Lecture 62 Nuclear Chemistry I

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Lecture 62 - Nuclear Chemistry I

• Assignment 16, Chapter 22

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Is it Chemistry or Physics?

• Chemistry Physics
• Nuclear fuel production, nuclear decay
• Energy waste disposal
• Bombs ? chain reactions
• Medicine biological effects
• of radiation
• Reactions new elements

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The Nucleus
d 10-13 cm
(d 10-8 cm for an atom)
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The Nucleons
mass (AMU) charge (esu)
proton 1 1
neutron 1 0
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In case you forgot...
many elements have several isotopes
92 protons 146 neutrons
92 protons 143 neutrons
Identical chemistries, different nuclear
reactions
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Forces in the Nucleus
electrostatic repulsion
proton (p)
neutron (n)
the nuclear force
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Chemical vs. Nuclear Energies

• CH4(g) 2 O2(g) CO2(g) 2 H2O(g)

DH -896 kJ/mol
-56 kJ/g
nuclear fission
235U
other nucei
DH -8.2 x 107 kJ/g
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Chemical vs. Nuclear Energies
2000 m3 CH4
1000 L
1 mol
x
x
y
m3
22.4 L
16 g
x
1.4 x 106 g CH4 / y
mol
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235U required 1.4 x 106 g x
8.2 x 107
1.0 g
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The Discovery of Radioactivity
-
b
a
g
U

b-
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a Particles

• positively charged
• massive
• accurate measurements 4He nuclei

2 protons 2 neutrons
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a Decay
238
4
234
U
He
Th
92
2
90
nucleons are conserved (238) charge is conserved
(92)
identities of atoms are not!
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b- Particles
negatively charged small mass
accurate measurements electrons
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b- Decay
0
234
e-
Pa
-1
91
90 p 144 n
91 p 143 n
i.e. a neutron is turned into a proton an
electron
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b Particles

• positively charged
• small mass
• accurate measurements positrons
• (positively charged electrons)

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b Decay
90
0
90
Mo
e
Nb
42
1
41
42 p 48 n
41 p 49 n
i.e. a proton is turned into a neutron a
positron
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Electron Capture (EC)
-











-
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Electron Capture
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Fe e-
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26 p 29 n
25 p 30 n
i.e. a proton captures an electron and is
turned into a neutron
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g ray Emission
g
nucleus after decay (high energy)
lower energy
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Radioactive Decay Rates

• rate k N

Number of radioactive nuclei
Decay constant
all radioactive decay is 1st order
N
ln
- kt
No
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Radioactive Decay Rates
0.693
t1/2
k
for example, t1/2 (238U) 4.51 x 109y
find the rate of decay in 1 kg 238U
rate k N
0.693

N
t1/2
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1000 g
x NAV
N
238 g/mol
2.5 x 1024
0.693
rate
x 2.5 x 1024
4.51 x 109 y
3.9 x 1014 y-1 1.2 x 107 s-1
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• stable decays very slowly
• non-radioactive does not decay

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Radioactive Decay Rates
radionuclide decay t1/2
238U a 4.51 x 109 y
235U fission 7.1 x 108 y
12C non-radioactive
14C b- 5715 y
60Co
b-, g
5.27 y
131I b- 8.07 d
241Am a 458 y