Download Chapter 21 Nuclear Chemistry

Chapter 21
Nuclear Chemistry
Nuclear
Chemistry
Radiocarbon Dating
Radioactive C-14 is formed in the upper
atmosphere by nuclear reactions initiated by
neutrons in cosmic radiation
14N + 1 n ---> 14C + 1H
o
The C-14 is oxidized to CO2, which circulates
through the biosphere.
When a plant dies, the C-14 is not replenished.
But the C-14 continues to decay with t1/2 =
5730 years.
Activity of a sample can be used to date the Nuclear
Chemistry
sample.
Nuclear Medicine: Imaging
Thyroid imaging using Tc-99m
Nuclear
Chemistry
Nuclear Reactions vs.
Normal Chemical Changes
• Nuclear reactions involve the
nucleus
• The nucleus opens, and protons and
neutrons are rearranged
• The opening of the nucleus releases
a tremendous amount of energy that
holds the nucleus together – called
binding energy
• “Normal” Chemical Reactions
involve electrons, not protons and Nuclear
Chemistry
neutrons
Food Irradiation
•Food can be irradiated with g rays
from 60Co or 137Cs.
•Irradiated milk has a shelf life of 3 mo.
without refrigeration.
•USDA has approved irradiation of
meats and eggs.
Nuclear
Chemistry
Mass Defect
• Some of the mass can be converted
into energy
• Shown by a very famous equation!
2
E=mc
Energy
Mass
Speed of light
Nuclear
Chemistry
The Nucleus
• Remember that the nucleus is comprised of
the two nucleons, protons and neutrons.
• The number of protons is the atomic number.
• The number of protons and neutrons together
is effectively the mass of the atom.
Nuclear
Chemistry
Isotopes
• Not all atoms of the same element have
the same mass due to different
numbers of neutrons in those atoms.
• There are three naturally occurring
isotopes of uranium:
Uranium-234
Uranium-235
Uranium-238
Nuclear
Chemistry
Radioactivity
• It is not uncommon for some nuclides of
an element to be unstable, or
radioactive.
• We refer to these as radionuclides.
• There are several ways radionuclides
can decay into a different nuclide.
Nuclear
Chemistry
Types of Radiation
(ά) – a positively
charged helium isotope • Alpha
we usually ignore the charge because it
involves electrons, not protons and
neutrons
•Beta (β) – an electron
•Gamma (γ) – pure energy;
called a ray rather than a
particle
4
2
He
0
1
0
0
e
g
Nuclear
Chemistry
Penetrating Ability
Nuclear
Chemistry
Other Nuclear Particles
• Neutron
1
0
n
• Positron – a positive
electron
0
1
e
•Proton – usually referred
to as hydrogen-1
•Any other elemental
isotope
1
1
H
Nuclear
Chemistry
Types of
Radioactive Decay
Nuclear
Chemistry
Alpha Decay:
Loss of an -particle (a helium nucleus)
4
2
238
92
U

He
234
90
U
+
4
2
He
Nuclear
Chemistry
Beta Decay:
Loss of a -particle (a high energy electron)
0
−1
131
53
I

0
or −1
131

54
e
Xe
+
0
−1
e
Nuclear
Chemistry
Positron Emission:
Loss of a positron (a particle that has the
same mass as but opposite charge than
an electron)
0
1
11
6
C
e
11

5
B
+
0
1
e
Nuclear
Chemistry
Gamma Emission:
Loss of a g-ray (high-energy radiation that
almost always accompanies the loss of a
nuclear particle)
0
0
g
Nuclear
Chemistry
Electron Capture (K-Capture)
Addition of an electron to a proton in the
nucleus
As a result, a proton is transformed into a
neutron.
1
1
p
+
0
−1
e
1

