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Chemistry 223 Nuclear Chemistry Part 2

Chemistry 223 Nuclear Chemistry Part 2. Radiometric Dating. in amount of radioactivity of a radionuclide is predictable & not affected by envrnmntal factors

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Chemistry 223 Nuclear Chemistry Part 2

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  1. Chemistry 223 Nuclear Chemistry Part 2

  2. Radiometric Dating • in amount of radioactivity of a radionuclide is predictable & not affected by envrnmntal factors • By measuring & comparing ratio of parent radioactive isotope to its stable daughter we can determine age of the object • (by using t½ & previous equations) Tro: Chemistry: A Molecular Approach

  3. Radiometric Dating • Mineral (geological) dating • compare amount of U-238 to Pb-206 in volcanic rocks and meteorites • Pb-206 from decay has unique “signature” • dates Earth: btwn 4.0 & 4.5 billion yrs. old • Can also compare amount of K-40 to Ar-40 Tro: Chemistry: A Molecular Approach

  4. Radiocarbon Dating • All things alive (or once alive) contain carbon • 3 isotopes of C exist in nature: C-12, C-13, and C–14, which is radioactive • C–14 radioactive half-life = _________ yrs • relatively short half-life - should have disappeared long ago Tro: Chemistry: A Molecular Approach

  5. Tro: Chemistry: A Molecular Approach

  6. While still living, C–14/C–12 is constant because organism replenishes C • CO2 in air source of all C in organism • Death: C–14/C–12 ratio decreases Tro: Chemistry: A Molecular Approach

  7. Measure: • C–14/C–12 ratio in (once living) artifact & compare to C–14/C–12 ratio in living organism date artifact • Limit ~ 50,000 years old. Why? Tro: Chemistry: A Molecular Approach

  8. Radiocarbon Dating Tro: Chemistry: A Molecular Approach

  9. t1/2 k + rate0, ratet t An ancient skull gives 4.50 dis/min∙g C. If a living organism gives 15.3 dis/min∙g C, how old is the skull? Given: Find: ratet1/2 = 4.50 dis/min∙gC, ratet1/2 = 15.3 dis/min∙gC time, yr Conceptual Plan: Relationships: Solve: Check: units are correct, the magnitude makes sense because it is less than 2 half-lives Tro: Chemistry: A Molecular Approach

  10. Clicker question: Archeologists have dated a civilization to 15,600 yrs ago. If a living sample gives 20.0 counts / min g C, what would be the # of counts per min g C for a rice grain found at the site? 3.03 counts / min g C 1.209 x 104counts / min g C 20.2 counts / min g C A rice grain? What are you nuts? Tro: Chemistry: A Molecular Approach

  11. t1/2 k + rate0, t ratet Given: Find: t = 15,600 yr, rate0 = 20.0 counts/min∙gC ratet, counts/min∙gC Conceptual Plan: Relationships: Solve: Check: units are correct, the magnitude makes sense because it is between 10 and 25% of the original activity Tro: Chemistry: A Molecular Approach

  12. Nonradioactive Nuclear Changes • A few unstable split iif hit just right by a neutron  two smaller nuclei = ____________ • Small nuclei accelerated until they overcome their charge repulsion & smash together  larger nucleus = _____________ • Both fission and fusion release enormous amounts of energy Lise Meitner Tro: Chemistry: A Molecular Approach

  13. Tro: Chemistry: A Molecular Approach

  14. Fission Chain Reaction • A chain reaction: when a reactant in the process is also a product of the process • in fission  3neutrons • only need small # of on to start chain rxtn • Many on’sproduced in fission: • ejected from U before hitting another U-235 • or absorbed by surrounding U-238 • Minimum amount of fissionable isotope needed to sustain chain rxtn = critical mass Tro: Chemistry: A Molecular Approach

  15. Tro: Chemistry: A Molecular Approach

  16. Fossil Nuclear Reaction in West Africa Pitchblende Uranium Oxide ore

  17. Fissionable isotopes: • U–235, Pu–239, &Pu–240 • Natural uranium is < 1% U–235 • rest mostly • not enough _______ to sustain chain reaction • To produce fissionable uranium, natural uranium must be enriched in U–235 • to ~ ___% for “weapons grade” • to ~ ___% for reactor grade Tro: Chemistry: A Molecular Approach

  18. Nuclear Power • Nuclear reactors use fission to generate electricity • about ____% of U.S. electricity • uses fission of U–235 to produce heat • Heat boils water, turning it to steam Tro: Chemistry: A Molecular Approach

  19. Nuclear Power Plants vs. Coal-Burning Power Plants • Use about ____ kg of fuel to generate enough electricity for 1 million people • No air pollution • Use ~ __________ kg of fuel to generate enough electricity for 1 million people • Produce NO2&SOx that add to acid rain • Produce CO2 that adds to the greenhouse effect Tro: Chemistry: A Molecular Approach

  20. Nuclear Power Plants - Core • fissionable material stored in long tubes, called _______________, arranged in a matrix • subcritical • Btwnfuel rods are ___________________ made of neutron-absorbing material • B and/or Cd • neutrons needed to sustain chain reaction • rods are placed in a material to slow down ejected neutrons = a _________________ • allows chain rxtnto occur below critical mass Tro: Chemistry: A Molecular Approach

  21. Nuclear Reactors If the neutron flow in a reactor is carefully regulated so that only enough heat is released to boil water, then the resulting steam can be used to produce electricity.

