242 



are affected in different degrees, and no simple rule accurately 

 defining the effect can be given. Roughly speaking, however, 

 a rise of 10 doubles the speed of every action. A rise of 100 

 will therefore make the speed roughly 1024 times greater. Hence, 

 when the chemist finds that two substances show no evidence of 

 interaction he infers that there must be either slow action or 

 none, and he seeks to settle the question quickly by heating the 

 mixture. 



The Influence of Temperature on a System in Equilib- 

 rium. In a reversible change the two opposing reactions are 

 different actions and their speeds are therefore affected in different 

 degrees by the same alteration in temperature. Hence, when the 

 temperature is changed, the relative amount of the two sets of 

 material present is altered and the equilibrium is displaced. 

 Thus, in Deacon's process, a rise of 40 in the temperature dis- 

 places the equilibrium backwards (p. 233), and diminishes the 

 yield of chlorine by 5 per cent. In the vapor of phosphorus penta- 

 chloride (p. 236), the displacement is in the opposite direction. 

 At 250, and 760 mm. pressure, 20 per cent of the material is 

 present as pentachloride and 80 per cent as trichloride and chlo- 

 rine. At 300 only 3 per cent of the pentachloride remains while 

 at 200 only a little more than 50 per cent is dissociated. Evi- 

 dently, here, raising the temperature favors the decomposition 

 of the pentachloride, and therefore increases the speed of its 

 dissociation more than it does the speed of the reunion of the 

 trichloride and chlorine. 



J^an-'t Hoff's Law. Now the facts mentioned above are con- 

 nected by a law which will answer many practical questions in 

 chenrstry. 



When phosphorus trichloride and chlorine combine (to form 

 PC1 5 ), heat is given out. Conversely, when phosphorus penta- 

 chloride dissociates, heat is absorbed: 



PC1 + 30,000 cal. <= PC1 3 + C1 2 , 



