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Dalle formule di struttura di Lewis ai concetti di numero di ossidazione, carica formale e carica effettiva di un atomo in una molecola
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I concetti di elettronegatività assoluta e di durezza assoluta degli atomi liberi, nel loro stato elettronico fondamentale, e il principio di equalizzazione dell’elettronegatività degli atomi nelle molecole, sono qui utilizzati insieme alle formule di struttura di Lewis e al modello di Gillespie (VSEPR) per determinare la probabile geometria molecolare, per una didattica della Chimica generale coerente ed efficace che a partire da una semplice formula analitica, permette di introdurre i concetti di numero di ossidazione, di carica formale e di carica effettiva per ogni atomo in una molecola.Abstract
The concepts of absolute electronegativity and absolute hardness of the free atoms, in their electronic ground state, and the principle of the electronegativity equalization for the atoms in molecules, are used together with the Lewis structures and the Gillespie model (VSEPR) for the definition of the molecular geometry, to present in the General chemistry course the concepts of oxidation number, of formal charge and effective charge for the atom in a molecule by means of a simple analytical formula
Il nuovo Sistema Internazionale di unità di misura (SI). La nuova definizione della quantità di sostanza e della mole
No full textIl 20 maggio 2019 è entrato in vigore il nuovo Sistema Internazionale di Unità di Misura (SI) nel quale le sette unità di misura fondamentali sono definite per mezzo di sette costanti di natura, con
valore numerico fissato. Da un punto di vista pratico le grandezze fisiche fondamentali e le unità di misura sono rimaste le stesse del precedente SI, ma con il nuovo SI è stato realizzato un sistema completamente libero da quantità fisiche basate su artefatti. L’unità di massa, il kilogrammo, non è più basato su un lingotto di una lega platino-iridio conservato presso il Bureau International de Poids et Mesures (BIPM) a Sèvres (Francia).
In questa nota riportiamo le definizioni delle sette unità di misura fondamentali e il valore delle costanti di natura che le caratterizzano, con particolare attenzione alla quantità di sostanza (quantità
chimica) e alla sua unità di misura, la mole, che sono di importanza fondamentale per la Chimica, alla stesso livello della Tavola Periodica degli elementi.Starting from May 20th, 2019 the new International System of Units (SI) should be in use in all the Nations. In this new SI the seven base quantities are defined by means of seven constants of nature with fixed numerical values. From a practical point of view the base quantities and the base units are the same of the previous SI but in the new SI it is realized a system free from physical units based on artifacts. The mass unit, the kilogram, is no more based on a platinum-iridium ingot maintained at the International Bureau of Weights and Measures (BIPM) in Sèvres (France).
In the present contribution we report the definitions of the seven base units and the numerical value of the seven constants of nature by which they are characterized. We devote particular attention to the amount of substance (chemical amount) and to its base unit, the mole, which are of fundamental importance for Chemistry, as well as the Periodic Table of the elements
Characterization by x-ray absorption, x-ray powder diffraction, and magnetic susceptibility of Cu-Zn-Co-Al-containing hydroxycarbonates, oxycarbonates, oxides, and their products of reduction
No full textCopper-zinc-cobalt-aluminium-containing crystalline hydroxycarbonates having hydrotalcite structure have been prepared by coprecipitation. X-ray powder diffraction (XRPD), magnetic susceptibility, and extended X-ray absorption fine structure (EXAFS) indicate that Cu2+, Zn2+, and Co2+ are present in an octahedral environment. Calcination of the hydroxycarbonates at 723 K produces quasi-amorphous oxycarbonates where Cu2+ and Co2+ still retain octahedral coordination and cobalt is almost completely oxidized to Co3+. The coordination of Zn2+, at this stage, is intermediate between the octahedral one of the precursors and the tetrahedral one of ZnO and Zn-based spinels. Further calcination at 973 K produces a mixture of crystalline oxides such as CuO, ZnO, CuAl2O4, ZnAl2O4, and ZnCo2O4. EXAFS analysis of these samples indicates that copper is mainly in a fourfold coordination (although two longer Cu-O distances are also detected), zinc is tetrahedral, and cobalt (as Co3+) is essentially octahedral. EXAFS and XANES investigations performed after in situ reduction (10% H2/N2, at 523 and 623 K) on the oxycarbonates and oxides reveal that the total Cu2+ → Cu0 reduction occurs only at 623 K in both series of samples, Co3+ is reduced to Co2+ only at 623 K in the oxycarbonates, and Zn2+ is never reduced. © 1996 Academic Press, Inc
