Thermochemistry of molten cryolite mixtures: III. The enthalpy of fusion and cryoscopy in the molten eutectic mixture 0.2l8Na3A1F6+0.782NaF

The enthalpy of fusion at the melting point of the eutectic mixture 0.218 Na₃AlF₆+0.782 NaF, 1162 K, has been determined by drop calorimetry and found to be 9086±100 cal mol^?1. The possibility of evaluating so-called “entropy models” on the basis of cryoscopy data for this eutectic melt with small additions of Al₂O₃ (1–2 mol%) is discussed. It is shown that if the enthalpy term in the equation

is neglected, such evaluations will lead to erroneous results.     

Melting and high-temperature solid state transitions in cobalt(II) halides

Melting and high temperature solid-state transitions in CoCl₂ and CoBr₂ are widely discussed. On the basis of DSC and conductometric measurements it was found that melting process of CoCl₂ is preceded by a solid-state transition appearing about 20 K below the melting point of CoCl₂. Due to deconvolution of the thermograms, the enthalpy of fusion and that of solid-state transition were found to be 36.4 and 9.6 kJ mol^–1, respectively. Melting points of CoCl₂ and CoBr₂ were established to be 999.0 and 949.7 K, respectively. Hitherto unknown enthalpy of fusion of CoBr₂ was determined to be 27.2 kJ mol^–1. A solid-state transition in CoBr₂ at 650 K has been confirmed.     

Analysis of a symmetric neopolyol ester

Quantitative thermal analysis was carried out for tetra[methyleneoxycarbonyl(2,4,4-trimethyl)pentyl]methane. The ester has a glass transition temperature of 219 K and a melting temperature of 304 K. The heat of fusion is 51.3 kJ mol^?1, and the increase in heat capacity at the glass transition is 250 J K^?1 mol^?1. The measured and calculated heat capacities of the solid and liquid states from 130 to 420 K are reported and a discussion of the glass and melting transitions is presented. The computation of the heat capacity made use of the Advanced Thermal Analysis System, ATHAS, using an approximate group-vibration spectrum and a Tarasov treatment of the skeletal vibrations. The experimental and calculated heat capacities of the solid ester were compared over the whole temperature range to detect changes in order and the presence of large-amplitude motion. An addition scheme for heat capacities of this and related esters was developed and used for the extrapolation of the heat capacity of the liquid state for this ester. The liquid heat capacity for the title ester is well represented by 691.1+1.668T [J K^?1 mol^?1]. A deficit in the entropy and enthalpy of fusion was observed relative to values estimated from empirical addition schemes, but no gradual disordering was noted outside the transition region. The final interpretation of this deficit of conformational entropy needs structure and mobility analysis by solid state¹³C NMR and X-ray diffraction. These analyses are reported in part II of this investigation.     

Thermodynamic study of the SOLID-LIQUID equilibria in the MIPO3-Cu(PO3)2 systems

Thermodynamic exploration of solid-liquid equilibria of the M^IPO₃-Cu(PO₃)₂ (with MI=Li, Na, K, Rb, Cs, Ag, Tl) systems is carried out with a semi-empirical equation of the liquidus curves. The enthalpies of fusion of pure polyphosphates and some intermediate compounds were determined from DTA curves. The temperature, enthalpy and entropy of fusion are calculated for each solid phase with the exception of silver polyphosphate and the intermediate compound Cs₄Cu(PO₃)₆ which have very limited crystallization fields. The calculated values of the melting enthalpies are approximately equal to the measured ones. The melting enthalpy of Cu(PO₃)₂ calculated from different binary systems shows a wide variation in the obtained values, 35-54 kJ mol^-1. The experimental value is 33.65 kJ mol^-1. The calculated temperatures and compositions in most binary systems are in good agreement with experimental determinations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Determination of specific heat capacity and standard molar combustion enthalpy of taurine by DSC

Using XRY-1C calorimeter, the standard molar enthalpy of taurine was determined to be ?2546.2?kJ?mol^?1 . The reliability of the instrument used was tested by using naphthalene as reference material; and through comparing the molar combustion enthalpy of naphthalene measured with its standard value found in literature, the absolute error and relative error were found to be 4.53?kJ?mol^?1 and 0.09%, respectively. The melting point and melting enthalpy of taurine were determined by Differential Scanning Calorimetry (DSC), which was found to be 588.45?K and ?22.197?kJ?mol^?1, respectively. Moreover, using the DSC method, the specific heat capacities C _p of taurine was measured and the relationship between C _p and temperature was established. The thermodynamic basic data obtained are available for the exploiting new synthesis method, engineering design and industry production of taurine.     

