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Chapter VIII Phenomenological chemical kinetics (empirical/classical) -- experimental aspects

Chapter VIII Phenomenological chemical kinetics (empirical/classical) -- experimental aspects. 8.6 6 determination of the reaction order. r = k [A]  [B]  [C] .

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Chapter VIII Phenomenological chemical kinetics (empirical/classical) -- experimental aspects

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  1. Chapter VIII Phenomenological chemical kinetics (empirical/classical) -- experimental aspects

  2. 8.6 6 determination of the reaction order r = k [A][B][C] Whenever we determine the order of a reaction, we can write out the rate equation of the reaction and tell the detail of the kinetic characteristics of the reaction according to the rate equation. Otherwise, the rate equation can provide useful information about the mechanism of the reaction. Therefore, determination of the order of the reaction is a work of great importance. • Integration method • Differential method • Partial order method • Isolation method Methods for determination of reaction order

  3. 6.1 Integration methods The integration methods are to use the integrated rate equation to determine the order of the reaction. Integration methods includes: 1) attempt method (trial-and–error ) 2) graphic method 3) half-life method

  4. The attempt method: the values of k can be calculated from the selected integrated equation from a knowledge of initial concentration (C0) and the concentration at various time intervals (C). If the reaction is of the selected order of reaction, the k at different intervals thus obtained will be the same. C2H5ONa + C2H5(CH3)2SI  NaI + C2H5O C2H5 + S(CH3)2 r = k[C2H5ONa][C2H5(CH3)2SI] A + B  P r = k [A][B]

  5. Table 1 kinetic data for C2H5ONa + C2H5(CH3)2SI at 337.10 K

  6. Table2 k of the reaction of different order ,  k t Therefore, the rate equation is: r = k[C2H5ONa][C2H5(CH3)2SI]

  7. Comment: • The attempt method is a laborious method • For reaction without simple order, it is impossible to ascertain reaction order using this method. • the experimental error may cause confusion sometimes.

  8. 2) The graphic method The linear relationship of reaction with different order is different.

  9. The rate equation of A  P can be expressed as r = k[A] Table 3 kinetic data for A  P.

  10. 3) half-life method the half-life of the reaction is proportional to the initial concentration of the reactant Graphic method S = 1 (1n) = 1, n = 2 Therefore, the reaction is of second order.

  11. Calculation NH4OCN  CO(NH2)2 n1 = 2.051, n2 = 2.019 n = 2.035  2

  12. 6.2 differential method Use the differential form of the rate equation to determine the order of the reaction. Graphic method Table 4 decomposition of CH3CHOCH4+CO.

  13. Linear fitting results: ln r = -1.593+1.865 lnc Linear correlation coefficient: 0.998 Therefore, the order of the reaction is 2.

  14. c r1 c1 r2 c2 r3 c3 c r4 c4 c0,1 r0,1 c0,2 t4 t4 t1 t2 t3 t r0,2 c0,3 Determination of reaction order through several parallel experiments. Determination of reaction order through one experiment. r0,3 Reaction order with respect to time: nt Reaction order with respect to concentration: nC t The method of initial rates is applicable of a wide variety of reactions and is particularly useful in reactions that are complicated by processes involving intermediate or products.

  15. calculation method nt= 2.534; 1.952; 2.327; 2.274  nt = 2.272  2

  16. Table 5 relationship between nC and nt. r = k[A][B]n = +  r = k[A][B][M]n = + + 

  17. 6.3 Partial order method To plot lnr versus lncA, if a linear relation can obtained,  = 0, =0. If no linear relation can be observed, adjust the value of  and  until a line can be obtained. The slope of this line is , and the corresponding value of  and  can be obtained simultaneously.

  18. 6.4 Isolation method When cB and cC were controlled • Three methods: • Fixation of concentration method; • Excess concentration method; • Application of stoichiometric ratio

  19. 1) Fix the concentration of other reactants Rate equation of 2NO + 2H2 N2 + 2H2O is r = k[NO] [H2].

  20. 2) Excess concentration C12H22O11 + H2O  C6H12O6 + C6H12O6 In excess concentration method, the concentration of B and C is madevery much larger than that of A。 This technique is particularly useful in determining rate constants for reactions involving water in aqueous solution. Pseudo order reaction.

  21. 3) Using stoichiometric ratio aA + bB  P Initial concentration 1 b/a Conversion concentration xbx/a Residual concentration 1-x (b/a)(1-x) r = k[A][B] = k (b/a)(1-x)+= k(b/a)(1-x)n Other methods: • Unit of k • Dependence of t1/2 on c0

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