Algunes reflexions al voltant del tractament de l'equilibri químic en el batxillerat

En aquest escrit, s'hi recorda el tractament que rebia l'equilibri químic en el batxillerat que va estudiar l'autor de l'article i es compara amb el que s'imparteix en el batxillerat actual. S'hi palesa que, en el batxillerat d'avui dia, l'equilibri químic rep un tractament força més extens i amb una càrrega conceptual més elevada que la d'aquells anys. Aquest fet pot comportar, si la matèria no es tracta amb el temps requerit, que els estudiants d'avui dia no acabin d'entendre gaire bé el significat ni la utilitat de les magnituds i les constants fisicoquímiques introduïdes i, a la vegada, es dificulti que puguin assolir completament uns altres objectius més elementals. D'altra banda, a partir de la revisió d'alguns dels textos de química més usats en els centres de batxillerat, en el present article es proposa tornar a considerar el tractament que reben certs aspectes concrets de l'equilibri químic i, en aquest sentit, s'hi discuteixen algunes de les inconveniències observades i s'hi plantegen alternatives per tractar els diferents aspectes d'una manera més adientThis paper compares the explanation of chemical equilibrium when the author was a student, to the current one in Catalonian high schools today. It can be seen that the current chemical equilibrium explanation is more extensive and it has a greater conceptual importance than in the past. This can cause a problem if not enough time is given to teaching the subject, leading to a poor understanding of the meaning and use of the physicochemical magnitudes and constants. At the same time, this problem can hinder the reaching of other basic goals. On the other hand, after the revision of the most used chemistry textbooks at high school level, a reconsideration of some aspects of the chemical equilibrium explanation is proposed. And so, some of the observed problems with the textbooks are discussed and alternative proposals for a clearer treatment of the subject are suggested

Els Qüestionaris com a recurs docent en l'aprenentatge autònom. Resultats d'una prova pilot en un curs de Química bàsica.

En aquest article es presenten uns nous qüestionaris on line dissenyats per millorar la comprensió dels continguts de l"assignatura Química Bàsica II i, alhora, propiciar el treball autònom dels estudiants. En el grup pilot que ha disposat dels qüestionaris, els resultats acadèmics dels estudiants que els han realitzat diverses vegades han estat millors que els dels alumnes que no els han utilitzat amb regularitat. Els resultats acadèmics del grup pilot també ha estat millor que els de cursos anteriors

Electrochemical Fenton-based treatment of tetracaine in synthetic and urban wastewater using active and non-active anodes

The electrochemical degradation of tetracaine hydrochloride has been studied in urban wastewater. Treatments in simulated matrix with similar ionic composition as well as in 0.050 M Na2SO4 were comparatively performed. The cell contained an air-diffusion cathode for H2O2 electrogeneration and an anode selected among active Pt, IrO2-based and RuO2-based materials and non-active boron-doped diamond (BDD). Electrochemical oxidation with electrogenerated H2O2 (EO-H2O2), electro-Fenton (EF) and photoelectro-Fenton (PEF) were comparatively assessed at pH 3.0 and constant current density. The pharmaceutical and its byproducts were oxidized by ¿OH formed from water oxidation at the anode surface and in the bulk from Fenton's reaction, which occurred upon addition of 0.50 mM Fe2+ in all media, along with active chlorine originated from the anodic oxidation of Cl- contained in the simulated matrix and urban wastewater. The PEF process was the most powerful treatment regardless of the electrolyte composition, owing to the additional photolysis of intermediates by UVA radiation. The use of BDD led to greater mineralization compared to other anodes, being feasible the total removal of all organics from urban wastewater by PEF at long electrolysis time. Chlorinated products were largely recalcitrant when Pt, IrO2-based or RuO2-based anodes were used, whereas they were effectively destroyed by BDD(¿OH). Tetracaine decay always obeyed a pseudo-first-order kinetics, being slightly faster with the RuO2-based anode in Cl- media because of the higher amounts of active chlorine produced. Total nitrogen and concentrations of NH4+, NO3-, ClO3-, ClO4- and active chlorine were determined to clarify the behavior of the different electrodes in PEF. Eight intermediates were identified by GC-MS and fumaric and oxalic acids were quantified as final carboxylic acids by ion-exclusion HPLC, allowing the proposal of a plausible reaction sequence for tetracaine mineralization by PEF in Cl--containing medium

Electrochemical treatment of butylated hydroxyanisole: electrocoagulation versus advanced oxidation

