THE DISPROPORTIONATION OF ZIRCONIUM TRICHLORIDE

S u m m a r y Equilibrium pressures of the reaction 2ZrC13(c) + ZrClz(c) + ZrC14(g) measured by a cooling curve modification of the dew-point technique are fitted over the range 613-723"~, by the relation loglopmm =-6246/T- 11'632+0.05 (S.D.). Derived thermochemical data are AHzg8 = 30.1 & 0.5 kcal and ASzg8 = 39.1 + 1 e.u., consistent with estimated AHf298ZrC13 =- 186 kcal/mole and AHfis8ZrCl2 =- 136 kcal/mole, Szg8ZrC13 = 36.4 e.u. and Szs8ZrCl2 = 28.3 e.u. The trihalides of the Group IV and V metals have all been shown to dispropor-tionate on heat'ing, so that their entropy and enthalpy of formation may be found by a study of the equilibrium pressures of the volatile higher halides. This assumes particular importance for zirconium and hafnium, which may be separated by selective disproportionation of the trichlorides.1 From preparative studies i t appears that ZrC13 becomes appreciably converted to ZrCla and ZrC14 above 330 " (Ruff and Walsteinz), 300 " (Newnhaml), or 450 " (Larsen and Leddy3). The preparation of pure ZrC13 by atomic hydrogen reduction (Newnham and Watts4