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.
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)
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
lattice-work 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 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. DH
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.
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