250 Van Name and Edgar — Velocities of Certain Reactions 



Table IX. 

 Mercury and Cupric Bromide. 



Tempei 



•ature 



25 c 







400 g. 



KBr per 



liter. 



c 



At 



V 



K 



c 



At 



V 





mol/liter 



min. 



cc. 



mol/liter 



min. 



cc. 



K 



1. r=160 







2. r= 



215 







0-0585 

 0-0529 

 0-0476 

 0-0420 

 0-0373 

 0-0330 

 0-0290 



10 

 10 

 11 

 10 

 10 

 10 



500 



480 

 460 

 440 

 420 

 400 

 380 



Av, 



4<85 

 4-86 

 5-02 

 4-92 

 4-96 

 4-94 



0-0774 

 0-0679 

 0-0593 

 0-0516 

 0-0446 

 0-0382 

 0-0326 



10 

 10 

 10 

 10 

 10 

 10 



510 

 490 

 470 

 450 

 430 

 410 

 390 



Av 



6-40 

 6-38 

 6-24 

 6-29 

 627 

 619 





4-93 



. 6-30 



insoluble bromides, and the reaction was followed by adding 

 samples of the liquid to a large excess of potassium iodide and 

 titrating the iodine set free. If the solution is exposed to the 

 air throughout the experiment, some oxidation of the cuprous 

 salt occurs, as the steady fall of the constant in the following 

 preliminary experiment will show: 



KBr per liter 



K 



400 g. 

 6-59, 6*54, 



•23. 



6-17, 



r = 220 

 5-62, 



5-79. 



This was avoided in the experiments of Table IX by saturat- 

 ing the cupric bromide solution with carbon dioxide at the 

 outset and conducting, throughout the experiment, a rapid 

 current of the gas into the upper part of the reaction vessel, so 

 that the liquid was covered at all times by a layer of carbon 

 dioxide. Under these conditions, as the results indicate, little 

 or no oxidation took place. 



Discussion. 



In Table X the values of the velocity constants are sum- 

 marized for convenient comparison. The agreement between 

 the constants obtained for different metals under like condi- 

 tions is unmistakable, and is especially striking with the metals 

 mercury, cadmium and zinc, which show a maximum variation 

 of less than two per cent. This can hardly be an accident and 

 points strongly to the conclusion that the reaction velocity is 

 independent of the metal. As compared with mercury the 

 constants for copper and silver are uniformly slightly lower, 

 the largest difference being about five per cent, which is more 

 than can reasonably be ascribed to experimental error alone. 



The difficulty encountered in the work with copper and with 

 silver in entirely preventing the formation of coatings of 

 insoluble iodide upon the metal at once suggests itself as a 

 probable explanation of the lower constants. As stated above, 



