Transcript CHAPTER 10

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CHAPTER 23
NUCLEAR CHEMISTRY
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THE NATURE OF RADIOACTIVITY
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Relative Penetration abilities
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NUCLEAR REACTIONS
• Nuclear reactions have a charge and mass
balance, and produce one or more new
elements by a nuclear change.
• The charge balance is performed using
the atomic numbers, Z, (subscripts) on the
symbol of the element or nuclear species.
• The mass balance is performed using the
mass numbers, A, (superscripts) on the
symbol of the element or nuclear species.
210
Po
----->
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NUCLEAR REACTIONS
x
206
X+
Pb Find the x's.
x
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Reactions Involving a and b Particles
a decay, occurs when the nucleus is too
massive.
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Np ----->
256
Lr ----->
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NUCLEAR REACTIONS
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• b, emission, ( nuclear electron) occurs when the n/p
ratio is too high; converts a neutron into a proton plus
an electron.
1
0
218
84
n
Po
--->
--->
1
1
p
218
85
0
+
-1
At
e
+
Try
214
83 Bi --->
0
e
-1
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Uranium-238 Decay
NUCLEAR REACTIONS
Positron Emission
and
Electron Capture (K Capture)
• These processes reduce the atomic number by one, by
converting a proton into a neutron.
• These processes occur when the p/n ratio is too large.
• The particle produced is called a positron, and has the
same mass as an electron, but has a positive charge.
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NUCLEAR REACTIONS
• Positron and electrons are anti matter and matter.
When they meet, they are annihilated and a photon
of light energy is emitted.
1
p
--->
1
1
n
0
0
+
e
+1
• In the electron capture process, a K (1s) electron is
captured by a nuclear proton.
1
1
p
+
0
-1
e
----->
1
0
n
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NUCLEAR REACTIONS
Try the following both ways:
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K
--->
Ca
--->
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Beta emission
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K 
b +
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e 
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0
1
Ar
Electron capture
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K +
0
1
Ar
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Beta emission
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Ca 
0
1
b +
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K
electron capture
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Ca + e 
0
1
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K
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STABILITY OF ATOMIC NUCLEI
• Region of stability and modes of decay.
• Notice that no elements above bismuth have
stable isotopes (too massive).
• Lower atomic numbers have equal numbers of
protons and neutrons.
• As the atomic number increases, so does the
n/p ratio for stable isotopes.
• Even numbers of protons and neutrons are
more stable than odd numbers.
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1
1
H
2
1
H
3
1
H
hydrogen deutrium tritium
STABILITY OF ATOMIC NUCLEI
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Binding Energy
• When protons and neutrons come together to form
a nucleus, the mass decreases.
• This mass decrease is changed into energy to hold
the nucleus together.
DE = (Dm)c2
• The binding energy per mole of nucleons
• The fusion vs fission split occurs at Fe-56, the most
stable nucleus.
• Calculations of energy in J/mole require the mass
in kilograms.
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RATES OF DISINTEGRATION
REACTIONS
• The time required for one-half of a pure
radioactive sample to decay is called the halflife, t1/2.
• A short half-life means that the isotope decays
quickly.
Half-Life
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HALF-LIFE is the time it takes for 1/2 a sample is disappear.
For 1st order reactions, the concept of HALF-LIFE is especially useful.
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Half-Life
• Reaction is 1st
order
decomposition of
H2O2.
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Half-Life
• Reaction after
654 min, 1 halflife.
• 1/2 of the
reactant
remains.
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Half-Life
• Reaction after
3 half-lives, or
1962 min.
• 1/8 of the
reactant
remains.
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Half-Lives of Radioactive Elements
Rate of decay of radioactive isotopes given in
terms of 1/2-life.
238U --> 234Th + He
4.5 x 109 y
14C --> 14N + beta
5730 y
131I
--> 131Xe + beta
8.05 d
Element 106 - seaborgium
263Sg
0.9 s
RATES OF DISINTEGRATION REACTIONS
• As before in the kinetics chapter,
t1/2 = (ln 2) / k.
