http://www.eng.uc.edu/STEP/
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January/February 2009
http://www.eng.uc.edu/STEP/
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January/February 2009
Lesson By Carol Clinton, Engineering Phd Fellow
In this lesson, students use physical manipulatives (cardboard models with repositionable adhesive paper
strips to simulate outer shell electrons) to gain an understanding of how atoms combine to form molecules
based on the configurations of electrons in the outer shells, how molecules break and atoms recombine into
new substances during chemical reaction, and what it means to balance the equation representing the reaction.
There are three reactions discussed. Exothermic (liberates
energy) and endothermic (requires energy) reactions can
have practical uses, such as hand-warmers, and medicinal
cool packs. Understanding whether to expect temperature
changes during reactions is important for chemical engineers
in designing processes (can cause explosions, runaway
reactions, etc). Ionization, another type of reaction, allows
“road salt” to function in lowering the freezing point of water
and keep highways safer.
Some objectives include that students will correctly identify
the atoms and their ratios that comprise desired molecules,
construct the target molecules, rearrange the atoms to create the desired end product chemical molecules,
and create the balanced chemical equations that represent the reactions.
To catch the students’ attention use the demonstration “Hot and cold packs.” Reaction 1: Put 1 tsp. CaCl2 in
a clear plastic zip bag with 10 ml of water. Pass around class to observe (dissolves and liberates heat). Reaction 2: Put 1 tsp NaHCO3 in a clear plastic zip bag with 10 ml water. Pass around class to observe (dissolves
and gets cold). Reaction 3: Pour one solution into the other bag and reseal. Pass around class (carefully) to
observe (bubbles of CO2 form and fill the bag – be careful not to let it pop). Extension: Add phenol red indicator and expand discussion and lesson to cover pH. The CO2 will form in a weakly acidic solution.
Classroom content will include the demonstration of cardboard models to construct a water molecule and
describe dissociation into Hydronium (H+) and Hydroxide ions (OH-), with the hydronium ion leaving its electron – resulting in a positive charge, and the hydroxide ion having the extra electron and therefore a negative
charge. Show how H2 and O2 must react in a 2:1 ratio to create 2 H2O molecules, and write the balanced
equation on the board.
Explain that these cardboard “atoms” are gross oversimplifications of actual atoms…MUCH larger than actual. Actual atoms
are 3D. Electrons are always in motion in clouds (not circular
tracks). Bond positions are not in co-planar 90 degree orientation. Electrons are the same, no matter where they begin.
The students use cardboard models to combine “atoms” to form
the starting molecules for reaction 1 (CaCl2 and H2O). Fit the
cardboard shapes together and “bond” with the re-positional adhesive strips.Take apart to show dissociation, leaving the electrons from Ca on the Cl atom creating Ca+2 and 2 Cl.- Repeat
for reactions 2 and 3. Reaction 3 will require balancing.
Project STEP
In use with two secondary classes, totaling 46 students, average improvement in balancing equations was
150%. Additionally, student feedback indicated that they liked the activity, learned a lot from the lesson, were
more interested in learning about engineering, and felt more confident in their abilities to learn science and
math. The exact chemistry of the reactions includes:
1.CaCl2 + H2O -> Ca+2 + 2 Cl- + H2O (which is actually H+ + OH-; or more correctly 2H2O is H3O+ +
OH-). The heat is from breaking the bonds in CaCl2.
2.NaHCO3 + H2O -> Na+ + HCO3- . Breaking this
bond requires energy.
3.CaCl2 + NaHCO3 -> CaCO3 + 2 NaCl + H+
H+ + NaHCO3 -> CO2 + H2O + Na+. It is the CO2
gas that forms bubbles and fills the bag. If you are
also using the pH indicator, this step forms a slightly
acidic solution which will change the indicator color.
This lesson requires preparation time to create the
atom and electron models. This is a good robust basic lesson, with interesting possible extensions into
pH, polymers, or other topics.
Lesson by Fee Mtshiya, Engineering Masters Fellow
This lesson is titled Molecular Shapes: Using Valence Shell Electron Pair Repulsion (VSEPR) Theory and
Lewis Structures. The primary objective in this lesson is to learn how to use the VSEPR theory to predict the
shape of molecules, based on knowledge of Lewis Structures. What should really stand out to the students in
the lesson is how a 2-Dimensional view of molecules moves to a 3-Dimensional view using the VSEPR theory
as a tool. One of the ideas that Project STEP emphasizes in its lessons is ‘Application to Real Life’ such that the students can make a clear connection between the subject matter and materials, objects, and events in their
everyday lives. In this regard, the catch for this lesson is looking at the hormone ethene using tomatoes and
pipe cleaners. Two tomatoes, one ripe and one green, are brought in and the teacher shows them to the students. The question being asked is, ‘What is the difference between the two?’ The difference is that the ripe
tomato has more of the hormone ethylene (C2H4) which tomatoes release as they ripen. The teacher then
puts together sections of pipe cleaners rolled into circles to represent the ethene molecule. Three colors of
pipe cleaners are used. One color represents carbon, another hydrogen, and the third the electrons that connect the two elements (these are smaller circles).
After a short but thorough explanation of what the VSEPR theory is and how it is used to predict molecular
shapes based of the Lewis Structure, the teacher presents the primary molecular structures to the class. The
students are divided into groups. One draws the Lewis Structure of a particular molecule on the board and the
other uses circular pipe cleaners of different colors to represent the molecule 3-dimensionally (similar to what
the teacher did in the catch). The teacher explains the electron spacing in the molecule, how they interact, the
name of the molecular shape, the bond angles, and any other important points to remember about that particular shape. After this, a power point slide of the correct molecular shape is shown and other slides are shown
with additional examples of molecules with that molecular shape.
The Project STEP: Track II Building Stemcinnati City newsletter is published six times per year. For subscriptions,
changes of address, or submitting news items and articles,
please contact the Editor, Andrea Burrows, at 513-5561029 or [email protected]
Program funded by the National Science
Foundation Grant #DGE 0538532 and
matching funds by the University of Cincinnati