nuclear chemistry
DESCRIPTION
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 : 4 2 He (helium nucleus) e.g. 238 92 U 234 90 Th + 4 2 He - PowerPoint PPT PresentationTRANSCRIPT
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 234
90Th + 42He
In air, -particles travel several cm.
In Al, -particles travel 10-3mm.
2. Emission of -particles: 0–1e (= electron)
e.g. 13153I 131
54Xe + 0–1e
-emission converts a neutron to a proton:
10n 1
1p + 1–1e
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.
4) Emission of positrons (= anti-electron, or +-particle): 0
+1e
e.g. 116C 11
5B + 01e
Positron emission converts a proton to a neutron:
11p 1
0n + 01e
Positrons have a short lifetime because they recombine with electrons and annihilate:
01e + 0
–1e 2 00
5) Electron Capture: an electron from the orbitals near the nucleus can be captured:
e.g. 8137Rb + 0
–1e 8136Kr
Electron capture converts a proton to a neutron:
11p + 0
–1e 10n
Fill in the blanks 239
94Pu 42He + ?
23491Pa 234
92U + ?
• 11p
• 0–1e
• 10n
• 42He
19277Ir + ? 192
76Os
189F 18
8O + ?
Sources of Exposure to RadiationA
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50
100
Rad
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Source
Natural Anthropogenic
200mrem(55%)
39mrem(11%)
27mrem(8%)
28mrem(8%)
40mrem(11%)
14mrem(4%)
11mrem(3%)
Roc
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Cos
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Ray
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Rad
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Med
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X-r
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Nuc
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Con
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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:
ln[N(t)/N0] = -kt
N(t) = N0e-kt
where N0 is the number of radioactive nuclei at t=0.
NUCLEAR DECAY KINETICS
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
Half-Life
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?
Radiocarbon Dating
Libby (1946) developed method of determining age using 146C. 14
6C is produced by cosmic radiation.
147N + 1
0n 146C + 1
1H 7.5 kg/year (~constant)
It decays: 14
6C 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?
Rules: 1) Up to atomic number 20, n=p is stable.
2) Above atomic number 20, n>p is stable.3) Above atomic number 84, all nuclei are unstable.4) 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
NUCLEAR STABILITY
NUCLEAR STABILITY
An isotope that is off the belt of stability can use four nuclear reactions to get to it:
1. 2. 3. positron emission4. electron capture
An isotope with a high n/p ratio is proton deficient.
To convert neutrons to protons, it can undergo -decay:
10n 1
1p + 0–1e
e.g. 9740Zr 97
41Nb + 0–1e
NUCLEAR STABILITY
i) Positron emission:1
1p 10n + 0
1e
e.g. 2011Na 20
10Ne + 01e
ii) Electron capture:1
1p + 0–1e 1
0n
Elements with atomic numbers greater than 84 undergo -decay in order to reduce both the numbers of neutrons and protons:
e.g. 23592U 231
90Th + 42He
An isotope with a low n/p ratio is neutron deficient.
To convert protons to neutrons, there are two possibilities:
NUCLEAR STABILITY contd.
238U DECAY
Cascade of and decay reactions
Moves diagonally down belt of stability
Eventually gets to stable isotope (206Pb)
2 11p + 2 1
0n 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
NUCLEAR BINDING ENERGY
E = (5.047x10-29kg) (3x108m/sec)2 = 4.543x10-12J/4
2He = 2.736x1012J/mole 4
2He (huge compared to E for chemical reaction)
Binding energy per nucleon:
42He: 1.14x10-12J
5626Fe: 1.41x10-12J (largest - most stable nucleus)
23892U: 1.22x10-12J
Nuclei with mass greater than ~200 amu can fall apart exothermically – nuclear fission.
Combining light nuclei can be exothermic – nuclear fusion.
NUCLEAR BINDING ENERGY contd.
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.
Fission
23592U + 1
0n 13752Te + 97
40Zr + 210n
14256Ba + 91
36Kr + 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.
NUCLEAR CHAIN REACTIONS
CRITICAL MASS
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.
NUCLEAR REACTORS
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.
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 don’t 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.
Breeder reactors are a second type of fission nuclear reactor.
A breeder reactor produces more fissionable material than it uses.
23994Pu and 233
92U are also fissionable nuclei and can be used in fission reactors.
23892U + 1
0n 23992U 239
93Np + 0–1e 239
94Pu + 0–1e
23290Th + 1
0n 23390Th 233
91Pa + 0-1e 233
92U + 0-1e
Breeder Reactors
Fusion “Chemistry of the stars”
The sun contains 73% H, and 26% He.
11H + 1
1H 21H + 0
+1e
11H + 2
1H 32He
32He + 3
2He 42He + 2
1H
32He + 1
1H 42He + 0
1e
Initiation of these reactions requires temperatures of 4x107K - not currently obtainable on a stable basis.
NUCLEAR REACTORS
Nuclear FusionTremendous 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 + 21
0n 42He
E= -2.73 x 1012 J/molBinding 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
H-bomb
63Li + 1
0n 31H + 4
2He
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.