EVERGREEN INS 2007 - Chemistry Lab II: “The emission spectrum of atomic hydrogen”

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Chemistry Lab II: “The emission spectrum of atomic hydrogen”You have learned about the Balmer series of the emission speNow if we plot a graph of 1/( versus 1/(n2)2 we should get a straight line with a negative slope and a positive intercept provided that we assign the correct sequence of integer vaExperiment 2: Emission spectra of various gasesIntroduction to Natural Science (2006/07) Winter 2007 Quarter Chemistry Lab II: “The emission spectrum of atomic hydrogen” Prepared by: Dr. Dharshi BopegederaExperiment 1: Recording and analyzing the Balmer series of the hydrogen spectrum using a “home made” spectrometer Do this lab in pairs. You will be given instructions on how to build and use the “home made” spectrometer. You have learned about the Balmer series of the emission spectrum of atomic hydrogen in class. Balmer series is in the visible region of the electromagnetic spectrum, hence easy to observe with the naked eye. While it is possible to record the emission spectrum of atomic hydrogen using a state of the art spectrometer, it is easy and more economical to use the “home made” spectrometer. Besides, you get to build and see the “guts” of the spectrometer! The hydrogen discharge tube is a quartz tube filled with H2 gas. Two electrodes (an anode and a cathode) are connected to the ends of the tube. When supplied with an electric charge, there is sufficient energy to break down the bond in the H2 molecules and to generate excited state H atoms. H2 → 2 H* H* = excited state H atoms These excited state H atoms emit energy, relaxing down to lower energy states. This excess energy is emitted in the form of light. In this experiment, we will be monitoring the visible light emitted by the excited H atoms. We can also monitor UV and infrared light emitted by the excited H atoms provided we have the appropriate detectors. Visible light is easy to monitor since we can use our eyes as the detector. The function of the diffraction grating is to disperse this emitted light into its respective wavelengths. A grating with approximately 600 grooves/mm is sufficient for this experiment. Gratings with a higher number of grooves/mm provide better resolution, but are more expensive. If we look at the hydrogen discharge tube through the grating, we will see several colored lines; these are the emitted visible light dispersed into different wavelengths by the diffraction grating. Pre-Lab: 1. Quantum theory predicts that the Rydberg constant (R) 3204h c 8me Rε= Equation 1 where m = mass of an electron, e = charge of an electron, c = speed of light, h = Planck’s constant, ε0 is the permitivity of a vacuum. Obtain these constant values from standard tables and calculate the value of R using Equation 1. Show all work. Obtain the value of R in cm-1 units. Carry the final answer to 6-8 significant figures. Lab Work: 1. Draw a block diagram of the “home made” spectrometer. Label all components.2. What kind of a grating did you use in this lab (how many grooves per millimeter)? 3. Record the emission spectrum of mercury in the visible region. Record the positions of the mercury lines in millimeters (on the meter ruler) to the highest number of significant figures possible. Record as many Hg lines as possible since this will improve the accuracy of your data. Mercury emission lines in the visible region are at the following wavelengths (nm) 404.6, 435.8, 546.0, 576.9, 579.1, and 695.9 4. Record the emission spectrum of hydrogen in the visible region. Record the positions of the hydrogen lines in millimeters (on the meter ruler) to the highest number of significant figures possible. You must be able to see at least 4 lines (red, blue-green, blue-violet and violet). The violet line may not be easy to observe but it will help your data analysis immensely if you spend time to record this line. Calculations: (Do the calculations on your own. You are welcome to get help) 2. Draw a calibration curve (use Microsoft Excel only) using the mercury data. Draw a smooth line of best fit (this may be a curve rather than a straight line) through the points. See sample calibration curve below. Wavelength (nm) Meter ruler reading (cm) 3. Use your calibration curve to determine the wavelengths of the hydrogen lines. 4. Now you have the wavelengths (λ values) of the hydrogen lines that belong to the Balmer series. You are trying to fit this data to the equation: ⎥⎦⎤⎢⎣⎡= n1 - n1 R 12221λ where n2 > n1 Equation 2 where λ is the wavelength, R is the Rydberg constant and n1, n2 are principal quantum numbers of the levels involved in the electronic transition. By fitting your data to this equation, you will be able to calculate the value of the Rydberg constant (R) and the correct assignment of the n2values. Recall that for the Balmer series n1=2. The n2 values for the different Balmer series lines are therefore greater than 2 (since n2 > n1). You can rearrange equation Equation 1 to obtain; nR nR - 12122+=λ Equation 3 Now if we plot a graph of 1/λ versus 1/(n2)2 we should get a straight line with a negative slope and a positive intercept provided that we assign the correct sequence of integer values for n2. 5. To find the correct sequence of integer values for n2, make three plots (using Microsoft Excel) for the following three trial sets of n2 values. • n2 = 3, 4, 5, 6 • n2 = 4, 5, 6, 7 • n2 = 5, 6, 7, 8 Think carefully about which hydrogen spectral line should be assigned the lowest n2 value in the above three sequences. 6. Be sure to have all three plots on the same graph so you can compare them easily. Do not place the graph on the worksheet. Instead use the “chart” feature in Excel. Your three plots should be labeled appropriately so that the reader can easily understand what you have done. Your worksheet should have your name and labeled columns so that the reader can easily follow your work. 7. When you are satisfied that you have a good graph, print it and the corresponding worksheet and attach to your lab notebook. Label your Excel graph and worksheet with your name. 8. Once you determine the correct sequence of integer values for n2, you can use that sequence for the rest of the data analysis. 9. Using Microsoft Excel, draw a graph of the correct data (i. e. a graph of 1/λ versus 1/(n2)2 where the n2 values are now the correct values as


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EVERGREEN INS 2007 - Chemistry Lab II: “The emission spectrum of atomic hydrogen”

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