrations appearing in the rate law are raised is called theoverall reaction order.

  44. For rate equationv = k[A]m[B]n mis the order of the reaction with respect to A,nis the order of the reaction with respect to B. m+n isoverall order of the reaction the exponents m and n are not necessarily related to the stoichiometric coefficients in the balanced equation, that is, in general it is not true that for a A + b B → c C + d D, m ≠ a and n ≠ b

  45. The rate law for the thermal decomposition(热分解)of acetaldehyde (CH3CHO) CH3CHO(g) →CH4(g) + CO(g)has been determined experimentally to bev =k[CH3CHO]3/2and not rate = k[CH3CHO] ×

  46. Example 5-1: Given the following data, what is the rate expression for the reaction between hydroxide ion and chlorine dioxide? 2ClO2(aq) + 2OH-(aq) →ClO3-(aq) + ClO2-(aq) + H2O [ClO2] (mol.L-1) [OH-] (mol.L-1) Rate (mol.L-1 s-1) 0.010 0.030 6.00×10-4 v1 0.010 0.075 1.50×10-3 v2 0.055 0.030 1.82×10-2 v3

  47. Solution: v = k[ClO2]m[OH-]n m=? By inspection, m = 2. The reaction is 2nd order in ClO2

  48. v = k[ClO2]m[OH-]n n=? By inspection, n = 1 • The overall rate expression is therefore • v = k[ClO2]2[OH-]

  49. 想一想: 反应速度与速率方程? 反应分子数和反应级数?

  50. First-order reactions A first-order reactionis a reaction whose rate depends on the reactant concentration raised to the first power. A →B the rate equation is Differential form: