in which the bracketed symbols represent the concentrations of the first experiment. The velocities of the two opposite reactions would be no longer equal, the combination of trichloride with bromine would be accelerated by the increased concentration of the former. Here, equilibrium would only be reëstablished when the trichloride and bromine had combined to a sufficient extent to make
(‹y› [PCl_{ 3}] − ‹x›) × ([Br_{ 2}]′ − ‹x›) / ([PCl_{ 3}Br_{ 2}] + ‹x›) = ‹k›_{ 2},
in which x represents the number of moles of additional phosphorus trichlordibromide formed in unit volume by the combination of bromine with phosphorus trichloride. The ‹net result› is seen to be that an ‹increase in the concentration of the one dissociation product eo ipso reduces the concentration of the other dissociation product›.
EXP. A third tube charged with the same quantity of phosphorus trichlordibromide as the tube mentioned above, and with an added excess of phosphorus trichloride, is placed in the warm water next to the tube containing the trichlordibromide. Its color is much paler than that of the latter, owing to the suppression of free bromine.[179]
The concentration of the free bromine, ([Br_{ 2}]′ − ‹x›), under the new conditions of equilibrium, is smaller than the original concentration [Br_{ 2}]′—a result confirmed by experience. ‹It is in our power, therefore, arbitrarily to change the concentration of a reacting component›, in a case of equilibrium, ‹and thus to affect the reactivity of the system;› for instance, for brominating purposes, the new system would be less effective than the original one, and it might be of especial service where bromination is to be avoided.
In the cases studied, are found the two fundamentally important relations expressed by the law of equilibrium: ‹the equilibrium constant is a measure of the stability of a certain system› and, in a way, of its ‹reactivity› at a given temperature; and the [p098] ‹concentration factors are variables›, which we may change to a very considerable extent, so as, to a certain degree, to subject the system to our own purposes. We shall repeatedly have occasion to refer to these two fundamental relations and we shall use them again and again in our analytical work.