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Nuclear Chemistry

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Title: Nuclear Chemistry


1
Nuclear Chemistry
The nuclei of some unstable isotopes change by
releasing energy and particles, collectively
known as radiation
Spontaneous nuclear reactions - five kinds 1)
Emission of ?-particles 42He (helium
nucleus) e.g. 23892U ? 23490Th
42He In air, ?-particles travel several cm. In
Al, ?-particles travel 10-3mm.
2
2. Emission of ?-particles 01e (
electron) e.g. 13153I ??? 13154Xe
01e ?-emission converts a neutron to a proton
10n ??? 11p 11e In air,
?-particles travel 10m. In Al, ?-particles travel
0.5mm.
3. Emission of ?-rays 00? ?-ray emission
changes neither atomic number nor mass. In Al,
? -particles travel 5-10 cm.
3
4) Emission of positrons ( anti-electron,
or ?-particle) 01e e.g. 116C ??? 115B
01e Positron emission converts a proton to a
neutron 11p ??? 10n 01e Positrons
have a short lifetime because they recombine with
electrons and annihilate 01e 01e ? 2
00?
4
  • Electron Capture an electron from the orbitals
    near the nucleus can be captured
  • e.g. 8137Rb 01e ? 8136Kr
  • Electron capture converts a proton to a neutron
  • 11p 01e ??? 10n

5
Fill in the blanks
  • 23994Pu ? 42He ?
  • 23491Pa ? 23492U ?
  • 11p
  • 01e
  • 10n
  • 42He
  • 19277Ir ? ? 19276Os
  • 189F ? 188O ?

6
Sources of Exposure to Radiation
7
NUCLEAR DECAY KINETICS
Because the mechanism is unimolecular, nuclear
decay is always a first order process. Decay
Rate -dN/dt kN where k is a constant, N
is the number of decaying nuclei. Integrated
rate law lnN(t)/N0 -kt N(t)
N0e-kt where N0 is the number of radioactive
nuclei at t0.
8
Half-Life
Half-Life the time required for half of a
radioactive sample to decay. N(t1/2) N0/2
ln(N/N0) -kt k 0.693/t1/2 t1/2
0.693/k Examples Isotope t1/2
Decay 23892U 4.5x109 yr
? 23592U 7.1x108 yr ? 146C 5.7x103 yr ?
9
Strontium-90, which is a fission product of
uranium, has a half-life of 28 years. This
isotope is a significant environmental concern.
What fraction of 90Sr produced today will remain
after 100 years?
10
Radiocarbon Dating
  • Libby (1946) developed method of determining age
    using 146C. 146C is produced by cosmic radiation.
  • 147N 10n ? 146C 11H 7.5 kg/year
    (constant)
  • It decays
  • 146C ? 147N 1-1e t 1/2 5.73 x 103years
  • Initially, in live plant C-14 has 14 dpm of C
  • (dpm disintegrations/min/g)
  • When the plant dies, the C-14 is not replaced and
    the disintegrations diminish.
  • Ex. The dead sea scrolls have 11 dpm. What is
    the age of the document?

11
NUCLEAR STABILITY
  • Rules
  • 1) Up to atomic number 20, np is stable.
  • 2) Above atomic number 20, ngtp is stable.
  • 3) Above atomic number 84, all nuclei are
    unstable.
  • Nuclei with 2, 8, 20, 28, 50, or 82 protons, or
    2, 8, 20, 28, 50, 82, or 126 neutrons are
    particularly stable. These are the nuclear
    equivalent of closed shell configurations (and
    are called magic numbers).
  • 5) Even numbers of protons and neutrons are more
    stable.
  • of Stable Nuclei
  • With This
  • Configuration Protons Neutrons
  • 157 Even Even
  • 52 Even Odd
  • 50 Odd Even
  • 5 Odd Odd

12
NUCLEAR STABILITY
  • An isotope that is off the belt of stability can
    use four nuclear reactions to get to it
  • ?
  • ?
  • positron emission
  • electron capture

