Page 201 - Instant notes
P. 201

Rate laws in action     187





        Therefore:




        which gives an overall observed rate of reaction of:




        Although this rate law is not first order, if the concentration of A is sufficiently high that
        the rate of deactivation collisions between A* and A is greater  than  the  rate  of
        unimolecular reaction of A*, then                (i.e.           ) and the
        rate law simplifies to:




        This expression is now a first order rate  law in which the observed first order rate
        constant, k uni, is a composite of rate constants for underlying elementary reactions:



        The Lindemann mechanism is easily adapted to the situation in which activation of A is
        dominated by collisions with molecules of a non-reactive diluent bath gas, M, rather than
        other molecules of A. The same kinetic approach for the elementary reactions:







        gives a rate of product formation:




        At any given pressure the concentration of  M is constant so the above rate law is
        equivalent to a first order rate law:
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