12/25/2010
Ch
Chapter 20
20
NUCLEAR CHEMISTRY
(Part II)
(Part II)
Dr. Al‐Saadi
1
Nuclear fission and nuclear reaction
 The graph above has very important implications for the use of nuclear processes as sources of energy.  Energy is released, that is, E is negative, when a process goes from a less stable to a more stable state nuclei  The higher a nuclide is on the curve, the more stable it is.  This means that two types of nuclear processes will be exothermic 1. Combining two light nuclei to form a heavier, more stable nucleus. This process is called fusion.
2. Splitting a heavy nucleus into two nuclei with smaller mass numbers. This process is called fission.
 Because of the large binding energies involved in holding the nucleus together, both these processes involve energy changes more than a million times larger than those associated with chemical reactions.
1
12/25/2010
Highest
g
stability
y
Nuclear fission
• 56Fe has highest Eb and is most stable
isotope.
•Energy sources:
Nuclear fusion –Fission for large radioactive
elements, such as U-235
–Fusion of veryy light
g nuclei such as
deuterium producing He. Not yet
accomplished.
–Atoms of Z=50-80 (intermediate
masses have the largest NBE.
Stability of nuclei increa
asing
The Binding Energy Per Nucleon as a Function of Mass Number
Fusion of light nuclei and fission of heavy nuclei are exothermic processes
 Nuclei of heavy atoms will gain more stability if they are fragmented (fission
g
(
into intermediate ones). They will )
y
give off energy when the fission occurs
 Nuclei of light atoms will gain more stability if they are fused together (fusion) to give atoms of intermediate NBE. Energy will be given off when fusion occurs. 2
12/25/2010
Both Fission and Fusion Produce More Stable Nuclides
Nuclear Fission
 Several isotopes of the heavy elements undergo fission if bombarded with neutrons of high enough fission if bombarded with neutrons
of high enough
energy
235
 In practice attention was paid to
92
and 239
U
94
Pu
Th discussion
The
di
i
will
ill focus
f
on
235
92
U
That is only 0.7% of the naturally occurring U
238
92
U
is most abundant isotope and does
not go fission
3
12/25/2010
235
92
U Fission
• 23592U + 10n  23692U*
• and 10-14 seconds later...
• 23692U*  9236Kr + 14156Ba + 3 10n + ENERGY
• 50 possible sets of fission products (sum of
atomic numbers = 92)
• 3 neutrons released for ONE 23592U (too many
for stability, thus fragmentation continues to
reach stability)
Fission Process
4
12/25/2010
Chain Fission Reactions
235
 Produced neutrons will attack more and more 92 U
forming chain reaction
 This chain reaction occurs in the atomic bomb. Energy is evolved in successive fissions that will lead to tremendous explosion
235
 For the chain reaction to occur must be large 92 U
(critical mass), thus most neutrons are captured
235
 Critical mass for is 1 to 10 Kg
92 U
• If the sample is too small most neutrons
escape braking the chain
Fission Produces a Chain Reaction
5
12/25/2010
6
12/25/2010
7
12/25/2010
8
12/25/2010
Nuclear Fission
A selfself-sustaining fission process is called a
chain reaction.
Neutrons
Causing
Event
Fission
subcritical
<1
critical
=1
supercritical
>1
Result
reaction stops
sustained reaction
violent explosion
9
12/25/2010
Fission Produces Two Neutrons
Nuclear reactors
 Because of the tremendous energies involved, it is desirable to develop the fission process as an energy source to produce electricity. d
l t i it
 To accomplish this, reactors were designed in which controlled fission can occur.  The resulting energy is used to heat water to produce steam to run turbine generators, in much the same way that a coal‐
burning power plant generates energy.
 A schematic diagram of a nuclear power plant is shown 10
12/25/2010
 In the reactor core, uranium that has been enriched to approximately 3% U‐235(natural uranium contains only 0.7% U‐235) is housed in cylinders.  A moderator surrounds the cylinders to slow down the neutrons so that the uranium fuel can capture them more efficiently. p
y
 Control rods, composed of substances that absorb neutrons, are used to regulate the power level of the reactor. The reactor is designed so that should a malfunction occur, the control are automatically inserted into the core to stop the reaction
 A liquid that is usually water is circulated through the core to extract the A liquid that is usually water is circulated through the core to extract the
heat generated  The energy can then passed on via a heat exchanger to water in the turbine system A Schematic Diagram of a Nuclear Power Plant
11
12/25/2010
A Schematic Diagram of a Reactor Core
12
12/25/2010
Breeder Reactors
Fissionable fuel is produced while the reactor runs 238
92
U
is changed (split) to fissionable 239
94 Pu
235
This reaction
involves absorption of neutrons
92 U
1
238
239
239
0 n 
9 Pu
2 U 
92 U
94
239
92
U 
239
93
Np 
239
93
Np 
239
94
0
1e
Pu 
0
1e
• As the reactor runs and U-235 is split some of the excess
neutrons are absorbed by U-238 to produce Pu-239
• Pu-239 is then separated and used to fuel another reactor
• This reactor, thus breeds nuclear fuel as it operates
Breeder Reactors
Fissionable fuel is produced while the reactor runs ( is split giving neutrons for the creation of
split, giving neutrons for the creation of ):
):
239
94 Pu
1
0n

