Chapter X Kinetics of Complex Reactions

1. Chapter X Kinetics of Complex Reactions §10.1 Typical complex reactions Levine: p.559 17.9

2. Complex reactions: reaction contains more than one elementary reaction Typical complex reactions 1) Opposing Reaction : 2) Parallel Reaction: 3) Consecutive Reaction:

3. 1.1 Opposing Reaction / reversible reaction majority of the reactions are reversible, i.e., the forwardand the backward / reverse reaction take place simultaneously. (1) kinetic equilibrium constant for opposing reaction consisting of elementary reactions: As reaction proceeds, r+ increases while r decreases. When r+ becomes equal to r, equilibrium is reached.

4. therefore In this way we arrive at a very important connection between the equilibrium constant and the rate coefficients of simple reactions. This relation, named as kinetic equilibrium constant, is correct only for elementaryreactions.

5. (2) rate equation For first-first order opposing reaction: The total rate is Under equilibrium conditions

6. which suggests that k+ and k can be determined by measuring x at t and at equilibrium concentration. Relaxation method. Similar to the rate equation of first-order reaction 1-2 opposing reaction 2-2 opposing reaction Principle of relaxation method for studying fast reaction

7. 1.2 Parallel reaction / Competing reaction When When The rate of parallel reaction is determined mainly by the faster one.

8. a t Integration of the equation yields: For production of B and C: x = y + z

9. c A B C t The composition of the final products is fixed. selectivity of the reaction.

10. logk log k logA1 logA1 B logA2 logA2 C C B 1/T 1/T Optimum temperature for better selectivity Example

11. Using catalyst to better selectivity The selectivity of the parallel reaction can be improved by adoption of appropriate catalyst.

12. Main reaction and Side reaction: reaction with higher k is taken as the main reaction, while others side reactions. Reaction that produces the demanded product is the main reaction. Selectivity:

13. CH4 + Cl2 CH3Cl CH2Cl2 CCl4 CHCl3 1.3 Consecutive reaction Some reactions proceed through the formation of intermediate. General reaction a = x + y + z

14. C C A B tmax t

15. C C A B tmax t shows that the intermediate’s concentration rises from zero to a maximum and then drops back to zero as A is depleted and C dominates in the mixture. If C is the demanded product, the reaction time should be prolonged. If B is the demanded product, the reaction should be interrupted at optimum time, i.e.,tmax. At tmax, the concentration of B = ?

17. y k1/k2 0 t When k2 >> k1, ymax would be very small, and the tmax would be very short.

18. Physical meaning of k2 >> k1 B is a active intermediate (Such as active atom: Cl, H, etc., radicals: CH3•, H2C:, C+, C-, etc., activated molecules: A*), it is difficult to form but easy to decompose to product. For consecutive reaction with large k2/k1 ratio, once the reaction take place, the active intermediate (B) rapidly attains its maximum concentration and its concentration keeps nearly unchanged during the whole reaction. Steady-state approximation

19. When k2 >> k1 The total rate is determined mainly by k1 When k2 << k1 The total rate is determined mainly by k2 The rate of the overall consecutive reaction depends only on the smaller rate constant (rate-determining step).

20. patient ! ?? !!   It’s a r.d.s  rate-determining step (r. d. s.): the step with the slowest rate.

21. Rate-determining step approximation The rate of the elementary step with the lowest rate constant, i.e., r.d.s., can be used to express the actual rate of the overall reaction. What is a eligible r. d. s.? Its activation energy should be 10 kJmol-1 more than that of other steps.