Lesson Review
| Author: | Thomas Knight |
| ETP Title: | Infrared Spectroscopy - an application for learing about Chemical Bonding |
| Organization: | Lawrence Berkeley National Laboratories |
| ETP Type: | |
| Grade Level(s): | 10,11,12 |
Subject Areas
- Science
California Standards
ChemistryChemical Bonds
2. Biological, chemical, and physical properties of matter result from the ability of atoms to form bonds from electrostatic forces between electrons and protons and between atoms and molecules. As a basis for understanding this concept:
f.* Students know how to predict the shape of simple molecules and their polarity from Lewis dot structures.
Investigation and Experimentation
1. Scientific progress is made by asking meaningful questions and conducting careful investigations. As a basis for understanding this concept and addressing the content in the other four strands, students should develop their own questions and perform investigations. Students will:
a. Select and use appropriate tools and technology (such as computer-linked probes, spreadsheets, and graphing calculators) to perform tests, collect data, analyze relationships, and display data.
d. Formulate explanations by using logic and evidence.
l. Analyze situations and solve problems that require combining and applying concepts from more than one area of science.
Lesson Abstract:
This ETP will teach students about chemical bonding through a real world application of Infrared Spectroscopy and trace forensic evidence. Students will learn about how to correlate infrared absorbance peaks to specific types of chemical bonds and how that can be used to identify molecular structure of unknowns. The final assessment will be a mock trial in which group members will serve as the expert witnesses for prosecution and defense, one of the lawyers.
The needs this ETP will fulfill in the classroom, teaching or school:
The connection between the ETP and Fellowship. :
Outline:
Title : Infrared Spectroscopy - an application for learning about Chemical Bonding
Company:
Subject area: Science
Grade Level: 9 – 12 (better for full year chemistry class)
Need:
This lesson will provide a tie between a high interest form of applied science to essential chemistry concepts. Students often simply learn molecular structure by rote. Too often in this era of CST scores, lessons focus on the answers to test questions rather than how science is actually done This ETP offers an experiemental connection to covalent bonding, structure and polarity and how it is actually measured. It trains students how to think graphically, an essential skill in scientific literacy.
Connection to Fellowship
This lesson directly involves the FTIR infrared spectrometers that I am working on this summer and stems directly from discussions from a team of visiting forensic scientists that I met while working at the same lab. In my fellowship, I needed to learn how FTIR works, what it tells us about molecular structure, and how different parts of the IR spectrum of a given molecule relate to specific bonds within it. I learned how to use the software that both runs the spectrometers and analyzes the resulting spectra. The measurement of the IR absorbance bands for Carbon Dioxide and Water are in every spectrum because they are very IR active and in the air inside the spectrometer. We used these molecules to calibrate the new instrument that I helped commission in my fellowship and to prove out its ability to function at extremely high resolution.
Prerequisites:
Students should be able to draw Lewis dot structures for CO2 and H2O and simple organic compounds.
Students should know the basic 3D structures and average bond angles and how this determines the dipole moment of the molecule.
Students should be familiar with the Electromagnetic Spectrum.
Students should be familiar with the concept of translational, rotational and vibrational kinetic energy.
Objectives:
Students will understand the difference between IR active and non-active vibrational modes of H2O and CO2.
Students will be able to identify peak absorbances for CO2 and H2O and relate them to the vibrational modes.
Students will be able to correlate peak absorbances in sample spectra to known absorbances from a list.
Students will be able to use spectral data to form and support an argument for trace evidence in a crime scene scenario
Outline:
Day 1 – Pre assessment and Introduction to FTIR spectroscopy
Day 2 - Lab Activity on Matching Bond types to spectra of simple molecules
Day 3 - Lab Activity on trace forensic evidence, post - assessment
Day 1
Warm-up questions:
1. Draw Lewis Dot structure of H2O, CO2 and CH4
2. Draw 3 D structure including Bond Angles.
3. Predict the polarity of the molecule and draw the dipole if present
Pre – assessment:
Hand – out the pre-assessment. Be sure that you make clear that there is more than one answer in several of the questions that is correct. After students complete this, review the correct answers with the entire class. For the questions that are review, this should not take much re-teaching, but if it is necessary, take time to do so before starting Activity #1. For questions that are pre-views, give the correct answer, explain that this is new information and then refer back the question when you teach the new material in the rest of the unit.
Activity #1 – Kinesthetic Model of Vibrational Modes of Molecules
Choose 3 volunteers from the class; assign one to be carbon and the other two to be oxygen. Using a pre-printed paper with “C” and “O” to identify the element each on represents, have them arrange themselves in a structure that represents CO2. Review what type of bond is formed between each one, who is more electronegative and the bond angle. Then, ask the group to represent how the molecule could be moving within itself. Try to get them to discover 3 different vibrational modes. Then, lead them through how to show each – the symmetric stretch, the asymmetric stretch and the bending modes.
Focus question: Which one of these 3 vibrational modes does not change the polarity of the molecule? Answer – the symmetric stretch. Since there is no change in the symmetry and the O atoms are equally electronegative, the polarity does not change.
Arrange the rest of the class into groups of 3 as well. This time, let them assign themselves to the atoms in H2O. Ask them to work together to find as many vibrational modes of water and decide whether or not they change in polarity. Have the students draw each vibrational mode that they discover and group them by non-change and polarity changing vibrations. There are 2 stretching modes – Symmetric and Assymetric, and 4 bending modes – rocking, scissoring, wagging and twisting.
