1 Reaction Kinetics (1) Xuan Cheng Xiamen University Physical Chemistry.

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1 1 Reaction Kinetics (1) Xuan Cheng Xiamen University Physical Chemistry

2 2 Reaction Kinetics For example -16.63 -237.19 Thermodynamics : spontaneous reactions (possible to take place) Thermodynamics: can’t answer (1) how to make them happen (2) how fast they will take place (3) the reaction mechanism Reaction Kinetics

3 3 Physical Chemistry Reaction Kinetics T, P, catalyst T, catalyst Chemical Kinetics: Chemical Kinetics The study of the rates and mechanisms of chemical reactions A reacting system is not in equilibrium Reaction Kinetics

4 4 Physical Chemistry Rate of Reaction Consider the homogeneous reaction (occurs entirely in one phase) (17.1) The rate of conversion J (17.2) At equilibrium, J = 0 Reaction Kinetics

5 5 Physical Chemistry Rate of Reaction (17.3) (17.4)* At constant V The rate of reaction r mol dm -3 s -1 mol cm -3 s -1 Reaction Kinetics

6 6 Physical Chemistry Rate Laws For many reactions (17.5)* k: rate constant (rate coefficient), k = f (T, P) , ,  : order or partial order  +  +  +  n : overall order The expression for r as a function of concentrations at a fixed temperature is called the rate law. Reaction Kinetics

7 7 Physical Chemistry Reaction Orders Zero-order First-order Second-order Third-order Negative First-order 1.5-order No simple order Reaction Kinetics

8 8 Physical Chemistry Rate Laws ReactionRate Law Reaction Kinetics

9 9 Physical Chemistry Reaction Mechanisms (17.7) intermediate (17.8) Step (a) Step (b) Step (c) Elementary reaction Reaction actually occurs Reaction Kinetics

10 10 Physical Chemistry Measurement of Reaction Rates Chemical Method The concentration of a reactant or product as a function of time Reaction vessels At constant T At intervals Slows down or stop the reaction Rapidly analyze chemical compositions of the mixture Cooling the sample removing a catalyst Diluting the mixture Adding a species Reaction Kinetics

11 11 Physical Chemistry 反应速率的测定方法 化学方法 不同时刻取出一定量反应物，设法用骤 冷、冲稀、加阻化剂、除去催化剂等方法使反 应立即停止，然后进行化学分析。 Reaction Kinetics

12 12 Physical Chemistry Measurement of Reaction Rates Physical Method Measures a physical property of the reaction system as a function of time. This allows the reaction to be followed continuously as it proceeds. More accurate and less tedious 物理方法 用各种物理性质测定方法（旋光、折射率、 电导率、电动势、粘度等）或现代谱仪（ IR 、 UV-VIS 、 ESR 、 NMR 、 ESCA 等）监测与浓度 有定量关系的物理量的变化，从而求得浓度变 化。物理方法有可能做原位反应。 Reaction Kinetics

13 13 Physical Chemistry Integration of Rate Laws Assumptions (a) The reaction is carried out at constant temperature (k is constant) (b) The volume is constant (r is given by (17.4)) (c) The reaction is “irreversible” (k o is very large) (17.4)* Integrate it to find [A] as a function of time: [A] = g(t) Reaction Kinetics

14 14 Physical Chemistry First-Order Reactions Suppose the reactionis first-order with (17.4)* (17.5)* (17.10) define (17.11) Reaction Kinetics

15 15 Physical Chemistry First-Order Reactions (17.12) (17.13) if at and (17.14)* The concentration of A (and r) decreases exponentially with time for a first-order reaction. (17.10) Integrated rate law Reaction Kinetics

16 16 First-order reaction: ln[A] vs t First-Order Reactions Physical Chemistry (17.14)* [A]/[A] o t First-order reaction: [A]/[A] o vs t Ln[A] t Reaction Kinetics

17 17 First-Order Reactions Physical Chemistry Half-life: (17.13) First-order reaction(17.15)* The time needed for [A] to drop to half its value Independent of [A] o A useful indication for the chemical reaction rate Reaction Kinetics

18 18 Second-Order Reactions Physical Chemistry (17.10) define(17.11) Reaction Kinetics

19 19 Second-Order Reactions Physical Chemistry (17.16) A plot of 1/[A] versus t gives a strait line of slope k A if (17.17) Reaction Kinetics

20 20 {[A] -1 } {t}{t} Second-order reaction: 1/[A] vs t Second-Order Reactions Physical Chemistry (17.16) Half-life: Second-order reaction with r = k[A] 2 Reaction Kinetics

21 21 Second-Order Reactions Physical Chemistry Second-order reaction with r = k[A] 2 First-order reaction with r = k[A] (17.18) Three variables Reaction Kinetics

22 22 Second-Order Reactions Physical Chemistry x=0 when t=0 Reaction Kinetics

23 23 Second-Order Reactions Physical Chemistry (17.22)’ Reaction Kinetics

24 24 Second-Order Reactions Physical Chemistry If [A] o =[B] o (17.23)’ Half-life: Reaction Kinetics

25 25 Third-Order Reactions Physical Chemistry (Problems 17.17 and 17.24) (17.24) Reaction Kinetics

26 26 nth-Order Reaction Physical Chemistry Consider (17.25) Integration gives (17.26) For n  1 (17.27) For n  1 (17.28) Reaction Kinetics

27 27 nth-Order Reaction Physical Chemistry (17.29) For n  1 For n = 1(17.30) Half-life: (17.26) For n = 0 Reaction Kinetics

28 28 Homework Physical Chemistry Page 591 Prob. 17.5 Prob. 17.6 Prob. 17.8 Prob. 17.10 Reaction Kinetics