水合烟酸铜Cu(Nic)2•H2O(s)(Nic＝烟酸)的合成、表征及热力学性质

近几十年来,烟酸盐类化合物或配合物由于优越的吸收率高和无毒副作用等特点使其在化妆品、药品和食品等领域作为营养添加剂具有重要应用前景。然而,这类化合物的基础热力学数据极其缺乏,从而限制了这类化合物的理论研究和应用开发的深入开展。为此,本论文利用室温固相合成方法和球磨技术合成了一种新化合物Cu(Nic)2•H2O(s),利用化学分析、元素分析、FTIR和X-射线粉末衍射技术表征了它的结构和组成,利用精密自动绝热热量计准确地测量了它在78-400 K温区的摩尔热容。在热容曲线的T ＝ 326-346 K温区观察到一个明显的固-液相变过程。利用相变温区三次重复实验热容的测量结果确定了此相变过程的峰温、相变焓和相变熵分别为：Tfus＝(341.290 ±0.873) K, DfusHm＝(13.582±0.012) kJ×mol-1, DfusSm＝(39.797±0.067) J×K-1×mol-1。通过最小二乘法将相变前和相变后的热容实验值分别拟合成了热容对温度的两个多项式方程。通过热容多项式方程的数值积分,得到了这个化合物的舒平热容值和相对于298.15 K的各种热力学函数值,并且将每隔5 K的热力学函数值列成了表格。     

Ene reaction between 4-phenyl-1,2,4-triazoline-3,5-dione and hex-1-ene. The enthalpies,entropies, and volumes of activation and reaction in solution

The rates of an ene reaction between 4-phenyl-1,2,4-triazoline-3,5-dione and hex-1-ene were studied in a temperature range of 15–40 °C and in a pressure range of 1–2013 bar. The enthalpy of reaction in 1,2-dichloroethane (?158.2±1.0 kJ mol^?1), the enthalpy (51.3±0.5 kJ mol^?1), entropy (122±2 J mol^?1 K^?1), and volume of activation (?31.0±1.0 cm3 mol^?1), and the volume of this reaction (?26.6±0.3 cm³ mol^?1) were determined. The high exothermic effect of the reaction suggests its irreversibility.     

2-氯-N,N-二甲基尼古丁酰胺的热容和热力学性质研究

Low-temperature heat capacities of 2-chloro-N,N-dimethylnicotinamide were precisely measured with a high-precision automated adiabatic calorimeter over the temperature range from 82 K to 380 K. The compound was observed to melt at （342.15±0.04） K. The molar enthalpy AfusionHm, and entropy of fusion, △fusionSm, as well as the chemical purity of the compound were determined to be （21387±7） J·mol^-1, （62.51±0.01） J·mol^-1·K^-1, （0.9946±0.0005） mass fraction, respectively. The extrapolated melting temperature for the pure compound obtained from fractional melting experiments was （342.25±0.024） K. The thermodynamic function data relative to the reference temperature 298.15 K were calculated based on the heat capacity measurements in the temperature range from 82 to 325 K. The thermal behavior of the compound was also investigated by different scanning calorimetry.     

Determination of thermodynamic formation values for some solid charge-transfer complexes of iodine

The standard Gibbs free energy, enthalpy, and entropy of complex formation of five solid molecular complexes of iodine have been determined by comparing the e.m.f.'s of galvanic cells having either solid iodine or the iodine complex as cathode. All of the complexes were found to have a negative enthalpy of formation, which was in the range ?5 to ?14 kJ mol^?1, except for one very weak complex. The relative stability of the complexes was largely determined by the standard entropy of formation which varied from +18 J K^?1 mol^?1, for the most stable of the complexes studied, to ?21 J K^?1 mol^?1.     

Measurement of high temperature heat content of silicon by drop calorimetry

Heat content of silicon has been measured in a temperature range of 700-1820 K using a drop calorimeter. Boron nitride was used as a sample crucible. The enthalpy of fusion and the melting point of silicon determined from the heat content-temperature plots are 48.31±0.18 kJ mol^-1 and 1687±5 K, respectively. The heat content and heat capacity equations were derived using the Shomate function for the solid region and the least square method for the liquid region, respectively, and compared with the literature values. This revised version was published online in August 2006 with corrections to the Cover Date.     

