A Historical/Philosophical A Historical/Philosophical Foundation for Teaching Foundation for Teaching Chemical EquilibriumChemical Equilibrium

Juan Quílez. IES Benicalap. 46015 Valencia. Spain j.quilez@terra.es

IHPST Calgary. June 24 -28, 2007

Why the History of Why the History of Chemistry?Chemistry?

Explanatory clues-Evolution of Chemical Concepts

’how we know’

Understanding Alternative Conceptions Nature of Chemistry

Issues of Instruction:a) general chemical problems

b) effective learning sequences

Science Textbooks’ References Science Textbooks’ References to the History of Scienceto the History of Science

A) Passing reference to the history of science.A) Passing reference to the history of science.

B) Historical distortions and mistakes.B) Historical distortions and mistakes.

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

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

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

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

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

ProposalsProposals

Causes

Teachers’ background

Implementation in the Classroom

Proposals

Elaboration of available HPS works

CHEMICAL EQUILIBRIUM

The Introduction of The Introduction of Chemical Equilibrium in the Chemical Equilibrium in the

ClassroomClassroomPrerequisites

• Mass and concentration.Mass and concentration.• Lack of mathematical Lack of mathematical

tooks and reasoning.tooks and reasoning.

Previous Knowledge

• Equilibrium = equal Equilibrium = equal opposing forcesopposing forces

• Equilibrium: Equilibrium: [[reactants]=[products]reactants]=[products]

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

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

Key Chemical Equilibrium Key Chemical Equilibrium IdeasIdeas

• Incomplete ReactionIncomplete Reaction

•Reversibility Reversibility

•Equilibrium ConstantEquilibrium Constant

•DynamicsDynamics

Chemical Equilibrium MisconceptionsChemical Equilibrium Misconceptions: : Causes and Possible RemediesCauses and Possible Remedies

a)a) Complete/reversible reactionsComplete/reversible reactions;;b)b) forward/backward reaction; forward/backward reaction; c)c) kinetic misunderstandingskinetic misunderstandings;;d)d) dynamic nature of chemical equilibrium;dynamic nature of chemical equilibrium;e)e) compartmentalised view of equilibrium;compartmentalised view of equilibrium;f)f) equilibrium as oscillating like a pendulum equilibrium as oscillating like a pendulum

Causes

Analogies &Pictorial

representations

Methodology‘rethoric of conclusions

Algorithmic procedures/

neglecting conceptual reasoning

Language

Teachers’misconceptions

Remedies

Development of novel curricula

Based on appropriate uses of the

history of chemical equilibrium

Historical Development of Chemical Historical Development of Chemical EquilibriumEquilibrium

The growth of chemical equilibrium from the first ideas of The growth of chemical equilibrium from the first ideas of ’chemical affinity’’chemical affinity’

• theoretical frames theoretical frames

• acceptation/rejection ideasacceptation/rejection ideas

• problems facedproblems faced

• controversial issuescontroversial issues

• knowledge that countedknowledge that counted

• different meanings different meanings

• evolutionevolution

1. The proposal for quantifying this property of chemicals

2. The different early kinetic molecular explanations given to equilibrium reactions.

3. The searching for finding the factors it depended on

4. The early thermodynamic foundation of chemical equilibrium

focused on

Theoretical grounds:

1) Incomplete reaction

2) Revesibility

3) Dynamics

Historical Historical reconstructionreconstructionI) Elective affinities

(forces)Complete reactions/

One direction

II) Berthollet CaCOCaCO33 + 2 NaCl → CaCl + 2 NaCl → CaCl22 + Na + Na22COCO33

Affinities/mass of reactantsStatic-equilibrium (forces)

Incomplete reaction - Reversibility

III) Guldberg and WaageConcentration of reactants/products

Static equilibrium (forces)(p-x)(p-x)aa(q-x)(q-x)bb = = ’(p’+x)’(p’+x)aa’(q’+x)’(q’+x)bb’’

