El principio de Le Chatelier a través de la historia y su formulación didáctica en la enseñanza del equilibrio químico

The historical evolution of the Le Chatelier's principle is connected with its use in the chemistry classroom. The apparent simplicity in which the Le Chatelier's principle was first formulated and the success achieved in the implementation of some important industrial processes gave it an initial acknowledgement still kept nowadays. Nevertheless, since the beginning of this century, different authors have pointed out the limited character of the qualitative rule and its vague and ambiguous formulation. Furthermore, thermodynamics gives for the Le Chatelier's principle some quantitative formulations that limit its applicability. However, most general chemistry textbooks have treated the Le Chatelier's qualitative rule as a universal and infallible principle, making suitable for teaching the statement given by Le Chatelier in 1888. This didactic teaching approach is a source of different misconceptions which are generated in the prediction of the evolution of a perturbed chemical equilibrium system. Hence, some authors have suggested avoiding the Le Chatelier's rule. This new option assumes to use exclusively the chemical equilibrium constant

Errores conceptuales en el estudio del equilibrio químico : nuevas aportaciones relacionadas con la incorrecta aplicación del principio de Le Chatelier

The limited usefulness of the Le Chatelier's principle and its vague and ambiguous formulation may be some of the sources of misconceptions in chemical equilibrium. To diagnose misconceptions a written test was administered to 170 first-year students. Responses revealed that mistakes emerged tl~rougha: ) misapplication and misunderstanding of the Le Chatelier's principle; b) utilization of rote-learning recall and algorithmic procedures; c) incorrect control of the variables involved and limited use of the chemical equilibriuni law; d) lack of mastery of the principles of chemical equilibrium and difficulty in transfering them to new situations

Above ground woody community attributes, biomass and carbon stocks along a rainfall gradient in the savannas of the central lowveld, South Africa

Enumeration of carbon stocks at benchmark sites is a necessary activity in assessing the potential carbon sequestration and possible generation of credits through restoration of intensively impacted sites. However, there is a lack of empirical studies throughout much of the savannas of sub-Saharan Africa, including South Africa. We report an estimation of species specific and site biomass and carbon stocks, and general vegetation structural attributes from three protected areas along a rainfall gradient in the central lowveld, South Africa. Estimates of biomass and carbon stocks were effected through destructive sampling to establish locally derived allometric equations. There was a gradient of increasing woody density, height of the canopy, number of species, density of regenerative stems and a greater proportion of stems in small size classes from the arid locality to the mesic locality, with the semi-arid locality being intermediate. The proportion of spinescent species decreased with increasing rainfall. The mesic locality was significantly more woody than either the arid or semi-arid sites, having double the biomass, four times the density and 40% higher basal area. Above ground carbon pools were also higher; carbon stocks were approximately 9 t/ha for the arid and semi-arid sites and 18 t/ha for the mesic site.

A comparative exploratory study of pre-service and in-service high school teachers’ pedagogical content knowledge in galvanic cells

