Showing papers on "Enthalpy published in 1973"

Selected Values of the Thermodynamic Properties of the Elements

Abstract: : Contents: Methods of evaluation, atomic weights, fundamental constants, symbols and units, general references, and properties of the elements.

Thermodynamic properties of strong electrolytes in aqueous solutions

Abstract: A generalized analytic correlation is presented for activity coefficient. osmotic coefficient, enthalpy, and heat capacity of single and multicomponent strong aqueous solutions. A good correlation for each salt to an ionic strength of six is obtained by assigning a single parameter “B” value to each salt. These B values are well approximated by assigning two parameters for each ion. Values presented for the common ions at 25°C allow the estimation of activity and osmotic coefficients of many unmeasured salt solutions.

Annealing effects in poly(vinylidene fluoride) as revealed by specific volume measurements, differential scanning calorimetry, and electron microscopy

Abstract: Specimens of poly(vinylidene fluoride), crystal form II, annealed at different temperatures between 130 and 180°C were characterized by specific volume measurements, differential scanning calorimetry (DSC), and electron microscopy. The degree of crystallinity calculated from the specific volume changed only by 15% i.e., from 50% to 65%. On the other hand, the melting behavior changed with annealing conditions. When a specimen was annealed above 170°C, two endothermic peaks appeared on either side of the annealing temperature. Results from DSC measurements made at different heating rates and electron microscopy showed that the two endotherms were caused by a bimodal distribution of lamellar thicknesses. The equilibrium melting point was found to be 210°C from the linear relation of the melting point and the annealing temperature. The equilibrium enthalpy and entropy of fusion were found to be 1.6 keal/mole and 3.3 eu/mole of repeat units by measurement on polymer–diluent mixtures. The surface free energy was found to be 5.1 kcal/mole of lamellar sequences from the plot of melting point versus reciprocal lamellar thickness obtained by electron microscopy. From a plot of enthalpy of fusion versus reciprocal lamellar thickness the surface enthalpy was found to be 20 keal/mole of lamellar sequences. These data lead to the estimate that a chain fold consists of about 30 repeat units.

Studies of hydrophobic bonding in aqueous alcohols: Enthalpy measurements and model calculations

Abstract: The coefficients which measure the contribution of a pair of solute molecules to the excess enthalpy have been measured in water at 25°C for all pairs of alcohols which can be formed from the series methyl to n-butyl plus t-butyl as well as for ethanol with some of the higher alcohols and with the n-alkyl sulfonates through octyl. The methylene-group contribution to these coefficients is readily identifiable in suitable cases. These data and the corresponding free-energy and volume coefficients, where they are known, are analyzed in terms of amodel which specifies the core repulsion and solvation-layer overlap terms in the potential for the interaction of two solute molecules. The latter term has an adjustable parameter, the so-called Gurney free-energy parameter which is adjusted for each solute pair to fit the free-energy data. Its temperature and pressure derivatives are adjusted to fit the enthalpy and volume data, respectively. These parameters are compared with the corresponding thermodynamic coefficients of solvation as far as possible.

Thermodynamic Properties of Nitrogen Including Liquid and Vapor Phases from 63K to 2000K with Pressures to 10,000 Bar,

Abstract: Tables of thermodynamic properties of nitrogen are presented for the liquid and vapor phases for temperatures from the freezing line to 2000 K and pressures to 10,000 bar. The tables include values of density, internal energy, enthalpy, entropy, isochoric heat capacity (Cv), isobaric heat capacity (Cp), velocity of sound, the isotherm derivative (∂P/∂ρ)τ, and the isochor derivative (∂P/∂T)ρ. The thermodynamic property tables are based on an equation of state, P=P (ρ,T), which accurately represents liquid and gaseous nitrogen for the range of pressures and temperatures covered by the tables. Comparisons of property values calculated from the equation of state with measured values for P‐ρ‐T, heat capacity, enthalpy, latent heat, and velocity of sound are included to illustrate the agreement between the experimental data and the tables of properties presented here. The coefficients of the equation of state were determined by a weighted least squares fit to selected P‐ρ‐T data and, simultaneously, to Cv data ...

