Standard State Partial Molar Heat Research Articles

Review of the Apparent Molar Heat Capacities of NaCl(aq), HCl(aq), and NaOH(aq) and Their Representation Using the Pitzer Model at Temperatures from (298.15 to 493.15) K

In this study, a new estimation of the standard state partial molar heat capacity (Cp0) of three binary systems [NaCl(aq), HCl(aq), and NaOH(aq)] for temperatures ranging from (298.15 to 493.15) K is performed. From experimental data (available in the literature to date) corresponding to the apparent molar heat capacities of the above-mentioned electrolytes, a procedure based on the assumption of Pitzer’s ion interaction model was applied to calculate these important Cp0 values at a given temperature. Results obtained for each electrolyte (or ion) were correlated as functions of temperature by commonly used polynomial equations. Use of the hydrogen ion convention enabled estimates of the temperature-dependent values of Cp0 of the individual ions Na+(aq), Cl−(aq), and OH−(aq). When used to calculate the temperature-dependent values of the Gibbs free energy of formation of OH−(aq), the formula for Cp0(T) of OH−(aq) given herein provides good agreement with values derived from independent measurements of the...

Standard state thermodynamic properties of aqueous sodium perrhenate using high dilution calorimetry up to 598.15 K

Standard state thermodynamic properties for aqueous sodium perrhenate at temperature in the range of (298.15 to 598.15) K and at p sat were determined by high dilution solution calorimetry down to 10 −4 m. Standard state partial molar heat capacities, C p , 2 ∘ , of aqueous sodium perrhenate calculated from present study are compared to literature values up to T = 398.15 K. The differences between C p , 2 ∘ of ReO 4 - ( aq ) and Cl −(aq) at lower temperature is much greater than that due to their internal molecular motions. Consequently, the perrhenate ion appears to have an ionic incomplete primary hydration shell as compared to the chloride ion. The ReO 4 - / Cl - difference in thermodynamic functions has now been well defined up to T = 598.15 K for other important high temperature calculations.

Apparent molar heat capacities of aqueous solutions of phosphoric acid and sulfur dioxide from 303 to 623 K and a pressure of 28 MPa

Heat capacities of aqueous solutions of phosphoric acid from 0.1 to 0.8 mol- kg-1 and sulfur dioxide from 0.2 to 0.9 mol-kg-1 have been measured with a flow heat-capacity calorimeter from 303 to 623 K and a pressure of 28 MPa. At the lowest molality single-solute solutions as well as mixtures of either H3PO4 or SO2 with HC1 were measured to repress dissociation. Calculated apparent molar heat capacities were corrected for dissociation reactions and the chemical relaxation effect. Experimental results for mixtures were analyzed using Young’s rule. Standard state partial molar heat capacities of H3PO4(aq) and SO2(aq) were obtained by extrapolation to infinite dilution. A few measurements of the densities of aqueous H3PO4 and SO2 were made at 25°C and a pressure of 28 MPa.

Standard state heat capacities of aqueous electrolytes and some related undissociated species

Uses of heat capacities of solutions of electrolytes are reviewed, with a particular emphasis on the standard state partial molar heat capacities and their applications to calculations of the effects of temperature on equilibrium constants, electrode potentials, enthalpies, and entropies. Methods of obtaining these standard partial molar heat capacities are summarized, followed by comparisons of values obtained in different ways. Many of the "best" such heat capacities are collected and then used as the basis for establishing single-ion heat capacities based on the convention that CpO(H+) = 0, followed by illustrations of the convenient use of these quantities. Finally, there is brief discussion of theoretical analysis of these standard partial molar heat capacities in relation to ion–solvent interactions. Key words: heat capacities, electrolytes; aqueous solutions, heat capacities; thermodynamics, electrolytes.

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).

Apparent molar heat capacities and volumes of aqueous HClO 4, HNO 3, (CH 3) 4NOH and K 2SO 4 at 298.15 K

Apparent molar heat capacities and volumes of HClO 4(aq), HNO 3(aq), (CH 3) 4NOH(aq) and K 2SO 4(aq) have been determined at 298.15 K. Infinite dilution standard state partial molar heat capacities and volumes have been calculated from these data. We recommend revised values for the conventional ionic partial molar heat capacities and volumes of ClO 4 −(aq), NO 3 −(aq) and SO 4 2−(aq).

Thermodynamics of aqueous EDTA systems: Apparent and partial molar heat capacities and volumes of aqueous EDTA4−, HEDTA3−, H2EDTA2−, NaEDTA3−, and KEDTA3− at 25 °C. Relaxation effects in mixed aqueous electrolyte solutions and calculations of temperature dependent equilibrium constants

Calorimetric and densimetric measurements have led to apparent molar heat capacities and volumes for aqueous solutions of the mixed electrolytes [(CH3)4N]4EDTA + (CH3)4NOH, Na4EDTA + NaOH, and K4EDTA + KOH, and single electrolytes Na2H2EDTA and [(CH3)4N]3[HEDTA] at 25 °C. We have analyzed these results in terms of Young's rule and Pitzer's ion interaction model to obtain standard state partial molar heat capacities and volumes of EDTA4−(aq), HEDTA3−(aq), H2EDTA2−(aq), NaEDTA3−(aq), and KEDTA3−(aq) at 25 °C. For these calculations it was also necessary to evaluate the "relaxation" contribution to the measured heat capacities of some solutions. The partial molar heat capacities obtained here have been used with enthalpies from previous investigations for calculations of several equilibrium constants over wide ranges of temperature; volumes can be used for similar calculations of the effects of pressure.

Thermodynamics of aqueous EDTA systems: Apparent and partial molar heat capacities and volumes of aqueous strontium and barium EDTA

Apparent molar heat capacities and volumes have been determined for Na2SrEDTA (aq) and Na2BaEDTA (aq). Standard state partial molar heat capacities and volumes have been calculated as well as the partial molar properties at 0.1 m ionic strength that are needed for various thermodynamic calculations. Selected enthalpies and stability constants from the literature have been combined with out heat capacities to generate predicted stability constants to 200°C.

Densities and apparent molar volumes of aqueous aluminum chloride. Analysis of apparent molar volumes and heat capacities of aqueous aluminum salts in terms of the Pitzer and Helgeson theoretical models

Densities of aqueous solutions of AlCl3 (containing dilute HCl) have been measured at 10, 25, 40, and 55 °C with results that have led to defined apparent molar volumes. We have used the Pitzer ion interaction model as the basis for analyzing these apparent molar volumes to obtain standard state (infinite dilution) partial molar volumes of AlCl3(aq) at each temperature. We have also made similar use of apparent molar heat capacities of aqueous solutions of AlCl3–HCl and Al(NO3)3–HNO3 from Hovey and Tremaine to obtain standard state partial molar heat capacities of AlCl3(aq) and Al(NO3)3(aq) at these same temperatures. Finally, the standard state partial molar volumes and heat capacities have been used with the Helgeson–Kirkham semi-theoretical equation of state for aqueous ions to provide a basis for estimating the thermodynamic properties of Al3+(aq) at high temperatures and pressures.