238
92
U
238
239
92 U  92 U
239
239
92 U  93 Np

239
239
93 Np  94 Pu

0
1e
0
 1e
13
12/25/2010
Fusion
 Large quantities of energy are produced by the fusion of two light nuclei to give a heavier one
fusion of two light nuclei to give a heavier one
1
1
H  13H  42 He  01n  Energy
 Stars and sun produce their energy through nuclear fusion.
fusion.  Our sun, which presently consists of 73% hydrogen, 26% helium, and 1 % other elements, gives off vast quantities of energy from the fusion of protons to form helium:
Proposed mechanism for
reactions on the sun
T  1X109 oC; E  1X1019 kJ/day
14
12/25/2010
How does fusion take place?
 The major stumbling block in having these fusion reactions feasible is that high energies are required to initiate fusion.  The forces that bind nucleons together to form a nucleus are effective only at very small distances (10‐13 cm).
 Thus, for two protons to bind together and thereby release energy, they must get very close together.  But protons, because they are identically charged, repel each other electrostatically.  This means that to get two protons (or two deuterons) close enough to bind together (the nuclear binding force is not electrostatic), they must be "shot" at each other at speeds )
"
"
high enough (106 m/s) to overcome the electrostatic repulsion.
 High temperatures are expected from various sources that are under study
Use of Isotopes
20.7
 Chemical analysis
o
Use of tracers • Sulfur
Sulfur‐35
35 in the determination of the structure in the determination of the structure
of thiosulfate
• Photosynthetic pathway using oxygen‐18 and 14 carbon
14‐carbon
Copyright
McGrawHill 2009
30
15
12/25/2010
 Isotopes in medicine
o
Use of tracers for diagnosis
• Sodium‐24 – blood flow
• Iodine‐131 –thyroid conditions
• Iodine ‐123 –
I di
123 b
brain imaging
i i
i
normal
o
Major advantage – easy to detect Alzheimer victim
31
Geiger Counter: Used to detect radiation
Copyright
McGrawHill 2009
32
16
12/25/2010
20.8 Biological Effects of Radiation
 Quantitative measures of radation
o
curie (Ci): fundamental unit of radioactivity
(Ci): fundamental unit of radioactivity
• Equivalent to 3.70 x 1010 nuclear disintegrations per second
o
rad (radiation absorbed dose)
• Considers activity
• Considers energy
• Considers type of radiation emitted
• 1 rad = 1 x 105 J/g of tissue irradiated
Copyright
McGrawHill 2009
33
o
RBE (relative biological effectiveness)
• Considers biological effect of radiation
–Part of body irradiated
–Type of radiation
o
rem (roentgen equivalent for man)
 Chemical basis for radiation damage
o
o
Copyright
McGrawHill 2009
Ionizing radiation produces radicals
Radicals (free radicals) –
(free radicals) molecular fragments with molecular fragments with
unpaired electrons
34
17
12/25/2010
Copyright
McGrawHill 2009
35
o
o
e and the hydroxyl radical can form other radicals In tissues radicals can attack and destroy membranes, enzymes, DNA, etc.
 Radiation damage
o
o
Copyright
McGrawHill 2009
Somatic (affect the organism within its lifetime)
Genetic (inheritable changes and gene mutations)
36
18
12/25/2010
Key Points