Crystallization of poly(vinylidene fluoride): Equilibrium melting point and heat of fusion of the α-polymorph

Samples of poly(vinylidene fluoride) were crystallized either (a) isothermally at a series of temperatures, or (b) in the presence of varying amounts of diluent (dimethylphthalate or dimethylacetamide). The α-polymorph was the only crystalline form present in these samples. Melting points of the first series (a) were determined by DSC and of the second (b) by dilatometry. The same equilibrium melting point for the α-polymorph (178°C) was obtained from analysis of the two sets of data. A value of 1425 cal mole^?1 (5.96 kJ mole^?1) for the heat of fusion of this polymorph was obtained from analysis of the polymer–diluent melting data. The heat of fusion and the entropy of fusion calculated therefrom correlated well with corresponding values for other fluoroethylene polymers. Thus, the equilibrium melting point and the heat of fusion could be predicted for a fluoropolymer for which data have not yet been reported–poly(trifluoroethylene). The melting point predicted, 222°C, agreed remarkably well with that determined here for an experimental sample, thereby lending support to the empirical correlations. The heat of fusion of poly(trifluoroethylene) is, therefore, expected to be approximately 1300 cal mole^?1.     

Chemical equilibria 6. Measurement of equilibrium constants for the dehydrogenation of 2-methylpropan-1-ol by a vapour-flow technique

Equilibrium constants for 2-methylpropan-1-ol + 2-methylpropanal + hydrogen have been calculated from measurements of the composition of mixtures formed by passing the vapour over a catalyst at several temperatures in the range 473 to 563 K. Equations relating the changes in enthalpy and entropy of the dehydrogenation reaction to temperature were derived from the equilibrium constants with the aid of heat capacities. By coupling these changes with other thermodynamic data, the standard enthalpy of formation and the standard entropy of 2-methylpropanal at 298.15 K were calculated to be ?(215.7 ± 1.3) kJ mol^?1 and (331.2 ± 1.7) J K^?1 mol^?1 respectively, in the gas state, and ?(247.3 ± 1.8) kJ mol^?1 and (238.3 ± 4.4) J K^?1 mol^?1 respectively, in the liquid state.     

Surface and bulk conduction and thermodynamic properties of ionic salt CsH<Subscript>5</Subscript>(PO<Subscript>4</Subscript>)<Subscript>2</Subscript>

Transport properties of ionic salt CsH₅(PO₄)₂ are studied by the impedance method. The salt’s bulk conductivity ranges from 10^?8 to 10^?4 S cm^?1 in the temperature interval 90 to 145°C. The apparent activation energy is high (1.6–2.0 eV). The conductivity is slightly anisotropic: it is maximum in the [001] direction and minimum in the [100] direction (～5.6 and 1 times × 10^?6 S cm^?1, respectively, at 130°C). The conductivity of polycrystalline samples is higher by 1–2 orders of magnitude, and the activation energy drops to 1.05 eV due to the formation of a pseudoliquid layer with a high proton mobility at the intercrystallite boundary. The salt’s thermodynamic properties are examined by differential scanning calorimetry and thermogravimetry. No phase transitions are discovered in the salt up to the melting point (151.6°C), with the melting enthalpy equal to ～34 kJ mol^?1. The crystallization occurs at lower temperatures (107°C) and the crystallization enthalpy (?18 kJ mol^?1) is lower than the melting enthalpy. The melting is accompanied by slow decomposition of the salt. Factors affecting the proton transport in the salt are analyzed.     

Some thermodynamic properties of benzocaine

The vaporization enthalpy of benzocaine, ethyl 4-aminobenzoate, has been evaluated using correlation gas chromatography at 298.15 K. The temperature dependence of retention time has also been used to evaluate the vapor pressure of the sub-cooled liquid from 298.15 K to the fusion temperature, 365.2 K, by correlation with the vapor pressures of the compounds used as standards. The vaporization enthalpy calculated from the vapor pressures of benzocaine at the melting point was combined with the experimental fusion enthalpy to evaluate the sublimation enthalpy at the fusion temperature. Application of the Clausius–Clapeyron equation together with the vapor pressure common to both phases permitted calculation of the vapor pressure of the solid at 298.15 K. Similar calculations were performed for two of the standards that were solids for comparisons with experimental data. Vaporization and sublimation enthalpies of (91.8 ± 4.2) and (112.9 ± 4.3) kJ mol^?1 are calculated for benzocaine at 298.15 K as are vapor pressures of 0.0083 and 0.0018 Pa for the sub-cooled liquid and crystalline material, respectively.     

