Ammonium chloride, gaseous dissociation

Gaseous Dissociation of Ammonium Chloride. Like all ammonium salts, ammonium chloride can be volatilized by applying heat. Some of the other salts are permanently decomposed by the process, but the vapor of ammonium chloride can be condensed again to the same solid substance. 

In the decomposition of ammonium chloride, according to the equation NH4C1 = NH3+HCl, we have only two phases, solid and gaseous. The dissociation pressure, therefore, is not determined until we have fixed another variable besides the temperature, e,g. the concentration of either of the gaseous products of decomposition. AVe have then determined not only the total pressure, but also the partial pressure and concentration of the other product of decomposition. 

Formation of solid ammonium chloride from the gaseous complexes may be divided into three steps. In the first, endothermic, step the complexes are dissociated to form neutral NH3 and HCl molecules in the second, undoubtedly also endothermic, protons are transferred from HCl to NH3 molecules to form the separated NH and Cl ions in the third, exothermic step the ions are combined to form a crystal which is stabilized by sttong Coulomb attfactions between cations and anions. The system NHz HCl, then, provides another demonstration of how intermolecular interactions is the solid phase may lead to more polar or ionic suiicture than in the gas phase. 

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