29 Jun 2015

Vibrational Modes and IR Spectra for Intro Chem

In-Class Activity

Submitted by Karen McFarlane Holman, Willamette University

In this activity, Introductory Chemistry students are given two possible structures of sulfur dioxide, and based on an assessment of given vibrational modes, they determine which of the modes are IR active (and thus, whether the molecule is a greenhouse gas).  They compare their assessment to experimental IR absorption peaks, and the students decide which structure is valid.

Learning Goals: 

A student should be able to…

• interpret vibrational modes from a minimal 2-D drawing and convert into a 3-D movement with their bodies.

• determine the dipole moment (relative direction and magnitude) for each extreme of the vibrational mode.

• assess whether the dipole moment is changing during a particular vibrational mode.

• compare theoretical results from their vibrational mode analysis to experimental IR spectra and deduce the correct structure.

Implementation Notes: 

I have used this activity in three different ways:  In-Class Activity, as a problem in a homework assignment, and (originally) as an exam question.  All three work well, but I now use it as an In-Class Activity because I appreciate that it gives immediate feedback regarding student understanding while I’m teaching this topic.

Time Required: 
Depending on how much time I have spent talking with them about H2O and CO2, this activity takes them about 10-20 minutes in class.
Evaluation Methods: 

When I use this LO as an In-Class Activity, I am walking around the room and engaging with the students to see them “dance”, discuss their results with partners and other groups, and come to their conclusions.  When it has been used as a problem in homework or in an exam, it was graded.

Evaluation Results: 

When I used it as a problem set as opposed to an In-Class Activity, the students generally did well (75-90% getting the problem correct).  However, I now only use this activity during class because I have found that it’s more effective to assess their understanding through active discussion because so many of the questions are yes/no.

Creative Commons License: 
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