13
NUCLEAR STABILITY
An isotope with a high n/p ratio is proton
deficient. To convert neutrons to protons, it
can undergo ?-decay 10n ? 11p 01e e.g.
9740Zr ? 9741Nb 01e
14
NUCLEAR STABILITY contd.
An isotope with a low n/p ratio is neutron
deficient. To convert protons to neutrons, there
are two possibilities
i) Positron emission 11p ? 10n 01e e.g.
2011Na ? 2010Ne 01e ii) Electron capture 11p
01e ?? 10n Elements with atomic numbers
greater than 84 undergo ?-decay in order to
reduce both the numbers of neutrons and
protons e.g. 23592U ? 23190Th 42He
15
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16
238U DECAY
Cascade of ? and ? decay reactions Moves
diagonally down belt of stability Eventually
gets to stable isotope (206Pb)
17
NUCLEAR BINDING ENERGY
2 11p 2 10n ?
42He 11p mass is 1.00728 amu 10n mass is
1.00867 amu 42He mass is 4.00150 amu Mass
defect (2)(1.00728 amu) (2)(1.00867 amu)
4.00150 amu 0.03040 amu 5.047x10-29
kg Binding energy is the energy required to
decompose the nucleus into nucleons (p and n) E
mc2 Probably better to write ?E
(?m)c2 ?E (5.047x10-29kg) (3x108m/sec)2
18
NUCLEAR BINDING ENERGY contd.
?E (5.047x10-29kg) (3x108m/sec)2
4.543x10-12J/42He 2.736x1012J/mole
42He (huge compared to ?E for
chemical reaction) Binding energy per nucleon
42He 1.14x10-12J 5626Fe 1.41x10-12J
(largest - most stable nucleus) 23892U 1.22x10-12
J Nuclei with mass greater than 200 amu can
fall apart exothermically nuclear
fission. Combining light nuclei can be
exothermic nuclear fusion.
19
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20
  • The rest masses of proton, neutron, and 12C
    nuclei are
  • 11p 1.007276470 amu
  • 11n 1.008664904 amu
  • 126C 12 amu (exact)
  • Practice problem
  • Calculate the binding energy/mole of 12C.
  • Calculate the binding energy/nucleon.
  • Compare to ?E for combustion of one mole C.

21
NUCLEAR CHAIN REACTIONS
Fission 23592U 10n ? 13752Te 9740Zr
210n ? 14256Ba 9136Kr 310n An
average of 2.4 neutrons are produced per
235U. Chain reactions Small most neutrons
are lost, subcritical mass. Medium constant
rate of fission, critical mass, nuclear
reactor. Large increasing rate of fission,
supercritical mass, bomb.
22
CRITICAL MASS
23
NUCLEAR REACTORS
Nuclear reactor fuel is 238U enriched with 3
235U. This amount of 235U is too small to go
supercritical. The fuel is in the form of UO2
pellets encased in Zr or steel rods. Liquid
circulating in the reactor core is heated and is
used to drive turbines. This liquid needs to be
cooled after use, so reactors are generally near
lakes and rivers.
24
NUCLEAR REACTORS
Cadmium or boron are used in control rods because
these elements absorb neutrons. Moderators are
used to slow down the emitted neutrons so that
they can be absorbed by adjacent fuel rods.
25
Nuclear Fission Bombs
  • Mainly U-235. Fortunately, U-235 is hard to
    purify
  • Uranium ore is concentrated and treated with
    Fluorine to form UF6. This is low boiling and
    can be evaporated at 56 oC.
  • 99.3 is non-fissionable U-238. Chemical
    reactions dont help separate isotopes.
  • Gaseous diffusion separates the heavier particles
    (UF6 with U-235 moves 0.4 faster than U-238)
  • Repeated diffusion over long barriers or
    centrifugation concentrates U-235
  • Breeder reactors- 238 U n ? 239 Pu 2e.
  • Under Glenn Seaborg, Plutonium bomb was produced
    at Hanford, WA.
  • Plutonium can be used for bombs or as a fuel
    source. However, small amounts of PuO2 dust in
    air causes lung cancer. Very toxic.

26
Breeder Reactors
Breeder reactors are a second type of fission
nuclear reactor. A breeder reactor produces more
fissionable material than it uses. 23994Pu and
23392U are also fissionable nuclei and can be
used in fission reactors. 23892U 10n ? 23992U
? 23993Np 01e ?? 23994Pu 01e 23290Th 10n
? 23390Th ??23391Pa 0-1e ??23392U 0-1e
27
NUCLEAR REACTORS
Fusion Chemistry of the stars The sun
contains 73 H, and 26 He. 11H 11H ? 21H
01e 11H 21H ? 32He 32He 32He ? 42He
21H 32He 11H ? 42He 01e Initiation of these
reactions requires temperatures of 4x107K - not
currently obtainable on a stable basis.
28
Nuclear Fusion
  • Tremendous amounts of energy are generated when
    light nuclei combine to form heavy nuclei-Sun
    (plasma 106 K)
  • Short range binding energies are able to overcome
    the proton-proton repulsion in the nuclei
  • 211H 210n ? 42He
  • ?E -2.73 x 1012 J/mol
  • Binding energy 2.15 x 108 kJ/mol
  • Note (covalent forces are only are fraction H-H
    bond E 436 kJ/mol)
  • The huge energy from 4 g of helium could keep a
    100 Watt bulb lit for 900 years

29
H-bomb
63Li 10n ? 31H 42He ?E -1.7
kJ/mol/ mol tritium The nucleons combine in a
high energy plasma (106 K). A U-235 or Pu-239
bomb is set off first. A 20-megaton bomb has 300
lbs Li-D as well as a fission/atomic bomb.
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