CHAPTER VII.
Velocity of Reaction.
Formation of the State of Equilibrium.— When a chemi- cal system is not in equilibrium it approaches this state with a greater or smaller velocity. Many reactions take place so very quickly that their course cannot be followed, whilst others proceed so slowly that their progress is not detectable. The influence of temperature in this respect is very great. For the establishment of an equilibrium in a mixture of hydrogen and iodine at 265° several months are required, at 350° about as many days, and at 440° about as many hours.
An explosive mixture of hydrogen and oxygen behaves in precisely the same way. Above 580° the reaction takes place with explosive violence, whilst at 155° it proceeds so slowly that it is only after months that combination has taken place to an appreciable extent [V. Meyer (i)]. At the normal temperature the mixture is apparently inactive, but in all probability this is not the case.
The following reactions, the velocities of which have been most fully studied, may serve as typical for others : the inversion of cane sugar under the influence of acids, the decomposition of ester into alcohol and acid, and the saponification of esters with bases.
Inversion of Cane Sugar. — The course of the inversion of cane sugar was first followed by Wilhelmy {2) in 1850. He found that the quantity of sugar inverted in unit time is, ceteris paribus, proportional to the amount of sugar in the solution. If the concentration of the sugar is A gram- molecules (mols), and, after time t, x mols are inverted, then
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