Nuclear Chemistry

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Nuclear Chemistry

• Chemical Reactions
• Reactions of atoms or molecule to create and/or
  break bonds.
• Nuclear Reactions
• Reactions of atoms and/or particles involving a
  nuclear change.

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Nuclear Reactions and Their Characteristics

• Nuclear Reactions
• Reactions of atomic nuclei
• Lead to change in the atom itself
• Usually results in the change of one element into
  another.
• The conclusion of alchemist attempts.
• Isotope
• Same atomic (Z) different mass (A).
• Z of protons A protons neutrons

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Nuclear Reactions and Their Characteristics
Mass of protons neutrons
Atomic of protons
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Nuclear Reactions and Their Characteristics
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Nuclear Reactions and Their Characteristics

• Characteristics of Nuclear Reactions
• Involve a change in nucleus.
• Different isotopes behave differently
• Rate is independent of T and P or catalysts
• Same reaction for elemental atoms or atoms in a
  chemical compound.
• Energy changes are very large!
• Can be on the order of 106 larger than a chemical
  reaction.

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Nuclear Reactions and Radioactivity

• Early studies showed emission of particles and
  energy from a radioactive nucleus (radionuclide).
• Alpha Emission (a)
• Emission of 42He2 from the nucleus.
• Alpha particle helium ion.
• Reduces mass by 4 and atomic by 2.

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Nuclear Reactions and Radioactivity

• Beta Emission (ß-)
• Emission of -01e from nucleus.
• 10n ? 11p -01e
• Beta particle electron
• Increases the atomic by 1 mass remains the
  same.
• Gamma Radiation (?)
• Emission of high energy electromagnetic radiation
  (? 1 x 10-13m).
• An energy releasing mechanism for most nuclear
  reactions.

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Nuclear Reactions and Radioactivity

• Positron Emission (ß)
• Emission of 01e from nucleus.
• 11p ? 10n 01e
• Positron (ß) positive electron
• Decreases the atomic by 1 mass remains the
  same.
• Electron Capture
• Capture of an inner-shell electron by the nucleus
• 11p -01e ? 10n
• X-Rays are emitted when another orbitals e falls
  into the vacancy.

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Nuclear WeaponsUniv. Alberta

• If E mc2 the energy contained in matter is 9 x
  1010 kJ/g for comparison with chemical reaction
  energies.
• In the actual fission of uranium-235, not all of
  the mass of the uranium-235 is converted into
  energy. Only about 0.1 is converted into energy
  the rest of it makes up the mass of the fission
  fragments. Thus fissioning uranium-235 produces
  only 9 x 1013 x 0.001 9 x 107 kJ / gram. This
  is still a very large value relative to the
  energies involved in chemical reactions.
• The explosion of one gram of the military
  explosive trinitrotoluene (TNT) will produce
  2.760 kJ. Thus the energy of one gram of
  fissioning uranium-235 is about equal to that of
  30 tonnes of exploding TNT. Clearly, the energy
  involved in nuclear devices is very much larger
  than the energy involved in the chemical
  reactions of chemical explosive devices. The
  first use of nuclear energy was a military use,
  the atomic bombs dropped by the United States on
  the Japanese cities of Hiroshima and Nagasaki in
  1945.
• It is traditional to consider the power of
  nuclear weapons in terms of the unit of tons of
  TNT required to produce the same energy. The
  nuclear weapons used on Hiroshima or Nagasaki in
  1945 were comparatively small atomic bombs rated
  at about 20 kilotons, or 20,000 tons of TNT
  equivalent

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Radioactive Decay Rates

• Rate of Radioactive Decay
• of altered nuclides per unit time.
• A 1st order reaction.
• Since it is a decomposition process
• A ? B C energy
• Rate k A
• Concentration is the number of radioactive nuclei
  (N)
• Rate k N

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Radioactive Decay Rates

• Integrated 1st order rate law
• Log (N/N0) -kt/2.303
• N of nuclei after time t.
• N0 of original nuclei.
• t time
• Half Life
• Time required for ½ of nuclei to decay.
• t½ 0.693/k
• K 0.693/ t½

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Radioactive Decay Rates
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Radioactive Decay Rates
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Radioactive Decay Rates

• Carbon Dating
• Atmospheric ratio of 14C to 12C is constant.
• 14C /12C ratio is constant for living organisms
  since C is continually replenished by CO2.
• Once the living tissue dies the dynamic
  equilibrium stops. 14C level begins to drop as
  the nuclear reaction proceeds.
• Age of the dead tissue can be determined by ½
  life calculation.
• Make the correlation to Potassium-40

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Southeast Missouri State Univ. Chem.
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Nuclear Stability

• Why are some nuclei stable and some are not?
• Definition of Stability.
• Non-radioactive.
• Measurable half-life.
• It has to do with neutrons.
• Acting as the glue that holds the protons from
  flying away from each other in the core.
• Certain specific arrangements or s make more
  stable nuclides.