Thermodynamic Properties of Helium 4 from 2 to 1500 K at Pressures to 108 Pa

Abstract: Tabular values of density, internal energy, enthalpy, entropy, heat capacity, and speed of sound for liquid and gaseous helium are presented for temperatures from 2 to 1500 kelvin at pressures from 1.0 × 104 to 1.0 × 108 pascals. Diagrams of temperature vs entropy are also given. The properties presented are calculated from an equation of state which was fitted to experimental P—V—T and other thermodynamic data from the world's literature. The equation of state was fitted to these data in three separate regions of pressure and temperature. The regional equations are forced to join smoothly at the preconceived boundaries. Extensive comparisons between the equation of state and experimental data have been made, and deviation plots are presented. A particularly careful determination of the second virial coefficient over the full temperature range 2–1500 kelvin is presented. The Joule‐Thomson inversion curve has been calculated and comparisons made with other sources. Equations for the density of the saturate...

Thermodynamics of binding of guest molecules to α- and β-cyclodextrins

Abstract: Values of ΔG, ΔH, and ΔS are reported for the interaction of eighteen molecules and ions with α-cyclodextrin and of four molecules with β-cyclodextrin. The conclusions of this study are (1) the cyclodextrin–ClO4– complex has been shown to bind a cation, (2)α- and β-cyclodextrin have equilibrium constants for binding the same guest that are similar but the enthalpy and entropy changes are quite different in some cases, and (3) changes in ΔH are largely compensated for by changes in ΔS and it is suggested that this effect is due principally to the nature of the solvent, i.e. water.

Ideal Gas Thermodynamic Properties of Ethane and Propane

Abstract: The thermodynamic properties (Cpo, So, Ho−H0o, (Ho−H0o)/T,−(Go−H0o)/T, ΔHfo, ΔGfo and log Kf) for ethane and propane in the ideal gaseous state in the temperature range from 0 to 1500 K and at 1 atm were calculated by statistical thermodynamic methods based on a rigid‐rotor harmonic‐oscillator model. The internal rotation contributions to thermodynamic functions were evaluated by using a partition function formed by summation of internal rotation energy levels. The calculated heat capacities and entropies compare favorably with available experimental data.

Heat capacity of seawater solutions from 5° to 35°C and 0.5 to 22‰ chlorinity

Abstract: The specific heat of seawater solutions from 0.5 to 22‰ chlorinity has been determined at 5°, 15°, 25°, and 35°C by using a new precision heat capacity calorimeter (Picker et al., 1971). The concentration dependence Cl (‰) of the specific heats cp has been fit to cp = cp° + ACp Cl (‰) + BCp Cl (‰)3/2, where cp° is the specific heat of pure water and ACp and BCp are temperature-dependent constants. The specific heats fit this equation with an average deviation of ±0.0002 joule deg−1 g−1 between 15° and 25°C and ±0.0005 joule deg−1 g−1 at 5°C. The specific heats determined in this study are in excellent agreement (±0.002 joule deg−1 g−1) with the results of Cox and Smith (1959) and Bromley et al. (1967, 1970). The apparent equivalent heat capacity ΦCp for sea salt has been determined from these heat capacities, and the concentration dependence (Im is the molal ionic strength) has been analyzed by ΦCp = ΦCp° + SCpIm1/2 + bCpIm, where ΦCp° is the infinite dilution equivalent heat capacity of sea salt (related to ion-water interactions), SCp is the Debye-Huckel limiting law slope (related to ideal ion-ion interactions), and bCp is an empirical constant (related to deviations from the Debye-Huckel limiting law). The relative apparent equivalent heat capacity ΦJ = ΦCp − ΦCp° calculated from our results is in good agreement with the temperature dependence of the enthalpy of seawater ΦJ = ∂ΦL/∂T calculated from the apparent equivalent enthalpy data ΦL of Millero et al. (1973). Young's (1954) rule, ΦCp = ∑EiφCp(i), has been used at 25°C to estimate the ΦCp of sea salt by using data on the major components of seawater in pure water. The estimated ΦCp and cp are in excellent agreement with the measured values.

Microcalorimetric determination of the enthalpy of sublimation of benzoic acid and anthracene

Abstract: A Calvet microcalorimeter has been utilized to determine the sublimation enthalpy of benzoic acid and anthracene. The vapor pressures measured by Knudsen effusion weight‐loss method over the temperature ranges 338–383°K and 353–432°K were, respectively, log10p(torr)=(12.175 ± 0.047) − (4501 ± 17)/T, for benzoic acid, and log10p(torr)=(12.616 ± 0.062) − (5277 ± 24)/T, for anthracene. The Gibbs energy functions for C6H5COOH and C14H10 gaseous molecules have been estimated.

