EVAPORATION AND CONDENSATION 



303 



Under these conditions the velocity of the reaction could be followed up 

 to the melting point of the filament. 



With carbon monoxide the reaction velocity reaches a maximum when 

 the filament temperature is about 900° K. and apparently becomes 

 negligibly small in comparison, when the temperature is raised to 1300° K. 

 or higher. 



With hydrogen, the rate of reaction also reaches its maximum value 

 at about 900° K. This rate does not decrease at higher temperatures, 

 however, but remains practically constant up to. the melting point of the 

 filament. 



From equation ( i ) the rate at which each gas comes into contact with 

 the filament may be calculated. By comparing this with the actual rate at 

 which the gas enters into reaction it is possible to determine e. Out of all 

 the hydrogen (or carbon monoxide) molecules which strike the filament 

 in a given time, let £1 represent the fraction which reacts with the oxygen. 

 Similarly let £0 be the corresponding fraction for the oxygen molecules 

 striking the surface. The following table gives the values of £1 and £2 

 calculated directly from the experimentally determined maximum rates of 

 reaction. 



In this table pi is the partial pressure of hydrogen or carbon monoxide 

 in bars and p2 is that of the oxygen. We see that when hydrogen is in large 

 excess, 53 per cent, of all oxygen molecules which strike the filament 

 react with hydrogen to form water. This may happen in either one or 

 two ways : 



Reaction 2H2 +02= 2H2O: 



1. The oxygen may react with hydrogen already present on the surface. 



2. The oxygen may condense on the surface and react with hydrogen 

 molecules which subsequently strike the surface. 



In either case the reflectivity of the oxygen molecules cannot exceed 

 47 per cent. (100 — 53). Similarly we may conclude that the reflectivity 

 of hydrogen molecules is not greater than 59 per cent. 



