Transcript Chapter 20 Nuclear Chemistry - TetuTeacher

Chapter 22
Nuclear Chemistry
Nuclear Chemistry
Nuclear (NOT Nucular) chemistry involves
changes in the nuclear composition
(protons and neutrons) of atoms that are
radioactive.
 Nuclear reactions can be used as energy
sources, for medical diagnosis, and for
medical treatment.

Chemical Reactions
Breaking/forming bonds via electrons
 Start and end with same number and type
of atoms (conservation of mass)
 DH ≈ -1 - 1000 kJ
 Rates affected by temperature, pressure,
concentration, catalyst

Nuclear Reactions
Change one element into another via
changes in nucleus (protons, neutrons)
 Change elements (lead CAN be turned
into gold)
 DH can be -1010 kJ
 Rates can only be affected by changing
concentrations

Definitions

Recall notations used for isotopes:
A
Z
X
A = mass number: # neutrons + # protons
 Z = atomic number: # protons

12
13
14
6
6
6
 Three isotopes of carbon. What’s different?
C

C
C
Protons and neutrons are collectively
called nucleons. The nucleus of a
specific isotope is called a nuclide.
Two types of Nuclear Reactions
Radioactive decay/emission: an
unstable atom emits a particle or energy.
This is a naturally occurring process and
we can’t stop or change it.
14
14
0
 Example: 6 C ® 7 N + -1e
 Transmutation: when atoms are
bombarded by particles, a reaction might
occur
14
4
17
1
 Example: 7 N + 2 He ® 8 O + 1H

22.1 Nuclear Reactions

Radiation arises from nuclear reactions:
parent nuclide  daughter nuclide + radiation
To balance, two conditions must be met:
• Conserve mass number (A)
• Conserve nuclear charge (Z)
 If we know two of the nuclear particles, we
can use these rules to identify the third
particle.

Nuclear Particles
4
2
Alpha particle: a = He ; written as He
0
 Beta particle (an e-): b = -1
e
0
 Gamma ray: g = high energy wave = 0 g
0
 Positron particle: b+ = +1 e
1
0
 Neutron: n = 0 n
1
+
 Proton: p = 1 p
 Table 22.1

4
2
2+
Penetrating Ability of Particles
Types of Radiation



Alpha least penetrating; stopped by aluminum
foil (> 10-3 cm), paper, skin
Heavy radioactive isotopes (radioisotopes) tend
to emit alpha particles
Predict products:
226 Ra 
88
Used in radiation therapy
232
90Th

Beta Emission/Decay, Positron
Emission/Decay

Beta - b=0-1e; more penetrating - stopped by
0.05 - 0.1 cm of aluminum



Beta emission: neutron is converted to a proton and
an electron
Positron emission: proton is converted to a neutron
and a positron
Predict products:




Beta emission: 239U 
Beta emission: 131I 
Positron emission: 207Po 
Positron emission: 40K 
Gamma Radiation (Emission)
Gamma - g = energy with no mass or
charge; most penetrating radiation
 Stopped by 5 - 11 cm of aluminum or thick
layer of concrete or lead
 Lead is commonly used to enclose
radioactive materials because radiation
does not penetrate readily
 In the 1950s, it was common to build thick
concrete bomb shelters

Gamma Radiation (Emission)

Gamma radiation is really a stream of
high-energy photons. Neither mass nor
atomic number changes. Only energy is
emitted:
Nucleus is highly energetic/unstable =
metastable (m or * after mass number)
11m
11
0
 .
5 B ® 5 B + 0g

 224*88Ra
 22488Ra + g
Electron Capture

Electron capture is the opposite of beta
emission. A proton combines with an
inner-shell electron to become a neutron:
197
80

Hg + e ® ____
0
-1
Worked example 22.1, Problems 22.1, 22.2
Group Work

Write balanced equations for:
1.Alpha emission from curium-242
2.Beta emission from magnesium-28
3.Positron emission from xenon-118
4.Electron capture by polonium-204

What particle is produced by decay of
thorium-214 to radium-210?
22.4 Nuclear Stability


Unstable isotopes are the radioactive ones they decay too rapidly to be measured.
“Stable” isotopes have measurable isotopes.
Every element up to Bi (except Tc) has one or
more stable isotopes. Elements above Bi are
all unstable.
The “Band of Stability” plots the stable atoms
(protons vs neutrons).
 Neutron/proton ratio helps predict stability.
Spontaneous Decay
Band of stability (and the higher “island of
stability”)

