Partial Molar Heat Capacities Research Articles

Heat capacities ofD-mannitol andD-sorbitol in water, dimethyl sulfoxide and formamide

Partial molar heat capacities at infinite dilution, C∞p, 2 have been determined calorimetrically for D-mannitol and D-sorbitol in water, dimethyl sulfoxide and formamide, at 298.15 K. The C∞p, 2 values obtained, which are similar for the three solvents, are much higher than the heat capacities for the pure compounds. The large ΔC∞p values for the dissolution process are believed to be partly due to the existence of several rotamers in solution. Sorbitol seems to be more sensitive than mannitol to specific solvent effects.

Calorimetric investigations of crystalline, molten, and supercooled Ca(NO 3) 2·4H 2O and of concentrated Ca(NO 3) 2(aq)

We have measured heat capacities of Ca(NO 3) 2·4.02H 2O in the crystalline, liquid, and supercooled-liquid states over the temperatures ranges 306.8 K to 318.8 K, 318.8 K to 351.4 K, and 306.8 K to 314.8 K, respectively. We have also measured the melting temperature (316.0 K) and the enthalpy of melting. Further measurements have been made on the heat capacities of concentrated Ca(NO 3) 2(aq). Results of all of these measurements have been used to calculate the entropy of melting, partial molar heat capacities of Ca(NO 3) 2 and H 2O in concentrated Ca(NO 3) 2(aq), and the glass-transition temperature.

Effects of complex formation in flowing fluids on the hydrothermal solubilities of minerals as a function of fluid pressure and temperature in the critical and supercritical regions of the system H 2O

Consideration of Darcy's law and its analogs for open channel flow, together with the thermodynamics of hydrolysis reactions in hydrothermal systems indicates that either fluid pressure and/or isothermal mineral solubilities or both may decrease or increase in the direction of fluid flow, depending on the volume of reaction and the permeability or aperture, cross-sectional area, and angle of flow as a function of distance along the flow path. Recent progress in theoretical geochemistry has led to improved equations of state which can be used to calculate the standard partial molal thermodynamic properties of both charged and neutral inorganic and organic aqueous species at pressures and temperatures to 1000°C and 5 kb ( Tanger and Helgeson, 1988; Shock and Helgeson, 1988, 1990; Shock, et al., 1989, 1992; Sverjensky et al., 1992). Thermodynamic properties generated from these revised equations of state for the hydrolysis of minerals to form aqueous complexes at high pressures and temperatures indicate that the signs of the standard partial molal volume, enthalpy, and heat capacity of reaction depend primarily on the number of ligands in the complexes, as well as their charge. If polyligand complexes and/or certain neutral aqueous species appear on the right side of the reaction, the isobaric and isothermal partial derivatives of the logarithm of the equilibrium constant ( log K) at P SAT may tend toward infinity and negative infinity, respectively, as fluid pressure and temperature increase in the liquid phase region and approach the critical point of H 2O. This behavior results in positive values of ( ∂ log K/ ∂T) p and negative values of ( ∂ log K/ ∂P) T at supercritical pressures and temperatures. For example, thermodynamic calculations indicate that values of log K for reactions representing hydrothermal sulfide solubilities in the acid pH range where the predominant sulfide species is H 2S (aq) decrease with increasing fluid pressure to an increasing degree with increasing temperature, which is consistent with experimental data reported by Hemley et al. (1986, 1992). In contrast, ( ∂ log K/ ∂T) p and ( ∂ log K/ ∂P) T in the supercritical region may be negative and positive, respectively, for reactions representing sulfide solubilities in hydrothermal solutions with higher pHs where HS − predominates over H 2S, but only if the chloride concentration is low. The opposite may be the case in concentrated alkali chloride solutions, regardless of the pH. Similar calculations indicate that log K for the incongruent reaction of K-feldspar or other aluminosilicates with supercritical hydrothermal solutions to form quartz and A1(OH) 4 −increases monotonically with increasing fluid pressure at constant temperature. However, the log K values maximize with increasing temperature at all pressures to at least ~3 kb, which is not true of the solubility of quartz. In contrast, values of log K for analogous reactions written in terms of Al 3+ or Al (OH) 2+ minimize with increasing temperature at constant pressure.

