Dissociation Pressure of Uranium Dicarbide

Vapor transport of uranium from UC_1.86 in a graphite crucible has been measured from 1930° to 2365°K by the Knudsen technique. In order to treat the data, unusual assumptions have been made concerning the thermal properties of carbides which hydrolyze to yield hydrocarbons. The gaseous uranium species is assumed to be monatomic. With the aid of these assumptions, the data have been treated by the third‐law method as well as by the second‐law method. Agreement between the two is considered good, and the ΔH₂₉₈° obtained for the process $${\mathrm{UC}}_{\text{1.86}}\mathrm{(s)=}\mathrm{U}\text{(g)+1.86\hspace{0.17em}}\mathrm{C}\mathrm{\hspace{0.17em}(}\mathrm{graphite})$$ is 140.15±1.82 kcal/mole. Reasonable estimations for reaction entropies within the system lead to an upper limit of 0.5 kcal/mole for ΔH₂₉₈° for the reaction $$\mathrm{UC}\text{(s)+0.86\hspace{0.17em}}\mathrm{C}\mathrm{\hspace{0.17em}(}\mathrm{graphite}\mathrm{)=}{\mathrm{UC}}_{\text{1.86}}\mathrm{(s),}$$ and hence a value of 119.8±2.8 kcal/mole is obtained as an upper limit for ΔH₂₉₈° of vaporization of uranium.