Showing papers on "Enthalpy published in 1974"
TL;DR: The thermal properties of five globular proteins with known spatial structure, ribonuclease, lysozyme, chymotrypsin, cytochrome c and myoglobin, are investigated by scanning microcalorimetry and it is shown that heat-denaturation of these proteins can be described to a first approximation by the two-state transition model.
1,332 citations
TL;DR: The JANAF Thermochemical Tables as discussed by the authors cover the thermodynamic properties over a wide temperature range with single phase tables for the crystal, liquid, and ideal gas state, and the properties given are heat capacity, entropy, Gibbs energy function, enthalpy, enthpy of formation and Gibbs energy of formation for formation of each compound from the elements in their standard reference states.
Abstract: The thermodynamic tabulations previously published as NSRDS‐NBS‐37 are extended by 154 new and revised tables. The JANAF Thermochemical Tables cover the thermodynamic properties over a wide temperature range with single phase tables for the crystal, liquid, and ideal gas state. The properties given are heat capacity, entropy, Gibbs energy function, enthalpy, enthalpy of formation, Gibbs energy of formation, and the logarithm of the equilibrium constant for formation of each compound from the elements in their standard reference states. Each tabulation lists all pertinent input data and contains a critical evaluation of the literature upon which these values are based. Literature references are given.
311 citations
TL;DR: In this paper, the entropy of transfer from water to nonaqueous solvents has been investigated and shown to be exothermic for cations, but endothermic with respect to anions.
Abstract: Standard molar free energies, enthalpies and entropies of transfer of some uni-univalent electrolytes from water to methanol, N-methylformamide, formamide, dimethyl sulphoxide, N,N-dimethyl- formamide, propylene carbonate, sulpholane, N-methylpyrrolidone and acetonitrile are presented. They have been divided into the corresponding thermodynamic properties for single ions by means of extrathermodynamic assumptions. Changes in the chemistry of anions on transfer from protic to dipolar aprotic solvents are mainly a function of enthalpy changes. There is a substantial loss of entropy on transferring both anions and cations from water to non-aqueous solvents. Entropies of transfer can be interpreted in terms of ordering and disordering solvent molecules and a uniquely extensive water structure. Evidence for solvation of the first and second kind in water is presented. Enthalpies of transfer from water are exothermic for cations but endothermic for many anions.
289 citations
TL;DR: In this paper, the authors measured the heat capacities of hexagonal ice with an adiabatic calorimeter and found that the anomalous temperature drifts and the corresponding C s T hump were observed around 100 K.
191 citations
01 Sep 1974
TL;DR: In this paper, the changes in enthalpy, entropy, and heat capacity that accompany micelle formation for sodium, octyl, decyl, and dodecyl sulfate, dimethyldecyl and dimethy-ldodecyl phosphine oxide, sodium dodecoyl sarcosinate, and bromide were determined by thermometric titrations between 21 and 35° in aqueous solution.
Abstract: The changes in enthalpy, entropy, and heat capacity that accompany micelle formation for sodium, octyl, decyl, and dodecyl sulfate, dimethyldecyl and dimethyldodecyl phosphine oxide, sodium dodecoyl sarcosinate, and dodecylpyridinium chloride and bromide were determined by thermometric titrations between 21 and 35° in aqueous solution. The enthalpy and entropy changes exhibited a correlation for a given chain length consistent with a compensation temperature of about 300°. In general, the enthalpy and entropy changes, which appear to be rather complex parameters, did not parallel the heat capacity change. The change in heat capacity for micellization appears mainly to reflect solvent effects. Sodium dodecyl sulfate was also studied in 6 M urea, 10% ethanol, 0.1 M sucrose, 0.1 M NaC1, 0.5 M NaCl, and 0.1 M NaBr. A comparison of enthalpy changes that have been determined by calorimetry and from the temperature dependence of the CMC indicates that the two approaches should not be assumed to yield identical results.
