A Modeling Approach to Chemistry
By Brenda Royce and Larry Dukerich
Since the summer of 1999, groups of modeling teachers have worked at Arizona State University
to organize the topics students ordinarily study in high school chemistry around a series of
particle models of increasing complexity. In 2004 we began an effort to develop a Modeling
Workshop for chemistry with a design parallel to that used in the Modeling Workshop in
mechanics. In June 2005 we conducted a pilot workshop in chemistry at ASU. Chemistry
modeling workshops are held each summer at ASU and at other locations nationwide.
Our curriculum design is influenced by the CHEM-Study approach that first appeared in the early
60's. Our work makes the particle models used to describe matter and the treatment of the role of
energy in change more explicit. The three questions that guide our approach to understanding
chemistry are:
1. How do we view matter? (Answer in terms of the particle you are using to describe matter)
2. How does it behave? (Provide an explanation of the behavior using this particle model)
3. What is the role of energy in the changes we observe?
At a password-protected page on the modeling website, workshop participants can download an
archive containing a prospective outline for a 1st year chemistry course, teacher notes, labs,
activities, sample worksheets, and other supporting documents. Our treatment of the role of
energy in both physical and chemical change is sufficiently different from the piecemeal
approach found in most curricula that teachers will want to review it thoroughly before teaching
it.
We use two evaluation instruments, both included on that web page. The Matter Concept
Inventory is an assessment of student understanding of the particle nature of matter – useful for
assigning groups at the beginning of the course. The Chemical Concepts Inventory is the 22-item
research-based instrument developed by Doug Mulford and published in the Journal of Chemical
Education. We also include a six-item supplemental inventory on energy and temperature
developed by chemistry education researcher Guy Ashkenazi, Ph.D.
What follows is the story line we use to uncover chemistry. The curriculum materials were tested
for two years at our high schools before being used in the pilot workshop. We owe much to ideas
contributed by chemistry teachers Joy Shrode, Consuelo Rogers and Carmela Minaya. If you
have questions, comments or concerns, please direct them to Larry Dukerich
<Ldukerich@mac.com> and Brenda Royce <BrendaR@csufresno.edu>.
Unit 1
Simple Particle: Describing Matter
Matter is composed of featureless spheres (BB’s) that have mass and volume. These particles are
essentially the “atoms” proposed by Democritus.
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Mass is a measure of how much stuff the object is made of. Matter is conserved during all kinds
of change because we are just rearranging the spheres. Volume is a measure of how much space
the object occupies. These are properties of an object.
Density describes how much stuff for each unit volume. This is a property of a substance.
Unit 2
Simple Particle Energy and States, part 1
Our BB’s are in constant, random, thermal motion, the speed of which depends on the
temperature. The BB's interact with one another by collisions.
Matter can exist in three phases, which are characterized by the arrangement of particles. This
arrangement affects the density and compressibility of each phase.
Particles of matter are in constant motion.
Thermal energy (Eth) is related to the motion of the particles and is measured by temperature.
Energy is transferred from particle to particle via collisions.
Pressure of gases is explained in terms of the collisions of the particles with the sides of the
container. There are functional relationships between the pressure of a gas and the volume,
temperature and the number of gas particles in a container.
Unit 3
Sticky Particle: Energy and States, part 2
Our BB's also exert attractions on one another. Energy is a conserved substance-like quantity
that is stored in various accounts and transferred in various ways.
Matter can exist in three phases - these are characterized by the arrangement of the particles and
the attractive forces that bind them. We call these particles “molecules” from the Latin (little
lumps of stuff). Energy is involved whenever the state (phase, temperature, etc) of the system
changes. Attractions between particles lower the energy of the system; the more tightly bound the
particles, the lower the energy due to interactions (Ei). During phase changes, changes in
interaction energy (Ei) result in a new arrangement or orientation of the particles. Energy can be
transferred between the system and surroundings via heating (collisions of countless microscopic
particles), working (due to forces between macroscopic bodies or due to expansion or contraction
of gases) and radiating (due to emission or absorption of photons).
Unit 4
Bonded Particle: Describing Substances, Electrical Nature of Matter
The particles that make up substances can be compounded from smaller particles. The fact that
compounds have definite composition leads us to Dalton’s model of the atom. Next we find that
these smaller particles (atoms) have the property of charge and some internal structure; we use
the Thomson model of the atom to account for our observations.
Matter is composed or pure substances or mixtures of these pure substances. The molecules of
pure substances have definite composition and properties whereas the composition and properties
of mixtures are variable. Molecules of pure substances can be broken down into simpler
particles (atoms or molecules)
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The energy required to separate the atoms in a compound is greater than that required to produce
a phase change.
Two kinds of charge exist in atoms. Charge plays a role in the attractive forces that hold solids
and liquids together and binds the atoms in molecules or crystal lattices.
