Standard Gibbs Energy Change Research Articles

Thermodynamics of ionization of trans-crotonic and adipic acids in water; the silver – silver adipate electrode

The ionization constant, K, of trans-crotonic acid, and the first and second ionization constants, K1 and K2, of adipic acid have been determined in water at seven different temperatures ranging from 15 to 45 °C with the help of the cells without liquid junction using silver – silver chloride and quinhydrone electrodes. The variations of pK, pK1, and pK2 with temperature have been expressed by the equations[Formula: see text]and[Formula: see text]From the temperature coefficient of the ionization constants, the standard enthalpy, entropy, Gibbs energy, and heat capacity changes of the corresponding dissociation processes have been calculated. Using the values of ionization constants of adipic acid, the standard potential of the Ag(s)/Ag2Ad(s)/Ad2− electrode, and the related standard thermodynamic quantities for the electrode process[Formula: see text]have been calculated in water at those temperatures.

Conditions of formation of alkali hydrogenphosphites

The Filippov's eutonic method was applied to the study of M2HPO3-H3PO3-H2O heterogeneous systems (M = Li, Na, K) at 25 °C. The values of the standard Gibbs energy changes for the reactions occurring in these systems were obtained. The highest changes in the Gibbs energy accompany the formation of hydrogenphosphites M(H2PO3), the energy increases from Li to K and decreases from hydrogenphosphite to tris(orthophosphite) and bis(orthophosphite).

Thermodynamic property changes in lanthanide(III) cation exchange reactions with poly(styrenesulfonic acid) type cation exchangers

Calorimetric and equilibrium distribution measurements at 298.1 K were employed to estimate standard Gibbs energy, enthalpy, and entropy changes in the ion-exchange reactions between 13 of the lanthanide(III) cations and yttrium(III) ion in dilute aqueous perchlorate solution and a strong-acid type cation exchanger. A plot of the lanthanide atomic number dependence of ..delta..G/sup 0/ for the reaction LnR/sub 3/ + Ce(III) = CeR/sub 3/ + Ln(III) showed discontinuities at Z = 64 (Gd) and between Z = 60 to 61 (Nd--Pm) and Z = 67 to 68 (Ho--Er) indicating a ''tetrad'' effect superimposed on the effect of the lanthanide contraction. No such Z dependence was found with the ..delta..H/sup 0/ values but rather a maximum exothermic effect was reached at approximately the middle of the rare earth series followed by a linear decrease in the heat evolved from Tb to Lu. The dependence of ..delta..G/sup 0/ on Z can be explained in terms of a slight modification of the effect of the lanthanide contraction by interelectronic repulsion effects in the Ln(III) cations. The unusual Z dependence of ..delta..H/sup 0/ can be rationalized by assuming that the coordination number for water in the primary hydration sphere of La(III) ions changes frommore » 9 to 8 and that strong ion-pair complexes are formed between the sulfonate groups of the ion exchanger and Ln(III) cations with only inner sphere hydration.« less

A study of copper(I) oxide in equilibrium with gold + copper alloys of low copper content

The equilibrium between copper(I) oxide and gold + copper alloys (25 to 30 moles per cent Cu) has been studied by low-pressure surface oxidation over the 810 to 900 K temperature range. Values of the standard Gibbs free energy were calculated from published heat capacities and the observed equilibrium oxygen pressures. The values: ΔG f o(Cu 2O, s, 298.15 K) = −(118.2 ± 1.2) kJ mol −1 and ΔH f o(Cu 2O, s, 298.15 K) = −(134.2 ± 1.1) kJ mol −1 were determined by extrapolation. By use of recently selected values for the standard entropies of oxygen and copper the value S o(Cu 2O, s, 298.15 K) = (115.1 ± 0.4) J K −1 mol −1 was obtained. Our results as well as others based upon pressure measurements differ significantly from the results obtained by electrochemical and calorimetric measurements. It is proposed that the deviations are primarily due to surface effects of the very small crystals at the alloy single-crystal surface. Hence, an experimental method is reported whereby standard Gibbs energy changes due to surface phenomena may be studied.