Aqueous partial molar heat capacities and volumes for NaTcO4 and NaReO4

A flow microcalorimeter/densimeter system has been commissioned to measure heat capacities and densities of solutions containing radioactive species as a function of temperature. Measurements were made for NaTcO4(aq) at six temperatures (189.15 K to 373.15 K for the heat capacities, 287.43 K to 396.67 K for the densities) over the molality range 0.01 to 0.29 mol-kg−1. Measurements for NaReO4(aq) (NaReO4 is a common nonradioactive analogue for NaTcO4) were made under similar conditions, but for eight temperatures and a more extensive range of molalities, 0.05 to 0.65 mol-kg−1. Heat capacities of NaCl(aq) reference solutions were also measured from 293.15 K to 398.15 K. The heat capacity and density data are analysed using Pitzer's ioninteraction model. Equations for the apparent molar heat capacities and volumes are reported. Values of the NaReO4(aq) partial molar heat capacities are compared to literature values based on integral heats of solution. The agreement between the two sets of NaReO4 results is good below 330 K, but only fair at the higher temperatures. Values of the partial molar volumes have also been derived. Using literature values and the results of our experiments, it is calculated that the disproportionation of hydrated TcO2(s) to form TcO 4 − (aq) and Tc(cr) occurs more readily at high temperatures. The uncertainties introduced by using thermodynamic values for ReO 4 − (aq), in the absence of values for TcO 4 − (aq), are discussed.

Heat capacities and volumes of methanol+acetonitrile mixtures at 5 and 25�C

Densities and heat capacities of methanol + acetonitrile mixtures were determined over the whole composition range at 5 and 25°C. Apparent and partial molar volumes and heat capacities for both components of the mixture were calculated from these data. These functions for acetonitrile run monotonously over the whole composition range of the mixture at both temperatures, while those for methanol exhibit extrema (volumes-minimum and heat capacities-maximum) at high acetonitrile content. The reasons of the observed behavior of the system are discussed.

Raman studies of hydration of hydroxy complexes and the effect on standard partial molar heat capacities

Raman spectroscopic studies of aqueous hydroxy complexes B(OH) 3, B(OH) 4 2−, Al(OH) 4 −, Zn(OH) 4 2−, and their deuterated analogues provide compelling evidence for the presence of strong hydrogen bonding of water in the outer solvation sphere to the oxygen of the hydroxy ligand in the first coordination sphere. The deuterated complexes are more strongly hydrogen bonded than the hydrogen complexes. The fact that these hydrogen-bonding effects for the hydroxy borates are much more pronounced than for the aluminate and zincate anions can account for the unexpectedly large differences in the standard partial molar heat capacities of B(OH) 4 − and A1(OH) 4 −. The extension of semi-empirical solvation models based on the Born approximation for aqueous hydrolyzed species will need to consider the rather large and specific effect of hydrogen bonding of water to the hydroxide ion.

Calculation of excess heat capacities for liquid mixtures

Experimental data of excess heat capacities for 29 binary liquid mixtures of various types have been correlated by the nrtl and uniquac models. For nine of these mixtures data at several isotherms is included. The parameters of the models, six for the nrtl and four for the uniquac, have been estimated by a minimization technique using Simplex and Fletcher techniques simultaneously. The partial molar excess heat capacities at infinite dilution are also calculated. Agreement between experimental and calculated values is satisfactory except for associating and highly non-ideal mixtures. The uniquac model performs better than the nrtl model.

Heat capacities of Fe2O3-bearing silicate liquids

Direct measurements of liquid heat capacity, using a Setaram HT1500 calorimeter in step-scanning mode, have been made in air on six compositions in the Na2O-FeO-Fe2O3-SiO2 system, two in the CaO-FeO-Fe2O3-SiO2 system and four of natural composition (basanite, andesite, dacite, and peralkaline rhyolite). The fitted standard deviations on our heat capacity measurements range from 0.6 to 3.6%. Step-scanning calorimetry is particularly useful when applied to iron-bearing silicate liquids because: (1) measurements are made over a small temperature interval (10K) through which the ferric-ferrous ratio of the liquid remains essentially constant during a single measurement; (2) the sample is held in equilibrium with an atmosphere that can be controlled; (3) heat capacity is measured directly and not derived from the slope of enthalpy measurements with temperature. Liquid compositions in the sodic and calcic systems were chosen because they contain large concentrations of Fe2O3 (up to 19 mol%), and their equilibrium ferric-ferrous ratios were known at every temperature of measurement. These measurement have been combined with heat capacity (Cp) data in the literature on iron-free silicate liquids to fit Cp as a function of composition. A model assuming no excess heat capacity (linear combination of partial molar heat capacities of oxide components) reproduces the liquid data within error (±2.2% on average). The derived partial molar heat capacity of the Fe2O3 component is 240.9 ±7.9 J/g.f.w.-K, with a standard error reduced by more than a factor of two from that in earlier studies. The model equation, based primarily on simple, synthetic compositions, predicts the heat capacity of the four magmatic liquids within 1.8% on average.

