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
Δm = mass of nucleons– actual mass
Find the mass defect of a copper-63 nucleus if the actual mass of a copper-63 nucleus is 62.914 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 or mass of nucleons is calculated:
29 protons (1.00728 amu/proton) + 34 neutrons (1.00867 amu/neutron) = 63.506 amu

Calculate the mass defect.
Δm = 63.506 amu - 62.914 amu = 0.592 amu
Nuclear 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, ΔE = Δmc2.
Example: Determine the binding energy of the copper-63 atom.

Convert the mass defect (calculated in the previous example) into kg.
(0.592 amu) x (1.6606 x 10-27 kg) = 9.835 x 10-28 kg
nucleus
amu
nucleus
Dang1

Convert this mass into energy using ΔE = Δmc2, where c = 3.00 x 108 m/s.
∆E = (9.835 x 10-28 kg) x (3.00 x 108 m)2 = 8.85 x 10-11 J
nucleus
s2
nucleus
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).
(8.85 x 10-11 J) x ( 1 kJ ) x (6.022 x 1023 nuclei) = 5.33 x 1010 kJ
nucleus
1000J
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.
(8.85 x 10-11 J) x [ 1 MeV
] x (
nucleus
1.602 x 10-13 J
Determine the binding energy per nucleon for Osmium-190.
mass of
190
Os = 189.95863 amu
1 nucleus ) =
63 nucleons
mass of neutron = 1.008664 amu
8.77 MeV
nucleon
mass of proton = 1.007276 amu
Consider the following nuclear reaction and determine how much energy gain or lost when U-238 undergoes alpha
emission.
The nuclei in the reaction have the following masses
U-238 = 238.0003 amu, Th-234 = 233.9942 amu and 42He = 4.0015 amu
Dang2
The binding energy per nucleon for most nuclei falls around 1.3 x 10-12 J (8 MeV). A plot of binding energy per
nucleon vs. mass number shows that the most stable nuclei occur around A = 50 to 60. Fe-56 is the most stable
nucleus in the universe. The existence of a maximum in this curve indicates that energy is released in either a
fission or fusion process in which more stable nuclei (i.e., closer to 56Fe) are produced.
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.
Dang3
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
Critical mass is the smallest mass of fissionable material in which a chain reaction can be sustained
-
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
Dang4
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
c) Silver-117 atoms undergo three beta emissions before they reach a stable nuclide. What is the final product?
22.4 – Detecting and Measuring Radioactivity
Dang5
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.
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
Radioactive carbon-14 is constantly being generated in the upper atmosphere by neutron bombardment:
Carbon-14 eventually enters the food chain via the formation of carbon dioxide and its uptake by plants via
photosynthesis. Eating these plants distributes carbon-14 throughout all living organisms
The half-life of carbon-14 is 5715 years:
Because the haf-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
Dang6
Example: A wood artifact is found to have a C-14 activity of 9.1 disintegrations per second. Given that the C-14
activity of an equal mass of fresh-cut wood has a constant value of 15.2 disintegrations per second, determine the
age of the artifact.
Example: A rock is found to contain 5.51 mg of U-238 and 1.63 mg of Pb-206. Determine the age of the rock (t ½
of U-238 is 4.51 x 109 yr)
Example: What is the age of a rock whose Ar-40/K-40 ratio is 1.15? The half-life is 1.28 x 109 years.
Dang7
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.
•
Dang8