Calorimetric Study of the Glassy State. VIII. Heat Capacity and Relaxational Phenomena of Isopropylbenzene

Abstract: The heat capacities of isopropylbenzene were measured with an adiabatic calorimeter for the crystal from 14 to 177.13 K (Tm), for the glassy state from 14 to around 126 K (glass transition temperature: Tg) and for the liquid from Tg to 313 K, with a sample of 99.93% purity. The heat and entropy of fusion were found to be 7326 J mol−1 and 41.36 J K−1 mol−1, respectively. Based on these data, a set of thermodynamic functions are tabulated at rounded temperatures. In addition to the primary glass transition phenomenon, a secondary enthalpy relaxation as well as a step-like heat capacity anomaly was observed at around 70 K. These facts were discussed in correlation with the β-relaxation already observed by dielectric measurement. Gonfigurational entropies of the supercooled liquid and of the glassy state were calculated to investigate a relation with relaxational properties. The agreement is found between the temperatures where catastrophe occurs in viscosity (T0), and where the configurational entropy vanish...

Hydrophobic interaction in water-ethanol mixtures

Abstract: The solubility, free energy, entropy, and enthalpy of solutions of methane and ethane were determined in various mixtures of water and ethanol. The hydrophobic interaction for the pair methane-methane was computed, in these mixtures, using an approximate expression derived previously. Also the entropy and the enthalpy associated with the hydrophobic interaction were evaluated from the experimental data. The main finding is the steep decrease in the strength of the hydrophobic interaction as the mixture becomes richer in alcohol. The variation of the corresponding entropy and enthalpy also shows an abrupt change at the composition ofxethanol≈0.2. Some interpretations of these findings in terms of structural changes in the solvent are discussed.

Heat Capacity of N-(o-Hydroxy-p-Methoxybenzylidene)-p-Butylaniline: A Glassy Nematic Liquid Crystal

Abstract: The heat capacity of N-(2-hydroxy-4-methoxybenzylildene)-4′-butylaniline(2,4-OHMBBA) with a purity of 99.393 mole % was measured between 13 and 375 K. The Melting point T m was 314.30 K, and the enthalpy and the entropy of fusion were 22.405 kJ mol−1 and 71.425 J K−1 mol−1, respectively. The temperature of transition from nematic to isotropic liquid was 33.65 ± 0.1 K, and the enthalpy and the entropy changes due to this mesophase transition were 887.4 ± 4.9 J mol−1 and 2.690 ± 0.015 J K−1 mol−1, respectively. A second-order character has been postulated for the mesophase transition. By cooling the nematic liquid crystal at a rate of − 12.2 K min−1 a frozen-in state of the stable nematic phase, i.e. a glassy state, was realized. The glass transition temperature Tg was 204 K and the heat capacity jump at T g was Δ C = 107 J K−1 mol−1. The activation enthalpy for the glass transition was estimated to be Δ H = 75 kJ mol−1. The residual entropy of the glassy state at O K was determined to be 12.69 ± 0...

On thermodynamics of solution by gas-liquid chromatography

Abstract: Discusses the standard and reference states commonly used in glc, and suggests the use of a more straightforward set of states for the evaluation of standard free energy, enthalpy, and entropy of s...

Adsorption of benzene and cyclohexane on faujasite-type zeolites. Part 1.—Thermodynamic properties at low coverage

Abstract: The adsorption of benzene and cyclohexane has been studied on X, Y and aluminium-deficient X and Y zeolites with various sodium ion contents. Equilibrium constants, enthalpies of adsorption, virial coefficients and surface interactions have been determined. At a constant temperature, equations correlating equilibrium constant of adsorption with either cation content or enthalpy of adsorption have been derived. Results for benzene are consistent with specific interactions of its π electrons with zeolite surface. These interactions allow the surface field changes with Na or Al atom content to be easily detected. Cyclohexane which has small interactions with surface sites indicates only small changes in properties with chemical composition. The two hydrocarbons show specific changes in adsorption properties near 25–30 sodium or aluminium atoms per unit cell.

Heats of thermally induced helix-coil transitions of DNA in aqueous solution.

Abstract: Thermal denaturation of calf thymus DNA at both alkaline and neutral pH values was studied by differential scanning calorimetry. It was shown that the dependence of the enthalpy of transition on pH and salt concentration could be accounted for on the basis of a heat capacity change of +40 cal deg−1(base pair)−1. In the pH range between 10.3 and 11.3, a release of 0.6 proton per base pair was calculated from the pH dependence of the melting temperature. The heat effect associated with the release of this proton was calculated to be 5 kcal mole−1.