En este estudio exploratorio se evalúa el conocimiento didáctico del contenido (CDC) sobre pilas galvánicas de profesores de secundaria en formación y en ejercicio. Además, este conocimiento se ha relacionado con los enfoques de enseñanza que prefieren. Para este propósito, se ha adaptado el instrumento CoRe de Loughran, Mulhall y Berry (2004) considerando dos situaciones: pila Daniell y pila voltaica simple. La muestra se compuso de 25 profesores en formación y de 15 profesores en ejercicio. El análisis de los resultados obtenidos hasta ahora sugiere que, en primer lugar, el CDC de los profesores de secundaria sobre pilas galvánicas no es el deseable; en segundo lugar, el CDC de los profesores en ejercicio parece significativamente mejor que el de los profesores en formación, y finalmente, no se observan diferencias significativas entre ambos grupos de profesores en las metodologías de enseñanza preferidas.En aquest estudi exploratori s'avalua el coneixement didàctic del contingut (CDC) sobre piles galvàniques de professors de secundària en formació i en exercici. A més, aquest coneixement s'ha relacionat amb els enfocaments d'ensenyament que prefereixen. Per a aquest propòsit, s'ha adaptat l'instrument CoRe de Loughran, Mulhall i Berry (2004) considerant dues situacions: pila Daniell i pila voltaica simple. La mostra es va compondre de 25 professors en formació i de 15 professors en exercici. L'anàlisi dels resultats obtinguts fins ara suggereix que, en primer lloc, el CDC dels professors de secundària sobre piles galvàniques no és el desitjable; en segon lloc, el CDC dels professors en exercici sembla significativament millor que el dels professors en formació, i finalment, no s'observen diferències significatives entre tots dos grups de professors en les metodologies d'ensenyament preferides.In this exploratory study pre-service and in-service high school teachers’ pedagogical content knowledge (PCK) in galvanic cells is assessed. In addition, this knowledge has been related to the instructional approaches they prefer. A written questionnaire, adapted from the Loughran, Mulhall, and Berry’s CoRe instrument (2004), considering two different situations (Daniell cell and simple voltaic cell), has been used for this purpose. The sample made up of 25 pre-service teachers and 15 in-service teachers. The analysis of the results achieved so far suggest that: a) high school teachers’ PCK on galvanic cells is not a desirable knowledge; b) in-service teachers seems to have a significantly better PCK than pre-service teachers; and c) there seem to be no significant differences in preferred teaching approaches between the two groups of teachers

Conceptual knowledge structure, working memory and science texts comprehension: A study with High School students

En este trabajo se estudian las relaciones existentes entre estructura de conocimiento conceptual, memoria de trabajo, nivel académico y comprensión de un texto de ciencias. A estudiantes de 4º de ESO y 2º de Bachillerato se les determinó la capacidad de su memoria de trabajo mediante el test de Daneman y Carpenter. A continuación, leyeron un texto y a partir de él elaboraron un mapa conceptual. Por último, se les administró un cuestionario de comprensión del texto. Los resultados revelan que: a) el nivel académico genera diferencias significativas en el test de memoria de trabajo, y en las puntuaciones del mapa conceptual y cuestionario de comprensión textual; b) las correlaciones entre las puntuaciones de las tres pruebas anteriores han resultado ser elevadas y significativas; c) las puntuaciones del mapa conceptual y del test de memoria de trabajo son predictores significativos de la puntuación del cuestionario de comprensión.In this paper we present a descriptive research that shows the relationships between conceptual knowledge structure, working memory, academic level and science text comprehension. High school students (10th and 12th grade students) have participated in this research. Students’ working memory capacity was measured by means of the Daneman and Carpenter’s test. Next, the students read a science text, and starting from the textual information, they constructed a concept map. Finally, they carried out a reading comprehension test. Results indicate that: a) the academic level produces significant differences in working memory, concept map, and reading comprehension tests scores; b) high and significant correlations have been found between the scores of these last three tests; c) concept map and memory working tests scores are significant predictors of reading comprehension test scores

Efectos de la similitud superficial y estructural sobre la transferencia a partir de análogos en problemas de alta y baja familiaridad : primeros resultados

Se desarrollan dos estudios exploratorios para analizar el éxito en la transferencia en resolución de problemas a partir de análogos. Las diferentes analogías entre problemas «ejemplo» y «propuestos » se suponen construidas a partir de dos variables: la superficie y la estructura. Los resultados muestran que cuando los contextos de los problemas son familiares para los estudiantes, las analogías superficiales pueden ocultar las diferencias estructurales. Sin embargo, cuando los contextos de los problemas son no familiares para los estudiantes, el único efecto significativo procede de la igualdad/diferencia estructural. El efecto global de la familiaridad de los problemas es una reducción sistemática del éxito en la transferencia en problemas no familiares, sea cual sea la relación entre problema «ejemplo » y problema «propuesto».Two exploratory studies were conducted in order to analyze the level of success in problem-solving transfer processes. Analogies between a «source» problem and different «target» problems were established through their Surface and Structural features. Results showed that when the context of the problems is familiar for students then Surface similarities can hide Structural differences. However, when the context of the problems is not familiar for students, the only significant effect came from the equal/different Structures. «Familiarity» with the context of the problems caused a global and systematic effect: non-familiar problems achieved lower success than familiar problems, no matter the relationship between source and target problems

