Showing papers on "Enthalpy published in 1976"
TL;DR: In this paper, an equation derived by Ritland relating the cooling rate and fictive temperature for glasses without memory is extended to those with memory, i.e. those which exhibit a spectrum of relaxation times.
Abstract: An equation derived by Ritland relating the cooling rate and fictive temperature for glasses without memory is extended to those with memory, i.e. those which exhibit a spectrum of relaxation times. Provided that the spectrum of relaxation times is temperature-independent, the limiting fictive temperature, T′f, obtained when a glass is cooled through the transition region, is shown to be related to the cooling rate, q, by d In |q|/d(1/T'f)=-Δh★/R
where R is the ideal gas constant and Δh★ the activation enthalpy for the relaxation times controlling the structural relaxation. Values of T′f vs q obtained from enthalpy measurements by differential scanning calorimetry are presented for B2O3, 0.4Ca(NO3)2—0.6KNO3, and borosilicate crown glasses; Δh★ is equal, within experimental error, to the activation enthalpy for shear viscosity. Values of T′f from volume and enthalpy measurements obtained at the same cooling rate for the borosilicate crown glass are equal.
1,065 citations
TL;DR: In this article, the enthalpy of dilution of all one-and two-solute aqueous mixtures of a series of compounds were measured from about 0.2 to 2.0 mole-kg−1 at 25°C.
Abstract: The enthalpy of dilution of all one-and two-solute aqueous mixtures of a series of compounds were measured from about 0.2 to 2.0 mole-kg−1 at 25°C. The compounds included in the study wereN-methylformamide,N-methylacetamide,N-methylpropionamide,N-butylacetamide, urea, ethylene glycol, pentaerythritol, glucose, and sucrose. The results of the enthalpy measurements were used to calculate the pairwise enthalpy of interaction for each compound with all the other compounds. A simple additivity principle is used to correlate the data. The principle assumes that each functional group on one molecule interacts with every functional group on the other molecule and that each of these interactions has a characteristic effect on the enthalpy that is independent of the positions of the functional groups in the two molecules. The resulting equation gives a rough but useful correlation of the results. Of the six interactions between the CH2, CONH, and CHOH functional groups, the CONH−CONH interaction is the strongest, the CHOH−CHOH interaction is the weakest, and the CH2−CH2 interaction is about equal in magnitude to the rest of the interactions. Thus, the CH2−CH2 and CONH−CONH are not the only interactions making important contributions to the enthalpy of a wide variety of systems.
324 citations
Journal Article•
189 citations
TL;DR: From the correlations found for the Gibbs energy change, enthalpy change, and heat capacity change, along with the use of an average minimum solubility temperature, an equation of state for the hydrophobic effect is derived.
Abstract: Several thermodynamic properties for the process of dissolution of pure hydrocarbons into water are found to be linearly related to the number of hydrogens on the hydrocarbon molecule. From the correlations found for the Gibbs energy change, enthalpy change, and heat capacity change, along with the use of an average minimum solubility temperature, an equation of state for the hydrophobic effect is derived. The entropy change upon dissolution per hydrocarbon hydrogen atom is close to -R ln 2. A model based upon a “tetrahedrally” localized water molecule with one corner defined by a carbon-hydrogen group and the other three corners defined by water molecules is used to estimate the observed entropy and heat capacity changes.
162 citations
Book•
01 Sep 1976
TL;DR: In this article, three possible conditions for free energy change of polymerization may be summarized as follows: at the ceiling temperature, Tc, ΔG is zero, so that where ΔHp and ΔSp are the enthalpy and entropy changes per monomer unit.
Abstract: I. THE CHILING TEMPERATURE CONCEPT A. Thermodynamic Approach The Gibbs free energy of a system at temperature T is defined as were H is the enthalpy and S the entropy of the system. The free energy change for any polymerization will be, therefore, When the polymer has a lower free energy than the initial monomer, a polymerization can occur spontaneously, and the sign of ΔG is negative. A positive sign for ΔG signifies, therefore, that the polymerization is not spontaneous. When the system is in equilibrium at a certain critical temperature, there is no tendency for polymerization, and, hence, ΔG = 0 [1–3]. This temperature is known as the ceiling temperature. These three possible conditions for free energy change of polymerization may be summarized as follows: At the ceiling temperature, Tc, ΔG is zero, so that where ΔHp and ΔSp are the enthalpy and entropy changes per monomer unit.
150 citations
TL;DR: Tests for the validity of the obtained functions of enthalpy and entropy are presented in the form of cyclic processes using experimental data obtained from thermodynamically different pathways.