This work compares the removal of butylated hydroxyanisole (BHA), a ubiquitous antioxidant in food and pharmaceuticals, from water either by electrocoagulation (EC) with an Fe|Fe cell or H2O2-based electrochemical advanced oxidation processes like electrochemical oxidation (EO-H2O2), electro-Fenton (EF) and photoelectro-Fenton (PEF) with an air-diffusion cathode. BHA degradation by EC was very poor, whereas the dissolved organic carbon (DOC) was more effectively abated in urban wastewater. The effect of pH, number of Fe|Fe pairs and current on the EC performance was examined. The additive was also slowly degraded by EO-H2O2 with a RuO2-based or BDD anode in 50 mM Na2SO4 solution. In the simulated matrix, BHA decay by EO-H2O2 was substantially enhanced owing to active chlorine generation from anodic oxidation of Cl−, whereas the OH-mediated oxidation at the BDD surface accounted for DOC decay. In EF and PEF, the OH produced in the bulk upgraded the mineralization, primordially using BDD. In raw urban wastewater at natural pH 7.9, the time course of BHA and DOC contents was affected by NOM oxidation, being accelerated in the order: EO-H2O2 < EF < PEF. The quickest decontamination of urban wastewater occurred in PEF at pH 3.0, because of the higher amounts of OH in the bulk along with UVA photolysis

Electro-Fenton process at mild pH using Fe(III)-EDDS as soluble catalyst and carbon felt as cathode

The feasibility of destruction of organic pollutants in water at near-neutral pH by homogeneous electro-Fenton (EF) process employing a soluble Fe(III)-EDDS complex as catalyst is demonstrated for the first time. The performance of the Fe(III)-EDDS-assisted EF process with carbon-felt or air-diffusion cathodes was evaluated from the degradation of butylated hydroxyanisole (BHA) in sulfate medium. The influence of applied current, pH and Fe(III):EDDS ratio and dosage on BHA decay and mineralization was related to the evolution of H2O2 and iron concentrations. Using Fe(III)-EDDS, up to 50% Fe(II) regeneration was achieved in 10 min, whereas only 23% was transformed using hydrated Fe3+. Almost total removal of BHA was achieved thanks to homogenous Fenton, heterogeneous Fenton with cathodically adsorbed Fe(III), and electrocatalysis. The mineralization partly corresponded to the gradual destruction of EDDS by hydroxyl radical (kabs = 5.22 × 109 M−1 s−1), and involved the formation of 5 oxidation and 6 dimerization or cyclization by-product

Electrochemical oxidation of anesthetic tetracaine in aqueous medium. Influence of the anode and matrix composition

The degradation of 150 mL of 0.561 mM tetracaine hydrochloride at pH 3.0 by electrochemical oxidation with electrogenerated H2O2 (EO-H2O2) has been studied at a low current density of 33.3 mA cm(2) in three different matrices: 0.050 M Na2SO4, real urban wastewater and a simulated matrix mimicking its electrolyte composition. Comparative trials were performed in an undivided cell with a 3 cm(2) borondoped diamond (BDD), Pt, IrO2-based or RuO2-based anode and a 3 cm(2) air-diffusion cathode that allowed continuous H2O2 electrogeneration. In 0.050 M Na2SO4, much faster and overall removal of tetracaine occurred using BDD because of the large oxidation ability of BDD((OH)-O-center dot) formed from anodic water oxidation. In either simulated matrix or real wastewater, the RuO2-based anode yielded the quickest tetracaine decay due to a large production of active chlorine from anodic oxidation of Cl . For the mineralization of the organic matter content, the BDD/air-diffusion cell was the best choice in all aqueous matrices, always reaching more than 50% of total organic carbon abatement after 360 min of electrolysis, as expected if BDD((OH)-O-center dot) mineralizes more easily the chloroderivatives formed from tetracaine oxidation in the presence of active chlorine. The initial N of tetracaine was partly transformed into NO3, although the total nitrogen of all solutions always decayed by the release of volatile by-products. In the Cl - containing matrices, significant amounts of ClO3 and ClO4 were obtained using BDD, whereas active chlorine was much largely produced using the RuO2-based anode. Five aromatic by-products, one of them being chlorinated, along with low concentrations of oxalic acid were identified. The change in toxicity during EO-H2O2 with BDD in the sulfate and simulated matrices was also assessed. (C) 2017 Elsevier B.V. All rights reserved

