L7 Atomic Spectra

L7 Atomic Spectra - CHEM 6BL Experiment 6: Atomic Spectra...

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Unformatted text preview: CHEM 6BL Experiment 6: Atomic Spectra Instructor: Dr. Sandrine Berniolles CHEM 6BL Course Schedule Date Lecture Lab & Period Exam Schedule Lab Schedule Prelab & Report Due Dates May M 2727-28 June 1 -3 ----------- 2nd lab E d E t 5/St t l b End Expt. 5/Start Expt. 6 (Parts I-III) I-III) Final Exam June 3 1st lab End Expt. 6 (Parts IIII-V) E6 Prelab + E5 P l b InIn-Lab Report E6 In-Lab InReport June 3 -4 ----------- 2nd lab CHECK-OUT CHECK- Last Regrades 1 Experiment #6: Atomic Spectra Spectroscopy Hugely important branch of chemistry Analysis of the light emitted or absorbed by substances Atomic Spectroscopy Spectroscopy applied to atoms 2 Experiment #6: Atomic Spectra Qualitative Data of absorption or emission can be of used to identify the element Data ntensity of absorption or emission can be used to determine the amount of element 3 Quantitative Page 1 CHEM 6BL Experiment 6: Atomic Spectra Instructor: Dr. Sandrine Berniolles The Nature of Light Light is electromagnetic radiation Light has a constant velocity c = 2.998 x 108 m/s 4 Electromagnetic Radiation Wavelength (): peak to peak distance ( Amplitude: strength of the oscillation 5 Electromagnetic Radiation Frequency (): number of oscillations per second ( Units: Herz (1 Hz = 1 s-1) 6 c = = c/ = c/ c/ c/ Page 2 CHEM 6BL Experiment 6: Atomic Spectra Instructor: Dr. Sandrine Berniolles Radiation Energy Each "unit" of light is known as a photon radiation carries energy: gy -19) = h = h c (J/photon 10 (J/photon Electromagnetic g Ephoton h = Planck's constant = 6.626 10-34 Js To get E per mole: Emole = Ephoton NA 7 Electromagnetic Radiation Spectrum 8 Electromagnetic Radiation Spectrum E/photon Radiation Frequency Type (1014 Hz) (10-9m=nm) (10-19 J) x- & -rays 103 3 103 Ultraviolet 8.6 350 5.7 Visible Violet 7.1 420 4.7 Green 5.7 530 3.8 Red 4.3 700 2.8 Infrared 3.0 1000 2.0 Microwaves 10-3 3 106 10-3 & Radio 9 Page 3 CHEM 6BL Experiment 6: Atomic Spectra Instructor: Dr. Sandrine Berniolles Experiment #6: Atomic Spectra When atoms are excited by heating or in an electric discharge, they emit light at discrete wavelengths At Atomic i emission spectrum: pattern of i i t tt f electromagnetic radiation emitted by excited atom element has its own, unique spectrum. Useful for chemical analysis 10 Each Atomic Emission Spectrum When an electron makes a transition from a higher energy level to a lower energy level ,the difference in the energy is emitted out as a photon . Each transition produces a photon of a certain energy and this appears as a line of certain wavelength on the spectrum. These spectrum. lines constitute the Emission spectrum. spectrum. p E = h = Ei - Ef h 11 Emission Lines 12 Page 4 CHEM 6BL Experiment 6: Atomic Spectra Instructor: Dr. Sandrine Berniolles Spectroscope 13 Atomic Emission Spectrum 14 Experiment #6: Atomic Spectra I. Spectroscope calibration Use mercury vapor lamp Obtain corrected from scale reading II. Examine one e- system: Hydrogen eExamine two e- system: Helium eExamine multi e- system: eAlkali & Alkaline Earth Metals 15 III. IV. V. Analyze an unknown mixture (2 metals) Page 5 CHEM 6BL Experiment 6: Atomic Spectra Instructor: Dr. Sandrine Berniolles Part II. Hydrogen Emission Spectrum 6 5 4 Energy 3 2 1 Ultra Violet Lyman Series Visible Balmer Series Infrared Paschen Series n IN LAB- Use H discharge tube 16 Hydrogen Emission Spectrum: Emission frequencies related to quantized transitions by the following equation: = RH 1 1 nf2 ni2 RH : Rydberg constant = 3.29 1015 Hz nf : "relaxed" electronic energy level ni : excited electronic energy level 17 Hydrogen Emission Spectrum: E E=h E = (RH h) 1 1 nf2 ni2 Ionization Energy of Hydrogen 18 (RH h) = 21.8 10-19 J/photon (RH h NA) = 1312.75 kJ/mole Page 6 CHEM 6BL Experiment 6: Atomic Spectra Instructor: Dr. Sandrine Berniolles Hydrogen Emission Spectrum: = c c 1/ = (RH / ) 1/ /c) 1 nf2 1 ni2 IN LAB: LAB: Use this equation with nf = 2 to determine which energy transition each of the observed visible lines represents (trial & error-based method) error19 Example: Hydrogen Spectra - What is the frequency, wavelength and energy of a n = 2 to n = 1 transition in hydrogen? Will it be visible? y g First - Determine the frequency (): = (3.29 1015 Hz) 1 1 12 22 20 = 2.47 1015 Hz (= s-1) Example: Hydrogen Spectra - What is the frequency, wavelength and energy of a n = 2 to n = 1 transition in hydrogen? Will it be visible? Second - Determine the wavelength (): c = = c/ = (2.998 108 m/s) / (2.47 1015 s-1) = 121 10-9 m = 121 nm (= UV) 21 Page 7 CHEM 6BL Experiment 6: Atomic Spectra Instructor: Dr. Sandrine Berniolles Example: Hydrogen Spectra - 6 5 4 Ener rgy 3 2 1 Ultra Violet Lyman Series Visible Balmer Series Infrared Paschen Series n 22 Example: Hydrogen Spectra - E What is the frequency, wavelength and energy of a n = 2 to n = 1 transition in hydrogen? Will it be visible? Third - Determine the Energy (E): (E): E=h E = (6.626 10-34 Js) (2.47 1015 s-1) = 16.4 10-19 J/photon 16. E = 986 kJ/mol 23 Part III. Helium Emission Spectrum Use same method as Part II to observe the emission spectrum of excited Helium Will use a helium discharge tube Will use quasi "Rydberg" equation to attempt to calculate energy states 24 Page 8 CHEM 6BL Experiment 6: Atomic Spectra Instructor: Dr. Sandrine Berniolles Parts IV-V. Alkali/Alkaline Earth Spectra IVObserve emission spectra of Alkali and Alkaline earth metals Excitation source: Bunsen burner Salt samples (Li, Na, K, Ca, Sr, Ba) These observations will allow you to identify an unknown mixture 25 Flame Test Sodium 26 Final Comments Tilt Bunsen burner when burning metal to avoid plugging it up "Unknown" has two metals! Lab report worksheet filled out in lab FOR MORE INFO... Read the Lab Manual! 27 Page 9 ...
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