 Nuclei and nuclear reactions
o
o
o
o
Radioactive decay
R
di ti d
Nuclear transmutations
Particles involved in nuclear reactions
Balancing nuclear reactions
 Nuclear stability
o
o
Type of interactions involved
Type
of interactions involved
Pattern of stability
• Magic numbers
• Odd/even numbers of nucleons
o
Nuclear binding energy
• Mass defect
Mass defect
• Einstein’s mass‐energy equivalence relationship
• Calculation nuclear binding energy
–Per mole of nucleons
–Per nucleon
 Natural radioactivity
o
Radioactive decay series
19
12/25/2010
o
o
Kinetics of radioactive decay
Dating based on radioactive decay
• Carbon‐14 dating
• Uranium‐238 datingg
• Potassium‐40 dating
 Nuclear Transmutation
o
o
Transuranium element
Particle accelerators
 Nuclear fission
Nuclear fission
o
Nuclear fission reactions
• Nuclear chain reactions
• Critical mass
o
Generation of electric power
• Light water reactors
Light water reactors
• Heavy water reactors
• Breeder reactors
o
Nuclear fusion
• Solar nuclear reactions
• Thermonuclear reactions
Th
l
i
• Potential for generation of electric power
• Thermonuclear bombs
20
12/25/2010
 Uses of Isotopes
o
o
Chemical Analysis
Medicine
 Biological effects of radiation
o
Units to measure radiation
Units to measure radiation
• curie
• rad
• RBE
• rem
o
o
o
Effect of free radicals
Effect
of free radicals
Somatic damage
Genetic damage
Effects of Radiation
Factors that make the biological effects
1. The energy of the radiation.
The higher the energy the more damage it can cause
The higher the energy the more damage it can cause. Radiation doses are measured in rads (radiation absorbed radiation absorbed dose), where 1rad corresponds to 10‐2 J of energy dose
deposited per kilogram of tissue.
2. The penetrating ability of the radiation.
The particles and rays produced in radioactive processes vary in their abilities to penetrate human tissue rays are vary in their abilities to penetrate human tissue: 
rays are
highly penetrating,  particles can penetrate approximately 1 cm, and  particles are stopped by the skin.
21
12/25/2010
3. The ionizing ability of the radiation
 Extraction of electrons from biomolecules to form ions is particularly detrimental to their functions. The ionizing ability of radiation varies dramatically. For example,  rays penetrate very deeply but cause only occasional ionization. On the other hand, 
particles, although not very penetrating, are very effective at causing ionization and produce a dense trail of damage.
 Thus ingestion of an  particle producer, such as plutonium, is particularly damaging.
4. The chemical properties of the radiation source
 When a radioactive nuclide is ingested into the body, its effectiveness in causing damage Wh
di ti
lid i i
t d i t th b d it ff ti
i
i d
depends on its residence time. For example, Kr‐85 and Sr‐90 are both ‐particle producers.  However, since krypton is chemically inert, it passes through the body quickly and does not have much time to do damage.
 Strontium, being chemically similar to calcium, can collect in bones, where it may cause leukemia and bone cancer.


The energy dose of the radiation and its effectiveness in causing biological damage form the source for the term rem (roentgen equivalent for man)
the source for the term rem (roentgen equivalent for man)
Number of rems = (number of rads X RBE (relative effectiveness of radiation in causing biological damage)
R i Q
Review Qs
Dr. Al‐Saadi
44
22
12/25/2010
In the following nuclear equation, identify the
missing product:
43
20
1)
2)
Ca +   __________ X
46
Ti
22
46
Sc
21
3)
4)
+
1
H
1
44
Ti
22
42
18
Ar
Dr. Al‐Saadi
45
ANSWER
2)
46
Sc
21
Make sure to memorize the abbreviations for the
subatomic
b t i particles.
ti l
23
12/25/2010
Identify the missing particle in the following
equation:
238
92
1)
2)
3)
242
P
Pu
94
234
Th
90
242
Th
90
4
U
2
He + ?
4)
234
U
92
5)
none of these
ANSWER
2)
234
90
Th
Just as chemical equations need the same
number of each type
yp of atom on each side,,
nuclear equations need the same number of
each type of nucleon on each side.
24
12/25/2010
QUESTION
Electron capture transforms
40
19
K into what
nuclide?
1)
2)
3)
40
Ca
20
40
Ar
18
4
2
4)
5)
40
–
K
19
39
Ca
20
He
ANSWER
2)
40
18
Ar
The electron is “captured” from the core
electrons
l t
swarming
i around
d th
the nucleus.
l
Remember to place the electron on the left side
of the reaction.
25
12/25/2010
QUESTION
Which of the following processes decreases the
atomic number by 1?
1) Gamma-ray production
2) Electron capture
3) Beta-particle production
4) Positron production
5) At least two of these processes decrease
the atomic number by 1.
ANSWER
5)
At least two of these processes decrease
the atomic number by 1.
Both electron capture and positron production
decrease the atomic number by 1.
26
12/25/2010
QUESTION
The rate constant for the beta decay of thorium–2
234 is 2.88  10 /day. What is the half-life of
this nuclide?
1) 53.1 days
2) 1.22 days
3) 0.693
0 693 days
4) 24.1 days
5) 101 days
ANSWER
4)
24.1 days
Half-life problems for nuclear processes are
simpler than chemical processes since nuclear
st
processes are always 1 order.
27
12/25/2010
QUESTION
T he nuclide
12
N is unstable. W hat type of
7
y w ould be expected?
p
radioactive decay
–
1) 
+
2) 
3) 
4) 
5)
1
n
0
ANSWER
2)

+
According to the band of stability graph
this nuclide is neutron -poor, so it
must do something to decrease the number of
protons or increase the number of neutrons.
28
12/25/2010
QUESTION
N uclides w ith too m any neutrons to be in the
band of stability are m ost likely to decay by w hat
m ode?
1) 
2) fission
+
3) 
4) electron capture
–
5) 
ANSWER
5)

–
This process is the opposite of positron emission
and allows the change of a neutron into a
proton.
29
12/25/2010
QUESTION
Which types of processes are likely when the
neutron-to-proton ratio in a nucleus is too low?
a.
b.
c.
d.
1)
4)
 decay
 decay
Positron production
Electron capture
a, b
b, c, d
2) b, c
5) b, d
3) c, d
ANSWER
3)
c, d
Section 18.1 Nuclear Stability and Radioactive
Decay (p. 841)
Beta decay will cause the neutron-to-proton ratio
to decrease even more.
30
12/25/2010
QUESTION
The number of half-lives needed for a
radioactive element to decay to about 6% of its
original activity is (choose nearest number):
1) 2
2) 3
3) 4
4) 5
5) 6
ANSWER
3)
4
100%  50%  25%  12.5%
12 5%  6.25%
6 25% .
Each arrow indicates a half-life.
31
12/25/2010
A radioactive element has a half-life of 1.0 hour.
How many hours will it take for the number of
atoms present to decay to 1/16th of the initial
value?
1) 16
2) 8
3) 4
4) 15
5) 2.77
ANSWER
3)
4
1  ½  ¼  1/8  1/16.
Each arrow indicates a half-life of 1.0 hour.
32
12/25/2010
Which statement is true about the following
reaction?
14
N
7
13.992
amu
1)
2)
3)
4)
5)
+
4
He
2
4.0015
amu

17
O
8
+
16.9986
amu
1
H
1
1.0073
amu
Energy is absorbed in the reaction.
Energy is released in the reaction.
No energy change is associated with the reaction
reaction.
Not enough information is given to determine the
energy change.
None of these.
ANSWER
1)
Energy is absorbed in the reaction.
The products have a combined mass greater
than the reactants. The addition of mass came
from the conversion of energy absorbed during
the process.
33
12/25/2010
QUESTION
If more than one neutron from each fission
event causes another fission event, the fission
situation is described as:
1)
2)
3)
4)
5)
critical.
subcritical.
supercritical.
moderated
moderated.
none of these.
ANSWER
3)
supercritical.
This type of event would be disastrous for a
nuclear power plant.
34