Relative Partial Molar Enthalpy Research Articles

Activity coefficients and relative partial molal enthalpies of substituted anilines in aqueous perchloric acid solutions

Activity coefficients and relative partial molal enthalpies have been deduced from measurements of the solubilities of 2,4-dinitroaniline, 2,6-dichloro-4-nitroaniline, and 2,4-dichloro-6-nitroaniline in aqueous perchloric acid at four temperatures. The results are discussed in relation to the Hammett acidity function H0 for aqueous perchloric acid and enthalpies of protonation of Hammett bases as a function of perchloric acid concentration.

Enthalpies of Dilution of Aqueous Electrolytes: Sulfuric Acid, Hydrochloric Acid, and Lithium Chloride.

Calorimetric measurements at 25 °C of the enthalpies of dilution of aqueous H2SO4 (0.00090 to 6.4 mol · kg-1), LiCl (0.026 to 6.7 mol · kg-1), and HCl (0.018 to 1.6 mol · kg-1) have been performed using two different isothermal calorimeters. The results of this work and that of three earlier calorimetric investigations and one Raman spectral investigation have been used to calculate values of the relative apparent molal enthalpies, and relative partial molal enthalpies for these electrolytes.

Thermochemistry of aqueous solutions of alkylated nucleic acid bases

Apparent molar relative enthalpies of dilution phi(L)of aqueous solutions of a series of alkylated diketopyrimidines: m(1)Ura, m(1,3)(2)Ura, m(1,3)(2)Thy, m(1,3,6)(3)Ura and e(1,3)(2)Thy were measured as a function of concentration of the solutes at three temperatures 298.15, 308.15 and 318.15 K. Dilution proved to be an endothermic process over the whole range of molalities m and temperatures studied for all compounds except the e(1,3)(2)Thy solution, the dilution of which, with the exception of the lowest concentrations (m > 0.2-0.3) was an exothermic process. Partial molar relative enthalpies of dilution -L(2)(m) derived from phi(L)(m) functions were analysed as if they were composed of two additive contributions: an endothermic one -L(2), (m(1)) and an exothermic one 1.2, (m(as)0), owing to the presence in the solutions of a free monomer m(1) or associated species m(as), respectively. Partial molar heat capacities of the solutes, evaluated by differentiation of -L(2)(m) functions in respect to temperature, decreased with the rise of concentration in the order of the tendency of the solutes to stacking association. Changes in heat content and in heat capacity of solutions upon their dilution are interpreted in terms of involvement of hydrophobic hydration and association of the solutes.

Hydrogen-diffusion-rate-limited hydriding and dehydriding kinetics

Hydriding and dehydriding kinetics are derived within the framework of the Johnson-Mehl-Avrami equation. Hydriding is considered to be rate limited by hydrogen diffusion through a growing hydride layer, and dehydriding by hydrogen diffusion through a growing metal layer. Incubation time effects due to surface contamination are taken into account by a delayed nucleation function. The dominant composition-dependent terms in the thermodynamics and in the mobility are taken into account to derive normative diffusivity for hydrogen. The temperature dependence of hydriding is defined by an activation energy (QD−ΔH̄H+ΔH12) where QD is the activation energy for diffusion of hydrogen in the hydride, ΔH̄H is the relative partial molar enthalpy of hydrogen in the hydride, and ΔH12 is the enthalpy of the hydrogen-pressure plateau reaction. For dehydriding the activation energy is (QD+ΔH̄H−ΔH12), but now QD and ΔH̄H refer to hydrogen in the metal. The solution thermodynamics contributions to the activation energy are shown to be very important. It is emphasized that in order to determine a meaningful activation energy, it is not the pressure that must be maintained constant but rather a factor T[1−(Pd/P)1/2] in hydriding and a factor [T[1−(P/Pd)1/2] in dehydriding, where Pd is the dissociation pressure of the hydride. Several hydriding studies from the literature are critically disccused in terms of these results.

