Chapter 21 Nuclear Chemistry

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Chapter 21Nuclear Chemistry
Chemistry, The Central Science, 10th
edition Theodore L. Brown H. Eugene LeMay, Jr.
and Bruce E. Bursten
John D. Bookstaver St. Charles Community
College St. Peters, MO ? 2006, Prentice Hall, Inc.
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February 3

• Nuclear chemistry
• HW
• 1,2,3,7,11,13,17,19,27,29 for tomorrow
• 31 to35 odd, 41,57,59,61

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The Nucleus

• Remember that the nucleus is comprised of the two
  nucleons, protons and neutrons.
• The number of protons is the atomic number.
• The number of protons and neutrons together is
  effectively the mass of the atom.

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Isotopes

• Not all atoms of the same element have the same
  mass due to different numbers of neutrons in
  those atoms.
• There are three naturally occurring isotopes of
  uranium
• Uranium-234
• Uranium-235
• Uranium-238

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Radioactivity

• It is not uncommon for some nuclides of an
  element to be unstable, or radioactive.
• We refer to these as radionuclides.
• There are several ways radionuclides can decay
  into a different nuclide.

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Types ofRadioactive Decay
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SeparationAlphaBetaGamma.MOV Separation of
Radiation
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Nuclear Reactions

• The chemical properties of the nucleus are
  independent of the state of chemical combination
  of the atom.
• In writing nuclear equations we are not concerned
  with the chemical form of the atom in which the
  nucleus resides.
• It makes no difference if the atom is as an
  element or a compound.
• Mass and charges MUST BE BALANCED!!!

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Alpha Decay

• Loss of an ?-particle (a helium nucleus)

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Alpha Decay

• Mass changes by 4
• The remaining fragment has 2 less protons
• Alpha radiation is the less penetrating of all
  the nuclear radiation (it is the most massive
  one!)

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Beta Decay

• Loss of a ?-particle (a high energy electron)

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Beta Decay

• Involves the conversion of a neutron in the
  nucleus into a proton and an electron.
• Beta radiation has high energies, can travel up
  to 300 cm in air.
• Can penetrate the skin

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Beta decay

• Write the reaction of decay for C-14

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Gamma Emission

• Loss of a ?-ray (high-energy radiation that
  almost always accompanies the loss of a nuclear
  particle)

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Positron Emission

• Loss of a positron ( particle with same mass,
  but opposite charge than an electron)

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Positron emission

• Involves the conversion of a proton to a neutron
  emitting a positron.
• The atomic number decreases by one, mass number
  remains the same.

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Electron Capture (K-Capture)

• Capture by the nucleus of an electron from the
  electron cloud surrounding the nucleus.
• As a result, a proton is transformed into a
  neutron.

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Electron capture

• Rb-81
• Note that the electron goes in the side of the
  reactants. Electron gets consumed.

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Patterns of nuclear Stability

• Any element with more than one proton ( all but
  hydrogen) will have repulsions between the
  protons in the nucleus.
• A strong nuclear force helps keep the nucleus
  from flying apart.

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Neutron-Proton Ratios

• Neutrons play a key role stabilizing the nucleus.
• The ratio of neutrons to protons is key to
  determine the stability of a nucleus .

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Neutron-Proton Ratios

• As nuclei get larger, it takes a greater number
  of neutrons to stabilize the nucleus.

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Neutron-Proton Ratios

• For smaller nuclei (Z ? 20) stable nuclei have a
  neutron-to-proton ratio close to 11.

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Stable Nuclei

• The shaded region in the figure shows what
  nuclides would be stable, the so-called belt of
  stability.

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Stable Nuclei

• Nuclei above this belt have too many neutrons.
• They tend to decay by emitting beta particles. (
  neutron becomes proton )

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Above the belt of stabilityBeta particle emission

• Too many neutrons. The nucleus emits Beta
  particles, decreasing the neutrons and increasing
  the number of protons.

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Stable Nuclei

• Nuclei below the belt have too many protons.
• They tend to become more stable by positron
  emission or electron capture (both lower the
  number of protons)

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Stable Nuclei

• Elements with low atomic number are stable if
  proton neutrons
• There are no stable nuclei with an atomic number
  greater than 83.
• These nuclei tend to decay by alpha emission.

