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Chemistry 223 Nuclear Chemistry Part 2. Radiometric Dating 2 Tro: Chemistry: A Molecular Approach in amount of radioactivity of a radionuclide is predictable.

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Presentation on theme: "Chemistry 223 Nuclear Chemistry Part 2. Radiometric Dating 2 Tro: Chemistry: A Molecular Approach in amount of radioactivity of a radionuclide is predictable."— Presentation transcript:

1 Chemistry 223 Nuclear Chemistry Part 2

2 Radiometric Dating 2 Tro: Chemistry: A Molecular Approach 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 t ½ & previous equations)

3 Radiometric Dating 3 Tro: Chemistry: A Molecular Approach 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

4 Radiocarbon Dating 4 Tro: Chemistry: A Molecular Approach 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

5 Tro: Chemistry: A Molecular Approach 5

6 6 While still living, C–14/C–12 is constant because organism replenishes C CO 2 in air source of all C in organism Death: C–14/C–12 ratio decreases

7 7 Tro: Chemistry: A Molecular Approach 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?

8 Radiocarbon Dating % C-14 (compared to living organism) Objects Age (in years) 100%0 90%870 80% % % % %11,500 10%19,000 5%24,800 1%38,100 8 Tro: Chemistry: A Molecular Approach

9 An ancient skull gives 4.50 dis/ming C. If a living organism gives 15.3 dis/ming C, how old is the skull? Tro: Chemistry: A Molecular Approach 9 units are correct, the magnitude makes sense because it is less than 2 half-lives Check: Solve: Conceptual Plan: Relationships: rate t1/2 = 4.50 dis/mingC, rate t1/2 = 15.3 dis/mingC time, yr Given: Find: t 1/2 krate 0, rate t t +

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? 10 Tro: Chemistry: A Molecular Approach A.3.03 counts / min g C B x 10 4 counts / min g C C.20.2 counts / min g C D.A rice grain? What are you nuts?

11 Tro: Chemistry: A Molecular Approach 11 units are correct, the magnitude makes sense because it is between 10 and 25% of the original activity Check: Solve: Conceptual Plan: Relationships: t = 15,600 yr, rate 0 = 20.0 counts/mingC rate t, counts/mingC Given: Find: t 1/2 krate 0, trate t +

12 Nonradioactive Nuclear Changes Lise Meitner 12 Tro: Chemistry: A Molecular Approach 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

13 13 Tro: Chemistry: A Molecular Approach

14 Fission Chain Reaction 14 Tro: Chemistry: A Molecular Approach A chain reaction: when a reactant in the process is also a product of the process in fission 3 neutrons only need small # of o n to start chain rxtn Many o n s produced 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

15 15 Tro: Chemistry: A Molecular Approach

16 Fossil Nuclear Reaction in West Africa Pitchblende Uranium Oxide ore

17 17 Tro: Chemistry: A Molecular Approach Fis sionable 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

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 18 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 NO 2 & SO x that add to acid rain Produce CO 2 that adds to the greenhouse effect 19 Tro: Chemistry: A Molecular Approach

20 Nuclear Power Plants - Core 20 Tro: Chemistry: A Molecular Approach fissionable material stored in long tubes, called _______________, arranged in a matrix subcritical Btwn fuel 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 rxtn to occur below critical mass

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 22 Tro: Chemistry: A Molecular Approach

23 23 PLWR - Core Cold Water Fuel Rods Hot Water Control Rods control rods made of n absorbing material. Allows rate of n flow thru reactor to be controlled. Because ns are required to continue chain rxtn, control rods control the rate of nuclear fission 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 2 dary cooling system.

25 Nuclear Reactors Light-water reactors: Heat generates steam for production of electricity. Control rods absorb ns & 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 Control rod storage ~800 o C water under pressure

27 Nuclear Reactors Heavy-water reactors: Deuterium ( 2 H) absorbs ns less effectively than ( 1 H), but is ~ 2x as effective at scattering neutrons. A nuclear reactor using D 2 O instead of H 2 O 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 29 Tro: Chemistry: A Molecular Approach Core melt-down Waste disposal Transporting waste How do we deal with nuclear power plants that are no longer safe to operate?

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

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

32 Where Does Energy from Fission Come from? During nuclear fission, some of the mass of the nucleus is converted into energy –E = mc 2 Each mole of U–235 that fissions produces about _________________ J of energy –a very exothermic chemical reaction produces _______________ J / mole 32 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 33 Tro: Chemistry: A Molecular Approach

34 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 amu Solve: Conceptual Plan: Relationships: mass C-16 = amu, mass p + = amu, mass n 0 = amu mass defect in amu, binding energy per nucleon in MeV Given: Find: m p+, m n0, m C-16 mass defect binding energy 35 Tro: Chemistry: A Molecular Approach

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37 Calculate the binding E per nucleon in Fe–56 (mass amu) 37 Tro: Chemistry: A Molecular Approach

38 Calculate the binding E per nucleon in Fe–56 (mass amu) 38 Tro: Chemistry: A Molecular Approach

39 Nuclear Fusion Fusion is combining light nuclei to make heavier, more stable nuclide Sun uses fusion of H isotopes to make helium as a power source 39 Tro: Chemistry: A Molecular Approach

40 Requires high input of E to initiate process need to overcome repulsion of pos nuclei Produces 10x energy per gram as fission No radioactive byproducts Unfortunately, only currently working application is the H-bomb 40 Tro: Chemistry: A Molecular Approach

41 Fusion 41 Tro: Chemistry: A Molecular Approach

42 Tokamak Fusion Reactor 42 Tro: Chemistry: A Molecular Approach

43 Making New Elements: Artificial Transmutation Tro: Chemistry: A Molecular Approach 43 High E particles can be smashed into target nuclei, resulting in production of new nuclei

44 Tro: Chemistry: A Molecular Approach 44 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

45 45 Artificial Transmutation Tro: Chemistry: A Molecular Approach 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

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49 Cyclotron source target 49 Tro: Chemistry: A Molecular Approach

50 Practice – Predict the other daughter nuclide and write a nuclear equation for each of the following Tro: Chemistry: A Molecular Approach 50 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


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