139 citations
TL;DR: The endotherm enthalpy changes ΔHD and temperatures TD of thermal denaturation of tropocollagen fibers were measured by DSC calorimetry as functions of water content and the effect of water uptake on theEnthalpy term is explained by water bridge formation within the collagen triple helix.
Abstract: The endotherm enthalpy changes ΔHD and temperatures TD of thermal denaturation of tropocollagen fibers were measured by DSC calorimetry as functions of water content. The denaturation temperatures decrease with increasing water content. The enthalpy change values increase sharply in the range 0–28% of water content, where a maximum of 14.3 cal g−1 is reached. The effect of water uptake on the enthalpy term is explained by water bridge formation within the collagen triple helix. Evidence is given for the existence of approximately three intercatenary water bridges per triplet at the enthalpy maximum, their H-bond energy amounting to approximately 4000 kcal/mol of protein. In the 30–60% range of water content, ΔHD decreases by 2 cal−1 probably due to interactions between secondary water structures and the stabilizing intrahelical water bonds.
The influence of two neutral potassium salts, with a structure-stabilizing and a structure-breaking anion (F− and I−), on the hydration dependence of ΔHD and TD was also studied. It was shown that the primary hydration is not influenced by these ions, but that TD and ΔHD are altered in an ion specific way in the presence of interface and bulk water. Hydrophobic interactions do not explain the experimental results. A reaction mechanism of the effects of ions upon the structural stability of collagen is proposed and discussed in terms of interactions of the medium water molecules with the intrahelical water bonds, and in terms of proton-donor/proton-acceptor equilibria between peptide groups, hydrated ions, and intrahelical water molecules.
121 citations
01 Jan 1974
TL;DR: It is not surprising, though nevertheless disappointing, that the availability of thermodynamic data for a system has frequently not led to a significantly improved understanding of the system.
Abstract: Calorimetry is concerned with the determination of the enthalpy change accompany ing transfer of a system from an initial state. to a final state, and has historically played a very important role in the accumulation of thermodynamic data. The recent upsurge of calorimetric activity in the biological sciences has been to a large extent concentrated on this type of activity, that is, the determination of the enthalpies of reasonably well-defined processes of biological interest which can then be combined with free energy data derived either from independent estimates of equilibrium constants, or from such estimates based on the calorimetric data themselves, to give entropy values. It is a truism to state that thermodynamics is not concerned with mechanism. The concepts of atoms and molecules and moles, although of course widely employed in chemical thermodynamics, are not essential to the full development of the science. It is therefore not surprising, though nevertheless disappointing, that the availability of thermodynamic data for a system has frequently not led to a significantly improved understanding of the system. For example, a detailed thermodynamic picture has been obtained for the binding of 2,4-dinitrophenyl (DNP) and 2,4,6-trinitrophenyl (TNP) haptens to rabbit anti-DNP and anti-TNP antibodies (Barisas et al 6, 7) and to the myeloma proteins MOPC 315 and MOpe 460
116 citations
TL;DR: In this paper, the internal rotation contribution to thermodynamic functions was calculated by using a partition function formed by summation of internal rotation energy levels for each of the above six chloroethanes in the ideal gaseous state in the temperature range from 0 to 1500 K and at 1 atm.
Abstract: The thermodynamic properties: Cpo, So, Ho−H0o,−(Go−H0o)/T, ΔHfo, ΔGfo, and log Kf for chloroethane, 1,1‐dichloroethane, 1,1,1‐trichloroethane, 1,1,1,2‐tetrachloroethane, pentachloroethane, and hexachloroethane in the ideal gaseous state in the temperature range from 0 to 1500 K and at 1 atm were evaluated by statistical thermodynamic methods based on a rigid‐rotor harmonic‐oscillator model. The internal rotation contributions to thermodynamic functions were calculated by using a partition function formed by summation of internal rotation energy levels. The internal rotation barrier heights (in kcal mol‐1) employed for generation of the energy levels for each of the above six chloroethanes are: 3.69, 3.54, 5.08, 10.38, 14.43, and 14.7, respectively. The calculated heat capacities and entropies are compared with available experimental data. The derived values of Cpo, So, and ΔHfo at 298.15 and 700 K are compared with those reported in the other major compilations.
112 citations
TL;DR: In this paper, thermal analysis and dilatometry at ambient pressure was performed on densified glasses, formed from the liquid state by cooling at 5°C/hr under elevated pressures of up to 517 MN/m2.
Abstract: Densified glasses, formed from the liquid state by cooling at 5°C/hr under elevated pressures of up to 517 MN/m2, were studied by thermal analysis and dilatometry at ambient pressure. The materials studied were polystyrene, poly(methyl methacrylate), phenolphthalein, sucrose, and a 1 : 1 mixture by weight of potassium and calcium nitrate. Enthalpies of the densified glasses were found to be up to 2–6 J/g greater than the enthalpies of the glasses formed at atmospheric pressure. The major enthalpy and volume relaxation on heating the densified glasses was found to be not correlated The upper temperature limit of the glass-transition region remained constant for all glasses cooled at various pressures, while the lower temperature limit of the polymeric glasses decreased up to 55°C with increasing pressure. The interpretation of the data suggests that an additional, significant volume relaxation occurred on depressurization at room temperature for all glasses analyzed.
104 citations
TL;DR: In this paper, the effects of added salt, chain length, and temperature on the critical micelle concentration (CMC) of sodium dodecylsulfate (SDS) were investigated.
93 citations
TL;DR: In this paper, a combined calorimetric and equilibrium method at 264, 300, and 437° (V), and at 356 and 440° (Nb) was used to study the high-temperature thermodynamic properties of V+H2 and Nb +H2 solid solutions, showing that the relative enthalpies and entropies of hydrogen in these solid solutions are very nearly independent of temperature in this temperature range.
Abstract: The high‐temperature thermodynamic properties of V+H2 and Nb+H2 solid solutions have been studied by a combined calorimetric and equilibrium method at 264, 300, and 437° (V), and at 356 and 440° (Nb). The results demonstrate experimentally that the relative partial enthalpies and entropies of hydrogen in these solid solutions are very nearly independent of temperature in this temperature range. The observed enthalpies are compared with data derived from equilibrium work, and with our earlier data at 430° for Ta+H2. From the equilibrium pressures and the partial enthalpies, we have calculated the partial entropies, also the partial excess entropies of hydrogen. The limiting values (at NH=0) of the excess entropies are compared with values derived from the sum of the contributions from the volume expansion and from vibrational and electronic heat capacity terms. These calculated values are systematically about 0.5–1.0 cal/deg · g‐atom lower than the experimental ones. This difference is too small to allow f...
TL;DR: In this paper, the association-dissociation equilibrium of beef-liver glutamate dehydrogenase was investigated in phosphate buffer of different ionic strengths and in the presence of the organic solvents ethylene glycol and dioxane.
Abstract: The association-dissociation equilibrium of beef-liver glutamate dehydrogenase was investigated in phosphate buffer of different ionic strengths and in the presence of the organic solvents ethylene glycol and dioxane. For some of the solvents the temperature dependence of the equilibrium constant was analyzed. The dependence of the apparent weight-average molecular weight on enzyme concentration shows always the same general shape as that found earlier in 0.067 M phosphate buffer at pH 7.6. Each curve can be fitted assuming an open association-dissociation equilibrium with one single equilibrium constant for all steps and with one single value for the second virial coefficient. Within the limit of the experimental error, the second virial coefficient is found to be the same for all systems (8–9 nmol × 1 × g-2), whereas the equilibrium constants vary by orders of magnitude.
At low ionic strength a very small temperature dependence was found with a reduced association at higher temperatures. At high ionic strength a temperature maximum of the equilibrium constant is observed. Preferential phosphate binding is indicated by the high values of the refractive index increment.