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Below the stability beltIncrease the number of
neutrons (by decreasing protons)

• Positron emission more common in lighter nuclei.
• Electron capture common for heavier nuclei.

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Radioactive Series

• Large radioactive nuclei cannot stabilize by
  undergoing only one nuclear transformation.
• They undergo a series of decays until they form a
  stable nuclide (often a nuclide of lead).

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Predicting modes of nuclear decay

• C-14
• Xe-118
• Pu-239
• In-120

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• beta decay
• Positron emission or electron capture
• Alpha decay (too heavy, loses mass)
• Beta decay (ratio too low, gains protons)

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MAGIC NUMBERS 2, 8, 20, 28, 50, or 82

• Nuclei with 2, 8, 20, 28, 50, or 82 protons or
  2, 8, 20, 28, 50, 82, or 126 neutrons tend to be
  more stable than nuclides with a different number
  of nucleons.

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Some Trends

• Nuclei with an even number of protons and
  neutrons tend to be more stable than nuclides
  that have odd numbers of these nucleons.

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Shell model of the nucleus

• Nucleons are described a residing in shells like
  the shells for electrons.
• The numbers 2,8,18,36,54,86 correspond to closed
  shells in nuclei.
• Evidence suggests that pair of protons and pairs
  of neutrons have special stability

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Transmutations

• To change one element into another.
• Only possible in nuclear reactions never in a
  chemical reaction.
• In order to modify the nucleus huge amount of
  energy are involved.
• These reactions are carried in particle
  accelerators or in nuclear reactors

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Nuclear transmutations

• Alpha particles have to move very fast to
  overcame electrostatic repulsions between them
  and the nucleus.
• Particle accelerators or smashers are used. They
  use magnetic fields to accelerate the particles.

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Particle Accelerators(only for charged
particles!)

• These particle accelerators are enormous, having
  circular tracks with radii that are miles long.

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Cyclotron

• Nuclear transformations can be induced by
  accelerating a particle and colliding it with the
  nuclide.

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Neutrons

• Can not be accelerated. They do not need it
  either (no charge!).
• Neutrons are products of natural decay, natural
  radioactive materials or are expelled of an
  artificial transmutation.
• Some neutron capture reactions are carried out in
  nuclear reactors where nuclei can be bombarded
  with neutrons.

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Representing artificial nuclear transmutations

• 14N 4He ? 7O 1H
• Target nucleus ( bombarding particle, ejected
  particle ) product nucleus
• 14N (a, p) 17O
• Write the balanced nuclear equations summarized
  as followed
• 16 O ( p, a) N
• 27Al (n, a)24 Na

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Measuring Radioactivity

• One can use a device like this Geiger counter to
  measure the amount of activity present in a
  radioactive sample.
• The ionizing radiation creates ions, which
  conduct a current that is detected by the
  instrument.

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Mass defect

• The mass of the nucleus is always smaller than
  the masses of the individual particles added up.
• The difference is the mass defect.
• That small amount translate to huge amounts of
  energy ?E (?m) c2
• That energy is the Binding energy of the nucleus,
  and is the energy needed to separate the nucleus.

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Energy in Nuclear Reactions

• For example, the mass change for the decay of 1
  mol of uranium-238 is -0.0046 g.
• The change in energy, ?E, is then
• ?E (?m) c2
• ?E (-4.6 ? 10-6 kg)(3.00 ? 108 m/s)2
• ?E -4.1 ? 1011 J This amount is 50,000 times
  greater than the combustion of 1 mol of CH4

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Types of nuclear reactionsfission and fusion

• The larger the binding energies, the more stable
  the nucleus is toward decomposition.
• Heavy nuclei gain stability (and give off energy)
  if they are fragmented into smaller nuclei.
  (FISSION)

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• Even greater amounts of energy are released if
  very light nuclei are combined or fused together.
  (FUSION)

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Nuclear Fission

• How does one tap all that energy?
• Nuclear fission is the type of reaction carried
  out in nuclear reactors.

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Nuclear Fission

• Bombardment of the radioactive nuclide with a
  neutron starts the process.
• Neutrons released in the transmutation strike
  other nuclei, causing their decay and the
  production of more neutrons.