Photoelectro-Fenton as post-treatment for electrocoagulated benzophenone-3-loaded synthetic and urban wastewater

The removal of benzophenone-3 (BP-3), a ubiquitous pollutant in municipal wastewater treatment fa-cilities, was optimal by means of a sequential electrocoagulation (EC)/UVA photoelectro-Fenton (PEF)treatment. Overall mineralization was attained upon combination of EC (Fe/Fe cell, 15 mA cm 2, 20 min)with PEF (boron-doped diamond/air-diffusion cell, 33.3 mA cm 2, 720 min), being superior to EC/electro-Fenton (EF) and requiring shorter time than single PEF. In EC, an Al/Al cell yielded the largest removal ofBP-3 in a simulated matrix at pH 11.0 due to precipitation of its neutral form caused by a substantial pHdrop, with optimum current density of 15 mA cm 2. EC of BP-3-loaded urban wastewater at natural pHwas quite effective also with a Fe/Fe cell, being preferred since it provided the required metal catalyst forsubsequent treatment. Among the electrochemical advanced oxidation processes tested, PEF was su-perior to electrochemical oxidation with electrogenerated H2O2(EO-H2O2) and EF, especially when usingthe boron-doped diamond instead of a RuO2-based anode, due to the oxidation of generated activechlorine and hydroxyl radicals, along with the photolytic action of UVA irradiation. GC-MS revealed theformation of 14 cyclic products in PEF treatment, two of them being also formed during EC

Electrochemical performance of carbon-supported Pt(Cu) electrocatalysts for low-temperature fuel cells

Pt(Cu) nanoparticles supported on carbon nanofibers (CNFs), multi-walled carbon nanotubes (MWCNTs) and Vulcan carbon XC72, have been synthesized by electroless deposition and galvanic exchange. The structural analyses show contracted Pt fcc lattices due to the formation of a PtCu alloy core covered by a Pt-rich shell, mean crystallite sizes of about 3 nm, as well as good dispersion and carbon attachment. The electrochemical surface areas (ECSAs) of Pt(Cu)/CNF and Pt(Cu)/XC72 are comparable to those of commercial Pt/C and PtCu/C. The Pt(Cu) electrocatalysts show more negative onset potentials for CO oxidation than Pt/C and PtCu/C, thus indicating their greater CO tolerance. Pt(Cu)/CNF and Pt(Cu)/MWCNT present the highest mass activity and specific activity for the O2 reduction, respectively, both with better relative stability than Pt(Cu)/XC72. Pt(Cu)/CNF and Pt(Cu)/MWCNT are then considered good cathode catalysts, yielding estimated savings of about 50 wt.% Pt, when applied to low-temperature fuel cells

Effects of the electrodeposition time in the synthesis of carbon-supported Pt(Cu) and Pt-Ru(Cu) core-shell electrocatalysts for polymer electrolyte fuel cells

Pt(Cu)/C and Pt-Ru(Cu)/C electrocatalysts with core-shell structure supported on Vulcan Carbon XC72R have been synthesized by potentiostatic deposition of Cu nanoparticles on the support, galvanic exchange with Pt and spontaneous deposition of Ru species. The duration of the electrodeposition time of the different species has been modified and the obtained electrocatalysts have been characterized using electrochemical and structural techniques. The High Resolution Transmission Electron Microscopy (HRTEM), Fast Fourier Transform (FFT) and Energy Dispersive X-ray (EDX) microanalyses allowed the determining of the effects of the electrodeposition time on the nanoparticle size and composition. The best conditions identified from Cyclic Voltammetry (CV) corresponded to onset potentials for CO and methanol oxidation on Pt-Ru(Cu)/C of 0.41 and 0.32 V vs. the Reversible Hydrogen Electrode (RHE), respectively, which were smaller by about 0.05 V than those determined for Ru-decorated commercial Pt/C. The CO oxidation peak potentials were about 0.1 V smaller when compared to commercial Pt/C and Pt-Ru/C. The positive effect of Cu was related to its electronic effect on the Pt shells and also to the generation of new active sites for CO oxidation. The synthesis conditions to obtain the best performance for CO and methanol oxidation on the core-shell Pt-Ru(Cu)/C electrocatalysts were identified. When compared to previous results in literature for methanol, ethanol and formic acid oxidation on Pt(Cu)/C catalysts, the present results suggest an additional positive effect of the deposited Ru species due to the introduction of the bifunctional mechanism for CO oxidatio