7_Modul 7_Reaksi rantai Kinetika Kimia.pptx

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1. Chain Reactions (Reaksi Rantai) Dr. Mohammad Masykuri, M.Si. Chemistry Education Study Program Teacher Training and Education Faculty Sebelas Maret Universit y ( UNS ) Website: http://masykuri.staf.fkip.uns.ac.id, email: [email protected] Kinetika Kimia

2. 2 Chain Reactions & Free-Radical Polymerizations A series of steps: a reactive intermediate is consumed Chain reaction Reactants are converted to products, Cycle is repeated , The intermediate is regenerated The structure of chain reactions Chain carriers: the intermediates responsible for the propagation of a chain reaction Radical chain carriers: the chain carriers are radicals Radicals: Species with unpaired electrons Ions, neutrons (in nuclear fission)

3. 3 Chain Reactions & Free-Radical Polymerizations A chain reaction leads to a complicated rate law. Derive the rate law for the formation of HBr according to the mechanism given below.

4. 4 The following radical chain mechanism has been proposed: (a) Initiation: (b) Propagation: (c) Retardation: (d) Termination: The third body M removes the energy of recombination; the constant concentration of M has been absorbed into the rate constant. Chain Reactions & Free-Radical Polymerizations

5. Chain Reactions & Free-Radical Polymerizations

6. 6 The net rates of the formation of the two intermediates are Chain Reactions & Free-Radical Polymerizations

7. 7 The steady-state concentrations of the intermediates are Chain Reactions & Free-Radical Polymerizations

8. 8 Branching chain reaction The chain produces more chain carriers than it consumes Combustion of hydrogen Chain-branching steps Initiation A highly exothermic reaction that is not a chain reaction may lead to an explosion! Chain Reactions & Free-Radical Polymerizations

9. 9 Explosion A chain-branching explosion may occur when there are chain-branching steps in a reaction, for then the number of chain carriers grows exponentially and the rate of reaction may cascade into an explosion. The reaction between hydrogen and oxygen Although the net reaction is very simple, the mechanism is very complex and has not yet been fully elucidated. Involving a chain reaction The chain carriers:  H,  O,  OH, and O 2 H Chain Reactions & Free-Radical Polymerizations

10. 10 Initiation: Propagation: A chain-branching explosion The occurrence of an explosion depends on the temperature and pressure of the system Chain Reactions & Free-Radical Polymerizations

11. 11 At very low pressure, the system is outside the explosion region and the mixture reacts smoothly. Increasing the pressure (along the vertical line) takes the system through the first explosion limit (if T > 730K). The mixture then explodes. The reaction is smooth when the pressure is above the second explosion limit. The explosion limits of the H 2 +O 2 reaction Thermal explosion Steady reaction explosion 600 700 800 900 4 3 2 5 6 lg ( P /Pa) T /K Upper limit Lower limit The pressure is increased to above the third explosion limit, a thermal explosion occurs. Chain Reactions & Free-Radical Polymerizations

12. 12 Rice-Herzfeld Mechanisms Simple rate laws can follow from quite complex chain mechanisms. (a) Initiation: (b) Propagation: (c) Retardation: (d) Termination: The Rice-Herzfeld mechanism for the pyrolysis of acetaldehyde is A chain reaction can lead to a simple rate law. Pyrolysis of acetaldehyde

13. 13 Rice-Herzfeld Mechanisms The net rates of the formation of the two intermediates are The sum of the two equation is

14. 14 The rate of formation of CH 4 is Rice-Herzfeld Mechanisms in agreement with the three-halves order observed experimentally. However, the true mechanism is more complicated than R-H mechanism. Other products (acetone, CH 3 COCH 3 , and propanaldehyde, CH 3 CH 2 CHO) can be formed.

15. 15 Free-Radical Polymerizations Let I and M stand for the initiator and monomer Chain polymerization Results in the rapid growth of an individual polymer chain for each activated monomer, and often occurs by a radical chain process. (a) Initiation (b) Propagation (c) Termination

16. 16 Free-Radical Polymerizations (a) Initiation (b) Propagation (fast) The rate-determining step is the formation of the radicals R  . The chain of reactions propagates quickly, f is the yield of the initiation step, the fraction of radicals that R  successfully initiate a chain. (17.99)

17. 17 Free-Radical Polymerizations (c) Termination Assume that the rate of termination is independent of the length of the chain, the rate of change of radical concentration by this process is The total radical concentration is approximately constant throughout the main part of the polymerization. (the rate at which radicals are formed by initiation  the rate at which they are removed by termination) (17.101)

18. 18 Free-Radical Polymerizations Applying the steady-state approximation The steady-state concentration of radical chains The rate of propagation of the chains (the monomer is consumed) (17.102) (17.103) The rate of polymerization is proportional to the square root of the initiator concentration.

19. 19 Free-Radical Polymerizations The degree of polymerization (DP) The number of monomers in the polymer (17.104) (17.105) (17.103) for termination by combination (17.104)

20. Sekian

21. Test II: Steady state dan Equilibrium Approx. Rabu, 6 Nov 2012 Open books/laptop