Der Sublimationsdruck von NbOCl3,f und die Normalentropie von NbOCl3,g

Sublimation Pressure of NbOCl_3,s and Standard Entropy of NbOCl_3,g The sublimation pressure of NbOCl₃ has been measured by means of the transportation method. The carrier-gas contained NbCl₅, so that the decomposition of NbOCl₃ is prevented: . Further at 1277 K the reaction Nb₂O₅ + 3 Cl₂ = 2 NbOCl_3,g + 1.5 O₂ has been measured by means of the transportation method. Considering ΔCp and ΔH°(298) follows .     

Zum chemischen Transport von CrOCl und Cr2O3 — Experimente und Modellrechnungen zur Beteiligung von CrOCl2,g

On the Chemical Transport of CrOCl and Cr₂O₃ - Experiments and Model Calculations for Participation of CrOCl_2,g . The migration of CrOCl in a temperature gradient (600°C→500°C) in the presence of chlorine is a result from an endothermic reaction . Above T₂ = 900°C several reactions are super imposed and Cr₂O₃, the product of the decomposition of CrOCl, migrates following the endothermic reaction . By continously monitoring the mass changes during the complete duration of the experiment the consecutive stationary deposition reactions could be registered separately and nonstationary changes in the gasphase could be recognized. The observed decomposition of solid CrOCl into Cr₂O_3,s as well as CrCl_3,g under equilibrium conditions is in accordance with thermochemical calculations assuming the heat of formation of CrOCl to be Δ_BH = - 135.3 ± 2 [kcal/mol]. Using this value the chemical transport of CrOCl with Cl₂, HCl, and HgCl₂ can be described.     

Chemischer Transport und Sublimationsverhalten von CrBr3 — Experimente und Modellrechnungen

On the Chemical Transport and Sublimation of CrBr₃ — Experiments and Model Calculations The migration of CrBr₃ in the presence of high concentrations of bromine (for example D(Br₂) = 0,05 mmol/ml; closed silica ampoules) in the investigated temperature range (T? = 625°C to 875°C; T? = 50°C) is a result from the endothermic reaction The chemical transport of CrBr₃ is superimposed with the sublimation. With low concentrations of D(Br₂) and high temperatures T? is the sublimation decisive participated. This is a result of the homogenous equilibrium between CrBr_3,g and CrBr_4,g (2a) The reaction (2a) in comparison with the chemical transport of CrCl₃ with Cl₂ (Gl. (2b)) is more shifted to CrBr_3,g.     

Beiträge zum thermischen Verhalten von Oxoniobaten der Übergangsmetalle. V. Zum Chemischen Transport von NiNb2O6 mit Cl2 und NH4Cl. Experimente und Modellrechnungen

Contributions on the Thermal Behaviour of Oxoniobates of the Transition Metals. V. Chemical Vapour Transport of NiNb₂O₆ with Cl₂ or NH₄Cl. Experiments and Calculations Well shaped crystals of NiNb₂O₆ were obtained by CVT using Cl₂ (added as PtCl₂) or NH₄Cl as transport agents (1020°C → 960°C). As a result of thermodynamic calculations the migration of NiNb₂O₆ in the temperature gradient in the presence of Cl₂ can be expressed by the heterogenous endothermic equilibrium (1). Assuming Δ_BH(NiNb₂O_{6, s}) = ?524.4 kcal/mol a satisfying agreement between thermodynamical calculation and experimental results can be reached. NH₄Cl is less suitable as transport agent, because Ni²⁺ is partly reduced to the metal by NH₃. The additionally H₂O produced by this reduction leads to a less favourable equilibrium position of (2) and to low deposition rates. .     

Zum chemischen Transport von Vanadiummonophosphid mit lod

Chemical transport of vanadiummonophosphide with iodine Well shaped crystals of vanadiummonophosphide can be grown by CVT using iodine as transport agent (e. g. → 900°C). As a result of thermodynamical calculations the evaporation and deposition of VP should be expressed by the following exothermic equilibrium At higher temperatures and low concentrations of Iodine, combined with desorption of moisture from the walls of the silica ampules endothermic deposition of VP according to has been observed. Assuming Δ_BH°₂₉₈(VP_s) = ?61 [kcal/mol] a satisfying agreement between thermodynamical calculations and experimental results can be reached.     

