Heat capacities of selected chlorohydrocarbons
Highlights
► Liquid heat capacities of seven chlorinated hydrocarbons were measured. ► Ideal-gas thermodynamic properties were calculated. ► Vapor pressure eqs. were developed by simultaneous correlation with heat capacities. ► Newly obtained data fill gaps in heat-capacity databases.
Introduction
Heat capacity is a basic thermophysical property, which is necessary for energy balances in chemical engineering and process control. Differences between ideal gas and liquid heat capacity can be also used for reliable extrapolation of vapor pressure down to the triple point [1]. Although there is a large database of critically evaluated liquid heat capacities [2], [3], [4] there is still significant amount of compounds (including those of large industrial and environmental importance) for which liquid heat capacity data are unreliable or not available at all [5]. To fill these gaps and to provide data for improvement of previously developed estimation method [6], liquid heat capacities for seven chlorinated hydrocarbons (1,1,2-trichloroethylene, (Z)- and (E)-1,2-dichloroethylene, 1,1-dichloropropane, 1,2-dichloropropane, 2,2-dichloropropane, and (trichloromethyl)benzene; see Table 1) in the temperature range from 260 K to 340 K were measured by Tian–Calvet calorimetry. The studied compounds except (trichloromethyl)benzene exhibit relatively high vapor pressures in the temperature range of calorimetric measurements. Literature vapor pressure data available at higher temperatures were thermodynamically extrapolated to enable evaluation of evaporation corrections. For performing the extrapolation, heat capacities of ideal gas are needed; they were calculated using the methods of statistical thermodynamics based on fundamental vibrational frequencies and molecular structure data calculated by density functional theory (DFT) at the B3LYP/6-311+G(2df,p) level of theory.
Section snippets
Simultaneous treatment of vapor pressures and related thermal data (SimCor)
Correction of measured heat capacities for sample vaporization must be evaluated when vapor pressure of sample is significant and there is vapor space above the sample. In this case the measured quantity is the heat capacity of two-phase system Ct and the saturation liquid heat capacity Csat (which is below normal boiling temperature practically indistinguishable from isobaric heat capacity ) can be obtained from [4], [7]where V is the volume of
Materials
Samples of chlorinated hydrocarbons were of commercial origin. All compounds were used as received. Their purity is summarized in Table 1. Samples were treated under dry nitrogen atmosphere during the cells filling.
Liquid heat capacity measurement
The heat capacity measurements were performed by Tian–Calvet calorimetry (μDSC IIIa, Setaram, France), where calorimetric vessel (volume 1 cm3) is surrounded by a series of thermocouples detecting heat flow from/to vessel. Heat capacity was measured by the well known three-step
Ideal-gas thermodynamic properties
Ideal gas heat capacities are required for thermodynamic extrapolation of vapor pressures to the temperature range of calorimetric determination of . The thermodynamic properties in the ideal gas state were calculated by statistical mechanics using the rigid rotor–harmonic oscillator (RRHO) approximation with correction for internal rotations. Molecular geometry optimizations and vibrational frequencies calculations were performed using the density functional theory (DFT) at the
Conclusions
The heat capacities of seven selected halogenated hydrocarbons were determined by highly sensitive Tian–Calvet calorimetry for the liquid phase (temperature range from 260 K to 340 K) and calculated by the methods of statistical thermodynamics in combination with quantum mechanics for the ideal gaseous phase (temperature range from 100 K to 1000 K). Due to relatively high vapor pressures of most samples, the liquid heat capacities were corrected for the sample vaporization. The liquid heat
Acknowledgement
This work is supported by the Ministry of Education of the Czech Republic under project ME10049 and grant MSM 604 613 7307.
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