For elements 1-20,
N = Z (N/Z = 1)

For elements > 20,
N > Z (N/Z < 1.6)
Radioactivity


Nuclides above the band (upper left) often
undergo beta decay; nuclides below the band
undergo positron emission, electron capture, or
alpha emission.
The heavier elements (93 and higher) are
synthetic and radioactive. Elements with molar
mass in parentheses on the periodic table are
radioactive. Why don’t these have a more precise
molar mass?
Spontaneous Nuclear Decay

Figure 22.5

Nuclei with higher
neutron/proton ratios
tend to emit beta
particles.
Nuclei with lower
neutron/proton ratios
tend to favor positron
emission, electron
capture, or alpha
emission.

Nuclear Stability
Figure 22.4
 Applet:
http://www.colorado.e
du/physics/2000/appl
ets/iso.html

Figure 22.6 Uranium Series
Kinetics of Radioactive Decay


The rate of decay, as well as the
type and energy of the radiation,
determines the damage caused
by radiation.
Nuclear decay follows first-order
kinetics, which gives a constant t1/2
over the course of the decay. Nuclear
decay rates are also independent of
temperature. Usually we cite t1/2
instead of a rate constant.
Decay of a Radionuclide
Successive half-lives have the same
value.
 The number of radioactive nuclei
decrease by a factor of 2 during each
half-life.
 Figure 22.2

Kinetics

ln (N / No) = -kt

N = number of atoms of a nuclide (same
as A from Ch. 12)
 t1/2
= 0.693 / k
 N = 1/2 No at t = t1/2

N
0.693t
Combining the equations: ln (
)=(
)
N0
t1
2

Table 22.2: Common Half-life values
Kinetics


Can use kinetics, just like in Chapter 12, to do
various calculations.
ln(N/No) = -0.693t / t1/2



How much nuclide is left after ___ time?
How much time has elapsed if decay is ___%
complete?
Use kinetics to determine storage time of
nuclear waste.
(Usually 10 half-lives: 2-10 = 0.000977, so 99.9023%
has decayed)
51Cr (t
1/2 = 27.8 days) is stored for > 10 months
Group Work

The decay constant for sodium-24, a
radioisotope used medically in blood studies, is
4.63 x 10–2 h–1. What is the half-life of 24Na?

Gold-128 undergoes beta decay to give
mercury-128 with a half-life of 2.7 days. What
percent of gold-128 is left after 14 days?

Worked example 22.2-5, Problems 22.3-8
C -14 Dating
http://www.youtube.com/watch?v=81dWTeregEA&feature=related
22.10 Archeological Dating




The decay of radioactive isotopes allows for the
relative age of once-living things to be
calculated.
Radiocarbon dating uses 14C
 Living things have a dynamic equilibrium of
14C in their system (ex: inhaling/exhaling)
When a plant or animal dies, it no longer
incorporates new 14C, and the 14C begins to
decay, causing the 14C content to become less
than that in the atmosphere.
14 C  14 N + 0 e6
7
-1
Radiometric Dating
Geological Dating


Geological dating is similar to archeological
dating, but uses longer-lived nuclides
Measure ratio of 40K to 40Ar in rocks
 4019K + 0-1e  4018Ar
 4019K
 40K


http://www.youtube.com/watch?v=1920gi3swe4
 4020Ca + 0-1e
has a half-life of 1.28x109 years
Rocks are crushed, 40Ar escapes, and the ratio
of 40K to 40Ar is measured.
This allowed the age of the earth to be
calculated to be 4.5 billion years.
Global Energy Sources


http://www.world-nuclear.org/info/inf01.html
http://www.nucleartourist.com/basics/why.htm

List of advantages/disadvantages
of each energy source.
Natural Sources of Radiation



Can damage tissue cells
Radiation comes continuously from many
sources besides nuclear power plants and
applications of isotopes
Natural radiation sources:
granite
soil
water
food
air
brick
concrete
cosmic rays (airplane flights)
radon in houses
Nuclear Reactions



Nuclear fission: bombarding nuclei of heavier
isotopes by neutrons splits them into lighter,
more stable nuclei.
Nuclear fusion: nuclei of light atoms are joined
together or fused. This requires very hot
conditions (i.e., the sun).
Bombardment/Transmutation: two elements
(usually one heavy, one light) are bombarded
by each other to create heavier elements
Chain Reaction



