Energy and Entropy Factors in Reaction Velocity

It is evident that if complicated changes in the modes of motion of molecules have to accompany a chemical transformation, the nonexponential term in the expression for the reaction rate will be small. That the probability factor will be low, that the partition function of the transition complex will be unfavourable, and that the entropy of activation wUl be small are all equivalent statements of the same fact. 

For the reasons so expressed, chemical reactions are frequently resolved into a series of steps for each one of which the entropy of activation is as large as possible. The overall transformation thus appears complex, especially if the observed rate is expressed in terms of the concentrations of the initial and final substances, but this apparent complexity is itself the resxdt of the tendency to proceed by the simplest possible stages. 

There are, of course, plenty of reactions which occur in a single chemical step. The unimolecular decompositions and some of the bimolecular reactions which have been discussed belong to this category, but there are even more examples of chemical changes which do not. 

The reaction between stannous salts and ferric salts occurs formally according to the equation  

The probability that a reaction should really take place in this way is, however, extremely small. Not only does the equation demand the simultaneous collision of three molecules, but of molecules with multiple positive charges which would exert strong mutual repulsions. The observed relations between the rate and the concentrations of the various ions present are consistent with the idea that the principal reacting species are actually the ferric ion and the complex ion SnClf , which participates in the equilibrium 

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