149 citations
TL;DR: In this article, a theoretical model is developed which predicts compositional dependence of the thermodynamic properties of intermetallic phases exhibiting the triple-defect B2 structure, and the model is applied to the following systems for which thermodynamic data are available.
135 citations
TL;DR: In this paper, the enthalpies and entropies of macrobicyclic ligands for complexation of alkali and alkaline-earth metal cations have been obtained from calorimetric measurements and from the previously determined stability constants.
Abstract: The enthalpies and entropies of complexation of alkali and alkaline-earth metal cations by several macrobicyclic ligands have been obtained from calorimetric measurements and from the previously determined stability constants [2]. Both enthalpy and entropy changes play an important role in the stability and selectivity of the complexes. Particularly noteworthy are the large enthalpies and the negative entropies of complexation obtained for the alkali cation complexes (Na+, K+, Rb+ and Cs+ cryptates). The Sr2+ and Ba2+ as well as [Li+ ⊂ 2.1.1]1 and [Na+ ⊂ 2.2.1] cryptates are of the enthalpy dominant type with also a favourable entropy change. The Ca2+ and [Li+ ⊂ 2.2.1] cryptates are entirely entropy stabilized with about zero heat of reaction. The high stability of the macrobicyclic complexes as compared to the macromonocylcic ones, the cryptate effect, is of enthalpic origin. The enthalpies of complexation display selectivity peaks, as do the stabilities, whereas the entropy changes do not. The high M2+/M+ selectivities found in terms of free energy, may be reversed when enthalpy is considered in view of the very different role played by the entropy term for M2+ and M+ cations. The enthalpies and entropies of ligation show that whereas the cryptate anions are similar in terms of entropy irrespective of which cation is included, the ligands, despite being more rigid than the hydration shell, are nevertheless able to adjust to some extent to the cation. This conclusion agrees with published X-rays data. The origin of the enthalpies and entropies of complexation is discussed in terms of structural features of the ligands and of solvation effects.
132 citations
TL;DR: Standard functions of enthalpy, entropy and the Gibbs energy of native and denatured lysozyme in the range of 0-100 degrees C and pH 1.5-7.0 are represented in three-dimensional projections.
130 citations
TL;DR: In this article, a method of evaluating the non-equilibrium in vapor enthalphy (due to vapor superheating) from experimentally obtained post-dryout data is presented.
122 citations
TL;DR: In this article, a computer-oriented strategy has been developed for simultaneous evaluation and correlation of thermodynamic data on groups of chemically related species, where internally consistent analytical representations of thermodynamics functions may be obtained by multiple regression on experimental data.
Abstract: A computer-oriented strategy has been developed for simultaneous evaluation and correlation of thermodynamic data on groups of chemically related species. A general empirical equation for the temperature dependence of the heat capacity of a species and derived functional relations for entropy, enthalpy, free energy, equilibrium constant, and electrochemical potential provides a mathematical model, whereby internally consistent analytical representations of thermodynamic functions may be obtained by multiple regression on experimental data. Programmining is simplified by defining the free energies of elements to be zero at 298.15/sup 0/K and the selected reference pressure. Conversion to other conventions is readily made after regression is completed. Error plots obtained from the initial parameter adjustment help to identify inconsistent data. Where justifiable, such data are rejected, and additional regressions are made to obtain an internally consistent description of the data. The species-related parameters from the final evaluation are then used with the mathematical model to calculate the thermochemical properties of the species considered. Application of this procedure is described, using, as examples, studies in the system Fe-C-H-O to derive thermodynamic functions for iron and its oxides and analysis of the system N-H-O to correlate the aqueous solubility of N/sub 2/ and NH/sub 3/ and themore » equilibrium between aqueous NH/sub 3/ and NH/sub 4//sup +/.« less
TL;DR: In this article, water vapour sorption in sugar beet root and its main components, namely the water insoluble fraction and sucrose, were calculated as a function of moisture content and several hypotheses were advanced in order to explain the values observed.
Abstract: Summary
Previously obtained water sorption isotherms were used for the calculation of thermodynamic functions of water vapour sorption in sugar beet root and its main components, namely the water insoluble fraction and sucrose. Integral molar enthalpy, entropy and free energy were calculated as a function of moisture content and several hypotheses were advanced in order to explain the values observed. These hypotheses were mainly concerned with configurational modifications of the adsorbents during the course of sorption.