• Use -dN/dt = A = kN to find the rate at
one point in time.
• Use ln A/Ao = -kt, or
ln N/No
= -kt when the problem involves two
times.
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Half-Life
Radioactive decay is a first order process.
Tritium ---> electron + helium
3H
0 e
3He
-1
If you have 1.50 mg of tritium, how much
is left after 49.2 years? t1/2 = 12.3 years
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Half-Life
Start with 1.50 mg of tritium, how much is left
after 49.2 years? t1/2 = 12.3 years
Solution
ln [A] / [A]0 = -kt
[A] = ?
[A]0 = 1.50 mg
Need k, so we calc k from:
t = 49.2 years
k = 0.693 / t1/2
Obtain k = 0.0564 y-1, (from: t1/2 = (ln 2) / k)
Now ln [A] / [A]0 = -kt = - (0.0564 y -1) • (49.2 y)
ln [A] / [A]0 = - 2.77
Take antilog: [A] / [A]0 = e-2.77 = 0.0627
0.0627 is the fraction remaining !
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Half-Life
Start with 1.50 mg of tritium, how much is left
after 49.2 years? t1/2 = 12.3 years
Solution
[A] / [A]0 = 0.0627
0.0627 is the fraction remaining !
Because [A]0 = 1.50 mg, [A] = 0.094 mg
But notice that 49.2 y = 4.00 half-lives
1.50 mg ---> 0.750 mg after 1
---> 0.375 mg after 2
---> 0.188 mg after 3
---> 0.094 mg after 4
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RATES OF DISINTEGRATION REACTIONS
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Rate of Radioactive Decay
• The rate of decay or activity, A, is directly proportional
to the number of atoms present:
-dN/dt = A = kN,
where k is the rate constant and N is the number of
atoms.
• From this equation, we can see that the rate law is first
order.
Therefore, ln [A/Ao] = -kt,
or
ln [N/No] = -kt.
RATES OF DISINTEGRATION REACTIONS
Radiochemical Dating
• C-14 dating is used to determine the carbon date of
substances that were once living.
• It is based on the assumption that the rate of C-14 in
the atmosphere is and has been constant based on
the conversion of N-14 to C-14 by cosmic neutron
bombardment.
• In the equation ln A/Ao = -kt,
Ao = 14
d/min.g, the baseline specific activity.
• Other methods involve Pb - U and K - Ar.
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Carbon-14 changes
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ARTIFICIAL
TRANSMUTATIONS
• The transuranium elements are made by
bombarding target atoms of uranium with
nuclei of other elements.
• The process is also used to produce other
isotopes of elements that do not naturally
occur, O-17, I-123, etc.
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NUCLEAR FISSION
• Fission is the process of splitting heavy nuclei
to produce lighter nuclei and energy.
• This is the process used in nuclear reactors.
• The most common element fissioned is U-235.
• This isotope must first be converted to U-236 by
a slow moving neutron
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NUCLEAR FISSION
Radioactive waste products and run-away
reactions are a concern with this type of power
production.
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This figure demonstrates the chain reaction
phenomenon of fission
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Fission reactor
NUCLEAR FUSION
• Nuclear fusion is the joining together of two
light nuclei to produce a heaver nucleus and
energy.
• The process occurs on the sun and in
Hydrogen Bombs.
• Attempts to use this process to produce
electrical energy has been unsuccessful to
date, but progress has been made.
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RADIATION EFFECTS AND UNITS
OF RADIATION
• Rontgen, rad, rem, and curie are common
units used.
• Exposure comes from natural sources,
about 65%; medical sources, about 32%;
and artificial sources, about 3%.
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APPLICATIONS OF
RADIOACTIVITY
• Food irradiation to reduce spoilage and kill
bacteria, mold, and yeasts.
• Radioactive tracers for following molecules.
• Radioactive isotopes for biomedical purposed
including cancer and medical imaging.
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