The Dissociation Energy of Carbon Monoxide and the Heat of Sublimation of Graphite

Abstract

For a number of years there has existed doubt about the value of the dissociation energy of carbon monoxide and about the heat of sublimation of graphite, a directly related quantity. The most popular values for the dissociation energy of carbon monoxide are 9.144 electron-volts, suggested by Herzberg [1] on the basis of predissociation phenomena in band spectra, 9.61 e. v., suggested by Hagstrum and Tate [2] on the basis of electron impact experiments (or the value 9.85 e. v. derivable from predissociation data [3]), and 11.11 e. v., suggested by Gaydon and Penney [4] from an analysis of spectroscopic data. These values together with thermochemical data lead to the values 124.9, 141.4, and 170.3 kcal./mole, respectively, for the heat of sublimation of graphite. Strong evidence for the last of these values has been presented by Brewer, Gilles, and Jenkins [5], who have reported 170.4 kcal./mole from a direct experimental determination. The value has, however, been criticized by other investigators [6,7], and has been defended by Brewer [8]. In this paper we communicate an argument which indicates that the higih values of about 170 kcal./mole for the heat of sublimation of carbon to C(g)(3P) and 11.11 e. v. for the dissociation energy of carbon monoxide are not correct, and which leads instead to the values 140 kcal./mole and 9.77 e. v., respectively.