A thermodynamic analysis of the solubility of nitrogen gas in manganese in the range 13–25 at.% N

The solubility of N 2(g) in Mn was measured in situ using TGA and DTG techniques. The temperature range was 650–1030 °C and the N 2 pressure range was 3.5–750 mmHg. A series of isobars was obtained covering the nitrogen concentration range 13–25 at.%. This range included the following phases: ϵ(≈Mn 4N) and ζ, ζ′ over a wide range of stability above 900 °C. The relative chemical potential of nitrogen dissolution for the ζ(ζ′) phase was found to be linear with N concentration (at.%) at constant temperature. The relative partial molar entropies and enthalpies of nitrogen dissolution were determined for the ϵ and ζ(ζ′) phases. Using these quantities a phase change ζ ag ζ′ was identified.

ChemInform Abstract: ABSORPTION OF HYDROGEN BY TITANIUM‐COBALT AND TITANIUM‐NICKEL INTERMETALLIC ALLOYS. PART 1. EXPERIMENTAL RESULTS

AbstractDie Druck‐Zusammensetzungs‐Temp.‐Beziehung der Systeme TiCo + H und TiNi + H werden im Druckbereich 10 kPa‐2 MPa und im Temp.‐Bereich 325‐600 K bestimmt und die entsprechenden partiellen molaren Enthalpien und Entropien abgeleitet.

Absorption of hydrogen by titanium–cobalt and titanium–nickel intermetallic alloys. Part 1.—Experimental results

Pressure–composition–temperature relationships are presented for the titanium–cobalt + hydrogen and the titanium–nickel + hydrogen systems for the ranges 10 kPa to 2 MPa and 325 to 600 K. The TiCo + H system exhibits three distinct two-phase regions, whereas the TiNi + H system contains no two-phase regions in this range of temperatures and pressures. Values for the relative partial molar enthalpy and entropy have been obtained from the equilibrium data.For TiCo, Δtext-decoration:overlineH⊖H2 is –45 kJ (mol H2)–1 at infinite dilution. The enthalpy then decreases to a value of –54 kJ (mol H2)–1 at the first plateau, and then increases to –48 kJ (mol H2)–1 at the second plateau, and –40 kJ (mol H2)–1 at the third plateau. The entropy decreases with hydrogen content to a value of –135 J K–1(mol H2)–1 at TiCoH1.0 and then remains fairly constant as the hydrogen content is increased.For TiNi, the enthalpy at infinite dilution is –58 kJ (mol H2)–1. The enthalpy then decreases to a miniumum of –60 kJ (mol H2)–1 at TiNiH0.9, before increasing rapidly at higher hydrogen contents. The entropy decreases to –140 J K–1(mol H2)–1 at TiNiH0.9, but then apparently increases at higher hydrogen contents. This ususual entropy effect is attributed to changes in the number of interstitial sites available to hydrogen, leading to an increase in the configurational entropy at higher hydrogen contents.Data are also presented which describe the temperature-programmed decomposition of the hydrides. The TiCo hydrides decompose in three stages corresponding to the three plateau regions of the isotherms. In contrast, the solubility of hydrogen in the TiNi alloy initially decreases with temperature, but at 700 K a reversal takes place and the solubility of hydrogen increases reaching a maximum at 850 K, before decreasing to zero at 950 K. These solubility effects may be caused by short range ordering of the atoms which surround tetrahedral interstitial sites, resulting in the creation of titanium-rich sites. Parallels are drawn with the martensitic transition which is known to occur in the TiNi intermetallic alloy.

Solubility of Hydrogen in CuTi, CuTi2, PdTi2, and Cu0.5Pd0.5Ti2. Reactions of Titanium Alloys with Gaseous Hydrogen)

AbstractCuTi2, PdTi2, and Cu0.5Pd0.5 Ti2 are isotypic (MoSi2‐structure type). The solubility of hydrogen in these alloys and in CuTi was measured by means of a volumetric method in the range 0.01 bar ≤ PH2 ≤ bar and 250°C ≤ T ≤ 850°C. The thermodynamic data were derived from the absorption isotherms found. The relative partial molar enthalpies of solution at infinite dilution ΔHH0 increase in the following order: α‐Ti (‐45.5kJ/mol H); Cu0.5Pd0.5Ti2 (‐40.5kJ/mol H); CuTi (‐37.5kJ/mol H); PdTi2 (‐37kJ/mol H and ‐33.5 kJ/mol H at 600850°C and 400700°C respectively); CuTi2 (‐33kJ/mol H). ‐ From the excess entropies and enthalpies it is concluded that the number of available interstitial sites for the H‐atoms is reduced in the alloys compared with α‐Ti, thus indicating a preferred occupation of interstices which are tetrahedrally surrounded by Ti‐atoms only. ‐ CuTi, CuTi2 and Cu0.5Pd0.5Ti2 show decomposition reactions by which titanium hydrides and other intermetallic compounds are formed. For the systems CuTi‐H and CuTi2‐H the decomposition steps could be analysed thermodynamically and roentgenographically up to the end products (copper and γ‐titanium hydride). ‐ The experimentally determined enthalpies of formation of CuTi and CuTi2, are compared with calculated ones. ‐ PdTi2 dissolves hydrogen gas up to the composition PdTi2H2 which can be explained by a complete occupation of the interstices formed by Ti‐tetrahedrons.

On the solubility of carbon in b.c.c. transition metals

Abstract The relative partial molar enthalpies of carbon in b.c.c. metals belonging to the first long period of the periodic table are given in terms of the electronic structure of the alloys. The calculation shows that the solubility of carbon decreases from titanium through iron when a self-consistent shift of the resonant state is taken into account.

Enthalpies of Lyotropic Liquid Crystalline Phases. : 2. The Partial Molar Enthalpies of the Lamellar and Hexagonal Phases in the System N-Pentanol-Sodium N-Octanoate-Water

Abstract The relative partial molar enthalpies of the components in the system N-pentanol-sodium N-octanoate-water (C5OH-NaC8-H2O) have been determined in the lamellar (D-) and hexagonal (E-) liquid crystalline phases formed in this system at 25°C. The enthalpies are correlated with Bragg spacings and earlier nmr studies. They indicate that (a) the minimum amount of water needed to hydrate the polar groups limits the region of existence of both D- and E-phase towards low water contents, (b) the interaction between -OH and -COO is especially important in stabilizing the D-phase containing large amounts of water, (c) the Bragg spacings and the enthalpies both change their concentration dependence markedly when the maximum amount of water that can be bound by primary hydration of the polar groups and the counter ions in the D-phase is exceeded, (d) enthalpies of transition from one phase to another are small compared to enthalpy changes with composition within the phases; the same holds for Bragg spacings. H...

The solubility of hydrogen in NbMo alloy

The solubility of hydrogen in Nb and in alloys of Nb and Mo containing an atomic fraction of Nb between 0.90 and 0.50 was measured in the temperature range 300 – 600 °C, at hydrogen pressures of 3 – 600 mmHg. From the results thermodynamic quantities of dissolved hydrogen were evaluated. It was found that the nature of the interaction between the hydrogen atoms changes from attractive to repulsive as the molybdenum content increases. The variation of the relative partial molar enthalpy of the dissolved hydrogen with the Mo content is discussed in terms of the electronic structure of the alloy.

Solution of hydrogen in palladium/copper alloys

The solution of hydrogen in Pd Cu fcc substitutional alloys has been examined in the region of low hydrogen contents. Relative partial molar enthalpies of absorption at infinite dilution are found to be somewhat less exothermic for the alloys than for palladium but the change with X Cu is small. Partial excess entropies of absorption decrease with X Cu but not as markedly as for the other Pd/group IB alloys. Relative electrical resistance relationships for Pd Cu alloys have been determined as a function of their hydrogen contents. Interstitial hydrogen increases the relative electrical resistance to a lesser extent with increase of X Cu, but the absolute increase of resistance is nearly invariant with X Cu.

The high temperature phase diagrams for Th-Mo, Th-Re, U-Mo, and U-Re; derived thermodynamic properties of refractory metal solutes in liquid thorium and uranium

Abstract The high temperature regions of the Th-Mo, Th-Re, U-Mo, and U-Re phase diagrams have been determined from 1100 to 2300°C by techniques developed in this laboratory. The present liquidus temperatures for the Th-Mo and U-Re systems are up to 700° higher than those previously reported. The likely reasons for these discrepancies are discussed. From the measured solubilities of molybdenum in liquid thorium are derived the limiting values of the relative partial molar enthalpy and excess entropy of molybdenum, −1130 ± 900 cal/g. atom and −2.2 ± 0.5 cal/g. atom/deg ., respectively; for molybdenum dissolved in liquid uranium the values are −7300 ± 300 cal/g. atom and −4.3 ± 0.2 cal/g. atom/deg ., respectively. Previously reported solubilities of tantalum and tungsten in liquid thorium and uranium are re-examined thermodynamically in the light of substantial departure from Henry's Law at solute concentrations less than 10 at.%. The measured activities of the solutes (Mo, Ta, W) indicate strong positive deviations from Raoultian behavior except in the case of the liquid U-Mo solution which exhibits an evolution from negative deviations to positive deviations with increasing temperature. In all cases, with the possible exception of U-Ta, the solutions do not approach regular solution behavior.

Thermodynamics of the solution of hydrogen in LaNi 5 at small hydrogen contents

Partial thermodynamic parameters for the solution of hydrogen in LaNi 5 have been determined by p-c-T methods on unactivated bulk samples of LaNi 5. The relative partial molar enthalpy of solution of hydrogen, at infinite dilution of hydrogen, is −33,9 kJ (g atom H) −1 (~425 K). This is more exothermic than the heat of absorption corresponding to the formation of the hydride phase from the hydrogen-saturated intermetallic compound. The partial molar enthalpy of solution becomes more endothermic with hydrogen content in the region of solubility in the pure intermetallic compound. The partial thermodynamic quantities at infinite dilution have been found to vary with the stoichiometry of the LaNi 5+ x where x varies from −0.2 to + 0.5. The partial molar enthalpy of absorption is most exothermic for the stoichiometric compound.

The effect of lattice defects on hydrogen solubility in palladium: I. Experimentally observed solubility enhancements and thermodynamics of absorption

Measurable, and reproducible solubility enhancements have been observed for hydrogen in cold-worked palladium in the low content α-phase (273–363 K). It is shown that the solubility of hydrogen is unaffected in palladium samples which had been quenched from near their melting points. It is concluded that the stress field about the dislocation array is the principal cause of the solubility enhancement. Absorption of hydrogen at the cores of dislocations is believed to be insignificant. From absorption isotherms measured at various temperatures, the relative partial molar enthalpies and entropies of solution of hydrogen into deformed palladium have been determined. The heat of solution of hydrogen by 78% deformed palladium is approximately 880 J/H more exothermic than for well-annealed palladium in the low hydrogen content α-phase.

