Measurement of Enthalpies of Evaporation of Bi and Bi2 by an Optical Absorption Technique

Abstract

The enthalpies of evaporation of Bi and Bi2 were determined by the second‐law method using an optical absorption technique to measure the change of vapor density with temperature for each species. The 3067‐Å line was used for Bi and the band head at 2731 Å was used for Bi2. The enthalpy of dissociation of Bi2 is the difference between the two enthalpies of evaporation. The evaporation values obtained wereBi(l)=Bi(g)ΔH∘298=49600±1100cal,Bi(l)=12Bi2(g)ΔH∘298=24700±1000cal,12Bi2(g)=Bi(g)ΔH∘298=24900±2100cal.The optical absorption technique was checked on lead, using the 2833‐Å line. For lead, the ΔH°v,298 measured was 47 240±850 cal, as compared with an accepted value of 46 600±300 cal.