Mejorando la efectividad instruccional del texto educativo en ciencias : primeros resultados

In this paper we compile and analyze the textual variables taking part in comprehension and learning processes on scientific texts. Next we propose a new classification which can be applicated to instructional text. This proposal is evaluated in an experiment with different groups of students and severa1 learning tasks. The first results are discussed

La necesidad de un cambio metodológico en la enseñanza del equilibrio químico : limitaciones del principio de Le Chatelier

This paper studies the didactic approach used in teaching Le Chatelier's Principle, showing that its usual formulation is ambiguous, and reports the way Chemistry textbooks use this principle in some specific cases with no mention of its limitations. The second part of this work analyses the strategies carried out by chemistry teachers when dealing with a specific equilibrium problem. It is shown how the usual methodology practised in both cases may lead to very important misconceptions, so a new methodological performance is proposed to achieve a real significative learning

Preguntas de los estudiantes de Educación Secundaria ante dispositivos experimentales

Las preguntas forman parte esencial del trabajo que desarrollan los científicos para construir nuevo conocimiento. Sin embargo, los alumnos formulan un número muy escaso de preguntas en las situaciones usuales de aula. En este trabajo se realiza una revisión de las investigaciones realizadas sobre génesis de preguntas y se describen tres estudios empíricos realizados con estudiantes de diferentes niveles educativos ante dispositivos experimentales. Los objetivos fueron: 1) Estimular y analizar las preguntas de los estudiantes cuando intentan comprender los dispositivos; 2) Contrastar la idea de que la mayoría de preguntas destinadas a obtener información tienen su origen en inferencias fallidas; 3) Estudiar la influencia del nivel de conocimientos sobre las preguntas formuladas. El procedimiento produjo una cantidad aceptable de preguntas para obtener información, motivadas por conciencia de incomprensión de los estudiantes. Los resultados obtenidos apoyan la taxonomía de preguntas propuestas y muestran que los estudiantes de mayor conocimiento formulan más preguntas destinadas a construir un modelo científico del funcionamiento de los dispositivos. Los resultados se replican de forma consistente.Questions are essential in the construction of new knowledge by scientists. Nevertheless, students ask very few questions in usual classroom situations. In this paper we review the research work on question generation and describe three empirical studies conducted with students of different educational levels facing experimental devices. The aims were: 1) To stimulate and to analyse the questions asked by students when trying to understand the devices; 2) To probe the idea that most of the information seeking questions are originated by failed inferences; 3) To study the influence of the level of students' knowledge on the questions asked. The procedure produced a significant quantity of information seeking questions, caused by the students' awareness of misunderstanding. The results obtained support the taxonomy of questions proposed and show that higher knowledge students ask significantly more questions addressed to the scientific model construction than lower knowledge students. The results are replicated consistently

El Conocimiento Didáctico del Contenido en ciencias: estado de la cuestión

This paper gives a descriptive overview of the literature related to Pedagogical Content Knowledge - PCK - in the sciences. It is expected that this review can contribute to a better understanding of PCK, pointing out what has been investigated about this concept. Specifically, we analyze: a) how PCK is defined, what are its main features and how it has been appropriated by teachers; b) the relationship between PCK, knowledge of the contents to be taught and students learning; c) how PCK was actually used in teachers' training and teachers' evaluation; and, d) the scientific areas in which PCK has been studied. It concludes that PCK is an essential tool for improving the quality of teacher training