The thermochemistry of ammonium compounds and the energy barrier opposing ammonium ion reorientation
1988; Royal Society of Chemistry; Issue: 2 Linguagem: Inglês
10.1039/dt9880000445
ISSN2050-5671
Autores Tópico(s)Thermal and Kinetic Analysis
ResumoThere is a good linear correlation between the energy barrier opposing cation reorientation in a compound (NH4)nX and the quantity Δ(Xn–)=1//n{ΔH⊖f[(NH4)nX, s]–ΔH⊖f(RbnX, s)}. At zero barrier height, Δ(Xn–)= 143 ± 3 kJ mol–1. There is a sense in which depressions below this threshold are a measure of cation–anion hydrogen bonding but this sense is an unconventional one. The estimated value of ΔH⊖f(NH4+, g) derived from the threshold is 638 ± 9 kJ mol–1. The correlation is also used to improve literature thermodynamic data on ammonium and alkali-metal perrhenates, e.g., ΔH⊖f(NH4ReO4, s)=–965 ± 5 kJ mol–1. Unknown enthalpies of formation of some ammonium and rubidium salts are also predicted, and the procedure is used to explore the thermodynamic stability of non-existent solid ammonium compounds. Both metallic ammonium and ammonium hydroxide are unstable at normal pressures. The former is most unlikely to be stabilized at high pressure, but it may be possible to synthesize the hydroxide in this way. The problem of the ionic radius of NH4+ is also discussed.
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