Thermodynamics of hydrogen-isotope-exchange reactions. Proton solvation inn-hexanol and water

This paper reports our results for the direct experimental determination of the equilibrium constant for the hydrogen-isotope-exchange reaction, 1/2D2(g)+HCl(hexOH)=1/2H2(g)+DCl(hexOD), where hexOH isn-hexanol and hexOD isn-hexanol with deuterium substitution in the alcohol function. The reaction was studied in electrochemical double cells without liquid junction for which the net cell reaction is the above isotope-exchange reaction. The experimentally determined value of e° (296.0°K) for this cell is 4.03±0.95 mV (strong electrolyte standard states, mole-fraction composition scale); the value of the equilibrium constant for the reaction is 1.17±0.05. The contributions of isotope-exchange and transfer effects to the magnitude of the standard Gibbs energy change for the above reaction and for the analogous reaction 1/2D2(g)+HCl(aq)=DCl(daq)+1/2H2(g) are considered. Our results support the conclusion of Heinzinger and Weston that the formulation of the solvated proton in water as H3O+, as opposed to H9O4 +, is sufficient for the interpretation of the thermodynamics of hydrogen-isotope-exchange reactions in water. We also find that the formulation of the solvated proton inn-hexanol as ROH 2 + is sufficient for the interpretation of our results on the thermodynamics of hydrogen-isotope-exchange inn-hexanol.

Thermodynamic properties of V nO 2 n−1 ( n = 4 to 9) at high temperatures and at 298.15 K

The thermodynamic properties of V n O 2 n−1 ( n = 4 to 9) were established in the temperature range 1200 to 1400 K. A solid-electrolyte cell of the type: Pt,Ni,NiO|0.85 ZrO 2·0.15 CaO| V n O 2n−1, V n−1 O 2(n−1) −1, Pt , was used in the experiments. The standard Gibbs energy, enthalpy, and entropy changes of the reactions 2 nV n−1 O 2( n−1)−1 + O 2 = 2( n−1)V n O 2 n−1 were calculated. Thermodynamic functions of formation from the elements for V n O 2 n−1 were calculated at 1300 K and at 298.15 K.

Differences in enthalpies of combustion and other related thermodynamic properties for regular and perdeuterated n-octane, isooctane, and benzene

Enthalpies of combustion of benzene, n-octane, and 2,2,4-trimethylpentane and their perdeuterated analogues in the liquid state at 298.15 K were measured by oxygen-bomb combustion calorimetry. Differences in enthalpies of combustion for these isotopic pairs were calculated. With the help of differences in the enthalpies of formation of H 2O and D 2O, the differences in the enthalpies of formation of these pairs at 298.15 K were derived. By use of additional estimated and statistically calculated data, the standard enthalpy and standard Gibbs energy changes for six exchange reactions of the hydrocarbons with D 2 and D 2O at 298.15 K in the gas phase were evaluated. Differences in zero-point energies of these gaseous isotopic pairs were calculated and compared with those obtained from spectroscopic data.

The thermodynamic properties of elementary phosphorus The heat capacities of two crystalline modifications of red phosphorus, of α and β white phosphorus, and of black phosphorus from 15 to 300 K

A study of time and temperature conditions for the preparation of several crystalline modifications of red phosphorus has been made. The heat capacities of two crystalline modifications of red phosphorus and the heat capacity of black phosphorus have been measured from 15 to 300 K. The heat capacity of white phosphorus has been measured from 15 to 320 K. White phosphorus has a first-order transition at 195.4 K. The enthalpy of transition, (124.6±0.2) cal mol −1, enthalpy of fusion, (157.4±0.5) cal mol −1, and the melting point, (317.3±0.1) K of white phosphorus were measured. The barrier to rotation of P 4 molecules in the α-phase of white phosphorus is low enough to allow the contribution to the heat capacity associated with hindered rotation to approach the classical value for free rotation at the melting point. The sublimation pressure of red phosphorus (form IV) was determined as (1.22±0.05) atm at 444.6 °C. The entropies of the two red forms, of black, and of white phosphorus were calculated from the heat capacities. Standard Gibbs energy and enthalpy changes for white and red (IV) phosphorus have been calculated from the entropies and other data concerning phosphorus.