One step in a chain reaction produces more neutrons than
it consumes. Each step gets faster.
Figure 22.8
Critical mass is the mass required for the chain reaction to
become self-sustaining.
Chain Reaction
Nuclear Reactor
The same fission process that leads to a
nuclear explosion can be used to
generate electric power.
 Nuclear Reactors “control” the fission
of 235U and use the energy produced to
heat water that drives steam turbines.
(Figure 22.9)

Nuclear Reactor

Figure
22.9
Reactor core

Can use B or Cd control rods to “control”
the rate of the fission reaction. They can
be raised and lowered as needed.
Nuclear Power




Energy released from 200 g U = 2x1013 J
Energy released from 1 ton coal = 5x107 J
It would take 1 million tons of coal to create the
same amount of energy.
Nuclear power plants don’t explode (like TNT
unless an uncontrolled nuclear chain reaction
occurs). When nuclear power plants have
meltdowns, the threat is from leakage of
radioactive material (water, airborne, soil
contamination, etc.)
Palo Verde Notes




14.2% of world’s energy production is nuclear;
20% of that from U.S.
104 reactors in U.S., 338 reactors outside U.S.;
300 NEW reactors projected by 2030
Palo Verde uses Boron control rods, runs at
max. capacity (control rods raised); reactor
solution is boric acid
Fukushima: inner and outer housing; outer
housing was damaged in tsunami; no inner
housing for the stored fuel rods - thus
contamination
Nuclear Reactor
Storage of fission products is a major
challenge.
 Chernobyl (1986) used no containment, so
the radiation leak was worse than from
Three Mile Island (1979).


Schematic
of underground
storage
system
Chernobyl
Ukraine, 1986
 Only 2 people
died in the
initial explosion; most died
from or were
affected by radiation exposure

Three Mile Island - PA; 1979

The accident started at 4 am when the water
pumps that supplied the steam generators of
the plant stopped abruptly. The subsequent
lack of steam was detected by the plant's safety
system which then automatically shut off the
steamturbine powering the generator. The three
primary causes of the Three Mile Island nuclear
accident included mistakes made by
employees, faulty plant design, and the
breakdown of key elements in the proper
operation of the plant.

http://www.eoearth.org/article/Three_Mile_Island,_Pennsylvania
Japan Nuclear Plant Disaster
 Worst accident since Chernobyl in 1986
 2nd- worst nuclear power accident ever
(Chernobyl was much worse)
 NOT caused directly by earthquake
 Loss of cooling of nuclear fuel due to
power loss following tsunami
 Similar logistical problems at most
U.S. plants involve powering of
pumps, NOT quake/tsunami risks
 Possible exception is Diablo
Canyon, CA, on coast, near San
Andreas Fault
9.0 Japan Quake - 3/11/11
Japan Nuclear Plant Disaster
 Does not appear any dangerous radiation will affect U.S.
(Hawaii or mainland)
 Buying iodine or Geiger counters in AZ is a waste of money
and time
 Online resources for Japan nuclear power plant:
 http://serc.carleton.edu/NAGTWorkshops/energy/nuclear.html
 Online resources for 9.0 Japan quake and tsunami:
 http://serc.carleton.edu/NAGTWorkshops/visualization/collections/jap
an2011.html
9.0 Japan Quake - 3/11/11
Nuclear Fusion






Nuclear Fusion is the formation of heavier
nuclei by the joining of lighter ones.
Fusion products are generally not radioactive.
Smaller atoms are fused together into one larger
atom: 11H + 21H  32He (i.e., in the sun)
Called thermonuclear because they require
extremely high temperatures.
Positive side: H is cheap, plentiful; He is waste
which isn’t toxic
Negative side: Still have trouble controlling the
reactions.
Nuclear Fusion

Iron is the end of fusion and the heaviest element found
in stars but it can become unstable through neutron
capture. Once it turns into Fe-59 it is unstable enough to
beginning decaying and will eventually become Co-59
which will become the unstable Co-60 and decay into Ni,
etc. Supernovas are able to form even heavier elements
due to the sheer amount of neutrons bombarding
elements before they have time to decay.
http://lifeng.lamost.org/courses/astrotoday/CHAISSON/A
T321/HTML/AT32104.HTM
Atomic vs Hydrogen Bombs



