Hydrocarbons standard entropies

Thinh, T.R and IVong, T.K. (1976). Estimation of Standard Heats of Formation, /SH-y, Standard Entropies of Formation, ASr Standard Free Energies of Formation, AGt, and Absolute Entropies, ASt of Hydrocarbons from Group Contributions An Accurate Approach. Can./. Chem.Eng., 54,344. 

Thinh, T.P. and Trong, T.K. (1976) Estimation of standard heats of formation, AHx, standard entropies of formation, ASx, standard free energies of formation, AG, and absolute entropies, ASx, °f hydrocarbons from group contributions an accurate approach. Can.]. Che-m. Eng., 54, 344—357. 

Predict which of the hydrocarbons below has the greater standard molar entropy at 25°C. Explain your reasoning. 

Molar entropies increase as the size and complexity of the molecule increases. Compare, for example, the standard molar entropies of the three two-carbon hydrocarbons ... 

The standard states of these materials are taken as the pure components at 298 °K and a pressure of 101.3 kPa. The following data on the absolute entropies of the hydrocarbons at 298 °K are available. 

It is well known that such quantities as the standard free energy, enthalpy and entropy display a remarkable tendency to be additive functions of independent contributions of part-structures of the molecule. This property, on which the mathematical simplicity of many extrathermodynamic relationships is largely based, is well illustrated, for example, by the enthalpies of formation at 298°K of several homologous series of gaseous hydrocarbons Y(CH2)mH, which are expressed by the relation (28) (Stull et al., 1969). In... 

In general, the standard enthalpy of micellization is large and negative, and an increase in temperature results in an increase in the c.m.c. the positive entropy of micellization relates to the increased mobility of hydrocarbon side chains deep within the micelle as well as the hydrophobic effect. Hoffmann and Ulbricht have provided a detailed account of the thermodynamics of micellization, and the interested reader will find that their tabulated thermodynamic values and treatment of models for micellar aggregation processes are especially worthwhile. 

Bekkedahl, N., and H. Matheson Heat capacity, entropy and free energy of rubber hydrocarbon. J. Research Natl. Bur. Standards 15, 503—515 (1935). 

It appears that there are two temperatures of a universal nature that describe the thermodynamic properties for the dissolution of liquid hydrocarbons into water. The first of these, 7h is the temperature at which the heat of solution is zero and has a value of approximately 20°C for a variety of liquids. The second universal temperature is Ts, where the standard-state entropy change is zero and, as noted, Ts is about 140°C. The standard-state free energy change can be expressed in terms of these two temperatures, requiring knowledge only of the heat capacity change for an individual substance... 

Curved Airrhenius plots and negative standard isotopic entropies have been observed for proton abstraction from a hydrocarbon acid (Bell et al., 1956) and for proton transfer to hydrocarbon anions (Caldin and Kasparian, 1965). A negative A8° value has also been associated with ku/kd for proton transfer to allylmercuric iodide (Kreevoy et al., 1966b). 

Equation (2.182) is commonly used to calculate the standard enthalpy of adsorption [83, 160, 171, 186, 187]. The constant K usually exhibits a weak dependence on temperature. The value of AH° calculated from Eq. (2.182) was found to be in the range of +10 to -20 kJ/mol for various surfactants. As mentioned above AG lies in the range -20 to -60 kJ/mol, hence the standard free energy of adsorption is mainly controlled by the adsorption entropy, see Eq. (2.180), and the value of TAS can amount to 10 to 50kJ/mol. The most significant contribution of entropy was found for the water/oil interface [160]. The increase of AS due to adsorption can be ascribed mainly to the disorder of water structure in the solution bulk [83, 160]. In solution the hydrocarbon chains of the surfactant molecules are surrounded by a structured water shell, while during the adsorption these shells are destructed. This leads to an increase in entropy of the system. The entropy also increases due to the transfer of hydrocarbon chains from the water phase to the gas phase and, especially, to the oil phase where they become more flexible. 

Point 4 is related to the molecular motion discussed in Section 19.3. In general, the number of degrees of freedom for a molecule increases with increasing number of atoms, and thus the number of possible microstates also increases. Figure 19.13 compares the standard molar entropies of three hydrocarbons in the gas phase. Notice how the entropy increases as the number of atoms in the molecule increases. 

The thermodynamics of solute-solvent interactions is most conveniently described in terms of unitary quantities. The unitary free energy and unitary entropy changes accompanying some process (such as the transfer of hydrocarbon from nonpolar solvent to water or the transfer of hydrocarbon from pure hydrocarbon to water) are the standard free-energy and entropy changes corrected for any translational entropy terms (the cratic entropy) that are not intrinsic to the interaction under consideration. The cratic entropy is simply the entropy of mixing the solute and solvent into an ideal solution. With the cratic contribution removed, the unitary free energy and entropy contain only contributions to the thermodynamics of the process that come from the interaction of the individual solute molecules with the solvent. 

Table 2.1 Change in standard molar Gibbs energy, enthalpy and entropy (all in kj mot ) for the transfer of hydrocarbons from pure liquids into water at 25 °C (Prausnitz, Lichtenthaler and de Azevedo, 1999 Gill and Wadso, 1976). Notice the large negative entropy changes due to the hydrophobic effect, in the case of n-butane, the entropy decrease amounts to 85% of the Gibbs energy of solubilization, while for other hydrocarbons the entropic contribution is even larger...

API Research Project 44 of the National Bureau of Standards pertains to nearly all important physical andl thermodynamic data on hydrocarbons such as boiling point, vapor pressure, critical constants, viscosity, entropy, heat of combustion, etc. 

A FIGURE 13-7 Standard molar entropies of some hydrocarbons... 

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