Adiabatic calorimetry and thermal analysis on acetaminophen

Molar heat capacities of acetaminophen were precisely measured with a small sample precision automated adiabatic calorimeter over the temperature range from 80 to 330 K. A solid-solid transition at 149.96 K was found from the C_p,m-T curve. The polynomial functions of C_p,.m(J K^-1 mol^-1) vs. T were established on the heat capacity measurements by means of the least square fitting method. Thermal decomposition processes of acetaminophen have been studied by thermogravimetry. And the thermal decomposition kinetics parameters, such as activation energy E, pre-exponential factor A and reaction order n, were calculated by TG-DTG techniques with the Freeman-Carroll method, Kissinger method and Ozawa method. Accordingly the thermal decomposition kinetics equation of acetaminophen is expressed as: dα/dt=2.67·10⁷e^-89630/RT(1-α)^0.23. The process of fusion has been investigated through DSC. The melting point, molar enthalpy and entropy of fusion are to be (441.89±0.04) K, 26.49±0.44 kJ mol^-1 and 59.80±1.01 J K^-1 mol^-1, respectively.     

4-硝基苯甲醇的低温热容和标准摩尔生成焓

用精密自动绝热量热计测定了4-硝基苯甲醇（4-NBA）在78 ~ 396 K温区的摩尔热容。其熔化温度、摩尔熔化焓及摩尔熔化熵分别为:(336.426 ± 0.088) K, (20.97 ± 0.13) kJ×mol-1 和 (57.24 ± 0.36) J×K-1×mol-1.根据热力学函数关系式,从热容值计算出了该物质在80 ~ 400 K温区的热力学函数值 [HT - H298.15 K] 和[ST - S298.15 K]. 用精密氧弹燃烧量热计测定了该物质在T＝298.15 K的恒容燃烧能和标准摩尔燃烧焓分别为 (C7H7NO3, s)＝- ( 3549.11 ± 1.47 ) kJ×mol-1 和 (C7H7NO3, s)＝- ( 3548.49 ± 1.47 ) kJ×mol-1. 利用标准摩尔燃烧焓和其他辅助热力学数据通过盖斯热化学循环, 计算出了该物质标准摩尔生成焓 (C7H7NO3, s)＝- (206.49 ± 2.52) kJ×mol-1 .     

Lithium sulfate: Calorimetric determination of the temperatures and enthalpies of high-temperature phase transitions

Using a high-temperature Calvet calorimeter as a differential enthalpic analyser Li₂SO₄ was investigated in the temperature range 800–1200 K. One transformation in the solid state was observed at 847 K with a corresponding enthalpy increment of 24.2 kJ mol^?1. The solid-liquid transition was found to occur at 1131 K with an enthalpy of fusion of 7.74 kJ mol^?1. Furthermore, the analysis of the corresponding thermograms supports the presence of the premelting effect which was evidenced by other techniques. A critical comparison with previous results in the literature is given.     

Differential scanning calorimetry of RbH2PO4 and CsH2PO4

The high-temperature phase behaviour of RbH₂PO₄ and CsH₂PO₄ have been studied. RbH₂PO₄ undergoes a single quasi-irreversible phase transition with an enthalpy of 4.665 kJ mol^?1. The transition is found to occur over the temperature range 86–111°C. CsH₂PO₄ undergoes two transitions at 149 and 230°C. The lower one is quasi-irreversible and has an enthalpy of 4.284 kJ mol^?1. The one at 230°C is reversible and has an enthalpy of 1.071 kJ mol^?1.     

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).     

Thermal properties of some cyclic disulfides: naphthalene disulfide and diphenylene disulfide

The specific heat, the melting heat and entropy, the vaporization heat of naphtalene disulfide (C₁₀H₆S₂) and of diphenylene disulfide (C₁₂H₈S₂) have been determined by differential scanning calorimetry (DSC).Over the temperature range examined the specific heat may be represented as follows:

where T is the temperature in degrees Kelvin, while melting heat, vaporization heat, melting entropy are for naphtalene disulfide: 3.10 kcal mol^?1, 6.42 kcal mol^?1, 7.87 cal deg^? mol^?1 and for diphenylene disulfide: 4.62 kcal mol^?1, 6.90 kcal mol^?1 and 11.87 cal deg^?1 mol^?1.