Physical Chemistry II Chapter X Mechanism of Complex Reactions 10.4 postulation of mechanism

Flow chart for establishment of mechanism In order to propose an reasonable mechanism, people have to collect as many as possible information about the reaction and have to follow many empirical rules. Moreover, hard work for certification have to be done. Flow chart for establishment of mechanism information collection mechanism postulation Verification and modification

useful information 1) Stiochiometric coefficient, reaction order, rate constant, rate equation, activation energy, pre-exponential factor. 2) Side reaction? Reversible reaction? Intermediate? What?

Mechanism postulation Experience, kinetic characteristics, molecular structure. H2 + Cl2  H2 + Br2 ? H2  2H or Br2  2Br? Verification and modification Usually consider three aspects of the mechansim 1) Rate aspect 2) Energy aspect 3) Structure aspect

Example: 12.3.1 Empirical rules for mechanism suggestion: Rule I: If rate equation can be expressed as: the element composition of the r.d.s is Example: Reaction: Rate equation: +4? +5? Element composition of r. d. s. is

? ? Proposed mechanism: r. d. s. This suggest that the proposed mechanism may be correct.

Rule II: Stiochiometric coefficient vs. reaction order 1) If reaction order > 3, before r. d. s., there are several rapid reaction. 2) Stiochiometric coefficient > reaction order, the reactant take part in the reaction after the r. d. s. 3) Molecule that does not appear in the stiochiometric equation but in the rate equation is a catalyst. If its partial order is positive, it reacts in r. d. s, or before r. d. s and replenished after r. d. s. If its partial order is negative, it is a negative catalyst and may be a product before r. d. s. or a reactant after r. d. s.

example Br- is a catalyst in r. d. s. H+ reacts before r. d. s., HNO2 take part in r.d.s in some form. C6H5NH2 reacts after r. d. s. Mechanism: Rapid equilibrium r.d.s. Rapid reaction

Using r. d. s and equilibrium approximation Is in the same form as the experimental rate equation:

Rule III: Fractional reaction order: such as 1/2, 1/3, suggests decomposition of molecules.

is just in the same form of the experimental rate equation Is the following mechanism possible?

Rule IV: For reaction without definite reaction order, the limiting conditions may first be used.

Using stationary-state approximation

The evidence for a mechanism is often indirect, and there is always the possibility that a different mechanism is also in accord with all the facts about the kinetics of the reaction. Whether or not a proposed mechanism is correct depends on the certification of experimental results. Many efforts have to be made to construct the mechanism building, unfortunately, this building may collapse over night on any new finding of experimental fact opposite to it. Mechanism: A great tower on sands

Exercise: H2 + Br2  2HBr Its rate equation is established by M. Bodenstein and S. C. Lind in 1906: The appearance of [Br]1/2 suggests the decomposition of Br2 and the [HBr]/[Br2] implies that the presence of the product decreases the rate of the reaction, i.e., the product acts as an inhibitor and the HBr and Br2 compete for the same intermediate. However, the inhibition is less if the concentration of bromine is high.