Thermodynamic properties of thiourea + hydrocarbon adducts from 12 to 300 K. The heat capacities and entropies of the adducts of c-C6H12, c-C7H14, c-C8H16, and (CH3)3C·C2H5

Heat capacity measurements on the adducts of thiourea with cyclohexane, cycloheptane, cyclo-octane, and 2,2-dimethylbutane have been made over the temperature range 12 to 300 K by means of an adiabatic calorimeter. All the samples showed several regions of anomalously high heat capacity. Some of the smaller anomalies are suggestive of displacive ferroelectric transitions. The curve for the cyclo-octane adduct showed a large hump with its maximum at 240 K and having an associated molar enthalpy of c-C₆H₁₆ of 7186 J mol⁻¹ and entropy of 31.86 J K⁻¹ mol⁻¹, which is here ascribed to conformational changes of the cyclo-octane; a smaller and more gradual hump in the curve for the cycloheptane adduct may be due to a similar cause.     

Low-temperature heat capacity and standard enthalpy of formation of neodymium glycine perchlorate complex [Nd2(Gly)6(H2O)4](ClO4)6·5H2O

Rare-earth perchlorate complex coordinated with glycine [Nd₂(Gly)₆(H₂O)₄](ClO₄)₆·5H₂O was synthesized and its structure was characterized by using thermogravimetric analysis (TG), differential thermal analysis (DTA), chemical analysis and elementary analysis. Its purity was 99.90%. Heat capacity measurement was carried out with a high-precision fully-automatic adiabatic calorimeter over the temperature range from 78 to 369 K. A solid-solid phase transformation peak was observed at 256.97 K, with the enthalpy and entropy of the phase transformation process are 4.438 kJ mol⁻¹ and 17.270 J K⁻¹ mol⁻¹, respectively. There is a big dehydrated peak appears at 330 K, its decomposition temperature, decomposition enthalpy and entropy are 320.606 K, 41.364 kJ mol⁻¹ and 129.018 J K⁻¹ mol⁻¹, respectively. The polynomial equations of heat capacity of this compound in different temperature ranges have been fitted. The standard enthalpy of formation was determined to be −8023.002 kJ mol⁻¹ with isoperibol reaction calorimeter at 298.15 K.     

Cesium chromate,Cs2CrO4: heat capacity and thermodynamic properties from 5 to 350 K

The heat capacity of a sample of Cs₂CrO₄ was determined in the temperature range 5 to 350 K by aneroid adiabatic calorimetry. The heat capacity at constant pressure C_p^o(298.15 K), the entropy S^o(298.15 K), the enthalpy {H^o(298.15 K) - H^o(0)} and the function $\text{? {G}{}^{\mathrm{o}}\text{(298.15}\text{K}\text{) - H}{}^{\mathrm{o}}\text{(0)}}\text{298.15}\text{K}$ were found to be (146.06 ± 0.15) J K^?1 mol^?1, (228.59 ± 0.23) J K^?1 mol^?1, (30161 ± 30) J mol^?1, and (127.43 ± 0.13) J K^?1 mol^?1, respectively. The heat capacity C_p^o(298.15 K) and entropy S^o(298.15 K) and entropy S^o(298.15 K) of Rb₂CrO₄ are estimated to be (146.0 ± 1.0) J K^?1 mol^?1 and (217.6 ± 3.0) J K^?1 mol^?1, respectively.     

