186 PHENOMENA, ATOMS, AND MOLECULES 



B = 0.005. Similarly, we find from (54) E = 0.26. With these values 

 we may calculate the correction term corresponding to the second term 

 of the second member of (61). The results are shown in the third column 

 of Table XIV. 



The resulting values for the heat of formation are given in the fourth 

 column. When these are compared with the values obtained at low pressures 

 (given in Table XIII), it is seen that the agreement is reasonably good. 



Averaging all the results gives q — 93000. It is felt, however, that a 

 properly weighted mean is closer to 90000. We shall therefore adopt for 

 our final value of q. 



q = 90000 calories. (62) 



This is the heat of formation of 2 grams of hydrogen molecules from 

 atoms at constant pressure and at 3000° K. At constant volume we obtain 

 (at 3000° K.) 



qo = 84000 calories. (63) 



Substituting the above value for q in (50) and integrating, neglecting 

 the small variation of g with the temperature, we find 



. „ 90000 19700 



log K = a - ^^ = a- -^Y^-^ (64) 



To determine the coefficient a we need to know the value of K at some 

 one temperature. Let us choose 2300° as the temperature at which the 

 low pressure results for K are most nearly free from experimental error. 

 At higher temperatures the term FW^ in the denominator of (43) intro- 

 duces some uncertainty, whereas at lower temperatures Wd becomes too 

 small compared with Wc (see Tables- II and III). By referring to Tables 

 IX, XI, XII, we thus obtain for T2 = 2300: 



Pres--..e 



K 



The mean of these is K = 0.12, but since the results at the lowest 

 pressures are more subject to error a more probable value is thought to be 



K = 0.16 at T2 = 2300. 



Substituting this in (64), we find 



a = 7.77. 



Our final equation for the dissociation constant of hydrogen is 



log K = 7.77 - 19700/T. (65) 



The values of K corresponding to various values of T as calculated 

 from this equation, are given in the second column of Table XV. From 



