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Engaging
All Of The Students.
In an introductory science curriculum the instructor must accommodate
students with very different preparation for the course. If
the course is for those with a strong high school science background
those with limited or substandard preparation will have trouble
competing. On the other hand, if the course is geared to the
students with a deficient background the better-prepared students
may look elsewhere for an intellectual challenge. With an experiment-centered,
guided-inquiry approach it is possible to present basic, even
routine material in a way that will make it seem new to all
participants. This is illustrated with the exercise "Mass
Relationships in Chemical Reactions." Students completing
an introductory chemistry course are generally expected to be
able to calculate the mass of product produced in a chemical
raction from a given mass of starting material(s). So, for example,
a student should be able to calculate the mass of silver chloride
produced from 1.0 gram of silver nitrate and an excess of sodium
chloride. Typically the instructor will present a general approach
based on converting mass of starting material to amount (in
units of Moles) followed by application of the balanced chemical
equation and finally conversion of the amount of product in
moles to the mass of product. Many students will have previously
worked this type of problem. Although most will not have a complete
grasp of what they are doing some will remember the computational
algorithm and are able to obtain the right answer. With the
Discovery approach, this type of problem can be introduced in
a laboratory frame of reference by asking the students to consider
an experiment in which a reaction is "calibrated"
by running it repeatedly, each time using successively more
starting material. Students are asked to predict the shape of
a plot of product mass as a function of mass of starting material.
They readily predict a linear relationship; the greater the
mass of starting material the greater the mass of product. They
also acknowledge that such a graph represents a calibration
curve that could be used to predict the mass of product from
any mass of starting material. The instructor engages the students
in a discussion of how the slope of the calibration curve is
related to the nature of the reaction. The exact student response
is not crucial, the goal is simply to have them begin to consider
that differences in unit masses of products or reactants will
contribute to each reaction curve having a unique slope. Their
initial predictions on which curves from a series of reactions
proposed by the instructor will have the greatest slope is used
to focus their interest on the outcome of the upcoming experimental
work. Students are assigned the experimental task of actually
preparing calibration curves for the series of reactions discussed
during the prelaboratory session. Teams are formed and each
is assigned a unique reaction in the series.
At the
end of the lab period or at the next lecture meeting student
data are pooled and the intructor leads a discussion of the
observed trends. The goal is to have students determine all
factors that affect the slope of the calibration curve. The
instructor assists by selecting appropriate reactions to compare
at each stage. This approach breaks the discovery into a series
of manageable insights for individual students. At some point
students realize that they have developed the ability to predict
the slope of any reaction calibration curve without actually
doing the experiment. Ultimately, this knowledge is used to
develop the traditional computational algorithm for predicting
the mass of product from mass of reactant for any chemical
reaction. Although students arrive at the traditional approach
to solving routine problems they may see it in a different
light. By building to the approach from experimental data,
students have a phenomenological basis for what is too often
seen as an abstract series of computations.
One of
the most important features of this experiment is that the
laboratory frame of reference makes the routine but essential
material appear new to the well-prepared student without making
it any more complex to the remaining students. Although the
approach does not necessarily simplify the material presented
to students with a weak background in science, these students
participate on a more equitable basis. We note that as intructors
we pay more attention and respond better to student feedback
when there is no longer a core of students who show an immediate
flash of understanding (actually familiarity) for each topic.
Students seem less likely to perceive their struggles as a
sign of inferiority in the discipline when none of their classmates
have all the answers.
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