228 OUTLINES OF PHYSICAL CHEMISTRY
between heterogeneous molecules can give rise to a trans- position of atoms. If now another gram-equivalent of bb' be dissolved in the solution (the concentration of bb' being thus doubled), then the molecules aa' will have twice the number of chances of colliding with bb' molecules, and the speed of the reaction will become 1x2 times as great as before. If now the concentration of aa' be doubled, this will again double the number of collisions favourable to double exchange, and the speed of the reaction will become 2x2 times greater than the initial speed. We see, there- fore, a priori that this speed becomes proportional to the product of the active masses.
It is to be understood that the reaction commences with a speed which is determined by the masses of the sub- stances brought together. But as the transposition goes on these masses diminish and the chemical phenomenon abates. We know several reactions the completion of which extends over a time sufficiently long to allow of an experi- mental study of their speeds.
Our study will now be divided into a static part, treat- ing of chemical equilibrium ; a dynamic part, relating to the speed of reactions; and a more general chapter, in which we shall again return to the theories of the constitu- tion of salt solutions.
��I. CHEMICAL EQUILIBEIUM
A. Reversible Reactions
By the action of acetic acid on ethyl alcohol, ethyl acetate and water are produced. And, inversely, water tends to saponify the ester formed and reproduce the original sub- stances. Both esterification and saponification are gene- rally incomplete reactions which mutually limit each other and lead to a state of equilibrium determined by the equality of the speeds of the inverse transformations. We
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