Activation of Reacting Molecules by Collisions The Lindemann Theory

Activation of Reacting Molecules by Collisions The Lindemann Theory 

A question that intrigued several kineticists around 1920 was the following. For bi-molecular reactions of the type A -1- B = Products collision theory gave at least a plausible conceptual picture If the collision between A and B is sufficiently vigorous, the energy barrier separating reactants and products can be crossed. How, though can one explain the case of monomolecular elementary reactions, e.g. an isomerization, such as cyclopropane to propylene, or the decomposition of a mol- 

According to the Lindemann-Christiansen hypothesis, formulated independently by both scientists in 1921, all molecules acquire and lose energy by collisions with surrounding molecules. This is expressed in the simplified form of the Lindemann mechanism, in which we use an asterisk to indicate a highly energetic or activated molecule, which has sufficient energy to cross the barrier towards the product side, and M is a molecule from the surroundings M may be from the same type as A  

Application of the steady state approximation to the energized intermediate A gives the concentration of this elusive species  

In the normal pressure regime, where the number of collisions is very high (according to the previous section 10 s L at 1 bar), the denominator will be dominated by the second term, and hence we find the normal result that the overall rate is first order in the concentration of A. However, at low enough pressures, the first term of the denominator becomes dominant. Suppose that A and M are the same species, then the rate of the unimolecular reaction from A to P becomes d[P] k+lT[A][A] 

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