Chemistry 223Nuclear Chemistry Part 2
Radiometric Dating
2Tro: Chemistry: A Molecular
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D 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)
Radiometric Dating
3Tro: Chemistry: A Molecular
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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
Radiocarbon Dating
4Tro: Chemistry: A Molecular
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• 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
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• 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
7Tro: Chemistry: A Molecular
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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?
Radiocarbon Dating% C-14 (compared to living
organism)Object’s Age (in years)
100% 0
90% 870
80% 1850
60% 4220
50% 5730
40% 7580
25% 11,500
10% 19,000
5% 24,800
1% 38,1008
Tro: Chemistry: A Molecular Approach
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?
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units are correct, the magnitude makes sense because it is less than 2 half-lives
Check:
Solve:
Conceptual Plan:
Relationships:
ratet1/2 = 4.50 dis/min∙gC, ratet1/2 = 15.3 dis/min∙gC
time, yr
Given:
Find:
t1/2 k rate0, ratet t+
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?
10Tro: Chemistry: A Molecular
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A. 3.03 counts / min g CB. 1.209 x 104 counts / min g CC. 20.2 counts / min g CD. A rice grain? What are you nuts?
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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, rate0 = 20.0 counts/min∙gC
ratet, counts/min∙gC
Given:
Find:
t1/2 k rate0, t ratet+
Nonradioactive Nuclear Changes
Lise Meitner
12Tro: Chemistry: A Molecular
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• 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
13Tro: Chemistry: A Molecular
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Fission Chain Reaction
14Tro: Chemistry: A Molecular
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A chain reaction: when a reactant in the process is also a product of the process
• in fission 3 neutrons• only need small # of on to start chain rxtn
Many on’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
15Tro: Chemistry: A Molecular
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Fossil Nuclear Reaction in West Africa
Pitchblende Uranium Oxide ore
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• 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
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
18Tro: Chemistry: A Molecular
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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
19Tro: Chemistry: A Molecular
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Nuclear Power Plants - Core
20Tro: Chemistry: A Molecular
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• fissionable material stored in long tubes, called _______________, arranged in a matrixsubcritical
• Btwn fuel rods are ___________________ made of neutron-absorbing materialB 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
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.
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PLWR - Core
ColdWater
FuelRods
HotWaterControl
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
Tro: Chemistry: A Molecular Approach
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.
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.
Control rod storage
~800 oC water under pressure
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
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.
Concerns about Nuclear Power
29Tro: Chemistry: A Molecular
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• Core melt-down
• Waste disposal
• Transporting waste• How do we deal with nuclear power
plants that are no longer safe to operate?
Chernobyl Reactor 1986Three Mile Island28 March thru early April
of 1979
Japan in 2011In what condition are American Nuclear Reactors?
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
32Tro: Chemistry: A Molecular
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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
33Tro: Chemistry: A Molecular
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34Tro: Chemistry: A Molecular
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Calculate mass defect & nuke binding E per nucleon (in MeV) for C–16, radioactive isotope of carbon with a mass of 16.014701 amu
Solve:
Conceptual Plan:
Relationships:
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
Given:
Find:
mp+, mn0, mC-16massdefect
binding energy
35Tro: Chemistry: A Molecular
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Calculate the binding E per nucleon in Fe–56(mass 55.93494 amu)
37Tro: Chemistry: A Molecular
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Calculate the binding E per nucleon in Fe–56(mass 55.93494 amu)
38Tro: Chemistry: A Molecular
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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
39Tro: Chemistry: A Molecular
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• 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
40Tro: Chemistry: A Molecular
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Fusion
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Tokamak Fusion Reactor
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Making New Elements:Artificial Transmutation
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High E particles can be smashed into target nuclei, resulting in production of new nuclei
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• 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
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Artificial Transmutation
Tro: Chemistry: A Molecular Approach
• Bombardment of one nucleus with another causing new atoms to be madecan also bombard with neutrons
• Reaction done in a particle accelerator
Tc-97 is made by bombarding Mo-96 with deuterium, releasing a neutron
Cyclotron
source
target
49Tro: Chemistry: A Molecular
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Practice – Predict the other daughter nuclide and write a nuclear equation for each of the following
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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