Low-temperature heat capacity of L- and DL-phenylglycines

Heat capacity of crystalline L- and DL-phenylglycines was measured in the temperature range from 6 to 305?K. For L-phenylglycine, no anomalies in the C _p (T) dependence were observed. For DL-phenylglycine, however, an anomaly in the temperature range 50?C75?K with a maximum at about 60?K was registered. The enthalpy and the entropy changes corresponding to this anomaly were estimated as 20?J?mol^?1 and 0.33?J?K^?1 mol^?1, respectively. In the temperature range 205?C225?K, an unusually large dispersion of the experimental points and a small change in the slope of the C _p (T) curve were noticed. Thermodynamic functions for L- and DL-phenylglycines in the temperature range 0?C305?K were calculated. At 298.15?K, the values of heat capacity, entropy, and enthalpy are equal to 179.1, 195.3?J?K^?1 mol^?1, and 28590?J?mol^?1 for L-phenylglycine and 177.7, 196.3?J?K^?1 mol^?1 and 28570?J?mol^?1 for DL-phenylglycine. For both L- and DL-phenylglycine, the C _p (T) at very low temperatures does not follow the Debye law C ?C T ³ . The heat capacity C _p (T) is slightly higher for L-phenylglycine, than for the racemic DL-crystal, with the exception of the phase transition region. The difference is smaller than was observed previously for the L-/DL-cysteines, and considerably smaller, than that for L-/DL- serines.     

Thermal properties of the pyrochlore, Y2Ti2O7

The thermal conductivity and heat capacity of high-purity single crystals of yttrium titanate, Y₂Ti₂O_7, have been determined over the temperature range 2 K?T?300 K. The experimental heat capacity is in very good agreement with an analysis based on three acoustic modes per unit cell (with the Debye characteristic temperature, θ_D, of ca. 970 K) and an assignment of the remaining 63 optic modes, as well as a correction for C_p−C_v. From the integrated heat capacity data, the enthalpy and entropy relative to absolute zero, are, respectively, H(T=298.15 K)−H₀=34.69 kJ mol⁻¹ and S(T=298.15 K)−S₀=211.2 J K⁻¹ mol⁻¹. The thermal conductivity shows a peak at ca. θ_D/50, characteristic of a highly purified crystal in which the phonon mean free path is about 10 μm in the defect/boundary low-temperature limit. The room-temperature thermal conductivity of Y₂Ti₂O₇ is 2.8 W m⁻¹ K⁻¹, close to the calculated theoretical thermal conductivity, κ_min, for fully coupled phonons at high temperatures.     

Thermodynamic properties of BaCeO3 and BaZrO3 at low temperatures

Specific heat capacities (C_p) of polycrystalline samples of BaCeO₃ and BaZrO₃ have been measured from about 1.6 K up to room temperature by means of adiabatic calorimetry. We provide corrected experimental data for the heat capacity of BaCeO₃ in the range T < 10 K and, for the first time, contribute experimental data below 53 K for BaZrO₃. Applying Debye's T³-law for T → 0 K, thermodynamic functions as molar entropy and enthalpy are derived by integration. We obtain C_p = 114.8 (±1.0) J mol⁻¹ K⁻¹, S° = 145.8 (±0.7) J mol⁻¹ K⁻¹ for BaCeO₃ and C_p = 107.0 (±1.0) J mol⁻¹ K⁻¹, S° = 125.5 (±0.6) J mol⁻¹ K⁻¹ for BaZrO₃ at 298.15 K. These results are in overall agreement with previously reported studies but slightly deviating, in both cases. Evaluations of C_p(T) yield Debye temperatures and identify deviations from the simple Debye-theory due to extra vibrational modes as well as anharmonicity. The anharmonicity turns out to be more pronounced at elevated temperatures for BaCeO₃. The characteristic Debye temperatures determined at T = 0 K are Θ₀ = 365 (±6) K for BaCeO₃ and Θ₀ = 402 (±9) K for BaZrO₃.     

Calorimetric study of the glassy state XII. Plural glass-transition phenomena of ethanol

Ethanol was found to give a metastable crystalline phase (crystal-II) when the liquid was cooled at a moderate rate. Glassy states of liquid and of newly found crystal-II were obtained in the calorimeter cell by controlling the cooling rate of the liquid. The heat capacities of these phases as well as that of the stable crystal-I were measured by an adiabatic calorimeter in the temperature range between 14 and 300 K. The glass transition temperature T_g, the heat-capacity jump at T_g, and the residual entropy were found to be 97 K, 35.3 J K^?1 mol^?1, and 8.93 J K^?1 mol^?1 for the glassy liquid, and 97 K, 22.8 J K^?1 mol^?1, and 4.24 J K^?1 mol^?1 for the glassy crystal-II, respectively. The values for the residual entropy are referred to the third-law entropy for crystal-I.The heat capacities reported previously for the supercooled liquid by Gibson et al. and by Parks and Kelley agree well with those for the metastable crystal-II. Those of the supercooled liquid connect smoothly with those obtained for the liquid above the melting temperature. Thus, ethanol is found to be another example of a low-molecular-weight compound which shows multiple glass-transition phenomena.     

Standard molar enthalpies of formation of KCdCl3 and K4CdCl6 by high-temperature drop calorimetry

Enthalpies of the synthesis reactions of the two compounds KCdCl₃(cr) and K₄CdCl₆(cr) from KCl(cr) and CdCl₂(cr) have been measured by drop calorimetry of solid samples of KCl, CdCl₂, KCdCl₃, and K₄CdCl₆ into melted mixtures of KCl and CdCl₂. For the two reactions: (1), CdCl₂(cr)+KCl(cr) = KCdCl₃(cr); and (2), CdCl₂(cr)+4KCl(cr) = K₄CdCl₆(cr), the experiments lead to the two standard molar enthalpies of reaction at 298.15 K: Δ₁H_m^o = ?(21.7±1.0) kJ·mol^?1 and Δ₂H_m^o = ?(39.0±3.8) kJ·mol^?1. These values are not in good agreement with those of previous workers.     

Determination of the enthalpy of fusion of K3TaF8 and K3TaOF6

The areas of the fusion and crystallization peaks of K₃TaF₈ and K₃TaOF₆ have been measured using the DSC mode of the high-temperature calorimeter (SETARAM 1800 K). On the basis of these quantities and the temperature dependence of the used calorimetric method sensitivity, the values of the enthalpy of fusion of K₃TaF₈ at temperature of fusion 1039 K: Δ_fusH_m(K₃TaF₈; 1039 K) = (52 ± 2) kJ mol⁻¹ and of K₃TaOF₆ at temperature of fusion 1055 K: Δ_fusH_m(K₃TaOF₆; 1055 K) = (62 ± 3) kJ mol⁻¹ have been determined.     

Heat capacity of iron-chromium spinels,Fe3−xCrxO4

The heat capacity of Fe_3?xCr_xO₄ with the composition x = 0.6, 0.8, and 1.0 was measured from 200 to 850 K. A γ-type heat capacity anomaly due to the ferri-paramagnetic transition was observed for all compositions. The transition temperatures were 652, 563, and 451 K for the compositions x = 0.6, 0.8, and 1.0, respectively. The variation of transition temperature with composition is discussed in terms of cation distribution. The magnetic contribution to the observed heat capacity was obtained by assuming that the heat capacity is expressed by the sum of the lattice heat capacity C_v(1), the dilation contribution d(d), and the magnetic contribution C(m). Entropy changes due to the transition were calculated from C(m) as 52.6, 49.7, and 46.3 J K^?1 mole^?1 for the compositions x = 0.6, 0.8, and 1.0, respectively, which are from 7 to 12 J K^?1 mole^?1 higher than the calculated values based on the assumption of randomization of unpaired spins on each ion. The difference between the observed and the calculated values is roughly explained by taking into account the orbital contribution of Fe²⁺ ions on octahedral and tetrahedral sites.     

Apparent molar heat capacities and volumes of silver nitrate and silver perchlorate in aqueous solution at 298 K

We have made calorimetric and density measurements leading to apparent molar heat capacities and volumes of dilute aqueous solutions of silver nitrate and silver perchlorate at 298 K. Resulting apparent molar properties at infinite dilution are the following: φ^o_c(AgNO₃) = ? 36.8 J K^?1 mol^?1, φ^o_v(AgNO₃) = 29.1 cm³ mol^?1, φ^o_c(AgClO₄) = 11.0 J K^?1 mol^?1, and φ^o_v(AgClO₄) = 43.5 cm³ mol^?1.     