0
n
Nuclear
Chemistry
Learning Check
What radioactive isotope is produced in
the following bombardment of boron?
10B
5
+ 4He
2
? +
1n
0
Nuclear
Chemistry
Write Nuclear Equations!
Write the nuclear equation for the beta
emitter Co-60.
Nuclear
Chemistry
Neutron-Proton Ratios
• Any element with more
than one proton (i.e.,
anything but hydrogen)
will have repulsions
between the protons in
the nucleus.
• A strong nuclear force
helps keep the nucleus
from flying apart.
Nuclear
Chemistry
Neutron-Proton Ratios
• Neutrons play a key role
stabilizing the nucleus.
• Therefore, the ratio of
neutrons to protons is an
important factor.
Nuclear
Chemistry
Neutron-Proton Ratios
For smaller nuclei
(Z  20) stable
nuclei have a
neutron-to-proton
ratio close to 1:1.
Nuclear
Chemistry
Neutron-Proton Ratios
As nuclei get
larger, it takes a
greater number of
neutrons to
stabilize the
nucleus.
Nuclear
Chemistry
Stable Nuclei
The shaded region in
the figure shows
what nuclides would
be stable, the socalled belt of stability.
Nuclear
Chemistry
Stable Nuclei
• Nuclei above this
belt have too many
neutrons.
• They tend to decay
by emitting beta
particles.
Nuclear
Chemistry
Stable Nuclei
• Nuclei below the
belt have too many
protons.
• They tend to
become more stable
by positron emission
or electron capture.
Nuclear
Chemistry
Stable Nuclei
• There are no stable nuclei with an
atomic number greater than 83.
• These nuclei tend to decay by alpha
emission.
Nuclear
Chemistry
Radioactive Series
• Large radioactive
nuclei cannot stabilize
by undergoing only
one nuclear
transformation.
• They undergo a series
of decays until they
form a stable nuclide
(often a nuclide of
lead).
Nuclear
Chemistry
Some Trends
Nuclei with 2, 8, 20,
28, 50, or 82
protons or 2, 8, 20,
28, 50, 82, or 126
neutrons tend to be
more stable than
nuclides with a
different number of
nucleons.
Nuclear
Chemistry
Some Trends
Nuclei with an even
number of protons
and neutrons tend to
be more stable than
nuclides that have
odd numbers of
these nucleons.
Nuclear
Chemistry
Nuclear Transformations
Nuclear
transformations
can be induced
by accelerating
a particle and
colliding it with
the nuclide.
Nuclear
Chemistry
Particle Accelerators
These particle accelerators are enormous,
having circular tracks with radii that are
miles long.
Nuclear
Chemistry
Half-Life
• HALF-LIFE is the time that it takes for
1/2 a sample to decompose.
• The rate of a nuclear transformation
depends only on the “reactant”
concentration.
Nuclear
Chemistry
Kinetics of Radioactive Decay
• Nuclear transmutation is a first-order
process.
• The kinetics of such a process, you will
recall, obey this equation:
A
= -kt
ln
A0
Nuclear
Chemistry
Half-Life
Decay of 20.0 mg of 15O. What remains after 3 half- Nuclear
Chemistry
lives? After 5 half-lives?
Kinetics of Radioactive Decay
• The half-life of such a process is:
0.693
= t1/2
k
• Comparing the amount of a radioactive
nuclide present at a given point in time
with the amount normally present, one
can find the age of an object.
Nuclear
Chemistry
Learning Check!
The half life of I-123 is 13 hr. How much
of a 64 mg sample of I-123 is left after
39 hours?
Nuclear
Chemistry
Kinetics of Radioactive Decay
A wooden object from an archeological site is
subjected to radiocarbon dating. The activity
of the sample that is due to 14C is measured
to be 11.6 disintegrations per second. The
activity of a carbon sample of equal mass
from fresh wood is 15.2 disintegrations per
second. The half-life of 14C is 5715 yr. What
is the age of the archeological sample?
Nuclear
Chemistry
Kinetics of Radioactive Decay
First we need to determine the rate
constant, k, for the process.
0.693
= t1/2
k
0.693
= 5715 yr
k
0.693
=k
5715 yr
1.21  10−4 yr−1 = k
Nuclear
Chemistry
Kinetics of Radioactive Decay
Now we can determine t:
A
= -kt
ln
A0
11.6
−4 yr−1) t
ln
=
-(1.21

