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.

 

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