The organic solvents investigated here (ethylene glycol, dioxane) cause a dissociation into the unimers (oligomers). The equilibrium constant decreases from 9 × 105 M-1 in aqueous phosphate buffer to 5.2 × 103 M−1 in 6 M ethylene glycol and to 2.4 × 105 M−1 in 2 M ethylene glycol. The effects obtained with 5% dioxane (0.57 M) are similar to that with 2 M ethylene glycol. In both cases a sharp decrease of the association with increasing temperature is observed.
The reaction enthalpy and the reaction entropy of the association reaction decrease with decreasing temperature in all solvents, the enthalpy being positive in the absence and negative in the presence of organic solvents. The free enthalpy of reaction, however, increases with increasing temperature in buffers of high and low ionic strengths but decreases in the presence of organic solvents. The association reaction at high ionic strength appears mainly to be entropy-driven whereas in the presence of the organic solvents the reaction seems to be energy-driven at all temperatures. It is suggested that the organic solvents interfere with the hydrophobic interactions between the unimers. The positive reaction enthalpy in the presence of high salt concentrations indicates that the attractive electrostatic forces are largely shielded, whereas shielding is imperfect for the repulsive forces which even seem to be enhanced. This could be due to a preferential binding of anions which would tend to increase the negative net charge and could explain the difference observed between the effects of phosphate and chloride solutions of identical ionic strength.
TL;DR: In this paper, the authors present precise experimental data for the adsorption (na) of nitrogen and the noble gases by two unactivated and two activated Saran type porous carbons over the temperature range 0 −125°C and at pressures (p) up to 1 atm.
Abstract: This paper presents precise experimental data for the adsorption (na) of nitrogen and the noble gases by two unactivated and two activated (10.8% and 22.0% burn-off) Saran type porous carbons over the temperature range 0–125°C and at pressures (p) up to 1 atm. The data are analysed in terms of virial type expansions of (i)na/p as a function of p and (ii) ln na/p as a function of na. The number of terms required in each equation to give an adequate representation of the data was determined for each system at each temperature by statistical analysis. Expansion (ii) was found to be superior in the sense that where there was a difference between the number of terms required, expansion (ii) required fewer terms than (i). These differences become more marked for the adsorption of the heavier gases at the lower temperatures. Henry's law constants obtained from (i) and (ii) were, however, in good agreement.The Henry's law constants and their temperature dependence were used to calculate differential enthalpies and standard differential entropies of adsorption (taking na= 10–3 mol g–1 as standard state) in the limit of ideal behaviour. Activation of the carbons decreased the Henry's law constants per unit surface area, and also the magnitude of the enthalpy of adsorption. The standard differential entropy of adsorption increases with activation, reflecting the larger “free volume” available to the adsorbed molecules which may be measured approximately either by the increase in micropore volume or by the increase in the monolayer equivalent surface area.
TL;DR: In this article, smoothed tabular values are presented for boiling point elevation (BPE), osmotic coefficient, water activity and minimum separation energy to 200°C and 12% sea salt.
Abstract: Based on new experimental data presented for the boiling point elevation of natural sea water solutions, smoothed tabular values are presented for boiling point elevation (BPE), osmotic coefficient, water activity, osmotic pressure, and minimum separation energy to 200°C and 12% sea salt.
The concentration of sodium chloride solutions having the same water activities as sea salt solutions is also presented.
Rigorous thermodynamics and the equations for strong electrolytes by Bromley (1973) were used for the correlation together with previously obtained heat capacity and enthalpy data and equations. A simplified equation is presented for boiling point elevation.
TL;DR: The thermodynamics of the hairpin helix-single strand transition of A6C6U6 has been analyzed by a staggering zipper model with consideration of single strand stacking and shows a specific influence of the C6 loop due to the stacking tendency of the cytosine residues.
TL;DR: Pressure and temperature data obtained, taken together with the literature value for the bilayer volume expansion during solid-fluid phase transition, and inserted into the Clausius-Clapeyron equation yield a ΔH value of 8.8 kcal/mole, which is within experimental error of the ΔHvalue obtained from differential scanning calorimetry and serves to support the validity of the data and the experimental technique.