Zur Thermodynamik der Metallcarbonate 3. Mitteilung [1]. Löslichkeitskonstanten und Freie Bildungsenthalpien von ZnCO3 und Zn5(OH)6(CO3)2 bei 25°

The solubilities of ZnCO₃ and Zn₅(OH)₆(CO₃)₂ have been investigated at 25°C in solutions of the constant ionic strength 0,2 M consisting primarily of sodium perchlorate. From experimental data the following values for equilibrium constants and GIBBS free energies of formation are deduced: A predominance area diagram for the ternary system Zn²⁺–H₂O–CO_2(g) including ZnO, ZnCO₃, Zn₅(OH)₆(CO₃₎₂, and Zn²⁺ is given.     

Chemischer Transport von Cr2O3 mit CrI3/I2 – Experimente und Modellrechnungen zur Beteiligung von CrOI2,g

On the Chemical Transport of Cr₂O₃ with CrI₃/I₂ – Experiments and Model-Calculations for Participation of CrOI_2,g Gaseous chromium oxyiodides that were unknown up to now cause the migration of Cr₂O₃ in the temperature gradient 1 000°C→900°C when iodine (e. g. 0.1 mmol/ml) and CrI₃ is added (eq. (1)). Transport agent for Cr₂O₃ is gaseous CrI₄. With a smaller concentration of iodine (D(I₂) ? 0.016 mmol/ml) and lower temperatures (e.g. T? = 850°C) the influence of H₂O (from the wall of the silica ampoule) becomes more important. Under these conditions the transport of Cr₂O₃ is a result from the endothermic reactions (2), (3) and (4). H_2,g has on the basis of the decomposition of HI_g a positive difference of the solubility and H_2,g should not to be considered as a transport agent. Because of the range of equilibrium-values the reaction 4 has to be taken into consideration. Estimated value of the enthalpie for CrOI_2,g is fixed more precisely by thermodynamic model calculation to Δ_fH°₂₉₈(CrOI_2,g) = ?51.4 kcal/mol. The estimated limit of error for the enthalpie of formation is smaller than ± 5 kcal/mol. Without an addition of CrI₃ is in the system Cr₂O₃/I₂ a migration of Cr₂O₃ not observable.     

Zum chemischen Transport im System Mn?O unter Berücksichtigung des Sauerstoffkoexistenzdruckes. II

Chemical Transport in the System Mn? O in Consideration of the Oxygen Coexistence Pressure. II The chemical transport of the coexistent phases Mn₂O₃? Mn₃O₄ and Mn₃O₄? MnO with TeCl₄ and SeCl₄ was analysed thermodynamically and experimentally. This two transport agents are able to transport the following phases:

1 Index (o) bedeutet obere, (u) untere Phasengrenze (index (o) – upper phase boundary, (u) – lower phase boundary)

     

Die Sublimation und thermische Zersetzung von TeJ4 und die Existenz von TeJ2 in der Gasphase

The Sublimation and the Thermal Decomposition of TeJ₄ and the Existence of TeJ₂ in the Gaseous Phase The sublimation and the decomposition of TeJ₄ have been investigated. For the sublimation and the decomposition reactions and the values of enthalpy and entropy were derived (see ?Inhaltsübersicht”?). The existence of TeJ₂ in the gaseous phase was demonstrated by equilibrium measurements and chemical transport experiments.     

Die Thermodynamik des Chemischen Transports von SnS2 mit I2

Experimente zum chemischen Transport von SnS₂ mit I₂ im Temperaturgefälle zeigten, daß der Transport unterhalb 900 K zur heißeren Zone hin erfolgt (Prozeß A), und daß er bei höheren Temperaturen zur Abscheidung von SnS₂ in der weniger heißen Zone führt (Prozeß B). Die eingehende thermodynamische Rechnung ergab, daß Prozeß A durch die exotherme Reaktion beherrscht wird, und daß im Prozeß B die endotherme Reaktion vorherrschend ist. Thermodynamics of the Chemical Transport of SnS₂ by I₂ The chemical transport of SnS₂ by I₂ in a temperature gradient has been investigated. Below 900 KSnS₂ is transported into the high temperature region (A) and at higher temperatures into the low temperature region (B). The thermodynamic discussion shows that A is governed by the exothermic reaction and B by the endothermic reaction .     