Molecular substances are composed of neutral molecules, whereas ionic substances are latticework structures of ions. These two kinds of substance have different physical properties.
Unit 5
Counting Bonded Particles: The Mole
From Avogadro's hypothesis we are able to count molecules by weighing macroscopic samples.
For gases at the same temperature and pressure we can deduce the following:
1. From combining volumes we can determine the ratio in which molecules react.
2. From masses of these gases we can determine the relative mass of individual molecules.
From these results it is possible to determine the molar masses of the elements; using these
masses and formulas of compounds, one can determine molar masses of compounds. The mole
was defined in terms of a readily weighable “lump” of a substance. These tools allow one to
relate “how much stuff” to “how many particles".
Unit 6
Rearranging Bonded Particles: Chemical Potential Energy
Chemical reactions involve the rearrangement of atoms in molecules to form new molecules.
This rearrangement of atoms results in a change in the chemical potential energy (Ech ) of the
system. This invariably produces changes in thermal energy (Eth ), and results in energy
transfers between system and surroundings.
Mass is conserved because the atoms in the products are the same as those found in the reactants.
This is represented symbolically as a balanced chemical equation. Because the grouping of atoms
into molecules is changed in a chemical reaction, the total number of molecules (or formula
units) in the products need not be the same as that in the reactants.
Substances store varying amounts of chemical potential energy (Ech ) due to the arrangement of
atoms. It is not possible to measure this amount of energy directly. However, rearrangement of
atoms during reaction produces changes in Eth; the resulting energy transfers (as Q) between
system and surroundings can be measured. From these one can deduce differences in the Ech of
reactants and products. Energy bar graphs are a useful tool for accounting for energy (stored and
transferred) during chemical change.
Unit 7
Relating "How Much" to "How Many" Bonded Particles: Stoichiometry I
Equations representing chemical reactions relate numbers of particles (molecules or formula
units) to weighable amounts of these particles.
Stoichiometry should not be reduced to a formulaic approach designed to “get the right answer”.
The fact that proportional relationships exist between the numbers of particles involved in a
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chemical reaction allows us to make predictions about “how much stuff” will be required or
produced. The reasoning of stoichiometry is best understood in the context of the whole reaction
process, which is organized in the BCA table. This table stresses the proportional relationships
that exist between moles of reactants and products, discouraging a formulaic approach designed
to “get the right answer”. Since we don't have “mole-meters”, conversions to or from moles are
simply about the convenience of dealing with quantities we can measure. These calculations are
secondary to the mole relationships indicated by the balanced chemical equation.
Unit 8
Relating "How Much" to "How Many" Bonded Particles: Stoichiometry II
Equations representing chemical reactions can also relate numbers of particles (molecules or
formula units) to volumes of gases, solutions and to the change in chemical potential energy.
Molar volumes of gases and molar concentrations of solutions are analogs to molar mass used in
the previous unit. They enable one to relate how much of a measured quantity to how many
particles are involved. The chemical potential energy involved in a reaction is proportional to the
number of particles involved. It may be included as a term in the balanced equation for a reaction
and treated in the same manner as reactants and products in the BCA table. H is used as our
best approximation of the change in Ech or Ei.
This ends part 1 of the course. These units, which focus on what is possible, form the core of the
Modeling Chemistry curriculum. An instructor has a choice as to which path to follow for the
remainder of the school year.
First option: We include materials that address topics/concepts found in many high school
curricula - but from a modeling perspective. These are tested materials (worksheets, labs,
activities, tests and quizzes, but they do not have fully fleshed out teacher notes sections as you
would find in part 1. Experienced chemistry teachers familiar with the Modeling approach may
be able to deduce the underlying structure well enough to find them useful.
Second option: We include three new units (currently under development with guidance from Dr.
Guy Ashkenazi) that take a different approach to concepts addressed in the 2nd half of a
chemistry course. These units pick up the story line where unit 8 left off; i.e., they focus on such
questions as:
1) how does energy "know" where to go, and
2) what factors affect whether a process is likely to occur?
There are detailed teacher notes for these units, but the materials should be considered beta
versions, since they have not been tested with students.
Unit 9n
Particle model of temperature. We know that when Ech of the system changes, the Eth also
changes, eventually resulting in a transfer of energy between the system and the surroundings.
We adopt a "kinetic" view of temperature to account for the direction of energy flow.
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Unit 10n
The probable direction of change. We adopt a "probability" view to account for the direction of
processes involving both structural and thermal change.
Unit 11n
Equilibrium. We return to a "kinetic" view to model a variety of processes as they approach and
reach the state of equilibrium.
from Modeling Instruction website ( http://modeling.asu.edu )
Participant Resources, Particle Models of Chemistry
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