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

Chemistry 223Nuclear Chemistry Part 2

Radiometric Dating

2Tro: Chemistry: A Molecular

Approach

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


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

Radiocarbon Dating


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

Tro: Chemistry: A Molecular Approach 5


Approach

• 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


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?

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?


Approach

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


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


Approach

Fission Chain Reaction


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 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


Approach

Fossil Nuclear Reaction in West Africa

Pitchblende Uranium Oxide ore


Approach

• 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


Approach

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


Approach

Nuclear Power Plants - Core


Approach

• 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.


Approach

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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


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


Approach

• 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


Approach

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


Approach


Approach

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


Approach

Calculate the binding E per nucleon in Fe–56(mass 55.93494 amu)


Approach

Calculate the binding E per nucleon in Fe–56(mass 55.93494 amu)


Approach

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


Approach

• 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


Approach

Fusion


Approach

Tokamak Fusion Reactor


Approach

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


• 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


Approach

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

Documents

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