Download Nuclear chemistry – the study of nuclear reactions and their uses in

Nuclear chemistry – the study of nuclear reactions and their uses in chemistry
1. Radioactivity
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Radioactivity is the spontaneous decay of a nucleus.
Nucleons – protons and neutrons
Nuclide is a nucleus with a specified number of protons and neutrons.
Nuclei that are radioactive are called radionuclides, and atoms containing these nuclei are called radioisotopes.
Emissions of radiation are one way an unstable nucleus is transformed to a more stable one with less energy.
i. The emitted radiation is the carrier of excess energy.
Alpha particles contain 2 protons, 2 neutrons, and no electrons (+2 charge)
Beta particles are high speed electrons emitted by an unstable nucleus.
Gamma radiation consists of high-energy photons. Doesn’t change the mass number or the atomic number
i. Represents the energy lost when the remaining nucleons reorganize into more stable arrangements.
Positron is a particle that has the same mass as an electron. But an opposite charge.
Electron capture is the capture by the nucleus of an electron from the electron cloud surrounding the nucleus.
2. Pattern of Nuclear stability
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Strong nuclear forces exist between nucleons.
The belt of stability, where all stable nuclei are found, ends at element 83; anything with 84 or more protons are radioactive.
Radioactive decay depends to a large extend on its neutron-to-proton ratio.
i. Nuclei above the belt of stability (high neutron-to-proton ratios).
1. Neutron rich nuclei can lower their ratio by emitting beta particles to move towards the belt of stability.
Decreases the number of neutrons and increases the number of protons in a nucleus.
ii. Nuclei below the belt of stability (low neutron-to-proton ratios).
1. Proton rich nuclei can increase their ratio by either positron emission or electron capture. Both kinds of
decay increase the number of neutrons and decrease the number of protons.
iii. Nuclei with atomic number greater than or equal to 84.
1. Undergo alpha emission to decrease the number of neutrons and protons.
Some nuclei can not gain stability by a single emission.
Radioactive series, or nuclear disintegration series, is a series of nuclear reactions that begins with an unstable nucleus and
terminates with a stable one.
Shell model of the nucleus corresponds to stable elements with closed electron configurations.
i. Nuclei with 2,8,20,28,50,82 protons or 2,8,20,28,50,82,126 (magic numbers) neutrons are generally more stable.
ii. Nuclei with even numbers of both protons and neutrons are generally more stable than those with off numbers.
3. Nuclear Transmutations
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Nuclear transmutation, the conversion of one nucleus to another, can be brought about by bombarding nuclei with either
charged particles or neutrons.
i. N-14 + α → O-17 + H+
Particle accelerators are used to overcome the electrostatic repulsion between nuclei.
i. The higher the nuclear charge on either the projectile or the target, the faster the projectile must be moving to bring
about a nuclear reaction.
ii. Neutrons are neutral and don’t need to be accelerated to cause a nuclear reaction.
Nuclear transmutations are used to produce the transuranium elements, those elements with atomic numbers greater than
that of uranium.
4. Rates of Radioactive Decay
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Radioactive decay is a first-order kinetic process.
i. Rate = kN (N = the number of radioactive nuclei; k = the decay constant.)
First order processes have a characteristic half-life, which is the time required for half of any given quantity of a substance
to react.
i. Half lifes are unaffected by external conditions.
ii. Half life can serve as a nuclear clock.
The rate at which a sample decays is called the activity, and is expressed at the number of disintegrations observed per unit
time (SI unit = Bq, Becquerel)
K = .693/t
Geiger count is a device used to measure and detect the amount of radioactivity.
i. Phosphors are substances that are excited by radiation and give off light as their electrons return to their lowerenergy state.
Scintillation counter is used to detect and measure radiation, based on the tiny flashed of light produced when radiation
strikes a suitable phosphor.
Radiotracer is used to trace the path of an element through a series of reactions.
5. Energy Changes in Nuclear Reactions
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E= mc2
i. If a system loses mass, it loses energy (exothermic); if it gains mass, it gains energy (endothermic).
ii. Since c2 is such a large number, small mass change result in a huge energy change.
Mass change and the associated energy changes in nuclear reactions are much greater than those in chemical reactions.
