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?
Explanatory clues-Evolution
of Chemical Concepts
’how we know’
Understanding
Alternative Conceptions
Issues of Instruction:
a) general chemical problems
b) effective learning sequences
Nature of Chemistry
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
Teachers’
background
Causes
Proposals
Elaboration of
available HPS works
Implementation
in the Classroom
CHEMICAL EQUILIBRIUM
The Introduction of Chemical
Equilibrium in the Classroom
Prerequisites
Previous Knowledge
• Mass and concentration.
• Lack of mathematical tooks
• Equilibrium = equal opposing
and reasoning.
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.
Key Chemical Equilibrium Ideas
• Incomplete Reaction
• Reversibility
• Equilibrium Constant
• Dynamics
a)
b)
c)
d)
e)
f)
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
Analogies &
Pictorial
representations
Methodology
Algorithmic
procedures/
neglecting conceptual
reasoning
Development
of novel curricula
Language
‘rethoric of conclusions
Teachers’
misconceptions
Based on appropriate
uses of the
history of chemical equilibrium
Historical Development of Chemical Equilibrium
The growth of chemical equilibrium from the first ideas of
’chemical affinity’
•
•
•
•
•
•
•
theoretical frames
acceptation/rejection ideas
problems faced
controversial issues
focused on
knowledge that counted
different meanings
evolution
1. The proposal for
2.
3.
4.
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
Theoretical grounds:
1) Incomplete reaction
2) Revesibility
3) Dynamics
Historical reconstruction
I) Elective affinities
(forces)
Complete reactions/
One direction
II) Berthollet
CaCO3 + 2 NaCl → CaCl2 + Na2CO3
Affinities/mass of reactants
Static-equilibrium (forces)
Incomplete reaction - Reversibility
IV) Pfaundler
Kinetic theory
Dynamical equilibrium
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
V) Van’t Hoff
Rates
Equilibrium constant
Kinetics
Thermodynamics
Implementation of the HPS Approach
when Teaching Chemical Equilibrium
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
III. 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:
a)
b)
c)
d)
e)
f)
g)
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 human
endeavour-the outcome of chemical equilibrium theory was due to
the contribution of several individuals, most of them contemporaries of
each other.
Scientific argumentation-to consider the evidence for and against
each model.
Controversial issues-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
Química 13, 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
Practice 5, 69-87.
• Quílez, J.: 2006, ’The Role of Theories in Early Studies of Chemical
Equilibria’, Bulletin for the History of Chemistry 31, 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)