Phonon spectra and phonon-dependent properties of AgSO4, an unusual sulfate of divalent silver

Phonon spectra of recently synthesized Ag(II)SO₄ have been measured using infrared absorption and Raman scattering spectroscopy, and theoretically predicted using density functional theory calculations. Excellent agreement between experimental and theoretical results with correlation coefficient of 1.05 allowed for full assignment of the experimentally observed vibrational bands, as well as calculation of standard vibrational entropy of AgSO₄ (118.2 J mol⁻¹ K⁻¹), vibrational heat capacity at constant volume (99.1 J mol⁻¹ K⁻¹), zero-point energy (48.3 kJ mol⁻¹). The experimental cut-off frequency of the phonon spectrum equals 1116 cm⁻¹ which translates to the Debye temperature of 1606 K. High frequencies of S–O stretching modes render sulfate connections of Ag(II) attractive precursors of high-T_C superconductors.     

Heat capacity measurements of MnxFe3−xO4

Heat capacities of Mn_xFe_3?xO₄ with the composition x = 1.0, 1.5, and 2.0 were measured from 200 to 740 K. λ-type heat capacity anomalies due to the ferri-paramagnetic transition were observed for all the compositions. The transition temperatures were 577, 471, and 385 K for the composition x = 1.0, 1.5 and 2.0, respectively, which are in good agreement with the results of magnetic measurements. The difference in heat capacities between the different samples was small except for the temperature range of the transition. The magnetic contribution to the observed heat capacity was obtained by assuming that the heat capacity can be expressed by the sum of the lattice heat capacity C_v (l), the dilation contribution C(d), and the magnetic contribution C(m). Entropy changes due to the transition were obtained from C(m) as 55.5, 50.7 and 49.2 J K^?1 mole^?1 for the composition x = 1.0, 1.5, and 2.0, respectively. The entropy changes were also calculated by assuming the randomization of unpaired electron spins on each ion, but they were from 6 to 10 J K^?1 mole^?1 smaller than the observed ones. The difference between the experimental and the calculated values is roughly explained by taking into account the cation exchange reaction between the tetrahedral and the octahedral sites in the spinel structure.     

Upper limits for the rate constant for the reaction Br + H2O2 → HBr + HO2

Upper limits for the rate constant for the reaction Br + H₂O₂ → HBr + HO₂ have been measured over the temperature range 298 to 417 K in a discharge flow, system using a mass spectrometer as a detector. Results are K₁< 1.5 × 10^?15 cm³ s^?1 at 298 K and K₁< 3.0 × 10^?15 cm³ s^?1 at 417 K, respectively. The implication to Stratospheric chemistry is discus     

Heat capacity and dual spin-transitions in the crossover system [Fe(2-pic)3]Cl2·EtOH

The heat capacity of [Fe(2-pic)₃]Cl₂·C₂H₅OH Crystal (2-pic: 2-picolylamine) has been measured with an adiabatic calorimeter between 13 and 315 K. Two phase transitions centered at 114.04 and 122.21 K were observed. This finding accords with recent prediction of possible existence of two-step spin-conversion (H. Köppen et al., Chem. Phys. Lett., 91 (1982) 348). The total transition enthalpy and entropy amounted to ΔH = 6.14 kJ mol^?1 and ΔS = 50.59 J K^?1 mol^?1. The transition entropy consists of the magnetic contribution (13.38 J K^?1 mol^?1), the orientational order-disorder phenomenon of the solvate ethanol molecule (8.97) and the change in the phonon system, in particular the change in stretching and deformation vibrations of the metal-ligand (28.24).     

Dehydration and linkage isomerization in K4[Ni(NO2)6]-H2O

The dehydration of K₄[Ni(NO₂)₆]-H₂O and the simultaneous linkage isomerization have been studied using differential scanning calorimetry. Two types of behavior were found for different samples. In one case, complete dehydration occurs and there is partial conversion of N-bonded nitrite to the O-bonded form. In the other case, only about 20% of the water is removed during the isomerization. The enthalpy change is estimated to be 3.5 kcal mol^?1 for each nitrite converted and 9.6 kcal mol^?1 for the dehydration.     

