A Historical/Philosophical Foundation for Teaching Chemical Equilibrium Juan Quílez. IES Benicalap . 46015 Valencia. Spain [email protected] IHPST Calgary. June 24 -28, 2007 Why the History of Chemistry? Understanding Alternative Conceptions Explanatory clues-Evolution
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Juan Quílez. IESBenicalap. 46015 Valencia. Spain [email protected]
IHPST Calgary. June 24 -28, 2007
Understanding Alternative Conceptions
of Chemical Concepts
’how we know’
Nature of Chemistry
Issues of Instruction:
a) general chemical problems
b) effective learning sequences
A) Passing reference to the history of science.
B) Historical distortions and mistakes.
Elaboration of available HPS works
Implementation in the Classroom
Prerequisites Causes and Proposals
Mass and concentration.
Lack of mathematical tooks and reasoning.
Equilibrium = equal opposing forces
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
Development of novel curricula
‘rethoric of conclusions
Based on appropriate
uses of thehistory of chemical equilibrium
theoretical frames Causes and Proposals
knowledge that counted
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 equilibriumHistorical Development of Chemical EquilibriumThe growth of chemical equilibrium from the first ideas of ’chemical affinity’
1) Incomplete reaction
I) Elective affinities
III) Guldberg and Waage
Concentration of reactants/products
Static equilibrium (forces)
(p-x)a(q-x)b = ’(p’+x)a’(q’+x)b’
CaCO3 + 2 NaCl → CaCl2 + Na2CO3
Affinities/mass of reactants
Incomplete reaction - Reversibility
V) Van’t Hoff
I) Students may develop a better understanding of the nature of chemistry:
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.
III) Teachers’ understanding of students’ ideas and their resistence to change: teaching implications