There are some very good animations of these vibrations available on the internet:
http://www.lsbu.ac.uk/water/vibrat.html (just on water)
http://www.chem.purdue.edu/gchelp/vibs/ (for several small molecules)
Activity #2
Review of vibrational modes, infrared light and how they interact to create a spectrum.
Please refer to the PowerPoint file in the resources section
"The Introductionto Infrared Spectroscopy PowerPoint” is a modified version of the one my mentor at Lawrence Berkeley National Laboratory, Dr. Michael Martin, uses to teach visting interns about the use of spectroscopy.
Day 2
Warm-up questions:
1. What are the characteristic vibrational modes of CO2?
2. Which energy absorptions correspond to these modes?
3. Where in the spectrum are you likely to find bonds that include H and why?
Activity #3
Assign each group 1 molecule. Have them draw the Lewis Dot structure and the 3 D structure including bond angles and dipoles. Then, using the Characteristic IR Band positions, have the students draw a graph showing the transmission spectra that they would predict. Here are some simple molecules to start with. See resources for their actual FTIR spectra. You may extend this to more difficult molecules if you like. You can find spectral data at http://webbook.nist.gov
1. Carbon Dioxide
2. Water
3. Ozone
4. Nitrous Oxide
5. Carbon Monoxide
6. Methane
7. Sulfur Dioxide
8. Ammonia
9. Nitric Oxide
10. Acetylene
11. Hydrogen Cyanide
Day 3
Warm-up questions
1. What types of trace evidence might you use FTIR for?
2. Why is FTIR a good technique for forensic trace evidence?
3. How would you use FTIR spectra to convince a jury that two samples were identical compounds?
Activity #4
Storyline: Your students will be acting as the crime lab. A former science teacher has been caught with several unknown white powders on his person and in a make-shift lab in his garage. It is the students’ job to determine the identity of these substances and to decide if a crime has been committed.
Assign each group an unknown substance from the crime scene. Provide the physical description of the unknown and the FTIR spectra. Have the students identify the characteristic peaks in the sample and then search the library (either using OMNIC software or on the internet – http://webbook.nist.gov should work well) to identify the identity of the unknown sample. Once each group has identified the sample, have them prepare their testimony for the mock – trial.
Post assessment:
Have students prepare to be subjected to questions from both the prosecuting attorney and cross-examination by the defendant’s attorney on the trace evidence involved in the case. (a presentation of the data to convince a jury of the guilt or innocence of suspect based on the spectral analysis of the unknown sample they are given.) Each student will have to prepare a written report showing answers to the questions, presentation of their spectral data from the Activity 4 worksheet, and any additional supporting documents they chose to include. Finally, hold a mock trial that assigns students to be expert witnesses, lawyers, jury and the teacher as the judge.
Here is a list of possible questions:
1. What is FTIR spectroscopy?
2. What does it tell us?
3. What does the machine actually do?
4. What is a covalent bond?
5. Why do these bonds absorb infrared radiation?
6. How do different molecules absorb different energy of light?
7. In the sample that was tested from the crime scene, what were the most important pieces of data?
8. How do these correlate to a known sample?
9. Which library did you use?
10. How unique is this spectrum compared to other unknowns. How sure are you of the identity of the unknown sample from the crime scene?
The assigned lawyers can develop their own questions.
Resources:
1. Intro to Spectroscopy PowerPoint (ppt) This is a modified version of a presentation that my mentor, Dr. Mike Martin uses when teaching new interns at Lawrence Berkeley National Laboratory
2. FTIR background primer - William Reusch of
3. Activity #2 – short PowerPoint on IR spectroscopy
4. Activity #3 – Student worksheet
5. Activity #3 - Characteristic IR Band Positions
6. Activity #3 – Sample Spectra of 10 compounds for Activity #3
7. Activity #4 – Student Worksheet
8. Activity #4 - Sample Spectra for unknown trace evidence
9. Pre-assessment
10. Post – assessment
11. Post - assessment rubric
Citations:
Webbook http://webbook.nist.gov author: The National Institute of Standards and Technology (NIST) © 2008 by the U.S. Secretary of Commerce on behalf of the United States of America. All rights reserved. Last modified: Thursday, August 06, 2009 3:52:09 PM
Water Stucture and Science, http://www.lsbu.ac.uk/water/vibrat.html, author: Martin Chaplin: martin.chaplin@lsbu.ac.uk last modified: Tuesday, July 28, 2009 8:14:10 AM
Vibration Modes of Small Molecules http://www.chem.purdue.edu/gchelp/vibs/ author: Purdue University Department of Chemistry, last modified:
Introduction to IR spectroscopy –excerpts from “CSEE IRI 2005: Introduction to Spectroscopy” by Dr. Michael Martin, Advanced Light Source Division, Lawrence Berkeley National Laboratory, MCMartin@lbl.gov
Characteristic IR Band Positions, http://infrared.als.lbl.gov/content/web-links/60-ir-band-positions,
Keywords:
chemical structure, infrared, spectroscopy, forensicsObjectives of the ETP:
Types of assessment or evaluation that will accompany this ETP:
Bibliographic or other resources your used in creating this curriculum:
Supply List:
Attachments:
