Showing papers on "Standard molar entropy published in 1987"

Nature of CO Adsorption during H 2 Oxidation in Relation to Modeling for CO Poisoning of a Fuel Cell Anode

Abstract: To understand the polarization loss due to poisoning by of a porous Pt anode under various conditions, poisoning losses have been measured in a half‐cell in the temperature range 110°–190°C in 100 weight percent for various mixtures of , , and gases. At a fixed current density, the poisoning loss, , was observed to vary linearly with ln . Deviation from linearity was observed at lower temperatures and higher current densities for high ratios. Considering only the linear portions, it has been possible to derive a general relationship for with temperature, concentration, and current density. The surface coverage by was calculated at various temperatures and was found to bear a linear relationship with ln . From the experimental Temkin isotherms, a general adsorption relationship has been obtained. The standard free energies for adsorption were calculated and were found to vary from −14.5 to −12.1 kcal/mol in the temperature range 130°–190°C. The standard entropy for adsorption was calculated to be −39 cal/mol K. Interpretations of the data indicate that adsorption occurs through 1:1 replacement of H by through the process of selective site poisoning. Under conditions where a nonlinearity in the poisoning relationship occurs, molecules undergo some orientation, favored by the increasing positive charges on the Pt surface.

Water adsorption on ZSM-5 and its aluminum free analog, silicalite

Abstract: The enthalpy, free energy, and entropy of water adsorption onto the zeolite ZSM-5 and its aluminum free analog, silicalite, were determined at 323.16 K. ZSM-5 contained a small number of strong adsorption sites as expected from its composition, but none were observed on silicalite. The sorbed water in both solids had a molar entropy very close to that of liquid water when more than about one-tenth of the sorption space was filled. The results support the contention that probably little water structuring occurs near a dehydroxylated silica surface.

Kinetic studies on human lactate dehydrogenase isoenzyme-catalyzed lactate-to-pyruvate reaction

Abstract: In order to evaluate the functional differences that may exist in human lactate dehydrogenase (LDH) isoenzymes widely used for clinical examination the kinetic and thermodynamic properties of the lactate to pyruvate reaction that they catalize were examined. Small but significant differences in the kinetic properties of the three isoenzymes were observed. The difference in the rate constants might affect the activity measurement of the individual isoenzyme as the initial velocity for the L-P reaction catalyzed will not be the same for an equal amount of enzyme. Equilibrium constants for the overall reaction in the presence and absence of pyruvate have been determined. On the basis of transition-state theory, the standard enthalpy and free-energy changes for formation of ternary activated complex were positive, while the standard entropy change was negative. Although the standard free-energy change was the same for activation by the three isoenzymes, the enthalpy and entropy changes for the LDH-3-catalyzed reaction were different from the respective values for others. A large positive value for the free-energy change and a negative value for the entropy change indicated unfavorable production of the activated complex (K infeq. sup╪ =1.89×10-16). The enzyme appears to stabilize and retain the activated complex until it dissociates into the products.

Thermodynamic study of oxygen in liquid InPb and InTl alloys

Abstract: The activity coefficients γ o of oxygen in liquid InPb and InTl alloys at 1073 and 1273 K have been measured over the entire composition range utilizing a modified coulometric titration method with the electrochemical cell Re, O in liquid InPb or InTl alloys ¦ZrO 2 (+CaO)¦air, Pt. The molar enthalpy change ΔH o ° and the molar entropy change ΔS o ° of oxygen dissolution for 1 2 O 2 (101 325 Pa) → O(1 mol.%) were evaluated from the results. The In γ o , ΔH o ° and ΔS o ° values exhibit a large negative deviation from the line corresponding to the additive equation.

The thermodynamics of vaporization and formation of samarium(III) oxide telluride

Abstract: The thermal decomposition of solid samarium(III) oxide telluride (SmO)2Te was studied over the temperature range 1788–1972 K by the Knudsen effusion weight-loss technique. The compound decomposes to yield B-Sm2O3(s) and a gas phase containing samarium and tellurium in a molar ratio of 2:3. Using enthalpy and entropy data from the literature for gaseous Sm, Te, SmTe and Te2, it could be established from thermodynamic calculations that, within the given temperature range, samarium and tellurium are the principal vapour species in equilibrium with solid (SmO)2Te and B-Sm2O3 (monoclinic modification), while SmTe and Te2 are of minor and very minor importance respectively. Using estimated values for the standard entropy and heat capacity of (SmO)2Te, the following second-law (index II) and third-law (index III) enthalpies and entropies for the reaction (SmO) 2 Te(s) → 2 3 Sm 2 O 3 (s) + 2 3 Sm(g) + Te(g) were found: ΔH298 °(II) = 679.0 ± 10.0 kJ mol−1; ΔH298°(III) = 686.7 ± 19.7 kJ mol−1; ΔS298°(II) = 208.9 ± 6.0 J mol−1 K−1; ΔS298°(III) = 213.1 J mol−1 K−1. A value of −1545 ± 15 kJ mol−1 was derived for the standard enthalpy of formation of (SmO)2Te. Possible trends of the enthalpies of formation of all compounds (LnO)2Te (Ln ≡ La-Lu) are discussed.

Thermodynamics of Ion Exchange on Ammonium 12-Molybdophosphate

Abstract: Standard free energies, enthalpies and entropies for the exchange of NH4 + ions on AMP are reported for eight cationic species For those ions which are weakly adsorbed (eg Li+, Na+, H3O +, Ag+ and K+)ΔG° values are positive, as expected, whereas for reactions involving Rb+, Cs+ and Tl+, ΔG° values are negative Variations in enthalpy and entropy terms for the various exchange reactions are discussed in terms of the energies required to remove the hydration shells of ingoing ions relative to that of the NH4 + ions and the energy term for the electrostatic interaction between the cations and the lattice anions

Relationship among Entropies,Volumes and Internal Pressures of Liquid Water and Aqueous Prototropic Ions

Abstract: According to the Webb theory of ionic hydration modified by the Onsager reaction field, an internal pressure of liquid water p in (H 2 O) is calculated by using exclusively the physical properties of water at the temperature T =298 K and the external pressure 1 atm; p in (H 2 O)=11.6 kbar, which is the pressure of liquid water as an elastic fluid equated to an electrostatic force per unit area at the surface of an ionic cavity. The cavity has been charged up to the elementary unit of e , and takes the same radius as that of the Onsager cavity for water, 1.93 A. The calculated value agrees exactly with the value of T S 0 / V 0 in terms of the standard molar entropy and volume of water, S 0 and V 0 , and also favorably with T Δ S i 0 / Δ V i 0 =10.9 kbar in terms of the standard entropy change and volume change in the self-ionization of liquid water, Δ S i 0 and Δ V i 0 . This relationship may be extended to the individual aqueous ions of H + and OH - .