Prediction of the excess heat capacities of 1-alkanol + alkane systems on the basis of pure-liquid properties

Abstract Moorthi, K. and Nagata, I., 1992. Prediction of the excess heat capacities of 1-alkanol + alkane systems on the basis of pure-liquid properties. Fluid Phase Equilibria, 72: 131-146. The partial molar excess heat capacities of 1-alkanols at infinite dilution within the range 283–313 K and the excess heat capacities, CpE, of 28 (C1–C12) 1-alkanol + alkane systems within the range 200–368 K were predicted by means of the athermal two-constant Mecke—Kempter model. The model parameters were determined on the basis of the properties of pure substances. For dilute solutions (less than 0.2 alcohol mole fraction) the excess heat capacities estimated agree well with the experimental values, the average deviation in CpE for 435 data points being 0.5 J K−1 mol−1. For the alcohol-rich solutions the average deviation in CpE is larger (1.5 J K−1 mol−1, based on 273 data points) and may indicate the presence of interactions which are not caused by hydrogen bonding. The results presented indicate that hydrogen bonding effects predominate in the excess heat capacities of 1-alkanol + alkane solutions.

High-temperature thermodynamics of aqueous alkali-metal salts

Heat capacities and densities of aqueous solutions of four alkali-metal halides (NaF, KF, KCl, and KI) and two alkali-metal sulfates (Na2SO4 and K2SO4) have been measured at temperatures from 298.15 K to 373.15 K and at the pressure 0.6 MPa over the molality ranges 0.03 mol·kg−1 to 0.44 mol·kg−1, 0.10 mol·kg−1 to 2.10 mol·kg−1, 0·03 mol·kg−1 to 2.01 mol·kg−1, 0.04 mol·kg−1 to 0.60 mol·kg−1, 0.05 mol·kg−1 to 1.53 mol·kg−1, and 0.03 mol·kg−1 to 0.40 mol·kg−1, respectively. The measurements were done using a flow differential-heat-capacity calorimeter in series with a vibrating-tube densimeter. Pitzer's ion-interaction model was used to analyse these results and literature values of comparable precision. Equations for the apparent molar heat capacities and volumes are reported, and consistent sets of ionic partial molar heat capacities and volumes at 298.15 K and 373.15 K, relative to the conventions: C∞p, 2(H+,qa) = 0 and V∞2(H+,aq) = 0, were calculated. Activity coefficients were also calculated for NaF and KF for the temperature range from 298.15 K to 473 K.

Calculation of the thermodynamic properties of aqueous species at high pressures and temperatures. Effective electrostatic radii, dissociation constants and standard partial molal properties to 1000 °C and 5 kbar

Within the framework of the revised HKF (H. C. Helgeson, D. H. Kirkham and G. C. Flowers, Am. J. Sci., 1981, 281, 1249) equations of state (J. C. Tanger IV and H. C. Helgeson, Am. J. Sci., 1988, 288, 19), prediction of the standard partial molal thermodynamic properties of aqueous ions and electrolytes at high pressures and temperatures requires values of the effective electrostatic radii of the ions (re), as well as provision for the temperature and pressure dependence of the relative permittivity of the solvent, H2O. Values of the relative permittivity of H2O, together with the Born functions needed to compute the standard partial molal Gibbs free energy, enthalpy, entropy, heat capacity and volume of solvation were calculated as a function of temperature and density from a modified version of the Uematsu–Franck equation (M. Uematsu and E. U. Franck, J. Phys. Chem. Ref. Data, 1980, 9, 1291). The temperature/pressure dependence of re is described in terms of a solvent function designated by g, which was evaluated in the present study at temperatures and pressures to 1000 °C and 5 kbar by regressing experimental standard partial molal heat capacities and volumes of NaCl reported in the literature together with published dissociation constants for NaClo at supercritical temperatures and pressures using the revised HKF equations of state for aqueous species. The calculated values of re decrease substantially with increasing temperature at constant pressure ⩽2 kbar, and with decreasing pressure at constant temperature 400 °C. The equations and parameters summarized below permit calculation of the standard partial molal properties of aqueous species from the revised HKF equations of state over a much more extensive range of temperature than was previously possible.

