Lec9-chem

Lec9-chem - Review Nuclear Stability (the band of...

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Review Nuclear Stability (the band of stability) Unstable Region: Too many protons Unstable Region: Too many neutrons
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0 e Types of nuclear decay: Review ß-particle a high-energy electron -1 0 e Positron Same mass as an electron, but with a positive charge 1 Neutron Proton + Electron 0 + 1 - 1 Proton Neutron + Positron + 1 0 + 1
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a-particle A Helium nucleus ejected from nuclear decay 4 He 2+ 2 ?-rays High-energy photons emitted from a nuclear decay 0 ? 0 Types of nuclear decay: Review 238 U 234 Th + 4 He 2+ 92 90 2 238 U 234 Th + 4 He 2+ + 2 0 ? 92 90 2 0
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Electron capture An inner electron is captured by the nucleus changing a proton to a neutron Types of nuclear decay: Review 201 Hg + 0 e 201 Au + 0 ? 80 79 -1 0
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Preview Another type of nuclear decay: Fission (fusion will be covered later) Biological damage induced by radiation Thermodynamic Stability and The Theory of Relativity
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The same is true with nucleons… n n p p n p p n The He 2+ ion is lower in energy (has less mass) than its individual nucleons.
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The Theory of Relativity (1905) Einstein E = mc 2 Energy Mass Speed of light Energy has mass!
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Let’s say that we want to make an oxygen nucleus from individual protons and neutrons 8 0 n 1 + 8 1 H + 1 8 O 8+ 16 We can use the Theory of Relativity to calculate the energy gain in this reaction Mass of ( ) = 2.67804 x10 -23 g 8 0 n 1 + 8 1 H + 1 8 O 8+ 16 Mass of ( ) = 2.65535 x10 -23 g ? m = -2.269 x 10 -25 g
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Where did this mass go? You have always been taught that matter is neither created nor destroyed! Have you been lied to all these years? No! Energy has mass!
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8 0 n 1 + 8 1 H + 1 8 O 8+ 16 Mass of ( ) = 2.67804 x10 -23 g 8 0 n 1 + 8 1 H + 1 8 O 8+ 16 Mass of ( ) = 2.65535 x10 -23 g ? m = -2.269 x 10 -25 g
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For the formation of one mole of loses: 8 O 8+ 16 (-2.269 x 10 -25 g)(6.022 x 10 23 ) = -0.1366 g / mol We can use this mass loss to calculate the energy gained by forming an oxygen nucleus. ? E = ? mc 2 ? E = -0.0001366 kg / mol (3.00 x 10 8 m / s) 2 ? E = -1.23 x 10 13 J / mol exothermic
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Notice the large amount of energy released in the nuclear process compared to a typical chemical reaction: Nuclear ~ 10 13 J / mol versus Chemical ~ 10 3 J / mol Nuclear energies are typically reported in MeV / nucleon.
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The next example is a measure in futility intended for educational purposes only! Just like my favorite poem: The more I study, the more I know The more I know, the more I forget The more I forget, the less I know So why study?
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Let’s convert our energy into MeV / neucleon 8 O 8+ 16 ? E = -1.23 x 10 13 J / mol = -2.04 x 10 -11 J / nucleus 6.022 x 10 23 nuclei / mol There are 1.60 x 10 -13 J in one MeV ? E = - 2.04 x 10 -11 J / nucleus = -1.28 x 10 2 MeV / nucleus 1.60 x 10 -13 J / MeV
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? E = -1.28 x 10 2 MeV / nucleus Finally we can divide this number by 16 since there are 8 protons and 8 neutrons in the nucleus to end up with MeV / nucleon ? E = -1.28 x 10 2 MeV/nucleus = -7.98 MeV / nucleon 16 nucleons / nucleus C
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Nuclear binding energy curve 4 He 2 235 U 92 238 U 92 56 Fe 26 Fe is the Most Stable Binding energy: 8.79 MeV C
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This note was uploaded on 03/21/2009 for the course CHEM 6C taught by Professor Hoeger during the Spring '08 term at UCSD.

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Lec9-chem - Review Nuclear Stability (the band of...

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