William L Masterton
Cecile N. Hurley
http://academic.cengage.com/chemistry/masterton
Chapter 19
Nuclear Reactions
Edward J. Neth • University of Connecticut
Outline
1.
2.
3.
4.
5.
Radioactivity
Rate of Radioactive Decay
Mass-Energy Relations
Nuclear Fission
Nuclear Fusion
Nuclear Reactions vs. Chemical Reactions
• In a chemical reaction
• Only the outer electron configuration of atoms and
molecules changes
• There is no change to the nucleus
• In a nuclear reaction
• Mass numbers may change
• Atomic numbers may change
• One element may be converted to another
Nuclear Symbols
• Recall that a nuclear symbol begins with the element symbol
• Mass number is at the top left
• Protons + neutrons
• Atomic number is at the bottom left
• Number of protons = number of electrons
12
6
C
14
6
C
Nuclear Equations
• Must always balance with respect to nuclear mass and charge
14
7
N n C H
1
0
14
6
1
1
• Notice
• Total mass on the left is 15 and the total mass on the right
is 15
• Total charge on the left is 7 and total charge on the right is 7
Radioactivity
• Radioactive nuclei spontaneously decompose
(decay) with the evolution of energy
• Radioactivity may be
• Natural; there are a few nuclei that are by nature
radioactive
• Induced; many nuclei can be made radioactive
by bombarding them with other particles
Five Modes of Radioactive Decay
• We will consider five modes of radioactive decay
• Alpha (α) particle emission
• Beta (β) particle emission
• Gamma (γ) radiation emission
• Positron emission
• K-electron capture
Alpha Particle Emission
• An alpha particle is a helium nucleus
• Mass is 4, charge is +2, atomic number 2
• Symbol is
4
or α
2 He
• When a nucleus emits an alpha particle, its mass decreases by
4 and its atomic number decreases by 2
238
92
U He Th
4
2
234
90
Beta Particle Emission
• Beta particles are high speed electrons
• Mass is zero, charge is -1
• Mass number does not change
• Effectively the conversion of a neutron into a proton with the
emission of an electron
• Atomic number increases by 1
Th e Pa
234
90
0
1
234
91
Gamma Radiation Emission
•
•
•
•
Gamma rays are photons
Mass number is zero
Charge is zero
No change in atomic number or mass number
Positron Emission
• Positrons are anti-electrons
• Mass 0
• Charge +1
• No change in mass number
• Effectively a conversion of a proton into a neutron
• Atomic number decreases by 1
40
19
K  e Ar
0
1
40
18
K-electron Capture
• Innermost electron (n=1) falls into the nucleus
• Effect is the same as for positron emission
• No change in the mass number
• Atomic number decreases by 1
82
37
Rb e Kr
0
1
82
36
Example 19.1
Induced Radioactivity - Bombardment
• More than 1,500 isotopes have been prepared in the
laboratory
• Stable nuclei are bombarded with
• Neutrons
• Charged particles (electron, positron, alpha)
• Other nuclei
• The result is a radioactive nucleus
Examples of Bombardment Reactions
27
13
Al  n  Al
28
13
Al  Si  e
1
0
28
14
• Aluminum-27 is converted
to radioactive aluminum-28
by neutron bombardment,
which decays by beta
emission
28
13
0
1
27
13
Al  He P  n
30
15
30
P 14
Si  01e
4
2
30
15
1
0
• Aluminum-27 is converted
to phosphorus-30 by alpha
particle bombardment; P30 decays by positron
emission
Transuranium Elements
• Elements beyond uranium are synthetic, having
been prepared by bombardment reactions
• Most nuclei produced have very short half-lives
• In some cases, only the decay products are
observed
• As of October, 2006 the heaviest element
reported is Element 118, Uuo-294
Table 19.1
Applications of Isotopes
• Medicine
• Some isotopes find use in medical diagnostics
and treatment
• Cancer treatment
• Iodine-131 for thyroid cancer
• Cobalt-60 for treatment of malignant cells
• Diagnostics
• PET, positron emission tomography: carbon-11
• Radioactive labeling
Table 19.2 – Medical Uses of Radioisotopes
Cobalt-60 Therapy
Chemical Applications
• Neutron activation analysis
• Sample bombarded by neutrons, inducing radioactivity
• Isotopes normally decay by gamma emission
• Activation of strontium in bones of fossils can indicate
something about the diet, since plants contain more
strontium than animals
84
38
Sr  n Sr
1
0
85
38
Commercial Applications
• Smoke detectors
• Americium-241
• Radioactive source ionizes air, which completes a
circuit; smoke particles open the circuit and trip
the alarm
Figure 19.1 – Smoke Detector
Food Irradiation
• Gamma radiation treatment
• Kills insects, larvae and parasites
• Food that is irradiated has a longer shelf life and
can be rid of parasites such as trichina in pork
Figure 19.1 – Irradiated Strawberries
Rate of Radioactive Decay
rate  kX
Xo
ln
 kt
X
0.693
k
t1
2
• Radioactive decay is a firstorder process
• The equations for first-order
reactions from Chapter 11
apply to radioactive decay
• k is the first-order rate
constant
• t1/2 is the half life
• X is the amount of sample
at time t
• X0 is the amount of sample
at t=0
Activity
• Activity is the rate of decay
• Number of atoms per unit time
• A = kN
• Units of activity
• 1 Becquerel (Bq) = 1 atom/sec
• 1 Curie (Ci) = 3.700 X 1010 atoms/sec
Example 19.2
Example 19.2, (Cont’d)
Figure 19.3 – Scintillation Counter
Age of Organic Material
• W.F. Libby, University of Chicago, 1950s
• Age of organic material related to the decay of carbon-14
• Carbon-14 forms in the upper atmosphere by bombardment
of nitrogen-14 by neutrons
N n C H
14
0
14
1
7
1
6
1
