GENERAL CHEMISTRY CHEM 152
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This 30 page Class Notes was uploaded by Carmela Kilback on Wednesday September 9, 2015. The Class Notes belongs to CHEM 152 at University of Washington taught by Philip Reid in Fall. Since its upload, it has received 14 views. For similar materials see /class/192566/chem-152-university-of-washington in Chemistry at University of Washington.
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Date Created: 09/09/15
Lecture 16 Bohr Model of the Atom 0 Reading Zumdahl 123 124 0 Outline Emission spectrum of atomic hydrogen The Bohr model Extension to higher atomic number Emission Photon Emission 0 Relaxation from one energy level to another by emitting a photon 0 With AB hCk 0 If 9 440 nm AB 45 X 103919 J Emission spectrum of H Continuous spectrum Quantized spectrum 1 Any AB is I Only certain E possible E 7 AE are allowed Emission spectrum of H cont Continuous spectrum Light Bulb Detector photographic plate Electric are white light source a Prism Detector photographic plate 7 Hydrogen Lamp Slit Quantized not continuous 410 nm 434 nm 486 nm 656 nm b Emission spectrum of H cont 6 AE 3 M hc AE 7 E 2 x2 hc AE 1 M Various energy levels in the hydrogen atom We can use the emission spectrum to determine the energy levels for the hydrogen atom Balmer Model 0 Joseph Balmer 1885 first noticed that the frequency of Visible lines in the H atom spectrum could be reproduced by voc i2 n345 n 0 The above equation predicts that as n increases the frequencies become more closely spaced Rydberg Model 0 Johann Rydberg extends the Balmer model by finding more emission lines outside the Visible region of the spectrum 1H13 m l 1 7397 n2n1ln12 n1 Ry329x1061m 0 This suggests that the energy levels of the H atom are proportional to ln2 The Bohr Model 0 Niels Bohr uses the emission spectrum of hydrogen to develop a quantum model for H 0 Central idea electron circles the nucleus in only certain allowed circular orbitals 0 Bohr postulates that there is Coulombic attraction between e and nucleus However classical physics is unable to explain why an H atom doesn t simply collapse The Bohr Model cont 0 Bohr model for the H atom is capable of reproducing the energy levels given by the empirical formulas of Balmer and Rydberg Z2 Z 2 atomic number 1 for H 2 n E 2178x10 18J n integer l 2 0 Ryx h 2l78 X 103918J The Bohr Model cont E Z2 E 2178x10 18J 2 n Z 0 Energy levels get Closer together Z as n increases quot3 0 at n infinity E O The Bohr Model cont 0 We can use the Bohr model to predict What AB is for any two energy levels AEE E final initial AE 2l78x10 18 21 2178x10 181 21 n nal ninitial 2 2 quot nal ninitial AE 2178x1018J 1 L The Bohr Model cont 0 Example At What wavelength Will emission from n 4 to n l for the H atom be observed AE 2 178x1018J 1 L nal itial l 4 AE 2l78x103918Jl a 204x103918J AE 204x10181 hf gt A 974x10 8m 974mm The Bohr Model cont 0 Example What is the longest wavelength of light that Will result in removal of the equot from H l 1 nal itial 00 l AE 2 178x1018J AE 2l78x103918J0 1 2178x103918J AE 2178x103918 hf gt A 913xlO 8m 913mm Extension to Higher Z 0 The Bohr model can be extended to any single electron system must keep track of Z atomic number 2 E 2178x10 18J Z z n Z 2 atomic number n integer l 2 0 Examples He Z 2 Li2 Z 3 etc Extension to Higher Z cont 0 Example At What wavelength Will emission from n 4 to n 1 for the He atom be observed AE 2178x1018J 2 2gL nal initial 2 1 4 AE 2178x103918J41 a 816x103918J AE 816x103918 hf gt A 243x10398m 243mm AH gt AH6 Where does this go wrong 0 The Bohr model s successes are limited 0 Doesn t work for multi electron atoms 0 The electron racetrack picture is incorrect 0 That said the Bohr model was a pioneering quantized picture of atomic energy levels Lecture 1 Energy 0 Reading Zumdahl 91 0 Outline Energy Kinetic and Potential System VS Surroundings Heat Work and Energy Energy Kinetic vs Potential 0 Potential Energy PE Energy due to position or composition Equals mgh in example 1 0 Kinetic Energy KE E Energy due to motion Equals mV22 in example Energy KE PE 0 Energy is the sum of kinetic energy and potential energy 0 Energy is readily converted between these two forms 0 If the system of interest is isolated no exchange With surroundings then total energy is constant Example Mass on a Spring 0 Initial PE 12 sz 0 At X O PE 0 0 KE l2mV2l2kX2 w l h v X Un1ts of Energy i m Joule lltgn12s2 Example Init PE 10 J M 10 kg VmaX 2PEM12 l4n1s First Law of Thermodynamics First Law Energy of the Universe is Constant E q W remember this q 2 heat Transferred between two bodies of differing temperature Note q 7 Temp W 2 work Force acting over a distance F X 1 Applying the First Law Need to differentiate between the system and SUITOUII HgS System That part of the universe you are interested ir ie you define it Surroundings System q transfer w transfer Surroundings The rest of the universe Conservation of Energy Total energy is conserved P 1mm Energy gained by the system must be lost by the Initial surroun 39ngs P 1alm Energy exchange can be in the form of q W or both Final Heat Exchange Exothermic Exothermic Reaction Chemical process in Which system evolves resulting in heat transfer to the surroundings Errata 39Emwr 0 q lt 0 heat is lost Potential energy Another Example of Exothermic System Surroundings 2 mol 02 1 mol CH4 Reactants APE Energy released39ith 2 mol H20 1 mol C02 Products Heat Exchange Endothermic 391quot ELK Elam Elvira a gum Endothermic Reaction Chemical process in Which system evolves resulting in heat transfer to the system 0 q gt 0 heat is gained Potential energy Another Example of Endothermic System Surroundings I 2 mol NO I Products 1 11101 N2 1 mol 0 Reactants Energy and Sign Convention 0 If system loses energy Equot Em Efinal lt E a EfinarE initial 1AEltO initia 0 If system gains energy Efinal gt E EfinalE39 initia initial 1AEgtO Heat and Work Sign Convention 0 If system gives heat 01f system does work q lt 0 q is negative vv lt 0 w is negative 0 If system gets heat 01f work done on system q gt 0 q is positive W gt O W is positive
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