Lecture 6

Lecture 6 - CHEM 7L Experiment 6: Atomic Spectra Experiment...

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Unformatted text preview: CHEM 7L Experiment 6: Atomic Spectra 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 1 Experiment #6: Atomic Spectra Qualitative Data – of absorption or emission can be used to identify the element used to identify the element Quantitative Data – ntensity of absorption or emission can be used to determine the amount of element 2 The Nature of Light Light is electromagnetic radiation Light has a constant velocity c = 2.998 x 108 m/s 3 Page 1 CHEM 7L Experiment 6: Atomic Spectra Electromagnetic Radiation Wavelength (): peak to peak distance Amplitude: strength of the oscillation 4 Electromagnetic Electromagnetic Radiation Frequency (): number of oscillations per second Units: Units: Herz (1 Hz = 1 s-1) 5 c = = c/ = c/ Radiation Energy Each “unit” of light is known as a photon photon Electromagnetic Ephoton radiation carries energy: = h = h c (J/photon 10-19) (J/photon h = Planck’s constant = 6.626 10-34 J·s/photon To get E per mole: Emole = Ephoton NA 6 Page 2 CHEM 7L Experiment 6: Atomic Spectra Electromagnetic Radiation Spectrum 7 Electromagnetic Radiation Spectrum Radiation Frequency λ E/photon 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 8 Experiment Experiment #6: Atomic Spectra When atoms are excited by heating or in an electric discharge, they emit light at discrete wavelengths Atomic At emission spectrum: pattern of electromagnetic electromagnetic radiation emitted by excited atom Each element has its own, unique spectrum. Useful for chemical analysis 9 Page 3 CHEM 7L Experiment 6: Atomic Spectra 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. E = hυ = Ei - Ef 10 Emission Lines 11 Spectroscope 12 Page 4 CHEM 7L Experiment 6: Atomic Spectra Atomic Emission Spectrum 13 Experiment #6: Atomic Spectra I. Spectroscope calibration Use mercury vapor lamp Obtain corrected λ from scale reading II. Examine one e- system: Hydrogen e- III. Examine two e- system: Helium e- IV. Examine multi e- system: eAlkali & Alkaline Earth Metals V. Analyze an unknown mixture (2 metals) 14 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 15 Page 5 CHEM 7L Experiment 6: Atomic Spectra 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 16 Hydrogen Emission Spectrum: E E=h E = (RH h) 1–1 nf2 ni2 (RH h) = 21.8 10-19 J/photon (RH h NA) = 1312.75 kJ/mole Ionization Energy of Hydrogen 17 Hydrogen Emission Spectrum: = c 1/ = (RH /c) 1/ (R 1 nf2 – 1 ni2 IN LAB: Use this equation with nf = 2 to determine which energy transition each of the observed visible lines represents 18 Page 6 CHEM 7L Experiment 6: Atomic Spectra Example: Hydrogen Spectra - What is the frequency, wavelength and energy of a n = 2 to n = 1 transition in hydrogen? Will it be visible? First - Determine the frequency (): = (3.29 1015 Hz) 1 –1 12 22 = 2.47 1015 Hz (= s-1) 19 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) 20 Example: Hydrogen Spectra - 6 5 4 3 Energy 2 1 Ultra Violet Lyman Series Visible Balmer Series Infrared Paschen Series n 21 Page 7 CHEM 7L Experiment 6: Atomic Spectra 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 J·s) (2.47 1015 s-1) = 16.4 10-19 J/photon 16. E = 986 kJ/mol 22 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 23 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 24 Page 8 CHEM 7L Experiment 6: Atomic Spectra Flame Test Sodium 25 Final Comments Tilt Bunsen burner when burning metal to avoid plugging it up "Unknown" has two metals! two Lab report worksheet filled out in lab FOR MORE INFO... Read Read the Lab Manual! 26 Page 9 ...
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This note was uploaded on 06/01/2011 for the course CHEM 7L taught by Professor Bernoilles during the Spring '11 term at UCSD.

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