394 CHEMICAL DYNAMICS 



acid first combines with the alcohol, forming a substance "which 

 can be isolated, namely ethyl-sulphuric acid and this compound 

 reacts with another molecule of alcohol, forming ether and re- 

 generating the sulphuric acid, in accordance with the equations 



HO X C 2 H 5 0. 



C 2 H 5 OH + S0 2 = H.OH + S0 2 



HO V 

 S0 2 + C 2 H 5 OH = S0 2 + C 2 H 5 - O - C 2 H 5 



and, at the end of the reaction, the sulphuric acid molecule which 

 is set free is unaltered and can carry another molecule of alcohol 

 through the same series of transformations. Hence, as in the 

 "typical" cases of catalysis cited above, the only things which 

 prevent the reaction from proceeding indefinitely, with the con- 

 version of an indefinite quantity of alcohol through the agency 

 of a limited quantity of sulphuric acid, are the failure of the 

 supply of alcohol, or the continued accumulation of the products; 

 thus, if the water which is formed in the above reaction be allowed 

 to remain in the system the sulphuric acid becomes, finally, so 

 highly diluted that the velocity of the transformation sinks to 

 a negligible magnitude, i.e., for practical purposes, the reaction 

 ceases. 



Another example of the same kind is the use of nitric oxide 

 as a "carrier" of oxygen in the manufacture of sulphuric acid. 

 The first reaction is the combination of NO with O to form 



nitrogen peroxide: 



2 NO + O 2 = 2 NO 2 . 



This is brought into contact with SO 2 and steam, sulphuric acid 

 is formed and nitric oxide regenerated : 



S0 2 + H 2 + NO 2 = H 2 S0 4 + NO 



and the NO is now free again to "carry" oxygen into the sul- 

 phurous, converting it into sulphuric acid. 



In a large number of instances in which the intermediate 

 reactions have not been defined and their occurrence established, 

 it has nevertheless been shown that the catalysor forms com- 

 pounds with the substrate or with modifications of the substrate, 



