|
See the lab locations, schedules, and sign-ups here!
The laboratory for Chemistry 81 is designed to provide you with realistic opportunities to compare theory and experiment. Atomic and molecular spectroscopy presents a body of experimental evidence with which quantum mechanics has been particularly successful. In addition, spectroscopic methods find highly practical application in modern chemistry. This lab offers the opportunity to become well acquainted with some of those methods. A final, but no less important objective of the lab is to introduce you to some of the techniques used in making physical measurements. In particular, all of the measurements you do will utilize rather complex instruments. An attempt will be made throughout to describe the instruments in terms of functional building blocks so that they will be more than black boxes to you.
There will be no single laboratory text for the course. The experiments will be described in handouts prepared for each experiment. In addition, lab lectures will be given whenever necessary. As usual, you should use a bound notebook for recording all data not contained in or written on our recorded spectra. This need not be a new notebook - the back pages of an old one will do.
The nature of the reports varies with the experiment. Instructions for writing reports will be contained in the individual handouts. In general, however, final reports will not be written in the notebooks; see "Laboratory Notebook and Reports" on this page for some general instructions.
You will be responsible for a minimum of three experiments of the four which are available, together with a computer lab which will be available toward the end of term. The four experiments include one in atomic (electronic) spectroscopy, two in vibration-rotation spectroscopy (choose one), and one in magnetic resonance spectroscopy. The atomic emission experiment will be done early in the term since it does not rely on theory much more detailed than that presented in general chemistry. A vibration-rotation experiment is best done about the middle of the term. The magnetic resonance experiment is done in the seventh or eight week.
You can scroll down to read summaries of each lab (the lab heading is a link to its page), or you can jump to each lab's main page through the links in the next line.
Lab 1 - Emission Spectrum of Atomic Na
Each pair of students will record the emission spectrum of Na using a computer-controlled monochromator. The sodium atom behaves approximately like a one-electron atom, and hence its spectrum has series in it like those found in the H atom spectrum. You will assign as many lines as possible to such series, and then show that quantitative agreement with formulas derived for one-electron atoms is not obtained. The corrections which must be made can be qualitatively explained by the effects of electron repulsion. Such experiments are the source of much of our knowledge of the structure of polyelectronic atoms. You will also calculate the Rydberg constant and the ionization energy for Na.
Lab 2a - Vibration-Rotation Spectra of HCl and DCl
This lab involves recording the fundamental vibration-rotation spectrum for HCl and DCl on an FTIR spectrometer. The spectra contain an enormous quantity of information and may be analyzed to yield accurate values of the mean bond lengths for HCl and DCl and values of the force constant for the stretching of the H-Cl chemical bond. Evidence for the existence of two isotopes of Cl, for rotational energy level degeneracy, and for vibrational anharmonicity is contained in each spectrum. For most students this is the vibration-rotation experiment of choice. For those students who would like to work with a trickier spectrum and the Lambda-9 spectrometer, the acetylene spectrum below is suggested. In both experiments, sufficient experimental data are obtained to allow meaningful error analyses.
Lab 2b - Vibrational-Rotation Spectra of C2H2 and C2D2
This experiment offers most of the joys of the HCl experiment plus some additional novelty. A vibration-rotation band occurring for both HCCH and DCCD in the near IR will be recorded using the Department's Lambda-9 spectrometer. Analysis of the rotational fine structure in the two spectra will follow the same scheme used in the HCl experiment and will allow calculation of C-H and C-C bond lengths. In addition, the presence of pairs of identical particles (nuclei) in both C2H2 and C2D2 allows one to see effects of the Pauli Exclusion Principle in its most general form.
Lab 3 - NMR (Nuclear Magnetic Resonance) of a Strongly Coupled 2-Spin System
The experimental portion of this lab will begin with a study of the operation of a high resolution NMR spectrometer. After instruction in its operation, each partner will run a spectrum of a two-spin system. The spectrum cannot be analyzed exactly for the chemical shifts and coupling constant unless the results of the theoretical portion of the lab are applied. The experiment instructions will guide you through the calculation of the spectrum to be expected for known chemical shifts and coupling constants. These results can then be used to analyze your spectrum.
Lab 4 - Computer Lab
In the last few years, the development of more efficient algorithms, the increased speed of computer hardware, and the availability of sophisticated graphical software on relatively inexpensive computer workstations have made electronic structure calculations accessible to the experimental research chemist.
There are several goals associated with this computer lab: 1) to provide a brief introduction to how electronic structure calculations are performed; 2) to illustrate the types of information one can now learn easily about molecular electronic structure; 3) to confirm some of the electronic structure results presented in lecture and in the text; and 4) to show how such techniques can be used to help solve chemical problems.
Laboratory Notebook and Reports
As usual, a bound, hard-cover, laboratory notebook is to be obtained before the start of laboratory work. All experimental data, except for notes made directly on spectra which you record, are to be entered in the notebook. The usual prohibition against using any intermediate record--scraps of paper, etc.--is in effect.
Reports on the experiment are not to be written up in the notebook but rather on 8-1/2 x 11 inch paper. The style for report writing explained in Shoemaker, Garland, and Nibler (SGN) (on Kresge library reserve) will be followed unless local instructions for a particular experiment ask for another format. Typed reports are not required but a high degree of legibility is. Shoemaker, Garland and Nibler discuss a sample report on pp. 12-22. Three exceptions to their discussion will apply to your reports. They are aimed at making the report more concise:
1) The introduction need not exceed 100 words and no outline of the pertinent theory is required since it is adequately described in references to the experiment.
2) Unless your experimental method was different than that outlined in the lab handouts, the Experimental section may be omitted completely.
3) Equations derived in the lab handouts or other references need not be re-derived in the Calculations section. SGN's remaining comments do apply.
Laboratory reports will be graded primarily on the adequacy with which data have been reduced to required results. The calculations must be accurate, carried out to a proper number of significant figures, and all calculation methods clearly indicated. Properly labeled results should be collected into tables whenever this mode of presentation is feasible. Copies of original spectra (or the originals themselves) should be included.
The importance of an error analysis as an integral part of any physical measurement is properly stressed in SGN. However, in order to make the time required for calculation as short as possible, the xeroxed instructions for each experiment are selective in the degree of error analysis completeness required. One or two experiments require no formal error analysis--in this case, intelligent use of the concept of significant figures will require anywhere from a severely truncated to a full-blown error analysis. SGN is an adequate reference for all error analysis procedures.
A separate report will be due from each student two weeks following the day on which an experiment was performed. Late reports will be assessed a penalty of 1/2 point for each day over the two-week deadline. Each report will be graded on a 0 to 10 point basis, but all three together with the assigned computer project will carry the weight of approximately 15% of your final grade. Failure to complete the laboratory portion of the course can result in failing the entire course no matter what your exam performance has been.
Standards of scientific integrity require that the report be your own work. Each member of an experimental pair should perform the necessary calculations independently. You are encouraged, however, to correct numerical errors and mistaken ideas by comparison of final and intermediate results with your partner or someone else. If such errors are detected, the results should be corrected independently. Simply copying someone else's work will be considered a violation of the Honor Principle.
|
|