236 



weights, p. 122) of the substance contained in a liter of the whole 

 mixture. There is the same number of molecules in a gram- 

 molecular weight of any substance (see p. 84) . Hence the number 

 of moles per liter defines the concentration of each substance .in 

 terms of this number of molecules in a liter as the unit of concen- 

 tration. 



As an example, the dissociation of phosphorus pentachloride 

 vapor into phosphorus trichloride and chlorine (p. 145) : 



may be considered. 



The law states that, in any mixture of the three substances, the 

 speed of the forward action, or decomposition, is proportional 

 to the concentration of PC1 5 molecules. It is therefore equal 

 to the concentration of PC1 5 molecules, which we may write 

 [PC1 5 ], multiplied by a constant, which we shall write KI. Mathe- 

 matically expressed, if Si is the speed of decomposition : 



Si = K! [PC1J 



In the same way the law states that the speed of the reverse 

 action, or combination, is proportional to the concentration of 

 PC1 3 molecules and also proportional to the concentration of C1 2 

 molecules. It is therefore equal to the concentration of PCla 

 molecules [PC1 3 ], multiplied by the concentration of C1 2 mole- 

 cules [Cy, multiplied by another constant, which we shall write 

 K 2 . If S 2 is the speed of combination, we have now: 



s 2 = K 2 [pcu [cy 



Each reaction, it will be noted, must slow up as the concen- 

 tration of the reacting substances diminishes. This is true of all 

 reactions. The further they proceed, the less rapid their speed. 

 The burning of a candle in a confined space becomes less brilliant 

 as the available oxygen is used up. The evolution of hydrogen 

 in the action of an acid on a metal becomes very slow when the 

 acid is nearly exhausted, owing to the diminution in the concen- 

 tratipn of hydrogen ion, 



