Thermal behavior of ammonia borane BH3NH3 (AB) has been studied by calorimetry, tensimetry and mass spectrometry methods. It is shown, that depending on vapor pressure in the system two competitive processes are taking place at 357K. At atmospheric pressure thermal decomposition with hydrogen evolution is the dominant process: BH3NH3(s) =1/n (BH2NH2)n(s) +H2(g) (1). At low pressures (circa 4mTorr) the major process is endothermic sublimation of AB:BH3NH3(s) =BH3NH3(g) (2). At intermediate pressures both processes occur simultaneously. Enthalpies for the processes (1) and (2) have been determined by drop-calorimetry method: (1)H357 . =.24.8 }2.3 kJmol.1 and subH357 .(BH3NH3) =76.3 }3.0 kJmol.1. Solid products after sublimation and decomposition have been characterized by IR and NMR spectroscopy; gaseous forms were studied by mass spectrometry. Activation energy of 94 +/- 11 kJ/mol for the process (1)in range 327-351K was determined by static tensimetry method. Based on the analysis of available thermodynamic characteristics, new values for the standard formation enthalpy of solid AB -133.4 +/- 5.2 kJ/mol and polyamidoborane -156.7 +/- 5.8 kJ/mol are recommended.
Compounds
#
Formula
Name
1
BH4N
boranamine
2
H2
hydrogen
3
BH6N
ammonia borane
4
C7H6O2
benzoic acid
Datasets
The table above is generated from the ThermoML associated json file (link above).
POMD and RXND refer to PureOrMixture and Reaction Datasets. The compound numbers are included in properties, variables, and phases, if specificied;
the numbers refer to the table of compounds on the left.
Type
Compound-#
Property
Variable
Constraint
Phase
Method
#Points
POMD
3
Molar enthalpy of vaporization or sublimation, kJ/mol ; Crystal
Temperature, K; Crystal
Crystal
Gas
Static calorimetry
1
POMD
4
Molar enthalpy of vaporization or sublimation, kJ/mol ; Crystal