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Vapor Pressure, Enthalpy, Evaporation Coefficient, and Enthalpy of Activation of the Beryllium Nitride (Be 3 N 2 ) Decomposition Reaction
Author(s) -
HOENIG CLARENCE L.,
SEARCY ALAN W.
Publication year - 1967
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1151-2916.1967.tb15162.x
Subject(s) - enthalpy , chemistry , enthalpy of sublimation , enthalpy of vaporization , thermodynamics , vapor pressure , partial pressure , sublimation (psychology) , atmospheric temperature range , evaporation , standard enthalpy of formation , analytical chemistry (journal) , oxygen , organic chemistry , psychotherapist , psychology , physics
Beryllium nitride (Be 3 N 2 ) vaporizes congruently in the range 1640° to 1960°K by the reaction Be, N 2 ( c ) = 3Be( g ) + N 2 ( g ). The equilibrium nitrogen partial pressure, in atmospheres, at the composition for congruent sublimation is given by the expression log P N2 = [(–1.952 ± 0.038) × 10 4 ] T −1 + (6.509 ± 0.207). The measured enthalpy of decomposition (370 ± 5 kcal at 298° K) yields an enthalpy of formation for Be 3 N 2 ( c ) of –136 ± 6 kcal/mole. The upper limit to the evaporation coefficient at 1600° to 2000°K can be set as 10 –4 by comparison of equilibrium data to Langmuir data obtained with a sample of 18% porosity. The apparent enthalpy of activation for the reaction is 409 ± 7 kcal/mole at 1800°K for the porous Langmuir specimen. An expression is developed to predict the temperature dependence of the reduced apparent pressures in Knudsen studies of substances with low evaporation coefficients in terms of the enthalpy of activation. The variation in temperature dependence of the Langmuir measurements and Knudsen measurements with three different‐sized orifices is consistent with predictions from this expression.