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Nuclear Fission

• This process continues in what we call a nuclear
  chain reaction.

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Nuclear Fission

• If there are not enough radioactive nuclides in
  the path of the ejected neutrons, the chain
  reaction will die out.

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Nuclear Fission

• Therefore, there must be a certain minimum
  amount of fissionable material present for the
  chain reaction to be sustained Critical Mass.

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Controlled vs Uncontrolled nuclear reaction

• Controlled reactions inside a nuclear power
  plant
• Uncontrolled reaction nuclear bomb

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Nuclear Reactors

• In nuclear reactors the heat generated by the
  reaction is used to produce steam that turns a
  turbine connected to a generator.

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Nuclear Reactors

• The reaction is kept in check by the use of
  control rods.
• These block the paths of some neutrons, keeping
  the system from reaching a dangerous
  supercritical mass.

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FUSION

• Combining small nucleii to form a larger one.
• Require millions of K of temperature

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Fusion

• 1H 1H ? 2H 1e energy
• 1H 2H ? 3He energy
• 3He 3He ? 4He 21H energy
• Reaction that occurs in the sun
• Temperature 107 K
• Heavier elements are synthesized in hotter stars
  108 K using Carbon as fuel

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Nuclear Fusion

• Fusion would be a superior method of generating
  power.
• The good news is that the products of the
  reaction are not radioactive.
• The bad news is that in order to achieve fusion,
  the material must be in the plasma state at
  several million kelvins.

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Nuclear Fusion(thermonuclear reactions)

• Tokamak apparati like the one shown at the right
  show promise for carrying out these reactions.
• They use magnetic fields to heat the material.
• 3 million K degrees were reached inside but is
  not enough to begin fusion which requires 40
  million K

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Rates of radioactive decayrate k NN is the
number of radioactive nuclei

• Activity rate at which a sample decays.
  Expressed in disintegrations per unit time.
• Becquerel (Bq) SI unit one nuclear
  disintegration per second.
• Curie (Ci) 3.7x1010 disintegrations per second,
  the rate of decay of 1g of Ra

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RADIOACTIVE DECAY

• As a radioactive sample decays, the amount of
  radiation emanating for the sample decays as
  well.
• After one half life, half of the emanations!

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Half-Life

• Half-life is defined as the time required for
  one-half of a reactant to react.
• Because A at t1/2 is one-half of the original
  A,
• At 0.5 A0.

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RADIOACTIVE DECAY

• Is a first order process. Its rate is
  proportional to the number of radioactive nuclei
  N in the sample rate k N

Time elapsed t k is the decay constant N0 is
the original amount Nt is the amount of sample
at time t
0.693 kt1/2
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Half life

• The half life of a reaction is useful to describe
  how fast it occurs.
• For a first order reaction (like nuclear decay!)
  it does not depend on the initial concentration
  of the reactants.
• HALF LIFE IS CONSTANT FOR A FIRST ORDER REACTION

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Half LifeDecay of 10.0 g sample of Sr-90t1/2
28.8 y
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Problem 1

• The half life of 210Pb 25 y
• 1) How much left of a sample of 50 mg will
  remain after 100 y?
• 2) Find number of half lives
• 3) Find fraction left
•  

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• 1- 6.25 g
• 2- 4 half lives
• 3- 1/16

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Problem 2

• How many years will take for 50mg of 210Pb to
  decay to 5 mg?
• Half life of 210Pb 25 y

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• 83 years

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Problem 3

• 90 of a radioisotope disintegrates in 36 hs.
  What is the half life?
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Problem 4

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• .953 g of Sr-90 remains after 2 y from a 1.000g
  sample.
• a) find the half life
• b) how much will remain after 5 y?
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• Half life 28.8 years
• Amount left (No) 0.89 g

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Radioactive Dating

• A rock contains .257 mg of Pb-206 for every mg of
  U-238.
• T1/2 4.5 x 10 9 y
• How old is the rock?

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• No we will assume that all the Pb-206 that is
  now present will come from the original U, plus
  the U that is still present
• (check the answer in textbook!)

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Calculating half life

• If 87.5 of a sample of I-131 decays in 24
  days, what is the half life of the
• I-131?