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[en] The interatomic distances in the transition states of radical hydrogen atom abstraction reactions X.+HY → XH+Y. determined by quantum chemical calculations are systematized and generalized. It is shown that depending on the reaction centre structure, these reactions can be subdivided into classes with the same X...Y interatomic distance in each class. The transition state geometries found by the methods of intersecting parabolas and intersecting Morse curves are also presented. The X...H...Y fragments are almost linear, the hydrogen atom position being determined by the reaction enthalpy. The effects of triplet repulsion, electronegativities and radii of X and Y atoms, the presence of adjoining π-bonds, and steric effects on the X...Y interatomic distances are analyzed and characterized. The bibliography includes 62 references.
[en] Effect of reaction heat due to uranium extraction is discussed by considering temperature profile in a pulsed column. A simulation code which treats heat generation and counter-current heat transfer was developed to estimate temperature profile. The agreement between the reported profile and simulation result is fairly good. The simulation analyses indicate the temperature profiles depend on the position of the extraction zone and the flow rates of both organic and aqueous phases. The results demonstrate that the authors' simulation code can provide the temperature profile in a pulsed column and is useful for monitoring the uranium extraction zone
[en] The heat of reaction and equilibrium pressure for both the oxidation and reduction reactions, (1/7)Pr7O12 + (1/7 - x/2)O2 = PrO/sub 2-x/, have been measured by means of a Tian-Calvet-type calorimeter and thermal balance. The results of equilibrium pressure measurements under isothermal conditions show a reproducible and unsymmetrical hysteresis loop. The unsymmetrical hysteresis loop is interpreted as due to a different pattern of intergrowth formed during oxidation and reduction. The ordered intermediate phases (Pr/sub n/O/sub 2n-2/) are intergrown coherently at the unit cell level with each other but not with the disordered α phase (PrO/sub 2-x/). The role of coherent intergrowth in both symmetric and unsymmetric hysteresis loops has been discussed. The partial molar enthalpy (-Δ anti H 0/sub 2/) is about 58 kcal/mole of O2 in the Pr7O12 phase; it increases slightly as O/Pr increases, then increases sharply to about 85 kcal/mole of O2 around PrO178. No difference in partial molar enthalpy is observed between the oxidation and reduction paths within the experimental error, even though different equilibrium pressures are observed depending upon the path. The slight increase in the partial molar enthalpy in the two-phase region is attributed to the interaction term of the different domains using the same regular solution model used to account for the hysteresis observed previously. The sharp change in -Δ anti H 0/sub 2/ around PrO178 is believed associated with the phase change from the coherently intergrown ordered phases to the disordered α phase. The difference between the partial molar entropy on oxidation and that on reduction is explained by both the regular solution model and the usual thermodynamic treatment using the data of isothermal hysteresis
[en] The heat of reaction and equilibrium pressure for the oxidation reaction TbO/sub 1.5 + x/ + (3/28 - x/2)O2 → (1/7)Tb7O12 have been measured by means of a Tian-Calvet-type calorimeter and a thermal balance. The results of the weight measurement show a reproducible hysteresis loop. The heat of reaction has been measured along the oxidation branch of the hysteresis loop. The partial molar enthalpy indicates four distinct compositional regions. First, the TbO/sub 1.5 + x/ region which can be described in terms of a point defect model and strong interaction between neighboring excess oxygen atoms. Second, the region between TbO154 and TbO161 is interpreted as the intrinsic hysteresis region and discussed in terms of the regular solution model. Third, the region between TbO161 and TbO170 is recognized as a pseudophase region. Fourth, a region of eta phase exists in which the partial thermodynamic quantities can be compared with those of CeO1714, PrO1714, and TbO1714. The partial molar enthalpy was also measured for a scanning loop which is also interpreted in terms of the four regimes
[en] Modification of a commercially available oxygen bomb calorimeter is described. This modification permits direct determination of the enthalpies of reaction of liquid metals that have melting points below about 310 K
[en] It has been shown using the calorimetric method that Cp2FexBr2, Cp2Fex2I2 complexes, which probably have an ionic structure, are formed as primary products in the process of ferrocene interaction with bromine and iodine in benzene solutions. Enthalpies of solid complex formation from ferrocene, bromine and iodine soluted in benzene are determined. They constitute values above -136.0 and -106.7 kJ/mol, respectively. Considerable values of enthalpies of ferrocene complex formation with bromine and iodine are explained by a significant contribution of the stabilization energy of a forming organometallic cation in the ionic complex to the total enthalpy
[en] The integration of a turbine expander into different types of high pressure, exothermic chemical synthesis processes is considered. In conventional systems, the reaction heat is often transferred to generate steam to drive steam turbines or used for heat integration. The heat is reduced in quality due to the temperature driving forces in the heat exchange equipment. Reaction heat can be utilised at the maximum possible temperature by placing a turbine expander directly after the reactor. The power generated from such combined power and chemical systems can either be exported or used to satisfy the process compressor requirements. A methodology is presented to lend structure to the development and analysis of the flow sheets for the mentioned systems. The methodology involves the consideration of various factors that were identified to impact on the flow sheet development. These factors are discussed and a brief overview of the flow sheet development for four different case studies is given. Process data generated from simulations are used
[en] Due to the complexity of solid state reactions such as cracking of polymers, it is very difficult to find the required heat of the involved reactions. In this study, using a differential scanning calorimetry (DSC) instrument, a new method is proposed to determine the heat of decomposition of polymers at constant temperatures. The integration of an isothermal DSC curve with respect to time gives the overall consumed heat by the process, which consists of two components: net heat of reaction and heat loss. Finding an effective solution to measure heat loss from DSC instrument is an important part of this new approach. The heat loss is determined as function of sample mass and operating temperature. The heat loss is employed to correct the DSC results and produce the net heat of decomposition reactions. This approach is new since, firstly, it uses high-certainty isothermal DSC measurements, and secondly, the heat loss values are calculated in the decomposition range. The procedure is employed to determine the required heat for pyrolysis of high-density polyethylene at constant temperatures of 400, 410, 420 and 430 °C, and the average value was calculated to be 1375 ± 233 kJ kg−1.