Navegando por Assunto "Entalpia"

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Acesso aberto (Open Access)
Determinação entalpimétrica de cálcio e magnésio em calcários usando-se um entalpímetro de fluxo contínuo
(Universidade Estadual de Campinas, 1986-03-10) MENDES, Afonso Silva; OLIVEIRA, Walace Alves de
Determination of calcium and magnesium in limestone using a flow enthalpimeter is described. The apparatus consists of a peristaltic pump, a proportional injector, two twin thermistors, a Wheatstone bridge and a strip-chart recorder. The injector, the 'flow manifolds and the emperature-sensing elqments are immersed in San insulated water bath. For the determination of calcium, preliminary studies were undertaken using the precipitation reaction with ammonium oxalate and the complaxation with Na2-EDTA and Mg-EDTA. After these studies, the experimental procedure chosen was based on complexation with Mg-EDTA. In order to test the method, determination of calcium was performed in synthetic, certified and other samples. Precision and accuracy was found to be equal to 1.0% and the analytical rate was 100 samples per hour. For the determination of magnesium, initial tests were conducted investigating the possibility of using the complexation reaction with Na2-EDTA and the previous separation of calcium by means of precipitation with ammonium oxalate and ammonium molybdate. In the final procedure, the complexation reaction with Na2-EDTA was chosen to give the amount of both calcium and magnesium. Then, magnesium is found by difference, since the percentage of calcium is known. Resuls of several determinations indicate an average error around 4% for samples with MgO higher than 10%. The uncertainty increases for samples having MgO lower than 10%. The analytical rate is about 100 samples per hour. The results of the enthalpimetric method were compared with those found by other techniques such as volumetry ( by complexation) atomic absorption and flame photometry.
Acesso aberto (Open Access)
. Predição das propriedades termodinâmicas do biodiesel e diesel comum, suas blendas e efeitos de aditivos via teoria do funcional da densidade e ensemble canônico.
(Universidade Federal do Pará, 2019-11-08) ROCHA, Adriano Santos da; CHAVES NETO, Antonio Maia de Jesus; http://lattes.cnpq.br/3507474637884699
In this study, we analyzed the thermodynamic properties of soy biodiesel, the most widely used biodiesel source among the world's largest consumers, and rapeseed biodiesel, a source with significant employment in the European Union and biodiesel additives. Also, the study of ordinary diesel was carried out to simulate the situations in which this fuel is in mixtures with different concentrations of biodiesel blends. The objective of this work is to study the thermodynamic properties of the fuels mentioned. Based, where possible, on experimental results to validate the expansion of theoretical results and point out the most appropriate methodology for this type of approach. To this end, computational simulations were carried out with theoretical support in the Density Functional Theory combined with the canonical ensemble model. The functional used in the simulations was type B3LYP with bases 6-31 + (d) and 6-311 + g (d), in addition to the composite method CBS-QB3. The calculated enthalpy values are higher in the biofuels in relation to diesel and the same happened with Entropy. When we analyze the change of Gibbs Free Energy, the largest variations were noted for biofuels. The coefficient of adiabatic expansion revealed lower degrees of freedom until 400 K temperature in both types of biodiesel, from that point all fuels have the same behavior for this property. The lowest relative error was found with the B3LYP/6-311 +g(d) method, with a value of 0.15% for the combustion of ordinary diesel. In relation to biofuels, the same method provided 0.48% relative error in the combustion of rapeseed biodiesel as the best result. Regarding the effects of temperature on these fuels, the results showed that both types of biodiesel are less susceptible to heating than ordinary diesel at all calculated points, always requiring more energy to raise their temperature. The results in the additives showed that the antioxidant pyrogallol that most interferes in the biodiesel properties.
Acesso aberto (Open Access)
Síntese, caracterização e termoquímica de adutos entre metil, N, N'-dimetil e tetrametiluréia com haletos de zinco, cádmio e mercúrio-II
(Universidade Estadual de Campinas, 1980) ASSUNÇÃO, Francisco Pereira; AIROLDI, Claúdio; http://lattes.cnpq.br/1481366755849386
The reactions of zinc, cadmium and mercury halides with the ligands methylurea (MU), N, N'-dimethylurea (DMU) and tetramethylureia (TMU) yielded the following adducts: 2nL2x2, MLX2 (M + Cd, Hg) (X =Cl, Br and L=MU, DMU and TMU), M(TMU) 2I2 (M + Zn, Cd),Cd (DMU) 3Br3 and Hg (DMU) 2Cl2. These adducts were characterized by elemental analyses, melting points, conductance measurements, thermogravimetric. ifra red spectra and mass spectra. From the values os standard enthalpies of dissolution of thehalides,ligands and adducts in suitable calorimetric solvents, the standard enthapy of the reactions was calculated: MX2(s) + nL (l,s) = MLnX2 (s); ΔHθR. For the obtention of the standard enthalpies of formation ΔHθf of the adducts ΔHθR values were used. It was also necessary to determine the standard enthalpies of formation of DMU and MU, which were obtained by means of combustion calorimetry. The standard enthalpies, ΔHθD and ΔHθM, for the equations below were also calculated, howerer, it was necessary to determine the standard enthapy of vaporation of TMU and the standard enthalpies of sublimation of DMU and MU. MLnX2(s) + Mx2(s) + nL(g) ; ΔHθD. MLnX2(s) + Mx2(s) + nL(g) ; -ΔHθM. The thermal data in conjution with the mass spectra of the adducts suggest that possibility these compouds do not exist, at least in appreciable amounts in the gaseous phase. However, the enthalpy of the metal-oxigen bond was estimated, based on the fact that this mode of coordenation has occured with these three ligands, in agreement with the infra red results. The ΔHθR parameter establiches the order of basicity, TMU>DMU>MU, and the acidity order of of the metals, Zn>Cd>Hg.