Activation Energy for Unimolecular Reactions

It follows from vast experimental data that all processes of unimolecular conversions can be divided into two classes, depending on the relation between the activation energy E and the reaction heat. One class comprises processes with an activation energy higher than the reaction heat (Fig. 28). Such are the reactions of cis-trans isomerization with their activation energies of tens of kj at a reaction heat of about 10 kJ. 

The decay of nitrous oxide N2O - 1 2 + 0 — 160 kJ with an activation energy 250 kJ is another example of a reaction of this class. 

The second class comprises unimolecular reactions with an activation energy E q equal to their heat (Fig. 28) [501]. 

Besides the decay to monoradicals, the biradical molecular decay, e.g. that of a ketene molecule CH2CO to CO and a biradical CH2, also falls into this category. 

The coincidence of activation energies and heats of such reactions affords (within experimental error) the possibility of direct measurements of bond energies by appropriate evaluation of the activation energies for the corresponding unimolecular reactions. This method has found wide application in thermochemistry. 

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