TRANSACT10>'S OF SECTION B. 615 



order, a bi-molecular with the equation of the second order, &c. Sometimes the 

 orders so found are in agreement with what we should expect from the chemical 

 equations, and sometimes they are not so, showing that the chemical equations do 

 not always represent fully the mechanism of a reaction. For example, we should 

 expect to find that both the inversion of cane sugar, in which one molecule of 

 sugar is changed into one of glucose and one of laevulose, and the change of ammo- 

 nium cyanate into urea 



/NH, 



NH.CNO = (JO are reactions of the first order. 



The determination of the reaction velocity shows in the manner above described 

 that this is true of the former, but that the latter is a reaction of the second 

 order. We must therefore assume that the two ions of ammonium cyanate NH^ and 

 CNO react as two molecules. 



Our object in this investigation was to determine the nature of the reaction 

 between bromic and hydrobromic acids. According to the chemical equation 

 5 HBr + HBrOj = 3 Ufi + 3 Br^ since there are in all 6 molecules on the left 



side of the equation, we should expect that only the equation — =}i(a-xY 



would give a constant value for 7c. This expectation is, however, not borne out 

 by the experimental results, they show that the reaction, whose velocity is being 

 measured, is only one of the second order instead of the sixth. We must therefore 

 assume that the first stage of the reduction is expressed by the chemical equation 

 HBr + HBr03 = HBrO + HBrOj, and that these acids when formed are instantly 

 decomposed by the hydrobromic acid present, thus : 



HBr + HBrO = H^O + Br^ and 

 8 HBr + HBrOj = 2 H,0 + 2 Br^. 



The reaction consists in the formation of bromine and water, and the experimental 

 method employed consists in titrating the liberated bromine by a standard solution of 

 sodium thiosulphate. In the first set of experiments the free bromic and hydro- 

 bromic acids were liberated from the solution of their salts by addition of a 

 definite large excess of sulphuric acid, the conditions being so arranged that its 

 concentration was the same in each experiment. The duration of an experiment 

 was noted from the time of addition of the sulphuric acid which started the 

 reaction. In the first series the solution was x^th normal with respect to KBr, 

 and ^^^jth normal with respect to /cBrOj and the mean value of h obtained from 



the integrated form of -^ = A; (5a — 5a;) (a - x) was 0-00423. A second series of 



experiments, in which the concentration of ZjBrOj was the same, but that of 

 KBr doubled, i.e., ^^ normal, gave as the mean value of It 0'00451 ; and when 

 the concentration of the KBrOg was also doubled the mean value of k was 0'00427. 

 These results are obtained by employing an equation of the second degree, so that 

 the reaction whose velocity is being measured must be looked upon as bi-molecular. 

 It consists in the production of HBrO and HBrO„ according to the equation 

 given above, viz., HBr + HBrO,, = HBrO + HBrO.,. 



This leads us to expect that the reaction which, in the presence of a large 

 excess of sulphuric acid — or of hydrogen ions — is bi-molecular, in its absence is of a 

 higher order, probably tetra-molecular. Because, in the light of the ionic theory, 



+ — — 

 the equation must be written thus — 2H + Br + BrOj = HBrO + HBrO.,, bromous 

 and hypobromous acids being, from analogy with the corresponding chlorine com- 

 pounds, very weak acids — i.e. very slightly ionised. This expectation was fully 

 verified by an examination of the reaction between hydrobromic and bromic acids 

 in the absence of sulphuric acid. 



We performed two series of experiments, in the first of which the bromic acid 



