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  22.6 Elementary reactions ã Elementary reactions : reactions which involve only a small number of molecules or ions.  A typical example: H + Br  2   → HBr + Br    ã Molecularity:  the number of molecules coming together to react in an elementary reaction. ã Molecularity and the reaction order are different  !!! Reaction order is an empirical quantity, and is obtained from the experimental rate law; Molecularity refers to an elementary reaction proposed as an individual step in a mechanism. It must be an integral.  ã  An elementary bimolecular reaction has a second-order rate law:  A + B → P   ã If a reaction is an elementary bimolecular process, then it has second-order reaction kinetics; However, if the kinetics are second- order, then the reaction might be complex. ]][[][ B Ak dt Ad    22.7 Consecutive elementary reactions ã  An example: 239 U → 239 Np → 239 Pu ã Consecutive unimolecular reaction  A → B → C  The rate of decomposition of A is: ã The intermediate B is formed from A, but also decays to C. The net formation rate of B is therefore: ã The substance C is produced from the unimolecular decay of B: ][][ Ak dt Ad  1  ][][ ][ B k Ak  dt B d  21   ][][ B k dt C d  2   ã Integrated solution for the first order reaction (A) is: ã Then one gets a new expression for the reactant B: the integrated solution for the above equation is: when assuming [B] 0  = 0. ã Based on the conservation law [A] + [B] + [C] = [A] 0 t k  e AA  1 0    ][][ ][e[A] ][ 1 k -0  B k k dt B d   t  21   0121  21 ])[(][  Ae e k k k B   t k t k     01221 12 1  ][][  Ak k e k e k  C  t k t k   

Treetops3 (Ingles)

Jul 23, 2017
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