TL;DR: In this article, the dynamic mechanical properties of glassy polymers, such as atactic polystyrene and poly(ethylene terephthalate), have been followed as a function of the excess enthalpy during isothermal annealing at selected temperatures below their respective glass transition temperatures.
Abstract: The dynamic mechanical properties of glassy polymers, such as atactic polystyrene and poly(ethylene terephthalate), have been followed as a function of the excess enthalpy during isothermal annealing at selected temperatures below their respective glass transition temperatures. For glassy polymers prepared from crystallizable as well as noncrystallizable polymers, the changes in modulus parallel the extent of enthalpy relaxation that occurs during the annealing period as a result of the nonequilibrium nature of the glassy state, i.e., the modulus increases with time and approaches a limiting value that increases with decreasing temperature. These results suggest that the observed changes in the dynamic mechanical moduli are associated with changes in the excess thermodynamic properties of the polymeric glasses, and not with structure formation other than the normal liquidlike packing of the chain segments. Thus the physical properties for corresponding equilibrium glassy states can be determined ...
TL;DR: In this paper, the authors measured the solubility of deoxygenated sickle cell hemoglobin (deoxy-Hb S), at varying pH and temperature over a range of concentrations encompassing those found in erythrocytes.
Abstract: Solubilities of deoxygenated sickle cell hemoglobin (deoxy-Hb S), at varying pH and temperature over a range of concentrations encompassing those found in erythrocytes, were measured. The technique involved ultracentrifugation, which gave values of the supernatant concentration and the mass of the sedimented material. The data establish that the solubility of doexy-Hb S is the saturation concentration and is independent of initial concentration. The mass of the pellet phase increases linearly with initial concentration. Moreover, the saturation concentration represents the critical concentration above which monomers are in equilibrium with polymers. These polymers are the putative cause of erythrocytes deformation associated with sickle cell anemia. The solubility-pH profiles of deoxy-Hb S at various temperatures, unlike those of other proteins, show no minima at the isoelectric pH but instead show a marked decrease in solubility below pH 7.0, indicating the predominance of polymerization over the expected increase in solubility. Deoxy-Hb S, within specified ranges of temperature and pH, possesses a negative temperature coefficient of solubility, a property characteristic of hydrophobic interactions. The saturation concentration is, however, temperature independent at conditions close to physiological. The enthalpy of polymerization (3.5 kcal/mol) is temperature independent from 6 degrees to 22 degrees for all pH values between 6.45 and 7.40. In the range of 22 degrees to 38 degrees, this parameter becomes less endothermic, having a value of 2.5 kcal/mol at pH 6.45 and a value of zero at pH 7.20. Such behavior of the system suggests a phase transition near 22 degreas. Within the range of conditions examined the polymerization is entropically driven.
TL;DR: In this paper, rate constants for proton-transfer reactions of the type XH/sup +/ + HCN(CH/sub 3/CN) + XH where X = H, D, OH, SH, NH/sub 2/H, and CH/sub 1/O have been measured at 297 +- 2K using the flowing afterglow technique.
Abstract: Rate constants for proton-transfer reactions of the type XH/sup +/ + HCN(CH/sub 3/CN) --> H/sub 2/CN/sup +/(CH/sub 3/CNH/sup +/) + X where X = H/sub 2/, N/sub 2/, CO, H/sub 2/O, and CO/sub 2/ and of the type HCN(CH/sub 3/CN) + X/sup -/ --> CN/sup -/(CH/sub 2/CN/sup -/) + XH where X = H, D, OH, SH, NH/sub 2/, C/sub 2/H, and CH/sub 3/O have been measured at 297 +- 2K using the flowing afterglow technique These reactions for which the measured rate constants span a range from 015 to 15 X 10/sup -8/cm/sup 3/ molecule/sup -1/s/sup -1/ represent some of the fastest ion-molecule reactions observed to date The rate constants are compared with the predictions of Langevin, average-dipole-orientation and locked-dipole theories of ion-molecule collisions The average-dipole-orientation theory provides the most realistic collision rates for these reactions although these appear to be underestimated by approx10 to approx40% The observed dependence of the efficiency for proton transfer on exothermicity indicates that the assumption of unit reaction probability resident in such comparisons may break down at low exothermicities
TL;DR: In this article, a Cahn Electrobalance was used to determine directly and very accurately the carbon content of α-Fe, Fe-0.48 wt pct Mo and Fe-1.16 wtpct Mo specimens which were equilibrated with a series of methane-hydrogen gas mixtures.