The Enthalpies of Binary Mixtures of Chalcogen Elements

Abstract: The enthalpies of binary mixtures among chalcogen elements were measured by means of calorimetry. In the system of sulfur+selenium, the positive enthalpy increment suggests the formation of a co-ring and a co-chain on mixing. In the selenium+tellurium and sulfur+tellurium systems, some local ordered structures were deduced from the minimum points of the enthalpy at the compositions of Te3Se2 and Te3S. In the latter system, the entropy of mixing was calculated from the reported phase diagram and the present enthalpy values. The entropy diagram also suggests that the local order is present.

Intrinsic acidities fo carboxylic acids from gas‐phase equilibria

Abstract: Recently, the authors reported results for the gas-phase proton transfer equilibria (B/sub 1/H/sup +/ + B/sub 2/ = B/sub 1/ + B/sub 2/H/sup +/) measured with a pulsed electron beam high-pressure mass spectrometer. The equilibrium constants and their temperature dependence allow the evaluation of free energy and enthalpy changes for the reactions and permit the measurement of the intrinsic basicity of the compounds in absence of a solvent. Similar measurements, but at much lower pressure and constant temperature, have been made by several groups, using ion cyclotron spectrometers. The present communication represents an extension of the proton transfer work to equilibria involving the acids HA. The results, measured with the same instrument, are given in tabular form. (13 refs.)

Ionic thermal current study of the α′ process in poly(hexamethylene adipamide)

Abstract: The ionic thermoconductivity (ITC) method has been used to study the α′ transition in the polyamide (nylon 66). Depending on the thermal history of the sample, the maximum of the thermocurrent peak attributed to the α′ relaxation is found somewhere between ca. 45°C and ca. 66°C; on one heating, it shifts to higher temperatures. The high sensitivity and resolving power of the ITC method permit resolution of this peak into four elementary “pure” activated processes, with constant activation energies in the relevant temperature range. Each of the four corresponding relaxation times follows an Arrhenius law, with a well-defined characteristic temperature T0 = 83°C at which all these relaxation times are equal. When this last result is interpreted in the light of Eyring's or Bauer's theory, it gives a linear relation between “apparent” activation entropy and activation enthalpy of the elementary processes. The characteristic temperature T0 is independent of the thermal history of the sample, and the temperature shift of the thermocurrent maximum can be interpreted by the observed variation of the relative intensities of the elementary processes, without modification of their other characteristic features.

Kinetics of thermal unfolding of lysozyme.

Abstract: A reversible unfolding of lysozyme has been studied by means of the temperature-jump method by measuring the absorbance change of pH indicator p-nitrophenol (p-NP). In the presence of 4.5M lithium bromide three relaxation processes were found. The fastest process with a relaxation time τ of about 30 μsec is found to come from the proton-transfer reaction between p-NP and His 15 in lysozyme. Only the slowest process (τ ≃ 15 sec) is a first-order reaction and attributable to the unfolding of lysozyme. These results suggest that the all-or-none model can be applied to the unfolding of lysozyme as a fairly good approximation. This shows good agreement with the result of Tanford et al. (J. Mol. Biol., 15, 489 (1966); J. Mol. Biol., 73, 185 (1973)). The enthalpy, entropy, and specific heat change between a native and a denatured form are found to be close to 53 kcal/mol, 168 cal/mole/°K, and 2.1 kcal/mol/°K, respectively. The enthalpy and entropy of activation and the specific heat change for the unfolding and the folding are also obtained from kinetic experiments. In the folding reaction, the enthalpy of activation is negative. In the unfolding reaction, the specific heat change is nearly equal to zero. These results suggest that a number of intramolecular hydrogen bonds are broken, but most residues remain tightly folded in the activated state. Our results in concentrated LiBr solutions are compared with those of Tanford in concentrated GuHCl solutions.

Equilibrium defect concentrations and their mobility in the crystalline lattice of cobaltous oxide

Abstract: Concentration and mobility of point defects in Co1−yO have been determined with the Rosenburg kinetic method as a function of temperature and equilibrium oxygen pressure in the temperature range 1000–1250°C and oxygen pressure range 3×10−3−1 atm. On the basis of these data the enthalpy of defect formation and activation energy of their mobility in crystalline lattices of cobaltous oxide have been calculated. The degree of ionization of the defects has been determined, and the self-diffusion coefficient of cobalt in cobaltous oxide and the activation energy of self-diffusion have been calculated.