LAWS OF ELECTROLYTIC DISSOCIATION 33 ' 



From which follows the equilibrium reaction i 



[CO,] [OH-] , , fr ^ iL \ r ,^ i^h 



[HCQ3-] = '^ i ^ i^^^^l -^ ^^X"^^ 



kw I 

 Substituting for [0H~] its value ^-^^ as derived from equation 



[h+] ^ i 



(3), page 12, we obtain ■ 



[CO,] X k^ , \ 



= Ki 1 



[H+1 [HCOai 



or 



[CO,] ki 



= k2 (1) 



[H+] [HCO3-] k. 



This equation represents the hypothetical chemical reaction of 



CO2 ;=i H+ + HCO3- 



and k2 could be used as a true "dissociation constant." But in 

 reality it is only an "apparent" dissociation constant, for the last 

 chemical reaction given above does not occur. 



But the formation of ions by carbonic acid in solution can be 

 accounted for in another way. The first step is assumed to be the 

 formation of a hydrate: 



CO2 + H2O ^ H2CO3 



and whose equilibrium state is 



[CO2] [H2O] 



[H2CO3] 



= k3 (2) 



By transposing the constant value H2O to the right side of the 

 equation and by combining it with ks to form a new constant we 

 obtain 



[CO.,] 



[H2CO3] 



= k4 (3) 



k4 has a very large value, i.e., there is only a small amount of H2CO3 

 in the presence of a great excess of CO2. The molecule H2CO3 

 dissociates electroly tically : 



H2CO3 ?=i H+ + HCO3- 



