The Factors of the Law of Chemical Equilibrium.
The following experiments may be used to illustrate the significance of the two classes of factors: Phosphorus pentabromide is partially decomposed by heat into the tribromide and bromine (a case of gaseous dissociation):
PBr5 ⇄ PBr3 + Br2.
Phosphorus trichlordibromide is decomposed more or less, in a similar fashion, into the components phosphorus trichloride and bromine, according to the equation
PCl3Br2 ⇄ PCl3 + Br2.
For the condition of equilibrium in the two cases we have
[PBr3] × [Br2]′ / [PBr5] = k1
and [PCl3] × [Br2]″ / [PCl3Br2] = k2.
Exp. Two tubes containing equivalent quantities of the two bromides are placed side by side in warm water.[178] The tube containing the trichlordibromide is found to be much more intensely colored by free bromine than that containing the pentabromide.
The intensity of the color of the bromine vapor shows that the concentration of bromine, [Br2]″, in the PCl3Br2 tube, is greater than the corresponding concentration, [Br2]′, in the PBr5 tube. As a molecule of pentahalide PX5 dissociates into one molecule of PX3 and one molecule of X2, [PCl3] equals [Br2]″ and is greater than [PBr3], which is equal to [Br2]′. Further, more of the pentabromide than of the trichlordibromide must be left undecomposed, i.e. [PCl3Br2] is smaller than PBr5. Since the factors in the numerator of the second equation are both larger, and the factor in the denominator smaller, than the corresponding factors in the first equation, k2 must be greater than k1. These constants are thus seen to be a measure of the chemical stability of these pentahalides. It is evident, too, that in reactions which depend on the presence of free bromine, such as the bromination of many organic compounds, the trichlordibromide should be more effective than the equivalent quantity of the pentabromide. [p097]
In the second place, if we were to introduce into either tube, for instance into the tube containing the phosphorus trichlordibromide, an excess of one of the dissociation products, say an excess of phosphorus trichloride, then the condition of equilibrium would necessarily be disturbed: