Showing papers on "Enthalpy published in 1983"

Thermochemical Data for Gaseous Monoxides

Abstract: Values for standard enthalpies of formation and dissociation energies for gaseous diatomic monoxides have been selected by critical assessment of experimental data from the literature. Gibbs energy functions, (−(G○T −H○298)/T), and enthalpy functions, (H○T −H○298), have been calculated from literature values for molecular parameters. Computer methods of storage, processing and retrieval are described and the resulting data are given in tables 4 to 11.

Enthalpy-entropy compensation and heat capacity changes for protein-ligand interactions: general thermodynamic models and data for the binding of nucleotides to ribonuclease A.

Abstract: General thermodynamic models are presented that can account for the existence of heat capacity changes and compensation between the enthalpy and entropy changes in protein-ligand interactions. The models involve the coupling between some type of transition in the state of the protein (or ligand) and the binding process. The coupled transition may be a proton dissociation, the binding of a second ligand, a change in the degree of aggregation, or a conformational change in either the protein or ligand. Both mandatory coupling and nonmandatory coupling between the binding process and the transition are considered. The model is also extended to include a multistate transition of the protein. Computer simulations show that apparently linear compensation plots (plots of delta H degrees vs. delta S degrees) with a slope approximately equal to the experimental temperature are to be expected for the binding of a ligand to a protein when such coupled reactions exist. Also heat capacity changes, which may be either positive or negative, are to be expected to accompany the reaction. Experimental thermodynamic data for the binding of cytidine 3'-phosphate to ribonuclease A are presented. These data demonstrate apparent enthalpy-entropy compensation when pH and ionic strength are varied. A negative heat capacity change, ranging from -145 (at mu = 1.0 M) to -225 cal/(mol X deg) (at mu = 0.05 M), is also observed for this protein-ligand interaction. The apparent compensation and heat capacity change data are interpreted according to the models presented.

Formation and decomposition of magnesium hydride

Abstract: The absorption of hydrogen in magnesium (purity, 99.8%–99.94%) was studied in the temperature range 260–425 °C and at pressures from the equilibrium value to 2 MPa above equilibrium. At constant temperature the absorption rate depends on the pressure whereas total absorption is attained in approximately the same time regardless of the pressure. The final composition is very close to stoichiometric MgH2 except at the lowest temperatures and highest pressures investigated when the reaction becomes extremely slow or ceases completely at 80%–90% of the stoichiometric composition. Accurate determination of the relative molar enthalpy gives a value of −70 kJ (mol H2)−1. No hysteresis in the ordinary sense was observed, but although no change in the plateau pressure occurred desorption of from 5%–15% of the remaining hydrogen required almost zero pressure. The actual value appears to be dependent on the material.

Thermal behaviour of carbohydrates studied by heat flow calorimetry

Abstract: The technique of heat flow calorimetry was used to study the thermal behaviour of different carbohydrates between 20°C and 270°C. The samples were analyzed by heating in sealed cells. The temperature range in which exothermic reactions, due to thermal decomposition, occurred varied widely depending on the type of carbohydrate investigated. Reaction enthalpies of 44 sugars and polysaccharides are given. Endothermic phenomena, such as fusion or vaporization of crystallized water, were also observed: fusion temperatures and enthalpies of 34 sugars and sugar alcohols are listed. Calorimetric curves showing crystallization of amorphous sucrose, cellobiose and lactose are also presented.

The Enthalpy Effect on Forming Diluted Solid Solutions of Two 4d and 5d Transition Metals

Abstract: Predictions of the enthalpies of solution in the solid state will be presented for arbitrary combinations of 4d and 5d transition metals. The approach is similar to the one used in a previous paper on combinations of two 4d metals. In addition to the contribution stemming from the chemical interaction, which is equal to the enthalpy of solution in the liquid state, the enthalpy of solution in the solid state contains two other terms. The first of these accounts for the elastic energy arising from the atomic size mismatch, where this size mismatch has been corrected for volume changes that accompany the alloy formation. The second term takes into account the variation of the structure dependent energy in relation to the number of d-electrons per atom. – The predictions are generally in good agreement with the solubilities found in the respective phase diagrams. For two systems, Zr in Pt and Pt in Nb, that on first sight present exceptions, preliminary experiments indicate that parts of the respective phase diagrams have to be revised. The structure-dependent contribution manifests itself not only in the limiting solubilities in the main crystal structures of the pure metals, but also in the existence of disordered intermediate phases and may also be responsible for the anomalously large entropy of fusion recently observed for some transition metals.

Thermodynamics and kinetics of protein incorporation into membranes

Abstract: The free energy and enthalpy of protein incorporation into membranes are calculated with special emphasis on the hitherto neglected effects of immobilization of protein and perturbation of lipid order in the membrane. The free energy change is found to be determined by the hydrophobic effect as the driving force for incorporation and the protein immobilization effect which leads to a considerable reduction of the free energy gained from the hydrophobic effect. For incorporation of a hydrophobic, bilayer-spanning alpha-helix, the free energy change obtained is of the order of -15 kcal/mol (1 cal = 4.184 J) in agreement with experimental results. The lipid perturbation effect yields only a small contribution to the free energy change due to an energy/entropy compensation inherent in lipid order. This effect dominates the enthalpy change, giving rise to values on the order of 100 kcal/mol with a pronounced temperature dependence around the lipid phase transition as observed experimentally. The kinetics of protein incorporation are even more strongly affected by the lipid perturbation effect, leading to an abrupt decrease of the rate of incorporation below the lipid phase transition.

The enthalpy of solution for solid binary alloys of two 4d-transition metals

Abstract: A demonstration is given how to derive the enthalpy of solid solution of alloys of two 4d transition metals for the three main crystallographic structures, fcc, hcp and bcc. Because of two additional terms the enthalpy of solution for solid phases differs from that in the corresponding liquid alloys. The first of these, accounting for elastic mismatch, can be estimated from Eshelby/Friedel-type elastic continuum theory. In our case we correct the atomic size mismatch for volume changes that accompany the formation of alloys. The second term accounts for the variation of structure dependent energies (i.e. the preference for either the fcc, bcc or hcp type of crystal structure) with the average number of electrons per atom. We derive an empirical curve for these structure dependent enthalpy terms, which is essentially based on theoretical results and gives a fair account of binary phase diagram characteristics.

The partial excess thermodynamic properties of hydrogen in palladium

Abstract: New measurements of the hydrogen chemical potential and the partial enthalpy of hydrogen in Pd-H alloys are reported. These results are combined with information available in the literature to obtain a consistent set of values for the partial excess enthalpy and entropy of hydrogen as a function of composition. The partial excess entropy of hydrogen varies from negative values at low hydrogen concentrations to positive values at high hydrogen concentrations and it is demonstrated that the principal contribution to this excess entropy is configurational. Problems associated with trying to interpret these properties in terms of pairwise interaction models are discussed.

Heat capacity, relative enthalpy, and calorimetric entropy of silicate minerals; an empirical method of prediction

Abstract: A procedure to estimate the standard molar heat capacity, relative enthalpy, and calorimetric entropy of silicate minerals has been devised and evaluated. These are estimated by summing, in appropriate proportions, fictive molar isobaric heat capacities, relative enthalpies, and entropies evaluated for structural components of the mineral phases such as MgO-4, MgO-6, or MgO-8 where the Mg is in 4-,6-, or 8-fold coordination, respectively. The fictive molar heat capacities and entropies were obtained from a large body of experimental calorimetric data on heat capacity, entropy, and relative enthalpy for minerals. The summation technique has a precision better than2To for heat capacity and relative enthalpy, and 5Vo for entropy, relative to the data base, between 29E and 1500 K. The accuracy of prediction of molar heat capacity and relative enthalpy for mineral phases and specific heat for rocks is expected to be within 3Tousing this technique. The accuracy of prediction of molar calorimetric entropy is expected to be within 5% using this technique. Tables of evaluated coefficients for the structural components of mineral phases are given. Three examples of calculations are given. (1) The estimated heat capacity of an illite is calculated and compared with experimental data which was not used in the evaluation. (2) The estimated relative enthalpy of acmite is calculated and compared with experimental data also not used in the evaluation. (3) The estimated calorimetric molar entropy of an illite is calculated and compared with data not used in the evaluation. In each of these cases, the estimated values deviate less than 2.5Vo from the observed values.

Equation of state and thermodynamic properties of liquid methanol from 298 to 489 K and pressures to 1040 bars

Abstract: The PVT properties of liquid methanol have been measured with a direct-weighing PVT apparatus, from 298 to 489 K and pressures to 1040 bar. The apparatus and the method are described. The results have been fitted to a 16-constant equation of state that has been used, together with published data for vapor pressure and saturated-vapor densities, to calculate the following properties of saturated and compressed liquid methanol over the experimental range: specific volume, isothermal compressibility, thermal expansion coefficient, thermal pressure coefficient, enthalpy, entropy, and constant-pressure heat capacity. The calculated properties are presented in tabular form, at round values of P and T.

Thermodynamics of aqueous calcium chloride

Abstract: Various published measurements of the osmotic coefficient and the apparent molal enthalpy of aqueous CaCl2 are examined and correlated. It is found that CaCl2 shows an anomalous behavior near 5 molal. Below that molality the various properties are well represented by the usual ion-interaction equation with simple temperature dependent expressions for the second and third virial parameters. Information from neutron diffraction and crystal structure is considered in discussing the anomalous behavior at high molality.

Heat Capacity and Enthalpy Behavior Near Phase Transitions in Some Alkylcyanobiphenyls

Abstract: An adiabatic scanning calorimeter has been used in a calorimetric investigation of alkyl-cyanobiphenyl (nCB) liquid crystals. Measurements near solid to smectic (KA), smectic A to nematic (AN) and nematic to isotropic (NI) or smectic A to isotropic (AI) phase transitions have been carried out for scanning rates as low as a few μKs−1. Results for the temperature dependence of the heat capacity as well as for the enthalpy (including latent heats) have been obtained for 8CB, 9CB, 10CB and 11CB. Latent heat values are reported for the different KA, NI and AI transitions. For the two compounds 8CB and 9CB, which show a AN transition, it was found that, within the resolution of the experiments, these transitions are second-order. The anomalous heat capacity observed near the AN transition is much larger for 9CB than for 8CB. The critical exponent α obtained from power law fits to the heat capacity data is 0.31 ± 0.03 for 8CB and 0.50 ± 0.05 for 9CB. It is suggested that the AN transition in 9CB must be...

Equilibrium vapor pressure measurements for the n-alcohols in the temperature range from −30°C to +30°C

Abstract: We measured the equilibrium vapor pressures pc ranging from 240 mmHg for methanol down to 5 · 10−3 mmHg for n-hexanol, using capacitance pressure transducers. Good agreement is observed between literature data obtained by mercury manometers and our data in the ranges of overlap. Our data are found to be well described by the empirical Dupre-Rankine formula. We analysed the thermodynamic basis of this relation and found its parameters to be interrelated. Once a reference temperature is chosen the number of adjustable parameters is reduced by one. The remaining parameters are physically relevant quantities, the molar vaporization enthalpy ΔHmv, and the difference of the molar specific heats of vapor and liquid phase ΔCmp. We evaluated these quantities from our measurements, and we give an overall fit of a combination of literature data and ours ranging from the boiling point to − 30 °C.

Fluid mixtures at high pressures

Abstract: In the present survey some important trends in the high pressure thermodynamics of fluid mixtures of non-electrolytes are reviewed. First the pressure dependence of excess functions such as the excess Gibbs energy G, the excess enthalpy 5E, the excess en— tropy 5E, and the excess heat capacity CE is discussed. They can be obtained from a knowledge of the xcess volume VE as a function of pressure, temperature, and composition. Experimental results demonstrate that the variations of E as a function of pressure can be important and that even a change of the sign of VE with increasing pressure has been found in some cases. The pressure dependence of HE values thus obtained agree well with literature values determined from direct f lowcalorimetric measurements. Until now most thermodynamic information has had to be deduced from high-pressure phase equilibria and critical phenomena where our knowledge is much better. The pressure dependence and critical phenomena of liquid-gas, liquid-liquid, and gasgas equilibria will be shortly reviewed. Mainly binary systems will be treated, but phase-separation phenomena in some ternary systems will also be considered. New developments during recent years have shown that the limits between liquid-gas, liquid-liquid, and gas-gas equilibria are not well defined and that continuous transitions occur. This continuity will be demonstrated on recent results for binary mixtures of hydrocarbons with tetrafluoromethane and nitrogen. Methods for the calculation and correlation of high-pressure phase equilibria in fluid mixtures under pressure are reviewed. They start from equations of state or from theories of mixtures using sometimes rather complicated mixing rules for the mixture parameters. Some results are presented and compared with experimental data. The significance of high-pressure phase equilibria in fluid mixtures for practical applications is briefly discussed e.g. f or fluid extraction, supercritical fluid chromatography (SFC), and for some high—pressure techniques and processes.

Heat capacity, enthalpy and crystallinity for a linear polyethylene obtained by DSC

Abstract: By means of computer-controlled DSC, heat-capacity measurements were performed on NBS SRM 1484 betweens-70°C and 250°C, in heating as well as cooling. By using stepwise and continuous measuring methods together with enthalpy calculations, an insight was obtained into the intrinsic consistency and the accuracy of the measurements. The reference states of polyethylene within the two-phase model were used to determine the crystallinity as a function of temperature, and to evaluate various other methods employed for the same purpose.

Thermodynamic analysis of drying foods

Abstract: Thermodynamic functions computed for food systems showing hysteresis indicate considerable differences. There appears to exist no qualitative agreement between the values obtained. Large differences in values between the adsorption an desorp-tion branches of isotherms make the absolute magnitude of the entropy and enthalpy changes uncertain. The desorption iso-steric heat was found to decrease with temperature while the adsorption branch of the isotherm for a few foods were found to exhibit temperature reversal effect giving higher values at low temperatures. Such trend may indicate either transition in the form of binding energy or the effect of temperature on structural alterations. In designing drying equipment, the isosteric heat of desorption will provide “a worst case” analysis as the true isosteric heat will lie somewhere between the adsorption and desorption isosteric heat. Any overcalculation in designing equipment by using the desorption branch of hysteresis data can only be in favor of increase...

Kinetics of radiation-induced segregation in Ni--12. 7 at. % Si

Abstract: The kinetics of radiation-induced segregation in Ni--12.7 at. % Si alloys was investigated using in situ, simultaneous Rutherford-backscattering spectrometry. It was observed that a precipitate layer of ..gamma..'--Ni/sub 3/Si grew at the specimen surface during 2.0-MeV He and 2.75-MeV Li irradiations. The thickness of the ..gamma..' layer was measured as a function of dose, dose rate, and temperature. For all of the irradiation conditions the ..gamma..' layer thickness grew proportionately to the square root of dose. The proportionality constant, or growth-rate constant, was dependent on temperature and dose rate. Below approx.570 /sup 0/C the growth-rate constant displayed Arrhenius behavior with an apparent activation enthalpy of 0.30 +- 0.04 eV, and it depended approximately on the -(1/4)th power of the dose rate. Above 590 /sup 0/C the growth-rate constant also displayed Arrhenius behavior but with an apparent activation enthalpy of -0.75 +- 0.15 eV; moreover, it was independent of dose rate in this temperature regime. A simple model for radiation-induced segregation is described which relates the segregation results to high-temperature point-defect properties in alloys. It shows that the apparent activation enthalpy of the growth-rate constant at low temperatures is equal to (1/4) the enthalpy of vacancy migration, and that at highmore » temperatures it is equal to -(1/2) the enthalpy of vacancy formation. The model also correctly predicts the observed dose-rate dependences of the growth-rate constant in the two temperature regimes.« less

Sub‐Sub‐Tg Enthalpy Relaxation in a B2O3 Glass

Abstract: Enthalpy relaxation processes in a B,O3 glass were investigated calorimetrically in the temperature region from well below to near the glass-transition temperature Tg. The low-temperature (TaTg- 100 K) or sub-sub-Tg anneals stabilize the glass structure. On heating, the annealed sample shows an excess endothermic peak above the annealing temperature Ta, and gradually recovers the original enthalpy without heating through Tg. The enthalpy relaxation evolves in a continuous manner with annealing time as seen in the recovery process. As Ta approaches Tg, a gradual transformation of the sub-sub-Tg behavior to that characteristic of the sub-Tg anneal occurs. The relaxation spectrum exhibits a broad distribution of relaxation times with activation energies of ∼ 105 J/mol. The so-called β distribution commonly used in the analysis of the sub-Tg relaxation processes is seen to be too narrow to describe the sub-sub-Tg behavior. Possible mechanisms for the sub-sub-Tg relaxation phenomena are proposed.

Thermodynamics of unsymmetrical electrolyte mixtures. Enthalpy and heat capacity

Abstract: There is a purely electrostatic contribution to the thermodynamic properties of electrolytes for the mixing of ions of different charge but the same sign. The previous treatment, which was limited to activity or osmotic coefficients, is extended to enthalpies and heat capacities and applied to the measurements of Cassel and Wood on the heat of mixing in the systems NaCl-BaCl/sub 2/ and NaCl-Na/sub 2/SO/sub 4/.

Kinetics of the gas-phase reaction CH3F + I2 ? CH2FI + HI: The C?H bond dissociation energy in methyl and methylene fluorides

Abstract: The kinetics of the gas-phase reaction of CH3F with I2 have been studied spectrophotometrically from 629 to 710 K, and were determined to be consistent with the following mechanism: (1) A least-squares analysis of the kinetic data taken in the initial stages of reaction resulted in where θ = 4.575T/1000 kcal/mol. The errors represent one standard deviation. The experimental activation energy E4 = 30.8 ± 0.2 kcal/mol was combined with the assumption E3 = 1 ± 1 kcal/mol and estimated heat capacities to obtain The enthalpy change at 298 K was combined with selected thermochemical data to derive The kinetic studies of ĊHF2 and CH2F2 have been reevaluated to yield These results are combined with literature data to yield the CH, CF, and CCl bond dissociation energies in their respective fluoromethanes, and the effect of α-fluorine substitution is discussed.