Apparent molar heat capacities and volumes of some alkylated derivatives of uracil and adenine in aqueous solution at 25�C

Densities and apparent molar heat capacities of some alkylated derivatives of uracil and adenine: 1-methyluracil, 1,3-dimethyluracil, 1,3-diethylthymine, 5,6-trimethylene-1,3-dimethyluracil, 5,6-tetramethylene-1,3-dimethyluracil, 5,6-pentamethylene-1,3-dimethyluracil, 2,9-dimethyladenine, 2-ethyl-9-methyladenine, 2-propyl-9-methyladenine, 8-ethyl-9-methyladenine, 6,8,9-trimethyladenine and 8-ethyl-6,9-dimethyladenine were determined using flow calorimetry and flow densimetry at 25°C. It was found that the partial molar volumes and heat capacities correlate linearly with the number of substituted methylene groups-CH 2 -as well as to the number of hydrogen atoms, n H , belonging to the skeleton of the molecule. In the case of alkylated uracils a difference was observed in the values at infinite dilution V 2 o and C p2 o , depending on the substitution of alkyl and cyclooligomethylene groups.

Some thermodynamic properties of dialkylsulphides and dialkyldisulphides in aqueous solution

Enthalpies of dissolution in water of dialkylsulphides: CH3(CH2)vS(CH2)vCH3, v = 0, 1, and 2, and dialkyldisulphides: CH3(CH2)vSS(CH2)vCH3, v = 0 and 1, were determined by microcalorimetry at the temperatures (288.15, 298.15, 308.15, and 328.15) K. Molar enthalpies of solvation and partial molar heat capacities at 298.15 K were derived for the infinitely dilute solutions. The partial molar heat capacities are discussed in terms of an empirical group scheme.

Thermodynamic properties of N-octyl-, N-decyl- and N-dodecylpyridinium chlorides in water

Densities, heat capacities and enthalpies of dilution at 25°C and osmotic coefficients at 37°C were measured for N-octyl-, N-decyl- and N-dodecyl-pyridinium chlorides in water over a wide concentration region. Conductivity measurements were performed in order to evaluate the cmc and the degree of counterion dissociation. Partial molar volumes, heat capacities, relative enthalpies and nonideal free energies and entropies at 25°C were derived from the experimental data as functions of the surfactant concentration. The changes with concentration of these properties are quite regular with the exception of the heat capacities which display anomalies at about 0.9, 0.25 and 0.12 mol-kg−1 for the octyl, decyl and dodecyl compounds, respectively. At these concentrations there were also changes in the slopes of the specific conductivity and of the product of the osmotic coefficients and the molality vs. concentration. These peculiarities can be ascribed to micelle structural transitions. The thermodynamic functions of micellization were graphically evaluated on the basis of the pseudo-phase transition model. These data have been compared to those for alkyltrimethylammonium bromides and alkylnicotinamide chlorides. It is shown that the introduction of the hydrophilic CONH2 group lowers the hydrophilic character of the pyridinium ring.

Calorimetric study of the digestion of gibbsite, Al(OH)3(cr), and thermodynamics of aqueous aluminate ion, Al(OH)4−(aq)

We have made calorimetric measurements of the enthalpies of solution of gibbsite, Al(OH)3(cr), in aqueous sodium hydroxide solutions at five temperatures from 100 to 150 °C. Results of these measurements have been used to obtain the standard enthalpies of formation of Na+(aq) + Al(OH)4−(aq) at the experimental temperatures. These results have also led to values of ΔCp0 for the reaction represented concisely by Al(OH)3(cr) + OH−(aq) = Al(OH)4−(aq), from which we have obtained standard state partial molar heat capacities of Na+(aq) + Al(OH)4−(aq). Combination of our results with those from earlier investigations has permitted calculation of thermodynamic properties of Na+(aq) + Al(OH)4−(aq) over a wide range of temperature and thence some generalizations about the usefulness of various equations for representing or predicting these thermodynamic properties. Key words: gibbsite, enthalpy of solution; sodium aluminate (aqueous), thermodynamic properties; heat capacities, Na+(aq) + Al(OH)4−(aq).

