CHM 123
Chapter 22
21.1 – Energy Changes During Nuclear Reactions:
*Recall: Activation energy is the potential energy barrier that must be surmounted before reactants can be converted
to products. The height of the barrier controls the rate of the reaction
Mass Defect: The loss in mass that occurs when protons and neutrons combine to form a nucleus. The loss in
mass is converted into energy that is released during the nuclear reaction and is thus a direct measure of the
binding energy holding the nucleons together
Find the mass defect of a copper-63 nucleus if the actual mass of a copper-63 nucleus is 62.91367 amu.
Copper has 29 protons and copper-63 also has (63 - 29) 34 neutrons.
The mass of a proton is 1.00728 amu and a neutron is 1.00867 amu.
The combined mass is calculated:
29 protons (1.00728 amu/proton) + 34 neutrons (1.00867 amu/neutron) = 63.50590 amu

Calculate the mass defect.
Δm = 63.50590 amu - 62.91367 amu = 0.59223 amu
Binding energy of a nucleus is the energy needed to break a nucleus into its individual protons and neutrons. The
larger binding energy, the more stable the nucleus
Conversion of Mass Defect into Energy
To convert the mass defect into energy:


Convert the mass defect into kilograms (1 amu = 1.6606 x 10-27 kg)
Convert the mass defect into its energy equivalent using Einstein's equation.
Example: Determine the binding energy of the copper-63 atom.

Convert the mass defect (calculated in the previous example) into kg.
(0.59223 amu) x (1.6606 x 10-27 kg) = 9.8346 x 10-28 kg
nucleus
amu
nucleus

Convert this mass into energy using ΔE = Δmc2, where c = 2.9979 x 108 m/s.
E = (9.8346 x 10-28 kg) x (2.9979 x 108 m2) = 8.8387 x 10-11 J
nucleus
s2
nucleus
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The energy calculated in the previous example is the nuclear binding energy. However, nuclear binding energy is
often expressed as kJ/mol of nuclei or as MeV/nucleon.

To convert the energy to kJ/mol of nuclei we will simply employ the conversion factors for converting
joules into kilojoules (1 kJ = 1000 J) and for converting individual particles into moles of particles
(Avogadro's Number).
(8.8387 x 10-11 J) x ( 1 kJ
nucleus
1000J