Gesamtdruckmessungen und Gasphasenzusammensetzung über Re2O7, ReO3 und ReO2

Total Pressure Measurements and Gas Phase Composition over Re₂O₇, ReO₃, and ReO₂ The total pressures over Re₂O₇, ReO₃, and ReO₂ have been determined by means of a membrane pressure gauge. The sublimation pressure over Re₂O₇ will be measureable at a temperature above 225°C and amounts 230 Torr at the melting point (at 315°C). The values are . ReO₃ decomposes at a temperature above 400°C according to with ΔH°_r,T = 214.6 ± 2kJ/mol and ΔS°_r,T = 263.2 ± 4J/K · mol. ReO₃ is not detectable in the gaseous phase in measurable quantities. ReO₂ decomposes at a temperature above 800°C according to with ΔH°_r,T = 387,0 ± 8.4 kJ/mol and ΔS°_r,T = 289.1 ± 12.5 J/K · mol.     

A pyrolysis mechanism for ammonia

The mechanism of NH₃ pyrolysis was investigated over a wide range of conditions behind reflected shock waves. Quantitative time-history measurements of the species NH and NH₂ were made using narrow-linewidth laser absorption. These records were used to establish an improved model mechanism for ammonia pyrolysis. The risetime and peak concentrations of NH and NH₂ in this experimental database have also been summarized graphically. Rate coefficients for several reactions which influence the NH and NH₂ profiles were fitted in the temperature range 2200 K to 2800 K. The reaction and the corresponding best fit rate coefficients are as follows: with a rate coefficient of 4.0 × 10¹³ exp(?3650/RT) cm³ mol^?1 s^?1, with a rate coefficient of 1.5 × 10¹⁵T^?0.5 cm³ mol^?1 s^?1 and with a rate coefficient of 5.0 × 10¹³ exp(?10000/RT) cm³ mol^?1 s^?1. The uncertainty in rate coefficient magnitude in each case is estimated to be ±50%. The temperature dependences of these rate coefficients are based on previous estimates. The experimental data from four earlier measurements of the dissociation reaction were reanalyzed in light of recent data for the rate of NH₃ + H → NH₂1 + H₂, and an improved rate coefficient of 2.2 × 10¹⁶ exp(?93470/RT) cm³ mol^?1 s^?1 in the temperature range 1740 to 3300 K was obtained. The uncertainty in the rate coefficient magnitude is estimated to be ± 15%.     

Beiträge zum chemischen Transport oxidischer Metallverbindungen. III. Der Transport von NiO und MgO über ihre Metallchloride

Transport effect of HCl on NiO and MgO according to between T₂ = 1000°C and T₁ = 800°C was calculated by the model of diffusion in dependence of total pressure; for comparison, the classical transport of α-Fe₂O₃ was analogously treated. By experimental determination of the transport rates at total pressures from 0.009 to 6 atm hitherto not considered influences of the amount and surface of the starting material, and of the transport time were found. These effects are explained by a (not in detail defined) term of ?sorption”? of the transport gas onto the powder of the starting material. For an explanation of the transport rates estimations of the diffusion coefficients of the gas pairs FeCl₃–HCl and NiCl₂–HCl were performed and the vapour pressure diagrams of NiCl₂ and MgCl₂ evaluated.     

Beiträge zum thermischen Verhalten von Oxoniobaten der Übergangsmetalle. IV. Zum Chemischen Transport von CoNb2O6 mit Cl2, NH4 und HgCl2. Experimente und Modellrechnungen

Contributions on the Thermal Behaviour of Oxoniobates of the Transition Metals. IV The Chemical Vapour Transport of CoNb₂O₆ with Cl₂, NH₄Cl, or HgCl₂. Experiments and Calculations Well shaped crystals of CoNb₂O₆ were obtained by CVT using Cl₂ (added as PtCl₂), NH₄Cl or HgCl₂ as transport agents (1020°C → 960°C). As a result of thermodynamic calculations the evaporation and deposition of CoNb₂O₆ in the presence of Cl₂ can be expressed by the heterogenous endothermic equilibrium (1). The endothermic reaction (2) is responsible for the CVT of CoNb₂O₆ if NH₄Cl is used as transport agent: The unfavourable site of the equilibrium (3) causes the small transport effect using HgCl₂ as transport agent. Assuming Δ_BH°₂₉₈(CoNb₂O_6,s) = ?524.7 kcal/mol a satisfying agreement between thermodynamical calculation and experimental results can be reached.     