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i. The mass change (Δm = mass of products – mass of reactants) for the radioactive decay of one mole of U-238 is
50,000 times greater than the combustion of one mole of CH 4
1. Th-234 (233.9942g) + α (4.0015g) – U-238 (238.003g) = -0.0046g (exothermic)
2. ΔE = Δmc2
3. ΔE = (2.9979E8m/s)2(-0.0046g)(1kg/1000g) = -4.1E11 J
All spontaneous nuclear reactions are exothermic.
The masses of nuclei are always less than the masses of the individual nucleons of which they are composed.
i. Helium-4 (2protons, 2 neutrons, 2 electrons) has a mass of 4.00150 amu
ii. The mass of 2 protons and 2 neutrons is 4.03188 amu.
1. The mass of the individual nuclei is .03038 amu greater that the He-4
2. This mass difference is called the mass defect.
Mass defect is readily understood if we consider that energy must be added to a nucleus in order to break it into separate
protons and neutrons.
i. The mass change for He-4 is .03038 amu
ii. The energy required is: ΔE = Δmc2 (where m is in kg)
The energy required to separate a nucleus into its individual nucleons is called the nuclear binding energy.
i. The larger the binding energy, the more stable the nucleus.
The trend shows that nuclei of intermediate mass numbers (such as Fe-56) are more tightly bound and stable than those with
either smaller or larger mass numbers.
i. Heavy nuclei gain stability and give off energy when they are fragmented into two mid-size nuclei
1. This process is called fission (exothermic process) and is used to generate energy in nuclear power plants.
ii. Greater amounts of energy are released if very light nuclei are combined or fused together to give more massive
nuclei
1. This process is call fusion (exothermic process) and is the essential energy producing process in the Sun.
6. Nuclear Fission
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Nuclear power plants and nuclear weapons depend on fission.
When heavy nuclei split due to the capture of neutrons, radioactive isotopes of many different elements are formed.
If one fission produces 2 neutrons, these 2 neutrons can produce four fissions, and so forth.
i. Reactions that multiple in this fashion are called chain reactions.
ii. The chain stops if neutrons are lost.
Critical mass – the amount of fissionable material large enough to maintain the chain reaction with a constant rate of fission.
i. When the critical mass of material is present, one neutron on average from each fission is subsequently effective in
producing another fission reaction.
A mass in excess of a critical mass is referred to as a supercritical mass.
i. Can lead to nuclear explosions.
Control rods are composed of material such as cadmium or boron that regulate the flux of the neutrons by absorbing them
and keeping the reaction chain self-sustaining and preventing the reactor core from overheating.
The reactor is started by a neutron-emitting source; it is stopped by inserting the control rods more deeply into the reactor
core (the site of fission)
Moderate – slows down neutrons so they can be captured more readily by the fuel.
Cooling liquid carries off the heat generated by the nuclear fission.
7. Nuclear Fusion
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Fusion is appealing as an energy source because of the availability of light isotopes and fusion products are generally not
radioactive.
Fusion reactions are known as thermonuclear reactions.
40 million Kelvin is required to fuse deuterium and tritium
i. High temperatures are required to overcome the electrostatic repulsion.
8. Biological effects of Radiation
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Ionizing radiation is energetic enough to remove an electron from water molecule
i. Harmful because most living tissue is 70% water.
ii. Alpha, beta, gamma are forms of ionizing radiation.
Non-ionizing radiation is generally of lower energy or slow-moving neutrons, such as radio waves.
Ionizing radiation generates free radicals, reactive substances with one or more unpaired electrons.
i. The formation of a single free radical can initiate a large number of chemical reactions that are ultimately able to
disrupt the normal operations of a cell.
Gray (Gy) and rad (Radiation Absorbed Dose) are two commonly used measure of exposure to radiation. 1 Gy = 100 rad
i. Gy is the absorption of 1 J of energy per kg of tissue
ii. Rad is the absorption of 1E-2 J of energy per kg of tissue. Used in medicine
Not all forms of radiation harm biological materials with the same efficiency
i. Relative biological effectiveness (RBE) is approximately 1 for gamma and beta and 10 for alpha radiation.
ii. The exact value of the RBE varies with does rate, total dose, and type of tissue affected.
iii. Number of rems (roentgen equivalent for man) = (number of rads)(RBE)
1. 600 rems is fatal; .5mrem is a dental x-ray; 360 mrem is an average yearly.
Radon is a product of nuclear disintegration and accounts for most of the 360 mrem; radioactive and can cause lung cancer.