  22. Tro: Chemistry: A Molecular Approach

  23. Control Rods control rods made of n absorbing material. Allows rate of n flow thru reactor to be controlled. Because n’s are required to continue chain rxtn, control rods control the rate of nuclear fission PLWR - Core Hot Water Fuel Rods Cold Water Tro: Chemistry: A Molecular Approach

  24. Nuclear Reactors • Light-water reactors: • Used to produce electricity • Fuel rods containing fissile isotope in stabilized form • (uranium oxide pellets encased in a corrosion- • resistant zirconium alloy) • suspended in a cooling bath - transfers heat • generated by fission rxtn to a 2dary cooling system.

  25. Nuclear Reactors • Light-water reactors: • Heat generates steam for production of electricity. • Control rods absorb n’s & control rate of • nuclear chain rxtn. • Pulling control rods out increases n flow, • allowing reactor to generate more heat. • inserting rods completely stops the rxtn.

  26. ~800 oC water under pressure Control rod storage

  27. Nuclear Reactors • Heavy-water reactors: • Deuterium (2H) absorbs n’s less effectively than (1H), • but is ~ 2x as effective at scattering neutrons. • A nuclear reactor using D2O instead of H2O • as the moderator is so efficient it can use un-enriched uranium as fuel, • which reduces operating costs & eliminates need for plants that produce enriched uranium

  28. Nuclear Reactors • Breeder reactors: • A nuclear fission reactor that produces • more fissionable fuel than it consumes. • Fuel produced is not the same as the fuel consumed. • Overall rxtn is conversion of • _______________________________ • which can be isolated chemically • and used to fuel a new reactor.

  29. Concerns about Nuclear Power • Core melt-down • Waste disposal • Transporting waste • How do we deal with nuclear power plants that are no longer safe to operate? Tro: Chemistry: A Molecular Approach

  30. Three Mile Island 28 March thru early April of 1979 Chernobyl Reactor 1986

  31. Japan in 2011 In what condition are American Nuclear Reactors?

  32. Where Does Energy fromFission Come from? • During nuclear fission, some of the mass of the nucleus is converted into energy • E = mc2 • Each mole of U–235 that fissions produces about _________________ J of energy • a very exothermic chemical reaction produces _______________ J / mole Tro: Chemistry: A Molecular Approach

  33. Mass Defect & Binding Energy • When nucleus forms, some mass of separate nucleons is converted into E • Diff in mass btwn separate nucleons & combined nucleus = the mass defect • E that is released when nucleus forms = the binding energy Tro: Chemistry: A Molecular Approach

  34. Tro: Chemistry: A Molecular Approach

  35. Calculate mass defect & nuke binding E per nucleon (in MeV) for C–16, radioactive isotope of carbon with a mass of 16.014701 amu mp+, mn0, mC-16 mass defect binding energy Given: Find: mass C-16 = 16.01470 amu, mass p+ = 1.00783 amu, mass n0 = 1.00866 amu mass defect in amu, binding energy per nucleon in MeV Conceptual Plan: Relationships: Solve: Tro: Chemistry: A Molecular Approach

  36. Calculate the binding Eper nucleon in Fe–56(mass 55.93494 amu) Tro: Chemistry: A Molecular Approach

  37. Calculate the binding Eper nucleon in Fe–56(mass 55.93494 amu) Tro: Chemistry: A Molecular Approach

  38. Nuclear Fusion • Fusion is combining light nuclei to make heavier, more stable nuclide • Sun uses fusion of Hisotopes to make helium as a power source Tro: Chemistry: A Molecular Approach

  39. Requires high input of E to initiate process • need to overcome repulsion of posnuclei • Produces 10x energy per gram as fission • No radioactive byproducts • Unfortunately, only currently working application is the H-bomb Tro: Chemistry: A Molecular Approach

  40. Fusion Tro: Chemistry: A Molecular Approach

  41. Tokamak Fusion Reactor Tro: Chemistry: A Molecular Approach

  42. Making New Elements:Artificial Transmutation High Eparticles can be smashed into target nuclei, resulting in production of new nuclei Tro: Chemistry: A Molecular Approach

  43. Particles may be radiation from other radionuclide, or charged particles that are accelerated • Rutherford made O–17 bombarding N–14 with alpha rays from radium • Cf–244 is made by bombarding U–238 with C–12 in a particle accelerator Tro: Chemistry: A Molecular Approach

  44. Artificial Transmutation • Bombardment of one nucleus with another causing new atoms to be made • can also bombard with neutrons • Reaction done in a particle accelerator • Tc-97 is made by bombarding Mo-96 with deuterium, releasing a neutron Tro: Chemistry: A Molecular Approach

  45. Cyclotron target source Tro: Chemistry: A Molecular Approach

  46. Practice – Predict the other daughter nuclide and write a nuclear equation for each of the following bombarding Ni–60 with a proton to make Co–57 bombarding N–14 with a neutron to make C–12 bombarding Cf–250 with B–11 producing 4 neutrons Tro: Chemistry: A Molecular Approach

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