The Thermal Transformations of the Aluminas and their Hydrates

on the transformation of a-into /3-o-nitroaniline provides a study of a monotropic system.They studied this conversion in the temperature range 273 to 313 °K.They observed an experimental energy (Arrhenius) of activation which is 17,300 cal./g.mole, and corresponds to AH* = 16,700 cal./g.mole.As they point out, this is very close to the measured heat of sublimation (19,000 cal./g.mole for the ß-form) and the transformation pro- ceeds essentially through the vapor phase.Their work also includes an instructive theoretical consideration of this process.It is instructive to examine equation 6a for the case of applied stresses.The case we shall consider is that of a pV term.This equation should then be writtenThe term AY * is the change in volume in going to the activated state and the term AY is the change in volume per mole of the products less the reactants.It should be noted that these terms may op- pose each other.An example is the graphite -* dia- mond transition.In this case the equilibrium is displaced in favor of diamond by high pressures but the term AY * is large and positive so that the rate of the transition is reduced by increasing pressure.21-23Another case similar to the above may be formulated about the well-known recrystallization often observed in metals.In this case, the larger grains of the metal grow at the expense of smaller ones.The driving energy is the high surface energy of the grains and the reaction goes in such a way as to yield large grains, thus minimizing the number of surface atoms and so the surface energy of the grains.Acknowledgment.