Equilibrium Constant

IV) PfaundlerKinetic theoryKinetic theory

Dynamical equilibrium

V) Van’t Hoff Rates

Equilibrium constantKinetics

Thermodynamics

Implementation of the HPS Implementation of the HPS Approach when Teaching Approach when Teaching Chemical EquilibriumChemical EquilibriumI.I. Students may develop a better Students may develop a better

understanding of the understanding of the nature of nature of chemistrychemistry

II.II. Historical sequence as a basis for the Historical sequence as a basis for the teaching the teaching the constructionconstruction of the main of the main chemical equilibrium conceptschemical equilibrium concepts

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

Implementation of the HPS Implementation of the HPS Approach when Teaching Chemical Approach when Teaching Chemical

EquilibriumEquilibrium

a)a) The importance of The importance of classificationclassification in chemistry-affinity tables. in chemistry-affinity tables. b)b) Scientists’ reluctance to Scientists’ reluctance to abandon a theoryabandon a theory on the basis of on the basis of

anomalous reactions alone-Bergman’s affinity. anomalous reactions alone-Bergman’s affinity. c)c) Rival theoriesRival theories coexist many years-Bergman’s/Berthollet’s coexist many years-Bergman’s/Berthollet’s

theory. theory. d)d) LanguageLanguage-different meanings of affinity. -different meanings of affinity. e)e) The construction of chemistry may be viewed as a The construction of chemistry may be viewed as a humanhuman

endeavourendeavour-the outcome of chemical equilibrium theory was -the outcome of chemical equilibrium theory was due to the contribution of several individuals, most of them due to the contribution of several individuals, most of them contemporaries of each other.contemporaries of each other.

f)f) ScientificScientific argumentationargumentation-to consider the evidence for and -to consider the evidence for and against each model.against each model.

g)g) ControversialControversial issuesissues-Berthollet’s theory/Proust’s law and -Berthollet’s theory/Proust’s law and Dalton’s theory.Dalton’s theory.

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

Implementation of the HPS Approach Implementation of the HPS Approach when Teaching Chemical Equilibriumwhen Teaching Chemical Equilibrium

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

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

FeFe3+3+(aq) + SCN(aq) + SCN--(aq) (aq) FeSCN FeSCN2+2+(aq)(aq) and and reversibilityreversibility: Dichromate : Dichromate Chromate Chromatec) Explaining chemical equilibrium reactions (Why is it that the reaction c) Explaining chemical equilibrium reactions (Why is it that the reaction

’stops’ when there are still reactants in the vessel of reaction?)’stops’ when there are still reactants in the vessel of reaction?)DynamismDynamism as an explanatory concept. as an explanatory concept.

d) d) Empirical derivationEmpirical derivation of the equilibrium constant. of the equilibrium constant. In advanced levels, a rigorous deduction of the equilibrium In advanced levels, a rigorous deduction of the equilibrium constant equation is based on thermodynamic grounds.constant equation is based on thermodynamic grounds.

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

Implementation of the HPS Approach Implementation of the HPS Approach when Teaching Chemical Equilibriumwhen Teaching Chemical Equilibrium

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

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

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

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

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

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

BibliographyBibliography• Quílez, J.: 2002, ’Aproximación a los Orígenes del Concepto Quílez, J.: 2002, ’Aproximación a los Orígenes del Concepto

de Equilibrio Químico: Algunas Implicaciones Didácticas’, de Equilibrio Químico: Algunas Implicaciones Didácticas’, Educación QuímicaEducación Química 1313, 101-112., 101-112.

• Quílez, J.: 2004, ’A Historical Approach to the Development Quílez, J.: 2004, ’A Historical Approach to the Development of Chemical Equilibrium Through the Evolution of the of Chemical Equilibrium Through the Evolution of the Affinity concept: Some Educational Suggestions’, Affinity concept: Some Educational Suggestions’, Chemistry Chemistry Education: Research and PracticeEducation: Research and Practice 55, 69-87., 69-87.

• Quílez, J.: 2006, ’The Role of Theories in Early Studies of Quílez, J.: 2006, ’The Role of Theories in Early Studies of Chemical Equilibria’, Chemical Equilibria’, Bulletin for the History of ChemistryBulletin for the History of Chemistry 3131, 45-57., 45-57.

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