Atomic weapons utilize the splitting of atoms
(235U) and are detonated through fission and its
resulting chain reaction. Hiroshima = 13 ktons
Hydrogen bombs are detonated through fusion.
They are 1000 times more explosive than
atomic bombs. Energy is released because of
overall loss of mass. Typical = 10 megatons!
Great site - summary of nuclear weapons:
http://library.thinkquest.org/3471/abomb.html
Nuclear Transmutation




Bombarding one element with another to
create a different element.
Occurs naturally in space, not on earth.
Preparation of Tc for medical imaging:
97 Mo + 2 H  97 Tc + 21 n
42
1
43
0
Synthesis of transuranium elements:
238 U + 4 He  239 Pu + 31 n
92
2
94
0
238 U + 12 C  246 Cf +41 n
92
6
98
0
238 U + 14 N  247 Es + 51 n
92
7
99
0
Bombardment Reactions




In September 1982, two new elements were
formed by bombardment with heavy nuclides:
58 Fe + 206 Pb  265
1 n
Hs
+
26
82
108
0
58 Fe + 209 Bi  266
1
26
83
109Mt + 0n
Prepared only a few atoms of each
The interest in preparing new elements is
generated in part by the prediction that there
will be another island of stability among
elements with Z~114 or 126, N~184.
Worked example 22.9, Problem 22.15-16
Particle Accelerators

Linear Accelerator
A beam of charged particles is accelerated
by passing it through a series of tubes
having alternating electrical charge.
 The tubes get successively longer because
the particles are moving faster.

Linear Accelerator
Australian Synchotron
Circular Accelerators

Circular accelerator
cyclotron
 A magnetic field is used to keep the particles
in a circular pathway.
 Alternating current on the “D”s is used to
accelerate the particles.
 Some cyclotrons have a radius of 1 km.
 The Large Hadron Collider (LHC) is 27
km/16.8 miles in diameter.

Cyclotron - LHC
Large Hadron
Collider:
(Switzerland and
France). People
thought that its use
in Sept ‘08 would
create a black hole.
Obviously not true!
Detection of Radioactive Decay
 Geiger

counter
activity = number of nuclei disintegrating
per unit time; units = Curie (Ci)
Biological Effects of Radiation

Calculate your radiation exposure (mrem):








Cosmic radiation at sea level
Add 1 for every 250 ft elevation
Radiation from earth
Building materials in houses
Radon gas from the ground
Radiation from food and water
Jet plane travel (9.5 mrem/hr)
If you smoke (~1300 mrem/yr)
27 mrem/year
4 (for Phoenix)
28
4
200
39
___
___
Biological Effects of Radiation
Average medical exposure
 Add 6 for each chest x-ray
 Add 245 for intestinal x-ray
 Smoke detectors
 Power plants


25-55 mrem/yr
____
____
10
1
Your total exposure this year: ___ mrem
Biological Effects of Radiation

Normal exposure = 200 mrem (0.2 rem) per
year, which produces no observable effects
 0-25 rem: no effect
 25-100 rem: temporary blood cell changes
 100-300 rem: radiation sickness
 400 rem: 50% chance of death
 > ~600 rem: 100% chance of death
 50,000 rem needed to kill bacteria
 1,000,000 rem needed to kill viruses
Applications of Nuclear Chem.
Archeological/geological dating (already
discussed)
 Nuclear medicine and radiotracers
 Food irradiation
 Small power generators for space
vehicles, submarines, and pacemakers
 Smoke alarms
 Warfare

Applications of Isotopes

Medical diagnoses
 99Tc for tumors in spleen, liver, brain,
thyroid
• tracer put into a metabolite that
concentrates in cancerous cells
 131I
or 123I in thyroid
 Cancer therapy -- destroy cells with g rays
 131I for thyroid cancers
 198Au for lung cancer
 32P for eye tumors
Imaging Procedures
X-Rays
 MRI: magnetic resonance imaging (no
radioisotopes used) - radio waves
stimulate H nuclei that give off a signal
that can be interpreted for anomalies
 PET scans: Positron Emission
Tomography

X-Ray
PET scans

Use radiotracers to
measure amounts
absorbed by the
body to determine
structure and
function of organs
and tissues.
Thyroid Imaging - PET Scan
Food Irradiation
Food can be irradiated with gamma rays
from 60Co or 137Cs.
 Irradiated milk has a shelf life of 3 mo.
without refrigeration.
 USDA has approved irradiation of meats
and eggs.
