Thermodynamic Properties of Neopentane from 4°K to the Melting Point and Comparison with Spectroscopic Data

Abstract

Abstract Heat capacities, latent heats and vapor pressures of solid and liquid neopentane were measured and the following values were obtained; heat of transition=628.7±0.3 cal/mol, T(trans. pt.)=140.498±0.05°K, heat of fusion=740.0±0.3 cal/mol, T(triple pt.)=256.750°K, and P(triple pt.)=268.47±0.13 Torr. The amount of impurity in the sample after purification was estimated from a plot of the equilibrium temperature against the reciprocal of the fraction melted. From the result the impurity present was found to be 0.0032 mol%. Despite of high purity, Cp value in the region below the melting point increases unexpectedly with temperature. This is interpreted as an effect of formation of vacancies in the crystals. The enthalpy required to form vacancies was estimated to be 3240±220 cal/mol from the analysis of the heat capacity curve in the region below the melting point. The molal entropy of the ideal gas at the boiling point, calculated from the calorimetric data, was 71.24±0.3 e. u. To bring this value into agreement with the value calculated from the infrared and Raman frequencies and the moments of inertia, requires the assumption of three equal potential energy maxima of 4.4 kcal/mol for hindered internal rotation of the methyl groups.