PS3_2011Ans - Name Ch 1b Solution Set for Problem Set Three...

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Unformatted text preview: Name Ch 1b, Solution Set for Problem Set Three Section Side 1 of 13 Due Friday, Jan. 21, 2011 Rec TA Rec TA 1. Rotational Spectroscopy, VSEPR & Permanent Dipole Moments (9 points) If a molecule has no permanent dipole moment, then (to first order) it will not exhibit a rotational absorption spectrum. In problem #1 of PS2, you evaluated two diatomic molecules and one linear, polyatomic molecule. Now consider a non-linear, polyatomic molecule that has one central atom and multiple terminal atoms that are all the same type. If that molecule has the proper symmetry, then it has no permanent dipole moment (and no rotational absorption spectrum). For each of the molecules below, use VSEPR to identify the molecular geometry and use that geometry to determine whether or not the molecule has a permanent dipole moment : (a) NH 3 (b) CH 4 (c) IF 5 Present your answers in a table with three columns (molecule; molecular geometry; dipole moment?) and one row for each molecule. You are not required to draw the structure, but you may need to if you’re having difficulties visualizing the molecule. If you draw the structure, you may also want to assign a dipole moment to each bond by drawing a vector from the negative charge to the positive charge (as done in OGN/OGC) or from the positive charge to the negative charge (as done in the lecture notes). Just be consistent. You can then find the total moment by taking a vector sum of the bond dipoles. Solution for Problem 1 Molecule Molecular Geometry Permanent Dipole Moment? (Yes/No) NH 3 Trigonal Pyramidal Yes CH 4 Tetrahedral No IF 5 Square Pyramidal Yes 1.5 pts each 1.5 pts each SIDE NOTE: Write the electronic structure geometry (including lone electron pairs) for each molecule. If the electronic structure geometry and molecular geometry are different, then the molecule has lone electron pairs that break symmetry and the molecule will automatically have a permanent dipole moment. Name Ch 1b, Solution Set for Problem Set Three Section Side 2 of 13 Due Friday, Jan. 21, 2011 Rec TA Rec TA 2. Vibrational Spectroscopy (8 points) You measure the infrared spectrum for ammonia, NH 3 , and find characteristic vibration frequencies at 3415 cm-1 , 3337 cm-1 , 1628 cm-1 and 950 cm-1 . You want to identify specific vibrational modes for each frequency; so you ask someone in Ch21 to draw the normal modes of ammonia for you and assign frequencies. Unfortunately, he runs out of time and all you have is the following drawing on a napkin: (a) How many vibrational degrees of freedom does NH 3 have? ( 2 points ) (b) We observe four vibrational frequencies, and all four normal vibrational modes for NH 3 are shown above. Why are there more vibrational degrees of freedom than normal modes? ( 2 points ) (c) Match each drawing with the following types of modes: symmetric stretch, asymmetric stretch, symmetric bend, asymmetric bend. ( 4 points ) Name Ch 1b, Solution Set for Problem Set Three Section Side 3 of 13 Due Friday, Jan. 21, 2011 Due Friday, Jan....
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This note was uploaded on 03/21/2011 for the course CHEMISTRY 1b taught by Professor Reisman;heath during the Winter '11 term at Caltech.

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PS3_2011Ans - Name Ch 1b Solution Set for Problem Set Three...

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