Functionalized metal oxide particles: a comparative study on their use as additives in polymer electrolyte membranes
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LaMO3 (M=Mn, Co, Fe) perovskite-type oxides. Structural, redox and catalytic properties towards oxidation of hexane
No full textPreparation and characterization of perovskite-type oxides containing La, Mn, Cu, and La, Co, Cu
No full textSynthesis and characterization of Nb 2 O 5 mesostructures with tunable morphology and their application in dye-sensitized solar cells
No full textMesoporous submicrometric particles of orthorhombic Nb2O5 were prepared by a very simple route
consisting in the hydrolysis of niobium ethoxide Nb(OEt)5 in an alcoholic medium containing 1-
hexadecylamine as structure-directing agent followed by a hydrothermal treatment. The effects
related to the variation of the length of the aliphatic chain of the alcohol solvent (C2 to C4) and the
ramification of the alcohol (primary, secondary and tertiary), as well as the reactants to solvent ratio
were analyzed in terms of morphology, crystal structure, specific surface area and porosity. The obtained
solids, once thermally treated to remove the organics, were thoroughly characterized by powder XRD,
UV-Vis spectroscopy (band gap measurement), photovoltage measurements (conduction band edge
determination), BET-BJH surface analysis and FESEM. By simply modifying the solvent and reactants/
solvent ratio, different morphologies spanning from nearly monodisperse beads to peanut-shaped particles,
sintered spheres aggregates, a mix of different morphologies can be achieved. The obtained materials
were tested as photoanodes in dye-sensitized solar cells. After the optimization of the thickness of
the photoanode and of its chemical treatment in order to improve the inter-particle connections, DSSCs
were prepared by using N719 dye and a non-volatile iodide-based electrolyte. The cells were tested by J-V
curves under AM 1.5 G illumination and dark, IPCE measurements and electrochemical impedance
spectroscopy. A remarkable efficiency value of 3.4% under 1 sun illumination was achieved by employing
peanut-shaped particles obtained by using 2-propanol as solvent with the highest used reactant/solvent
ratio
Hexane total oxidation on LaMO3 (M = Mn, Co, Fe) perovskite-type oxides
No full textLaMO3 (M = Mn, Fe, Co) perovskites were prepared by calcining the citrate precursors at 800 degreesC for 5 h. PdO 3%/Al2O3, a largely used combustion catalyst, was also studied for comparison. The catalytic behavior was examined in the hexane total oxidation. The following activity scale was found: LaFeO3 > LaCoO3 > LaMnO3. >3% PdO/Al2O3, the three perovskites activating the C-H bond cleavage of hexane better than PdO/Al2O3. At temperature around 250-300 degreesC mobile surface oxygen species are involved in the oxidation process, whereas at T > 300 degreesC oxygen mobile species are no more present and so, particularly for LaCoO3 and PdO/Al2O3, the activation mechanism becomes more energetic. The activity values were compared with those obtained from the total oxidation of methane. (C) 2001 Elsevier Science B.V. All rights reserved
The reducibility of highly stable Ni-containing species in catalysts derived from hydrotalcite-type precursors
No full textA study was conducted on the speciation and reducibility of Ni in catalysts derived from hydrotalcite-type
(HT) precursors intercalated by silicates. Silicate and nickel contents in Ni/Mg/Al HT precursors varied and
the products obtained by thermal decomposition in the 773–1373 K range were characterized. Sample
properties were related to the amount of silicates and nickel. The former altered the formation of the
spinel-type phase and decreased the ratio between MgO and MgAl2O4 phases in which the active
species were stabilized. On the other hand, the Ni distribution depended on the Ni-loading. Nicontaining
species in the spinel phase were more abundant for high Ni-loaded catalysts, and were
readily reduced by H2 treatment at 1023 K, whereas those in the Ni1xMgxO solid solution remained
partially unreduced
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