Heat capacity measurements of SnSe and SnSe2

The heat capacities of SnSe and SnSe₂ were measured in the temperature range 230–580 K using a computer interfaced differential scanning calorimeter. From these measurements, the Debye temperatures of SnSe and SnSe₂ were calculated as a function of temperature. An estimated Debye temperature of 220 K for SnSe was used to calculate the absolute entropy of SnSe at 298 K to be 85.2 ± 6.0 J K^?1 mole^?1. In the light of other work, the suitability of Debye temperatures for estimating low temperature heat capacities of SnSe₂ is questioned.     

Thermodynamic properties of hydrated sodium l-threonate Na(C4H7O5)·H2O(s)

Low-temperature heat capacities of the compound Na(C₄H₇O₅)·H₂O(s) have been measured with an automated adiabatic calorimeter. A solid-solid phase transition and dehydration occur at 290-318 K and 367-373 K, respectively. The enthalpy and entropy of the solid-solid transition are Δ_transH_m = (5.75 ± 0.01) kJ mol⁻¹ and Δ_transS_m = (18.47 ± 0.02) J K⁻¹ mol⁻¹. The enthalpy and entropy of the dehydration are Δ_dH_m = (15.35 ± 0.03) kJ mol⁻¹ and Δ_dS_m = (41.35 ± 0.08) J K⁻¹ mol⁻¹. Experimental values of heat capacities for the solids (I and II) and the solid-liquid mixture (III) have been fitted to polynomial equations.     

Thermodynamics of BaNd2Fe2O7(s) and BaNdFeO4(s) in the system BaNdFeO

Two compounds, BaNd₂Fe₂O₇(s) and BaNdFeO₄(s) in the quaternary system BaNdFeO were prepared by citrate-nitrate gel combustion route and characterized by X-ray diffraction analysis. Heat capacities of these two oxides were measured in two different temperature ranges: (i) 130-325 K and (ii) 310-845 K, using a heat flux type differential scanning calorimeter. Two different types of solid-state electrochemical cells with CaF₂(s) as the solid electrolyte were employed to measure the e.m.f. as a function of temperature. The standard molar Gibbs energies of formation of these quaternary oxides were calculated as a function of temperature from the e.m.f. data. The standard molar enthalpies of formation from elements at 298.15 K, ΔfHm° (298.15 K) and the standard entropies, Sm° (298.15 K) of these oxides were calculated by the second law method. The values of ΔfHm° (298.15 K) and Sm° (298.15 K) obtained for BaNd₂Fe₂O₇(s) are: −2756.9 kJ mol⁻¹ and 234.0 J K⁻¹ mol⁻¹ whereas those for BaNdFeO₄(s) are: −2061.5 kJ mol⁻¹ and 91.6 J K⁻¹ mol⁻¹, respectively.     

Heat capacity, enthalpy and entropy of strontium niobate Sr2Nb2O7 and calcium niobate Ca2Nb2O7

The heat capacity and the enthalpy increments of strontium niobate Sr₂Nb₂O₇ and calcium niobate Ca₂Nb₂O₇ were measured by the relaxation time method (2–300 K), DSC (260–360 K) and drop calorimetry (720–1370 K). Temperature dependencies of the molar heat capacity in the form C_pm = 248.0 + 0.04350T − 3.948 × 10⁶/T² J K⁻¹ mol⁻¹ for Sr₂Nb₂O₇ and C_pm = 257.2 + 0.03621T − 4.434 × 10⁶/T² J K⁻¹ mol⁻¹ for Ca₂Nb₂O₇ were derived by the least-square method from the experimental data. The molar entropies at 298.15 K, S_m°(298.15 K) = 238.5 ± 1.3 J K⁻¹ mol⁻¹ for Sr₂Nb₂O₇ and S_m°(298.15 K) = 212.4 ± 1.2 J K⁻¹ mol⁻¹ for Ca₂Nb₂O₇, were evaluated from the low-temperature heat capacity measurements.