Abstract: A Cahn Electrobalance has been used to determine directly and very accurately the carbon content of α-Fe, Fe-0.48 wt pct Mo and Fe-1.16 wt pct Mo specimens which were equilibrated with a series of methane-hydrogen gas mixtures. The equilibria investigated involved the ferrite phases of the alloys between 682 and 848‡C. The experimental results permitted direct calculation of the activities of carbon in the samples, relative to graphite as unity, and of other thermodynamic functions, without the necessity for any correction factors. The results have been compared with the experimental measurements of a number of other investigators. In ferrite, the partial molar enthalpy and entropy of solution of carbon are found to be 26,800 cal/mole, (112, 130 J/mole), and 30.59 cal/K-mole, (127.99 J/K-mole) respectively at temperatures below about 727‡C. Above this temperature, the values are 25,200 cal/mole and 29.13 cal/K-mole, respectively. The addition of molybdenum results in an increase in these properties below 727‡C and a decrease in the values above 753‡C, and the changes are found to be proportional to the molybdenum content. The solubility of carbon in α-Fe is found to be 0.0176 wt pct at the eutectoid temperature. Molybdenum increases the solubility relative to the Fe-C system at temperatures above the eutectoid and decreases it below the eutectoid.
TL;DR: In this article, a calculation of the formation energy and volume for a vacancy in transition metals is presented, using a tight binding scheme for the d band and a Born-Mayer type potential to account for the repulsive part of the energy at small distances.
TL;DR: It was shown that this corresponds to an equilibrium between activated and nonactivated forms and not to the presence of a population of complexes unable to undergo activation, suggesting that breakage of some noncovalent bonds could take place during activation.
TL;DR: In this paper, the authors have argued that the entropy of ionization of a vacancy in a semiconductor should be closely approximated by the enthalpy of the forbidden band gap.
Abstract: In the preceding paper the authors have argued that the entropy of ionization of a vacancy in a semiconductor should be closely approximated by the entropy of the forbidden band gap; $\ensuremath{\Delta}{S}_{c\ensuremath{
u}}=\ensuremath{\Delta}{S}_{I}({V}^{+})=\ensuremath{\Delta}{S}_{I}({V}^{\ensuremath{-}})$. Here this hypothesis is combined with the hypothesis that the entropy of formation of the neutral vacancy, $\ensuremath{\Delta}S({V}^{x})$, is only the entropy of its Jahn-Teller distortion. Thus $\ensuremath{\Delta}{S}_{I}({V}^{\ifmmode\pm\else\textpm\fi{}})\ensuremath{\gg}\ensuremath{\Delta}S({V}^{x})=k\mathrm{ln}3$ for Si and Ge. Empirical values for the enthalpy of ionization at low temperature, $\ensuremath{\Delta}{H}_{I}({V}^{\ifmmode\pm\else\textpm\fi{}})$, are combined with the values for $\ensuremath{\Delta}H({V}^{x})$ predicted by the macroscopic theory of Phillips and Van Vechten in order to predict the result of quenching experiments taking proper account of the charge states of the vacancies. Agreement with the experiments of Logan, Hiraki, and Elstner and Kamprath is quite satisfactory.
TL;DR: The activation and overall thermodynamic parameters as well as the individual thermodynamic values for the different steps were combined to construct a self-consistent energy profile for the reaction.
Abstract: The electron transfer reaction between the "blue" single copper protein azurin (from Pseudomonas aeruginosa) and the hexacyanoiron (II/III) couple has been studied. Equilibrium constants for the reduction of azurin were measured spectrophotometrically in the temperature range 5-33 degrees C (K = 1.1 X 10(-2) at 25 degrees C, deltaH degrees = 10.9 kcal/mol, 0.1 M potassium phosphate, pH 7.0, I = 0.22). The enthalpy change was also determined by microcalorimetry and from the analysis of chemical relaxation amplitudes. Following a temperature-jump perturbation of this equilibrium, only a single relaxation was observed. The reciprocal of the relaxation time increased linearly as oxidized azurin was reacted with increasing amounts of ferrocyanide, yet reached saturation when reduced azurin was titrated with ferricyanide. This behavior as well as the analysis of the relaxation amplitudes led to the following scheme for this system: see article. At 25 degrees C the rate constants for the electron transfer were k+3=6.4s-1 and k-3=45s-1, the association constants K1=54 M-1 and K2-1=610 M-1. The activation and overall thermodynamic parameters as well as the individual thermodynamic values for the different steps were combined to construct a self-consistent energy profile for the reaction.
TL;DR: In this paper, the formation constants of Cu(OH), Cu2(OH)2 and Cu3(OH4) were determined by potentiometric measurements and the enthalpy changes were obtained by direct calorimetry.
