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A Historical/Philosophical Foundation for Teaching Chemical Equilibrium Juan Quílez. IES Benicalap . 46015 Valencia. Spain j.quilez@terra.es IHPST Calgary. June 24 -28, 2007 Why the History of Chemistry? Understanding Alternative Conceptions Explanatory clues-Evolution

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A Historical/Philosophical Foundation for Teaching Chemical Equilibrium

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A Historical/Philosophical Foundation for Teaching Chemical Equilibrium

Juan Quílez. IESBenicalap. 46015 Valencia. Spain j.quilez@terra.es

IHPST Calgary. June 24 -28, 2007


Why the History of Chemistry?

Understanding Alternative Conceptions

Explanatory clues-Evolution

of Chemical Concepts

’how we know’

Nature of Chemistry

Issues of Instruction:

a) general chemical problems

b) effective learning sequences


Science Textbooks’ References to the History of Science

A) Passing reference to the history of science.

B) Historical distortions and mistakes.

  • The past is usually interpreted in terms of present ideas and values.

  • They normally make reference to ’great scientists’ that worked in isolation using an inductive method of discovery.

  • They usually present the last products of science, outlined as the final outcome of a cumulative process.

  • Their authors de-emphasise science as a way of thinking.


Inadequate Way in Which HPS is Dealt with in Textbooks: Causes and Proposals

Proposals

Teachers’ background

Causes

Elaboration of available HPS works

Implementation in the Classroom

CHEMICAL EQUILIBRIUM


Prerequisites

Mass and concentration.

Lack of mathematical tooks and reasoning.

Previous Knowledge

Equilibrium = equal opposing forces

Equilibrium: [reactants]=[products]

Newton’s third law-based reasoning applies to chemical equilibrium shifts. Linear causal reasoning

Chemical reactions always proceed to completion, taking place in one direction only.

The Introduction of Chemical Equilibrium in the Classroom


Key Chemical Equilibrium Ideas

  • Incomplete Reaction

  • Reversibility

  • Equilibrium Constant

  • Dynamics


Chemical Equilibrium Misconceptions: Causes and Possible Remedies

  • Complete/reversible reactions;

  • forward/backward reaction;

  • kinetic misunderstandings;

  • dynamic nature of chemical equilibrium;

  • compartmentalised view of equilibrium;

  • equilibrium as oscillating like a pendulum

Remedies

Causes

Algorithmic procedures/

neglecting conceptual

reasoning

Analogies &

Pictorial

representations

Development of novel curricula

Language

Methodology

‘rethoric of conclusions

Based on appropriate

uses of thehistory of chemical equilibrium

Teachers’misconceptions


theoretical frames

acceptation/rejection ideas

problems faced

controversial issues

knowledge that counted

different meanings

evolution

The proposal for quantifying this property of chemicals

The different early kinetic molecular explanations given to equilibrium reactions.

The searching for finding the factors it depended on

The early thermodynamic foundation of chemical equilibrium

Historical Development of Chemical EquilibriumThe growth of chemical equilibrium from the first ideas of ’chemical affinity’

focused on

Theoretical grounds:

1) Incomplete reaction

2) Revesibility

3) Dynamics


Historical reconstruction

I) Elective affinities

(forces)

Complete reactions/

One direction

III) Guldberg and Waage

Concentration of reactants/products

Static equilibrium (forces)

(p-x)a(q-x)b = ’(p’+x)a’(q’+x)b’

Equilibrium Constant

II) Berthollet

CaCO3 + 2 NaCl → CaCl2 + Na2CO3

Affinities/mass of reactants

Static-equilibrium (forces)

Incomplete reaction - Reversibility

V) Van’t Hoff

Rates

Equilibrium constantKinetics

Thermodynamics

IV) Pfaundler

Kinetic theory

Dynamical equilibrium


Implementation of the HPS Approach when Teaching Chemical Equilibrium

  • Students may develop a better understanding of the nature of chemistry

  • Historical sequence as a basis for the teaching the construction of the main chemical equilibrium concepts

  • Teachers’ understanding of students’ ideas and their resistence to change: teaching implications


Implementation of the HPS Approach when Teaching Chemical Equilibrium

I) Students may develop a better understanding of the nature of chemistry:

  • The importance of classification in chemistry-affinity tables.

  • Scientists’ reluctance to abandon a theory on the basis of anomalous reactions alone-Bergman’s affinity.

  • Rival theories coexist many years-Bergman’s/Berthollet’s theory.

  • Language-different meanings of affinity.

  • The construction of chemistry may be viewed as a humanendeavour-the outcome of chemical equilibrium theory was due to the contribution of several individuals, most of them contemporaries of each other.

  • Scientificargumentation-to consider the evidence for and against each model.

  • Controversialissues-Berthollet’s theory/Proust’s law and Dalton’s theory.


Implementation of the HPS Approach when Teaching Chemical Equilibrium

II) Historical sequence as a basis for the teaching the construction of the main chemical equilibrium concepts

a) The kinetic introduction of chemical equilibrium is questionable from an epistemological point of view (i.e. students are exposed to the answers before having given the question).

b) Challenging student previous ideas about chemical reactions in order to develop the concepts of incompleteness:

Fe3+(aq) + SCN-(aq)  FeSCN2+(aq)

and reversibility: Dichromate  Chromate

c) Explaining chemical equilibrium reactions (Why is it that the reaction ’stops’ when there are still reactants in the vessel of reaction?)

Dynamism as an explanatory concept.

d) Empirical derivation of the equilibrium constant.

In advanced levels, a rigorous deduction of the equilibrium constant equation is based on thermodynamic grounds.


Implementation of the HPS Approach when Teaching Chemical Equilibrium

III) Teachers’ understanding of students’ ideas and their resistence to change: teaching implications

  • Some of the misunderstandings that student hold when dealing with chemical equilibrium parallel some of the ideas of 19th century scientists.

  • Teachers may use the history of chemical equilibrium to encourage students’ conceptual change. It means that students may be helped by the teacher in order to challenge early models of chemical reaction and, ultimately, their own conceptions.

  • The historical dimension can be used to achieve determined affective objectives because the episodes of resistence to conceptual change in science clearly show that even well-known scientist can be mistaken. Thus, it is comforting to perceive that others have thought in a similar manner-that to hold such thoughts is not to be guilty or mere stupidity.

  • The laboratory can be used to replicate some 19th century chemical equilibrium experiments.

  • The historical dimension may add fundamental insights when planning alternative teaching approaches to questioned/controversial educational issues (e.g. Le Chatelier’s principle may be reshaped on the grounds of the equilibrium law).


Bibliography

  • Quílez, J.: 2002, ’Aproximación a los Orígenes del Concepto de Equilibrio Químico: Algunas Implicaciones Didácticas’, Educación Química13, 101-112.

  • Quílez, J.: 2004, ’A Historical Approach to the Development of Chemical Equilibrium Through the Evolution of the Affinity concept: Some Educational Suggestions’, Chemistry Education: Research and Practice5, 69-87.

  • Quílez, J.: 2006, ’The Role of Theories in Early Studies of Chemical Equilibria’, Bulletin for the History of Chemistry31, 45-57.

  • Quílez, J.: 2006, ’From Chemical Forces to Chemical Rates: a Historical/Philosophical Foundation for the Teaching of Chemical Equilibrium’, Science & Education (on line)


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