Dodecyltrimethylammonium bromide in water-urea mixtures: volumes, heat capacities, and conductivities

Densities, heat capacities, and conductivities of dodecyltrimethylammonium bromide (DTAB)-water-urea mixtures over a wide range of urea and surfactant concentrations were measured at 298 K. The partial molar volumes (V 2 ) and heat capacities (C p2 ) of DTAB in urea solutions at fixed compositions were derived. The profiles of C p2 as a function of DTAB concentration show anomalies at about 0.30-0.35 mol kg −1 , depending on the urea molality (m u ), whereas those of V 2 do not

A study of the electrolytic solution process using the scaled particle theory. 5. Heat capacity of aqueous ions at elevated temperatures

A theory composite of the scaled particle theory and the Born model of solvent continuum has been used to theoretically calculate the standard heat capacity of hydration as well as the partial molal heat capacity of aqueous ions and electrolytes at elevated temperatures. The uncertainties in the second temperature derivatives of solvent dielectric constant at various temperatures present a barrier to an accurate heat capacity prediction by the theory. Nevertheless, the agreement between the predicted standard heat capacity of electrolytes in solution and the corresponding experimental data, particularly at higher temperatures, is encouraging. Moreover, the composite theory seems to provide the most accurate thermodynamic predictions to date for aqueous electrolytes at higher temperatures without involving any arbitrary adjustable parameter. We therefore use this theory to find the proper ionic scale of the partial molal heat capacities at elevated temperatures. Key words: scaled particle theory, solvent continuum model of Born, standard heat capacity of aqueous ions, absolute scale for hydration thermodynamic quantities.

Aqueous solutions containing amino acids and peptides. Part 27.—Partial molar heat capacities and partial molar volumes of some N-acetyl amino acid amides, some N-acetyl peptide amides and two peptides at 25 °C

Partial molar heat capacities, in water at 25 °C, of the compounds N-acetylglycinamide, N-acetyl-L-alaninamide, N-acetylglycylglycinamide, N-acetylglycyl-L-alaninamide, N-acetyl-L-alanylglycinamide, N-acetyl-L-alanyl-L-alaninamide, glycyl-DL-alanine and DL-alanylglycine have been obtained. The information obtained, for the amides, is discussed in terms of group-additivity approaches, and group contributions are evaluated. The values for these differ in some instances, and particularly that for the heat capacity of the peptide group, from those obtained in an earlier study. Comparison of the parent amino acids and peptides with the amides shows that there is a marked contribution to the heat capacities of the zwitterionic compounds from electrostatic sources.Some results are presented of the partial molar volumes, also at 25 °C, of N-acetylglycyl-L-alaninamide, N-acetyl-L-alanylglycinamide and DL-alanylglycine.A correlation is evident between the heat capacity and volumetric interaction coefficients, for amino acids, peptides and N-acetylamides, obtained from the molality dependences of the apparent molar properties, and this has been rationalised using a simple electrostatic approach.

Heat capacities of maltose, maltotriose, maltotetrose and α-, β-, and γ-cyclodextrin in the solid state and in dilute aqueous solution

Maltose, maltotriose, maltotetrose, and α-, β-, and γ-cyclodextrin have been studied by microcalorimetric techniques. Heat capacities have been determined at 298.15 K for the dried pure compounds and for defined hydrates. Partial molar heat capacities in water at infinite dilution have been determined at 298.15 K for all substances. Enthalpies of solution in water of hydrated forms of the cyclodextrins have been determined at 288.15 K, 298.15 K, and 308.15 K.

Apparent and partial molar heat capacities and volumes of chromates (CrO42-(aq), HCrO4-(aq), and Cr2O72-(aq)) at 25.degree.C: chemical relaxation and calculation of equilibrium constants for high temperatures

Apparent and partial molar heat capacities and volumes of chromates (CrO42-(aq), HCrO4-(aq), and Cr2O72-(aq)) at 25.degree.C: chemical relaxation and calculation of equilibrium constants for high temperatures