WHAT is the evidence? WHY did I choose this
evidence? HOW does the evidence show
growth?
I chose
to show my growth in my understanding of the rotational energy of a
molecules and how the vibrational and rotational energy of a molecule
are related to the fine detail of IR spectroscopy by presenting various
pieces of evidence taken from MCE Chem507 coursework. Growth is
shown by contrasting my baseline evidence with my later evidence.
My baseline evidence is my incorrect answer to a quiz question (which
exposed my inaccurate understanding of the rigid rotor model of
rotational energy) and my correct, but rudimentary understanding of
rotational spectra on an in-class POGIL (ChemActivity 21). My
later evidence is my revised answer to the quiz question in my baseline
and the notes I created and put on the board during the March 7th class
before our quiz (on which I scored a 37/40 = 92.5%) to help myself and
my classmates further grasp how energy transitions associated with
rotation were related to spectra.
I
chose to document my growth in the areas of spectroscopy mentioned
above because I feel that I now have a much better grasp of the
underlying energetic phenomena that produce spectra. Before
taking Dr. PhilIips and Ms. Carroll's course, I had never really
grasped the significance and relevance of the selection rules and
formulas for vibrational and rotational energy levels.
Furthermore, before this class, I would not have been capable of
explaining or teaching others how these formulas and rules were related
to the spectra. In contrast, I now feel that I grasp, and
in fact, am fairly confident of my understanding and my ability to
explain how the two are related--this shows a significant growth in
content understanding.
Please see below each piece of evidence for a more detailed discussion
of related content. You may click on
any piece of evidence for a larger image.
EVIDENCE #1
Baseline
Evidence (click on pic for larger
image)
MCE Chem507:
Molecular Spec Quiz #5, Question 3 (Feb
7, 2009)
|
Later
Evidence (click on pic for larger
image)
Revised answer
to baseline MCE quiz question (June
2009)
|
In this piece of baseline evidence, my answer to this question on a
quiz exhibits a fundamental error in my understanding of the rotational
motion corresponding to a molecule's rotational energy. Probably
as a result of confusing rotational energy levels associated with
quantum mechanics with rotations related to conformational changes in
organic chemistry, I had thought that rotation was the rotation around
a bond, rather than rotation around the center of mass--I therefore
chose I2 as having the
greatest rotational energy since the other molecules would not rotate
along their bonds (because it would require breaking pi
bonds). I did not choose Xe or He because atoms do not have
rotational energy since there is not energy change when they rotate on
their center of mass--the nucleus (that much I knew!)
I had worked with the rotational energy formulas successfully in the
spectroscopy class, but I never really questioned how the newly learned
information was incompatible with my previously and incorrectly learned
ideas of molecular rotation. This misconception was resistant to
correction until brought to my attention by my poor performance on this
quiz question.
|
In this later evidence--my revised answers to the baseline quiz
question, I show a coherent understanding of the molecular rotation
that fits (and does not conflict with) the quantum mechanical formulas
for a molecule's rotational energy levels. The rotational energy
of a molecule corresponds to its rotation around the molecule's center
of mass.
As shown in the revised answer, as molecular mass and bond length
decrease, the rotational inertia decreases and the rotational energy of
a molecule increases. The correct answer choice would be N2
because it has the lowest molecular mass and a triple bond that
shortens its bondlength.
|
EVIDENCE #2
Baseline Evidence (click on pic for larger
image)
Answers
to ChemActivity21: Rotational and Vibrational Spectra of Molecules,
Model 2 (Feb 7, 2009)
|
Later Evidence (click on pic for larger
image)
Notes I created and put on board before a
quiz to help fellow classmates understand rotovibrational spectra (March 7, 2009)
|
In this baseline evidence--my answers to a
ChemActivity that introduced us to the concept of rotational energy
transitions, I have an understanding of the magnitude of rotational
energy transitions, but I have not yet connected that awareness to the
actual rotovibrational spectral lines seen in a high resolution IR
spectra.
While not included here, we completed Model 4 of this activity in which
we ranked rotovibrational transitions from low energy to high
energy. The completion of that exercise gave me a qualitative
understanding of why certain transitions were higher or lower energy,
but my understanding was still too shallow to be able to represent the
magnitude of these transitions symbolically in mathematics that would
more precisely define the differences in magnitude in a way that would
be meaningful and more broadly applicable.
|
In this later evidence--my own notes on the fine
rotovibrational spectra, I show my growth in understanding by my
ability to synthesize, correlate, and come up with my own way of
connecting and organizing information related to rotovibrational energy
transitions and the associated spectral lines.
Since the rotational energy of a molecule is given by:
the magnitude of an energy transition for adjacent
rotational energy levels is given in terms of Be.
For example,
- Energy (J=0 --> J=1) = 2 Be
- Energy (J=1 --> J=2) = 4 Be
- Energy (J=2 --> J=3) = 6 Be
- Energy (J=3 -->
J=4) = 8 Be
When vibrational/ rotational transition (v=0 --> v= 1, and change in
J is +1 or - 1) occur simultaneously, it is possible to determine
the energy in terms of Be
and the energy of the fundamental vibrational transition (from v=0
--> v=1), which we can represent by the letter V. It then
becomes clear what transitions are most and least energetic if we put
the magnitude of the energy transitions in terms of V and Be
:
- Energy (v=0 --> v=1; J=4 -->
J=3) = V - 8 Be
(lowest energy transition)
- Energy (v=0 --> v=1; J=3 -->
J=2) = V - 6 Be
- Energy (v=0 --> v=1; J=2 -->
J=1) = V - 4 Be
- Energy (v=0 --> v=1; J=1 -->
J=0) = V - 2 Be
- Energy (v=0 --> v=1; J=0 -->
J=1) = V + 2 Be
- Energy (v=0 --> v=1; J=1 -->
J=2) = V + 4 Be
- Energy (v=0 --> v=1; J=2 -->
J=3) = V + 6 Be
- Energy (v=0 --> v=1; J=3 -->
J=4) = V + 8 Be
(highest energy transition)
|
| Updated
June 28, 2009 |
Return to top
|
|
Back
to 1:
A
B
C
