Heat of dissociation of As4(g) and the heat of formation of As2(g)

The equilibrium ${\mathrm{As}}_4(\mathrm{g}\text{)=2}{\mathrm{As}}_2(\mathrm{g})$ has been studied in detail over solid phase MoAs₂ and Mo₂As₃ using the Knudsen effusion technique combined with a mass spectrometer. A ``see‐through'' ion source and a liquid‐nitrogen cooled condensation plate were used to eliminate the spurious As₄(g) which has been encountered by other investigators. Excellent agreement was obtained between the second‐ and third‐law enthalpies. From equilibrium constants measured between 807 and 1050°K, the following third‐law heats were determined: ${\mathrm{As}}_4(\mathrm{g}\text{)=2}{\mathrm{As}}_2(\mathrm{g}{\mathrm{),\hspace{0.17em}\Delta \; H}}_0{\text{°=53.95± 1.1,\hspace{0.17em}Δ H}}_{298}{\text{°=54.26± 0.7,\hspace{0.17em}Δ H}}_0{°}_{\mathrm{(f)}}[{\mathrm{As}}_2(\mathrm{g}{\text{)]=45.65± 0.7,\hspace{0.17em}Δ H}}_{\text{298(f)}}{\mathrm{°[As}}_2(\mathrm{g}\text{)]=45.45± 0.7}$ , all in kilocalorie mole⁻¹, the errors being estimates of total error. The results obtained when GaAs, Mo₅As₄, and Zn₃As₂ were used as source phases are briefly described. The results of previous investigations of the ${\mathrm{As}}_4(\mathrm{g}\text{)=2}{\mathrm{As}}_2(\mathrm{g})$ equilibrium are critically discussed.