10
15.2
ln 0.763 = -(1.21  10−4 yr−1) t
6310 yr = t
Nuclear
Chemistry
Measuring Radioactivity
• One can use a device like this Geiger counter to
measure the amount of activity present in a
radioactive sample.
• The ionizing radiation creates ions, which conduct
a current that is detected by the instrument.
Nuclear
Chemistry
Effects of Radiation
Nuclear
Chemistry
Nuclear
Chemistry
Energy in Nuclear Reactions
• There is a tremendous amount of
energy stored in nuclei.
• Einstein’s famous equation, E = mc2,
relates directly to the calculation of this
energy.
Nuclear
Chemistry
Energy in Nuclear Reactions
• In the types of chemical reactions we
have encountered previously, the
amount of mass converted to energy
has been minimal.
• However, these energies are many
thousands of times greater in nuclear
reactions.
Nuclear
Chemistry
Energy in Nuclear Reactions
For example, the mass change for the decay
of 1 mol of uranium-238 is −0.0046 g.
The change in energy, E, is then
E = (m) c2
E = (−4.6  10−6 kg)(3.00  108 m/s)2
E = −4.1  1011 J
Nuclear
Chemistry
Nuclear Fission
• How does one tap all that energy?
• Nuclear fission is the type of reaction carried
out in nuclear reactors.
Nuclear
Chemistry
Nuclear Fission
• Bombardment of the radioactive nuclide with
a neutron starts the process.
• Neutrons released in the transmutation strike
other nuclei, causing their decay and the
Nuclear
production of more neutrons.
Chemistry
Nuclear Fission
This process continues in what we call a
nuclear chain reaction.
Nuclear
Chemistry
Nuclear Fission
If there are not enough radioactive nuclides in the
path of the ejected neutrons, the chain reaction
will die out.
Nuclear
Chemistry
Nuclear Fission
Therefore, there must be a certain minimum
amount of fissionable material present for the
chain reaction to be sustained: Critical Mass.
Nuclear
Chemistry
Nuclear Reactors
In nuclear reactors the heat generated by the
reaction is used to produce steam that turns a
turbine connected to a generator.
Nuclear
Chemistry
Nuclear Fission & POWER
• Currently about 103
nuclear power plants
in the U.S. and about
435 worldwide.
• 20% of the world’s
energy comes from
nuclear.
Nuclear
Chemistry
Figure 19.6: Diagram of a nuclear power plant.
Nuclear
Chemistry
Nuclear Reactors
• The reaction is kept in
check by the use of
control rods.
• These block the paths of
some neutrons, keeping
the system from reaching
a dangerous supercritical
mass.
Nuclear
Chemistry
Nuclear power plants
• All power plants produce steam which turns turbines
to produce energy. Sources of heat to produce
steam: heat from fission, coal, oil, natural gas, water
• Nuclear power plant
• 1 pellet of U-235 produces as much energy as 149
gallons of oil, 157 gallons gasoline, 1780 pounds of
coal
• A pellet is about as big as a pencil eraser
• One reactor contains 193 assemblies of rods,
260 rods / assembly w/ 230 pellets per rod
Nuclear
Chemistry
1 million gallons of steam go through the turbine
per minute. Turbine spins at 1800 rpm’s.
395,000 gallons of water in the core are heated to
619 F with a pressure of 2235 psi
• Every 1.5 years 1/3 of the core is replaced.
Takes 40 days to refuel – carried down man
made “canals” to a huge swimming pool on site
• Standing outside of a containment building you
would get 5 milirem exposure of radiation: 10
mrem exposure from x-rays, 15 mrem from 2
Nuclear
packs of cigarettes
Chemistry
Nuclear Fusion
• Fusion would be a
superior method of
generating power.
 The good news is that the
products of the reaction are
not radioactive.
 The bad news is that in
order to achieve fusion, the
material must be in the
plasma state at several
million kelvins.
Nuclear
Chemistry
Nuclear Fusion
• Tokamak apparati like the
one shown at the right
show promise for carrying
out these reactions.
• They use magnetic fields
to heat the material.
Nuclear
Chemistry