Reaction of CF3 radicals with group IV tetramethyls

Arrhenius parameters have been measured for the abstraction of hydrogen from the C Si, Ge, and Sn tetramethyls: The rate constants correlate with the proton chemical shift, which is related to a polar effect. In all cases except carbon, a hot-molecule β-fluorine rearrangement-elimination reaction occurs following radical combination: We suggest the occurrence of a radical exchange reaction for the Si, Sn, and Ge systems, with k_exchange (CF₃ + Sn(Me)₄) ～ 10⁷ ml m^?1 s^?1.     

Thermal stability of alcohols

3,3-Dimethylbutanol-2 (3,3-DMB-ol-2) and 2,3-dimethylbutanol-2 (2,3-DMB-ol-2) have been decomposed in comparative-rate single-pulse shock-tube experiments. The mechanisms of the decompositions are The rate expressions are They lead to D(iC₃H₇? H) – D((CH₃)₂(OH) C? H) = 8.3 kJ and D(C₂H₅? H) – D(CH₃(OH) CH? H) = 24.2 kJ. These data, in conjunction with reasonable assumptions, give and The rate expressions for the decomposition of 2,3-DMB-1 and 3,3-DMB-1 are and      

Chlorine-photosensitized reactions in the Cl2 + O2 + 1,1,1,2-C2H2Cl4 system

The gas-phase photochlorination (λ = 436 nm) of the 1,1,1,2-C₂H₂Cl₄ has been studied in the absence and the presence of oxygen at temperatures between 360 and 420°K. Activation energies have been estimated for the following reaction steps: The dissociation energy D(CCl₃CHCl? O₂) ± (24.8 ± 1.5) kcal/mole has also been estimated from the difference in activation energy of the direct and reverse reactions The mechanism is discussed and the rate parameters are compared to those obtained for a series of other chlorinated ethanes.     

The self-inhibited pyrolysis of isobutane

The pyrolysis of isobutane was investigated in the ranges of 770° to 855°K and 20 to 150 Torr at up to 4% decomposition. The reaction is homogeneous and strongly self-inhibited. A simple Rice-Herzfeld chain terminated by the recombination of methyl radicals is proposed for the initial, uninhibited reaction. Self-inhibition is due to abstraction of hydrogen atoms from product isobutene giving resonance-stabilized 2-methylallyl radicals which participate in termination reactions. The reaction chains are shown to be long. It is suggested that a previously published rate constant for the initiation reaction (1) is incorrect and the value k₁ = 10^16.8 exp (?81700 cal mol^?1/RT)s^?1 is recommended. The values of the rate constants for the reactions (4i) (4t) (8) are estimated to be and From a recalculation of previously published data on the pyrolysis of isobutane at lower temperatures and higher pressures, the value k_11c, = 10^9.6 cm³ mol^?1 s^?1 is obtained for the rate constant of recombination of t-butyl. A calculation which is independent of any assumed rate constants or thermochemistry shows that the predominant chain termination reaction is the recombination of two methyl radicals in the conditions of the present work and the recombination of two t-butyl radicals in those of our previous study at lower temperatures and higher pressures.     

The kinetics and the mechanism of the thermal gas-phase reaction between NO2 and 1,1-dichlorodifluoroethylene,CF2CCl2

The kinetics of the gas phase reaction between NO₂ and CF₂CCl₂ has been investigated in the temperature range from 50 to 80°C. The reaction is homogeneous. Three products are formed: O₂NCF₂CCl₂NO₂ and equimolecular amounts of CINO and of O₂NCF₂C(O)Cl. The rate of consumption of the reactants is independent of the total pressure, the reaction products, and added inert gases and can be represented by a second-order reaction: However, the distribution of the products is influenced by the pressure of the present gases, which favor the formation of the dinitro-compound in a specific way. The effect of CF₂CCl₂ is the greatest. In the absence of added gases, the ratio of O₂NCF₂CCl₂NO₂ to that of O₂NCF₂C(O)Cl is proportional to (CF₂CCl₂ + γ_P products). The experimental results can be explaned by the following mechanism: P and X represent the products and the added gases: