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.
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.
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.