TL;DR: In this paper, the activation energy obtained for the oxidative vaporization reaction corresponded numerically with the thermochemical enthalpy of the reaction, and a theoretical equation was given for calculating the rate from thermodynamic data using boundary-layer theory.
Abstract: Rates of oxidative vaporization of Cr2O3 on preoxidized resistively heated chromium were determined in flowing oxygen at a pressure of 0.115 Torr for temperatures from 1000 to 1300 C. Reaction controlled rates were obtained from experimental rates by a gold calibration technique, and these rates were shown to agree with those predicted by thermochemical analysis. The activation energy obtained for the oxidative vaporization reaction corresponded numerically with the thermochemical enthalpy of the reaction. A theoretical equation is given for calculating the rate from thermodynamic data using boundary-layer theory.
TL;DR: The heat capacity of SnCl2·2H2O and SnCl 2·2D2O crystals, substances with layer structure, was measured from 13 to 300 K.
Abstract: The heat capacity of SnCl2·2H2O and SnCl2·2D2O crystals, substances with layer structure, was measured from 13 to 300 K. Anomalies due to ordering of the hydrogen (deuterium) position were observed at TC(H2O)=217.94 K and at Tc(D2O)=234.47±0.05 K. The total entropy change of the transition was estimated to be 4.6 J K−1 mol−1 for both compounds. An exact calculation of the high temperature entropy based on the two-dimensional lattice model gave 3.13 J K−1 mol−1. The critical indices α± of the divergent heat capacity anomalies are as follows: α+(H2O)=0.53, α−H2O)=0.48, α+(D2O)=0.57, α−(D2O)=0.54. Another heat capacity anomaly was found at 140–160 K where the heat capacity values depended on the thermal history of the specimen. The enthalpy relaxation observed at the temperature region was analysed by use of the first-order rate equation, giving the activation enthalpies ΔHa(H2O)=49.9 kJ mol−1 and ΔHa(D2O)=49.8 kJ mol−1. The concept of long-range-ordered glassy state was introduced for description of the non...
TL;DR: An internally consistent correlation was made of the extant experimental data on the entropy, heat capacity, enthalpy of formation, and isomerization equilibria of alkane hydrocarbons as mentioned in this paper.
Abstract: An internally consistent correlation was made of the extant experimental data on the entropy, heat capacity, enthalpy of formation, and isomerization equilibria of alkane hydrocarbons. Of the data selected as reliable, all 41 values of entropy, 98 out of 100 values of heat capacity, 52 out of 54 values of enthalpy of formation, and all 15 values of enthalpy of isomerization from equilibrium data were fit within the assigned experimental uncertainty. Working formulas, tables, and equations are given whereby thermodynamic properties for selected temperatures between 200 and 1500 K can be calculated for any alkane hydrocarbon with a tractably small number of molecular conformations (say ≤34=81). Compounds with more conformations can be included by use of appropriate substitution methods. Values so obtained for the entropy, heat capacity, and enthalpy of formation at 298.15 K are tabulated for all alkanes C3–C10. Variance‐covariance matrices from least squares adjustment of constants provide estimates of the ...
TL;DR: In this article, the Gibbs energy, enthalpy, and entropy of solution for 1 atm partial pressure of gas and 298.15 K have been calculated from the experimental results, compared with the results obtained through application of the scaled particle theory to gas solubilities.
TL;DR: A Tian-Calvet type microcalorimeter has been used at 1353 K to measure directly the partial molar enthalpy of solution of O 2 in cerium oxides of compositions ranging from CeO 1.5 to CeO 2 as mentioned in this paper.
TL;DR: In this article, Gibbs-Duhem integration was used to calculate the integral free energies of formation in the hexagonal δ′-NiAs phase and the interaction energy between iron vacancies was found to be 820 cal·g−1.