Pressure-composition-temperature relationships in niobium alloy-hydrogen systems

Pressure-composition-temperature relationships have been determined for the solution of hydrogen in a series of alloys of niobium with Ti, Mo, Cu, Pd, Ni, Sn, and Al, over the temperature range 623–1123 K and at pressures up to 1 atm. All the niobium alloys, except with titanium, have a lower affinity for hydrogen than has pure niobium. This is reflected both as a decrease in the solubility of hydrogen at any reference temperature and pressure, and also as an increase in the relative partial molar enthalpy of solution of hydrogen at infinite dilution (ΔH̄∞H).The extent to which each alloying element affects the solubility of hydrogen in niobium cannot be accounted for on the basis of changes in the electron/atom ratio of the metal. Instead, the results are interpreted as evidence that the band structure and density of states of niobium are modified in different ways, depending on whether the alloying metal is a transition or non-transition element.It is shown that the large changes in ΔH̄∞H between titanium, niobium and molybdenum, are not due to the differences in the density of states of the metals. The difference in the density of states of titanium and molybdenum could account for only about 10 K J mol−1 of the observed 210 K J mol−1 change in ΔH̄∞H.

The effect of lattice defects on Hydrogen solubility in palladium: II. Interpretation of solubility enhancements

It is proposed that the stress-field of the dislocation array is the principal cause of the solubility enhancement observed in the low hydrogen content, α-phase of cold-worked palladium. With an assumed uniform dislocation density of 9 × 1011cm−2, the observed solubility enhancement of 1.65 (298 K) for heavily cold-worked palladium can be reproduced if the core radius is assumed to be 2 × Burgers vector. The temperature dependence of the solubility enhancement is also reasonably well predicted. The significance of measured relative partial molar enthalpies and entropies of absorption of hydrogen into cold-worked palladium is examined.

The thermodynamic functions of hydrogen in solid solutions of the group V metals Nb, Ta and V

The large volume of available data for the isotherms characterizing the dissolution of H from H 2-gas into the Group V metals Nb, Ta and V has been reviewed. Both the temperature and composition variation of the partial molar and relative partial molar enthalpies and entropies have been discussed in terms of theoretical models in which the H-atoms in solution can possess degrees of translational freedom. The relative invariance of the relative partial enthalpy Δ H u to temperature and the change in H u with temperature are in accord with this concept. The variation of H u with solute composition has been used in conjunction with recent elastic data for HNb, HTa and HV solid solutions to discuss the form of the HH interaction in these systems.

Terminal solid solubility of hydrogen in the zirconium–2.5 weight % niobium alloy

The terminal solid solubility (TSS) of hydrogen in the Zr–2.5 wt. % Nb alloy has been determined by measuring the decomposition pressures of several Zr–2.5 wt.% Nb alloys containing hydrogen in the temperature range 425 to 900°C. The TSS boundaries α/(α+δ) and α/(α+β) were determined from the points of inflection of the van't Hoff decomposition pressure against temperature plots. The TSS of hydrogen in Zr–2.5 wt. % Nb alloy is about 1 to 2.5 atom % more than that in unalloyed zirconium (the maximum difference being at the eutectoid temperature of the Zr/H2 system). The decomposition pressure against temperature relation in the α and (α+δ) phase regions and the temperature against concentration relation along the TSS boundary have also been derived. The mean relative partial molar enthalpy of hydrogen in the α-phase and the mean enthalpy of formation of δ-hydride from the saturated α-phase in the alloy are –24.6 kcal (mol H2)–1 and –40.9 kcal (mol H2)–1 respectively.

Thermodynamics of hydrochloric acid in water + 50 mass per cent of ethylene carbonate from e.m.f. measurements at 293.15 to 328.15 K

Electromotive force measurements of cells of the type: Pt|H 2(g)HCl(m) in H 2 + ethylene carbonate|AgCl|Ag at 8 temperatures from 293.15 to 328.15 K have been used to derive (a) the standard e.m.f. of the cell in water + 50 mass per cent ethylene carbonate, (b) the activity coefficient of hydrochloric acid, (c) the relative partial molar enthalpy and relative partial molar heat capacity of HCl at 298.15 K, and (d) the thermodynamic quantities for the transfer of HCl from the standard state in water to the standard state in water + 50 mass per cent ethylene carbonate. The molality m of the acid ranged from 0.0098 to 0.25 mol kg −1. The standard e.m.f. was determined by an extrapolation method making use of the extended terms of the Debye-Hückel theory.