300 PHENOMENA, ATOMS, AND MOLECULES 



not throw much hght on the reflectivity of gas molecules, but if Millikan's 

 interpretation of his experiments is correct they would seem to indicate 

 that a reflectivity as high as lO per cent, occurs in some cases. 



STATISTICS OF CHEMICAL REACTIONS 



(a) Heterogeneous Reactions. — -The rate at which the molecules of a 

 gas come into contact with a solid surface immersed in it, may be calculated 

 by equation (i). Suppose a heterogeneous chemical reaction occurs at the 

 surface of the solid body. Then, if we can measure the rate at which the 

 reaction occurs, we may be able to calculate by Equation i what fraction 

 of the collisions between the gas molecules and the solid body results in a 

 chemical reaction. Let us caU this fraction e. 



In order to avoid complications due to the accumulation of the products 

 of the reaction close to the surface it will be necessary either to work with 

 very low pressures of gases or to choose reactions which take place very 

 slowly. 



The value of e may in general have any value less than unity. It is 

 evident that the reflectivity of the gas molecules striking the surface must 

 be less than i — s and therefore, by a determination of £, we obtain an 

 upper limit for the reflectivity. 



Strutt ^^ has measured 8 for two heterogeneous reactions. In the decom- 

 position of ozone in contact with a silver surface he found s equal to 

 unity, while in the case of the conversion of atomic nitrogen into molecular 

 nitrogen in contact with copper oxide, he obtained e = .002. From this we 

 may conclude that the reflectivity of ozone molecules from a silver surface 

 is small. In the second case, however, it is not necessary to conclude that 

 the reflectivity is 0.998. We shall see from the general discussion below that 

 the small value of e is probably caused by a large portion of the surface 

 being in an inactive condition, perhaps due to its being covered with a layer 

 of adsorbed nitrogen molecules. 



In a study of the rate of attack of a heated tungsten filament by oxygen 

 at low pressure,^** the writer determined the values of e for the reaction 

 2W + 3O2 = 2WO3. The fraction £ was found to be independent of the 

 pressure and to increase rapidly with temperature from .001 at 1270° K. 

 to 0.15 at 2770° K. 



Some recent work has shown that at higher temperatures e continues to 

 increase so that at 3000° K. it is over 0.40. Extrapolation indicates that at 

 still higher temperatures e would reach a limiting value of about 0.7. 



From these results we may conclude that the reflectivity of oxygen 



^^ Proc. Roy. Soc, A., 87, 302, 1912. 

 ^^ Jour. Amer. Chem. Soc, 35, 115, 1913- 