Abstract: Vapor pressures of tellurium of iron—tellurium alloys were determined between 550 and 900°C and between 0 and 67 at % Te by an isopiestic method. Activities and partial molar enthalpies of tellurium were calculated and integral free energies of formation were obtained by aGibbs-Duhem integration. Two statistical models were used to calculate activities in the hexagonal δ′-NiAs phase. Excellent agreement with the experimental values was obtained by assuming random distribution of iron atoms and iron vacancies in the defected (00 1/2) layers of the lattice. The interaction energy between iron vacancies was found to be 820 cal·g-atom−1.
TL;DR: In this paper, the standard enthalpy of molybdates has been determined in a solution calorimeter, when combined with auxiliary thermochemical values, yielded the standard entropy of formation, ΔH f o (Li 2 MoO 4, c, 298.15 K) = −(363.26 ± 0.12) kcal th mol −1.
TL;DR: In this article, the relationship between nitrogen lone pair interactions and thermodynamic parameters associated with amine basicity is delineated and the use of BH/sup +/ homolytic bond dissociation energies as a measure of nitrogen lone-pair interactions and of resonance stabilization is discussed.
Abstract: Determination of gas phase basicities (defined as the enthalpy change for the reaction BH/sup +/ ..-->.. B + H/sup +/) of diazabicyclooctane and quinuclidine using ion cyclotron resonance spectroscopy is reported. The relationship between nitrogen lone pair interactions and thermodynamic parameters associated with amine basicity are delineated. The use of BH/sup +/ homolytic bond dissociation energies as a measure of nitrogen lone pair interactions and of resonance stabilization is discussed. (DDA)
01 Mar 1974-Metallurgical and Materials Transactions B-process Metallurgy and Materials Processing Science
TL;DR: In this paper, the vapor pressure of zinc in equilibrium with α,β1, and γ-phase Ni-Zn alloys was determined based on measurement of the attenuation of a resonance line of zinc of wavelength 3076A.
Abstract: Atomic absorption spectroscopy was used to determine the vapor pressure of zinc in equilibrium with α,β1, and γ-phase Ni-Zn alloys. This technique is based upon measurement of the attenuation of a resonance line of zinc of wavelength 3076A. Thermodynamic activity, relative partial molar enthalpy and entropy, and excess properties of zinc were calculated. Lattice parameters were also measured for the alloys from Debye-Scherrer patterns, and trends in the thermodynamic data were correlated with composition variation in the lattice parameters. Thermodynamic properties in all three phases showed negative deviation from ideality. All three phases showed a large, negative excess entropy. An attempt was made to partition the thermodynamic properties of the α-phase alloys into magnetic, vibrational, and electronic contributions, and a model based on the rigid band approximation was applied to the excess free energy of zinc. Thermodynamic properties of the β1 alloys were found to be in good agreement with a model based on a substitutional defect in the equiatomic alloy. Thermodynamic behavior of the γ-phase alloys was correlated with magnetic susceptibility and lattice effects—behavior in this electron compound seems complex. Magnetic contributions due to destruction in the alloys of the magnetic moment of nickel appear to make a strong contribution to the entropy of alloys in this system.
TL;DR: In this article, the effects of water and Li+ addition upon the thermodynamical potential separation between successive electron transfers in a series of dinitro compounds were investigated in terms of ion-pairing and prefferential solvation.
TL;DR: In this paper, the changes in free energy, enthalpy and entropy for the formation of uranyl(VI)-monochloroacetate, -β-chloropropionate and -acetate have been determined at 25°C and in aqueous perchlorate medium of ionic strength 1.00 M.
TL;DR: In this paper, the thermodynamic properties of molten polyethylene are calculated for the temperature range 100° to 300°C and for pressures up to 3000 atmospheres, based on limited experimental data augmented by a corresponding-state correlation and an empirical equation of state.
Abstract: Thermodynamic properties of molten polyethylene are calculated for the temperature range 100° to 300°C and for pressures up to 3000 atmospheres. The calculations are based on limited experimental data augmented by a corresponding-states correlation and an empirical equation of state. Specific volume, internal energy, enthalpy, entropy, and solubility parameter are presented as functions of temperature and pressure. The results of this study may be useful for engineering design in the high-pressure polyethylene process.