Standard molar entropy

About: Standard molar entropy is a research topic. Over the lifetime, 1586 publications have been published within this topic receiving 29886 citations.

Thermochemical property estimation of hydrogenated silicon clusters

Abstract: The thermochemical properties for selected hydrogenated silicon clusters (Si(x)H(y), x = 3-13, y = 0-18) were calculated using quantum chemical calculations and statistical thermodynamics. Standard enthalpy of formation at 298 K and standard entropy and constant pressure heat capacity at various temperatures, i.e., 298-6000 K, were calculated for 162 hydrogenated silicon clusters using G3//B3LYP. The hydrogenated silicon clusters contained ten to twenty fused Si-Si bonds, i.e., bonds participating in more than one three- to six-membered ring. The hydrogenated silicon clusters in this study involved different degrees of hydrogenation, i.e., the ratio of hydrogen to silicon atoms varied widely depending on the size of the cluster and/or degree of multifunctionality. A group additivity database composed of atom-centered groups and ring corrections, as well as bond-centered groups, was created to predict thermochemical properties most accurately. For the training set molecules, the average absolute deviation (AAD) comparing the G3//B3LYP values to the values obtained from the revised group additivity database for standard enthalpy of formation and entropy at 298 K and constant pressure heat capacity at 500, 1000, and 1500 K were 3.2%, 1.9%, 0.40%, 0.43%, and 0.53%, respectively. Sensitivity analysis of the revised group additivity parameter database revealed that the group parameters were able to predict the thermochemical properties of molecules that were not used in the training set within an AAD of 3.8% for standard enthalpy of formation at 298 K.

Thermodynamic data and kinetics of evolution for dilute solution of hydrogen in tantalum

Abstract: Using ultra-high vacuum techniques the equilibrium pressure Pe of hydrogen dissolved in a tantalum wire has been measured over the pressure-temerature-concentration ranges 10–7–10–2 torr (1 torr = 133.32 N m–2), 311–484 K and 0.19–3.33 atomic %(or 0.0019–0.0333 H/Ta atomic ratio) respectively. Ideal solution behaviour was observed for hydrogen concentrations below about 2.6 atomic % described by the expression C=√Pe(2.4±0.1)× 10–3 exp [(4.3±0.1)× 103/T], where C is in atomic %, Pe is in torr, and T is the temperature in K. The relative partial molar heat of solution ΔH° is –(36.0 ± 0.8)× 103 J g-atom–1 and the relative partial molar entropy, ΔS°, is –(50 ± 2) J deg.–1 g-atom–1. The rate of evolution of hydrogen dissolved in a tantalum wire in a continuosly pumped vacuum system was also studied over the pressure and temperature ranges 10–4–10–7 torr and 396–513 K respectively. The rate-limiting process was identified as the recombination of H atoms at the metal surface rather than diffusion in the metal bulk. The activation energy for this process is –(33.0 ± 1.5)× 103 J g-atom–1.

Thermodynamic Properties of Gaseous Carbon Disulfide.

Abstract: Efficient analytical representations of the thermodynamic properties for carbon disulfide remain open challenges in the communality of science and engineering. We present two analytical representations of the entropy and Gibbs free energy for gaseous carbon disulfide which we find to be of satisfactory accuracy and convenient for future use. The proposed two analytical representations merely rely on five molecular constants of the carbon disulfide molecule and avoid applications of a large number of experimental spectroscopy data. In the temperature range from 300 to 6000 K, the average relative deviations of the predicted molar entropy and reduced Gibbs free energy values from the National Institute of Standards and Technology database are 0.250 and 0.108%, respectively.