Thermal Properties Based upon Heat Capacity and Decomposition Pressure of Tetramethylammonium Hydrogen Dichloride. Nature of the Hydrogen Bond in the [Cl–H–Cl]— Ion

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Abstract
The heat capacity of tetramethylammonium hydrogen dichloride was determined from 5° to 350°K by adiabatic calorimetry and the derived thermodynamic properties were calculated. No thermal anomalies were found. Molal values of heat capacity, entropy, and free‐energy function at 298.15°K are 49.07, 60.63, and —31.72 cal mole—1 °K—1, respectively. The decomposition pressure of tetramethylammonium hydrogen dichloride, measured from 340° to 380°K with a Bourdon gauge as the null indicator, can be represented by the equation log10Pmm=—28.4677–1698.5 T—1+15.502 log10T—0.012256 T. Extrapolated values at 298.15° and 410.8°K are 1.8 and 760 mm Hg, respectively. Entropy increments for the decomposition reaction were calculated from the equilibrium data and compared with third‐law entropy increment values derived from the thermal data. Excellent agreement was obtained. This provides strong support for the location of the equilibrium position of the hydrogen ion midway between the two chloride ions.