TL;DR: In this article, a precision differential flow calorimeter for the measurement of enthalpies of mixing of liquids has been constructed for benzene + cyclohexane, which has been made at temperatures between 28015 K and 39315 K. The standard deviation of the 29815 K measurements is 021 J mol −1, and agreement with the results of best isothermal dilution calorimeters is excellent
TL;DR: In this paper, a wide range of copper(II) polyamine complexes have been investigated by EPR and electronic spectroscopy in aqueous solution, and all the compounds are found to be 4 + 2 or 4 + I species with a dx2-y2 ground state.
TL;DR: In this article, the authors measured the temperatures of combustion and standard enthalpies of 22 experimental organic explosives and one standard explosive, TNT, and compared the calculated velocities with experimental veloities where suitable values of the latter were available.
Abstract: As part of a study of thermally stable explosives, the heats of combustion and standard enthalpies of formation of 22 experimental organic explosives and of one standard explosive, TNT, were determined. The results of these measurements, together with the reported crystal densities, were used to calculate the detonation velocities and pressures by two different methods. The calculated velocities were compared with experimental velocities where suitable values of the latter were available. (auth)
TL;DR: A Cahn Electrobalance has been used to determine directly and very accurately the carbon content of iron, iron-048 wt pct molybdenum and iron-116 wtpct Mo alloy as mentioned in this paper, which were equilibrated with a series of methane-hydrogen gas mixtures.
Abstract: A Cahn Electrobalance has been used to determine directly and very accurately the carbon content of iron, iron-048 wt pct molybdenum and iron-116 wt pct molybdenum specimens which were equilibrated with a series of methane-hydrogen gas mixtures of constant composition The equilibria investigated involved the austenite phases of the alloys at 783, 813 and 848‡C The experimental results permit direct calculation of the activities of carbon in the samples, relative to graphite as unity, and of the enthalpy and entropy of solution of carbon The results are compared with the experimental measurements of a number of other investigators The results are in excellent agreement with those of Smith and Schenck and Kaiser for the Fe-C system at 800‡C, and indicate -HC/M values of 9700 ± 500 cal/mole for pure Fe, 10,030 ± 500 cal/mole for an Fe-048 wt pct Mo alloy, and 10,150 ± 500 cal/mole for an Fe-116 wt pct Mo alloy The effect of molybdenum in austenite is to decrease the activity coefficient of carbon in austenite
TL;DR: In this paper, a method for the analysis of dilution heats of micellar systems is proposed for the simultaneous determination of critical mousellar concentration and micellization enthalpy from measurements performed at a single temperature.
Abstract: A method is proposed for the analysis of dilution heats of micellar systems. Simultaneous determination of critical micellar concentration and of micellization enthalpy can be derived from measurements performed at a single temperature.
TL;DR: The ionization constant of liquid D2O has been measured over the temperature range 298 to 523 K using an aqueous electrolyte concentration cell as discussed by the authors, and the standard free energy, enthalpy, entro...
Abstract: The ionization constant of liquid D2O has been measured over the temperature range 298 to 523 K using an aqueous electrolyte concentration cell. Values for the standard free energy, enthalpy, entro...
TL;DR: In this paper, mixing calorimetry has been used to determine the excess enthalpy HE as a function of composition for liquid mixtures of nitrogen + methane at 91.5 and 105.0 K. Good agreement is found between the present results and a number of other recent studies.
01 Oct 1976
TL;DR: In this paper, the surface pressure versus mean area curves of mixed monolayers of cholesterol with n-long chain fatty acids were obtained at various compositions and temperatures, and it was observed that the area occupied by the expanded monolayer of fatty acid becomes somewhat smaller by addition of cholesterol.
Abstract: The surface pressure versus mean area curves of the mixed monolayers of cholesterol with n-long chain fatty acids was obtained at various compositions and temperatures. It was observed that the area occupied by the expanded monolayer of fatty acid becomes somewhat smaller by addition of cholesterol. The surface pressure of the transition from expanded to condensed state varied with the composition and temperature, whereas the area occupied by the condensed monolayer composed of cholesterol and fatty acid satisfied the additivity rule. The two-dimensional phase diagram of the cholesterol-myristic acid system was constructed and the partial molar enthalpy of myristic acid was evaluated. It was concluded that cholesterol and fatty acid are miscible in the expanded state and immiscible in the condensed state and that the interaction between them is not specific but large enough to form the homogeneous expanded monolayer.