212 PHENOMENA, ATOMS, AND MOLECULES 



SECOND HYPOTHESIS 



In this hypothesis we assume that hydrogen molecules striking the 

 surface condense as such, each molecule occupying only one elementary 

 space. The atoms which strike the surface condense as atoms. Interaction 

 between the atoms and molecules on the surface occurs. 



The rate of condensation of atomic hydrogen is ai6\ii while the rate 

 at which it evaporates from the surface is Vi^i. Hence we have 



2C0 = j^i^i — aidfx. (icj) 



This is identical with Equation 4. The rate of condensation of molecular 

 hydrogen is a2^|i2 whereas the rate of evaporation is V2^2- This gives 



w = a26fJi2 — V262. (20) 



The rate at which hydrogen atoms combine will be proportional to the 

 square of ^1, since the chance that two atoms shall be in adjacent spaces 

 is proportional to 61^. We may thus place the rate of formation of 

 molecular hydrogen from the atoms, equal to /?i^i^ where j5i is the rate of 

 formation when the surface is wholly covered with atomic hydrogen. 



The rate at which hydrogen molecules dissociate into atoms on the 

 surface will be proportional to the product of 6 by 62 since a hydrogen 

 molecule must be adjacent to a vacant space in order that it can dissociate. 

 The chance that a hydrogen molecule and a vacant space shall be adjacent 

 to each other is evidently proportional to Od-z- We may, therefore, place 

 the rate of dissociation of hydrogen equal to /52^^2- 



We thus obtain a third equation 



,^ = ^^662 — Mi'- (21) 



Finally we have the condition (in the absence of catalytic poisons) 



-^ Ox -^ do = I. (22) 



If we substitute (0 = in Equations 19, 20 and 21, we obtain the 

 values of }.ii, [I2, etc., corresponding to equilibrium conditions. 



(23) 

 (24) 

 (2.0 