• Carbon-14 incorporates itself into living things
• Steady-state while the organism is alive
• Once an organism dies, C-14 level falls due to radioactive decay
• The original rate of decay is 15.3 atoms/min
• Half-life of C-14 is 5730 yr
Example 19.3
The Shroud of Turin
• A sample of 0.1 g of the Shroud of Turin was
analyzed for its C-14 content
• Evidence showed the flax used to weave the
shroud dated from the 14th century
• Could not have been the burial cloth of Christ
Mass-Energy Relations
• The energy change accompanying a nuclear reaction can be
calculated from the equation
E  c m
2
• Where
• Δm = change in mass = mass of products minus mass of
reactants
• ΔE = change in energy = energy of products – energy of
reactants
• c is the speed of light
Change in Mass
• In any spontaneous nuclear reaction, the products
weigh less than the reactants
• Therefore, the energy of the products is less than
the energy of the reactants
• There is a release of energy when the reaction
takes place
Units
m
c  3.00 X 10
s
8
m2
E  9.00 X 10
 m
2
s
m2 m2
J
1J  1kg 2 ;1 2  1
s
s
kg
J
J
E  9.00 X 1016
m  9.00 X 1010
m
kg
kg
16
Example 19.4
Example 19.4, (Cont’d)
Nuclear Binding Energy
• The nucleus weighs less than the sum of the
individual masses of the neutrons and protons
• This is called the mass defect
• The mass defect leads to the binding energy,
which holds the nucleus together
Binding Energy of Lithium-6
• Mass of one mole: 6.01348 g
• Mass of nucleons:
• (3 X 1.00867)+(3 X 1.00728) = 6.04785g
• Mass defect: 6.04785 - 6.01348 = 0.03437g/mol
• ΔE = 9.00 X 1010 kJ/g X 0.03437g = 3.09 X 109
kJ/mol
Example 19.5
Figure 19.4
Nuclear Stability and the Binding Energy
• Binding energy per mole of nucleons
• Divide the binding energy by the number of
nucleons
• For Li-6 this is
• 3.09 X 109 kJ/mol Li-6 X 1 mol Li-6/6 mol nucleons =
5.15 X 108 kJ/mol
• Release of the binding energy
• Nuclear fission: split large nucleus into smaller ones
• Nuclear fusion: fuse small nuclei into larger ones
Nuclear Fission
• Discovery, 1938
• Otto Hahn
• Lise Meitner
• World War II
• The Manhattan Project – produced the first atomic
bomb
• First nuclear explosion, July 16, 1945
• Hiroshima, August 6, 1945
• Nagasaki, August 9, 1945
The Fission Process
• Uranium-235 is 0.7% of naturally occurring uranium
• U-235 undergoes fission
• Splits into two unequal fragments
• Releases more neutrons than are consumed
The Fission Process (Cont’d)
• The first products of nuclear fission are radioactive and
decay by beta emission
• Note that in the fission process, more neutrons are produced
than consumed
• A chain reaction results
• Energy is released due to the conversion of mass into
energy
Chain Reactions
• To sustain a chain reaction, the sample of fissile
material must be large enough to contain the
neutrons that are generated
• Samples that are too small will not sustain a chain
reaction
• The sample that will sustain a chain reaction is
called a critical mass
Nuclear Reactors
• About 20% of the electricity generated in the US
comes from the fission of U-235 in nuclear reactors
• US reactors are called light water reactors
• UO2 pellets in a zirconium alloy tube
• Control rods are used to moderate the reaction
• Can be inserted to absorb neutrons
• Prevent a runaway chain reaction
• Tremendous amount of heat is produced, which turns
water to steam and turns a turbine to produce electricity
• Ordinary water is used both to cool the reaction and to slow the
neutrons
• Most reactors use ordinary (light) water
Heavy Water Reactors
• Canadian reactors (CANDU)
• Use D2O (2H2O) as a moderator
• The use of D2O allows the use of natural uranium
without enrichment
• Enrichment is the process of increasing the U-235
content to a few percent from 0.7%
• Enrichment is an expensive, technologically demanding
process
• Done by gaseous effusion
• UF6
Nuclear Energy and History
• In the 1970s it was assumed that nuclear reactors
would replace fossil fuels (oil, gas, coal) as the major
source of electricity
• In France, this has indeed happened
• In the US, this has not happened
• Accident at Three Mile Island, Chernobyl
• Disposal of radioactive waste
Figure 19.5 – Pressurized Water Reactor
Nuclear Fusion
• Light isotopes such as hydrogen are unstable with
respect toward fusion into heavier isotopes
• Considerably more energy is released in fusing light
nuclei than in splitting heavy nuclei
Example 19.6
Example 19.6 (Cont’d)
Issues with Nuclear Fusion
• As an energy source, nuclear fusion has several
advantages over fission
• Light isotopes are more abundant than heavy
ones
• Greater energy release
• Non-radioactive products
• Disadvantages
• Large activation energies
• High temperatures are difficult to contain
Nuclear Fusion and Stars
Figure 19.6
Key Concepts
1. Write balanced nuclear reactions
2. Relate activity to rate constant and number of
atoms
3. Relate activity to age of objects.
4. Relate m and E in a nuclear reaction
5. Calculate binding energies
Key Concepts
1. Draw a diagram for a voltaic cell, labeling the
electrodes and diagramming current flow.
2. Use standard potentials to
Compare relative strengths of oxidizing and
reducing agents.
Calculate E and/or reaction spontaneity.
3. Relate E° to ΔG° and K.
4. Use the Nernst equation to relate voltage to
concentration.
5. Relate mass of product to charge, energy or current
in electrolysis reactions.