) x (6.022 x 1023 nuclei) = 5.3227 x 1010 kJ
mol
mol of nuclei
To convert the binding energy to MeV (megaelectron volts) per nucleon we will employ the
conversion factor for converting joules into MeV (1 MeV = 1.602 x 10-13 J) and the number of nucleons
(protons and neutrons) which make up the nucleus.
(8.8387 x 10-11 J) x [ 1 MeV
nucleus
1.602 x 10-13 J
] x ( 1 nucleus ) =
8.758 MeV
63 nucleons
nucleon
Because of the interconversion
of mass and energy, the law of
conservation of mass and the
law of conservation of energy
must be combined. The
combination of mass and energy
must be conserved
Binding energy per nucleon generally increases from small atoms to atoms with mass number around 56. Thus
fusing atoms to form medium-size atoms (nuclear fusion) release energy.
Binding energy per nucleon generally decreases from atoms with mass number around 56 and larger. Thus splitting
larger atoms to form medium-sized atoms (nuclear fission) also releases energy.
22.2 Nuclear Fission and Nuclear Fusion
Nuclear fission and nuclear fusion are reactions in which nuclei attain size closer to the intermediate range (A = 50)
In nuclear fission,
•
a large nucleus is bombarded with a small particle.
•
the nucleus splits into smaller nuclei and several neutrons.
•
large amounts of energy are released.
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When a neutron bombards 235U,
•
an unstable nucleus of 236U
undergoes fission (splits).
•
smaller nuclei are produced, such
as Kr-91 and Ba-142.
•
neutrons are released to bombard
more 235U.
The result is a chain reaction that continues to occur even if the external supply of bombarding neutrons is cut off.
If the sample is too small, many of the neutrons leave the
sample before they have a chance to be absorbed
-
-
If the sample is much larger than this supercritical
mass), the number of nuclei that split multiplies
rapidly. A rapid chain reaction results in the splitting
of most of the fissionable nuclei and in the explosive
release of an enormous amount of energy.
-
Nuclear fission reactor is a device that permits a
controlled chain reaction of nuclear fissions. In power
plants, a nuclear reactor is used to produce heat, which
in turn is used to produce steam to drive an electric
generator.
Nuclear fusion
• occurs at extremely high
temperatures (100000000 °C).
• combines small nuclei into
larger nuclei.
• releases large amounts of
energy.
• occurs continuously in the
sun and stars.
Critical mass is the smallest mass of fissionable
material in which a chain reaction can be
sustained
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It’s the fusion reaction of hydrogen nuclei to produce helium that powers our sun and other stars. To get the nuclei
to react, the bombarding nucleus must have enough kinetic energy to overcome the repulsion of electric charges of
the nuclei.
Advantages: Relatively cheap fuel source and the products are nontoxic.
Disadvantage: A temperature of 40 million Kelvin is needed to initiate the process.
22.3 – Nuclear Transmutation
Nuclear Transmutation: The change of one element into another. Such transmutation is often brought about by
bombardment of an atom with a high energy particle such as a proton, neutron or α-particle. In the ensuing
collision energy between particle and atom, an unstable nucleus is momentarily created, a nuclear change occurs,
and a different element is produced. The first nuclear transmutation was accomplished in 1917 by Ernest
Rutherford
Examples:
a) Supply the missing atomic symbol to complete the equation for the following nuclear fission reaction.
b) In another fission reaction, U-235 bombarded with a neutron produces Sr-94, another small nucleus, and 3
neutrons. Write the complete equation for the fission reaction
22.4 – Detecting and Measuring Radioactivity
Radioactive emissions are invisible. We can’t see, hear, smell, touch or taste them, no matter how high the dose.
We can, however, detect radiation by measuring its ionizing properties.
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Ionizing Radiation: Radiation that knocks an electron from a molecule, thereby ionizing it.
-
Including not only α-particle, β-particle, γ-particle but also X-ray and cosmic ray
Radiation intensity is expressed in different ways, depending on what is being measured. See Table 22.3 for some
units measure the number of nuclear decay events.
22.5 – Application of Nuclear Chemistry
Dating
Because the half-life of any nuclide is constant, the amount of substance remaining in an artifact can serve as a
nuclear clock to determine ages of objects. When an animal is alive it maintains a caron-14 to carbon-12 ratio
identical to that in the atmosphere. When it dies, this ratio decreases. By measuring this ratio and contrast it to the
atmosphere, we can get an approximate age.
Radioactive decay is a first order process
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The half-life of a radioisotope is the
time for the radiation level to
decrease (decay) to one half of the
original value
Example: Phosphorous-32, a radioisotope used in the treatment of leukemia, has a half-life of 14.3 days. A
biochemist has a vial containing a compound of phosphorous-32. If the compound is used in an experiment 5.5
day after the compound was prepared, what fraction of the radioactive isotope originally present remains?
-
Suppose the sample in the vial originally contained 0.28 g of phosphorous-32. How many grams remain
after 5.5 day?
Example: The half-life of radon-222, a radioactive gas of concern as health hazard in some homes, is 3.823 days.
What is the decay constant of Rn-222?
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Medical Application:
Radioisotopes with short half-lives are used in nuclear medicine because
•
They have the same chemistry in the body as the nonradioactive atoms.
•
In the organs of the body, they give off radiation that exposes a photographic plate (scan), giving an image
of an organ.
•
In Vivo Procedures
•
•
•
Determination of whole-blood volume using red blood cells labeled with chromium-51.
Therapeutic Procedures
•
Irradiation of tumors using gamma rays emitted from cobalt-60.
•
Beta emission of iodine-131 to treat thyroid disease.
Imaging Procedures
•
Use of a radiopharmaceutical agent known to concentrate in a specific tissue or organ. These target
areas show up as “hot” spots compared to a “cold” background. Technetium-99m is the most widely
used radioisotope.
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