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Nuclear Stability

• Neutrons
• All isotopes larger than Bi-209 are radioactive.
• Proton-proton repulsions become so great that the
  glue cannot stabilize the core. All nuclides
  are radioactive above Z 83
• There are 264 stable nuclides.
• Paired protons and paired neutrons appear to be
  most stable.

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Nuclear Stability

• 264 stable nuclides
• Stable protons neutrons
• 157 even even
• 52 even odd
• 50 odd even
• 5 odd odd

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Nuclear Stability

• Magic Numbers are most stable.
• Protons 2, 8, 20, 28, 50, 82
• Neutrons 2, 8, 20, 28, 50, 82, 126
• These numbers are proposed to be associated with
  completed nuclear shells of protons and neutrons.
• Similar to the shells of electronic orbitals.
• The description is extremely complex for general
  chemistry description (more like 3rd year quantum
  physics).

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Computational Science Orientation Program Oak
Ridge National Laboratory
Multi-body problem
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Oak Ridge National Laboratory
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Southeast Missouri State Univ. Chem.
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Southeast Missouri State Univ. Chem.
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Energy Changes During Nuclear Reactions

• Every reaction (even normal chemical reactions)
  produce products that do not precisely conserve
  mass.
• The laws of mass conservation and energy
  conservation must be combined to produce a
  consistent explanation.

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Energy Changes During Nuclear Reactions

• E mc2
• E is in J or eV
• J 1Kgm2/s2 eV 1.6 x 10-19 J
• The mass of any nuclide is less than the mass of
  the individual neutrons and protons.
• E.g. helium-4
• Neutron 1.00866 amu
• Proton 1.00728 amu

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Energy Changes During Nuclear Reactions

• The difference between actual nuclear mass and
  neutron proton mass is the mass defect.
• For helium-4 this is 0.03038 g/mol or
• 2.73 x 109 kJ/mol as calculated by Einstein's
  equation.
• This number can also be used to calculate a
  binding energy (a measure of nuclear stability)
  often expressed as eV/nucleon. 7.08 MeV/nucleon
  for helium-4.
• For a chemical reaction this can be calculated
  from the ?E for the reaction (either or ).
  This amounts to about 1 x 10-9 g / 100kJ of
  energy (absorbed or released from a chemical
  reaction)

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Nuclear Fission and Fusion

• Fission
• The fragmenting of heavy nuclei into smaller
  ones.
• Fusion
• The joining together of light nulei.

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Nuclear Fission and Fusion

• Chain Reaction
• Self sustaining series of nuclear fissions
  created by absorption of neutrons from previous
  fissions.
• Continues even when outside neutron source is cut
  off.
• Critical Mass
• Smallest mass of radioactive material that can
  sustain a chain reaction.
• Critical mass for 235U is 56 Kg.

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Nuclear Reactors

• Corrections from Friday.
• Boron and/or Cadmium rods mixed with Uranium
  rods.
• Uranium rods are mixture of 238U and 235U.
• Kept at 235U level of 3.

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Fusion

• A very attractive power source.
• Same as the sun.
• Uses abundant materials
• Produces no radioactive products or pollutants.
• Extremely high activation energy (T 4 x 107K)
• Containment is a key problem.
• Some short reactions have been sustained (1s)
  using magnetic fields.

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Nuclear Transmutation

• The creation of new elements by bombarding them
  with high energy particles.
• Uranium-238 Helium-4 ? Plutonium-241.

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Biological Effects

• Units of radiation dose.
• Curie dose equal to 1g of radium.
• Rad (Radiation Absorbed Dose)
• absorption of 0.01J/Kg
• Sievert (Sv) SI unit measuring amount of
  tissue damage caused by 1J/Kg.
• Rem (Roentgen Equivalent in Man) 0.01SV

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Web Sites

• http//www.uic.com.au/education.htm
• http//www.ems.psu.edu/radovic/Chernobyl.html