1 chemistry 100 chapter 21 nuclear chemistry. 2 nuclear equations nucleons: particles in the...
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Chemistry 100 Chapter 21
Nuclear Chemistry
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Nuclear Equations
Nucleons: particles in the nucleus:– p+: proton– n0: neutron.
Mass number: the sume of the number of p+ and n0.
Atomic number: the number of p+. Nuclear equations, the total number of
nucleons is conserved:
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Sample Nuclear Equations
HeThU 42
23490
23892
42He - particle
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147
146 NC
0-1 -
particle
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Three Types Of Decay Processes
-radiation – the loss of 4
2He from the nucleus,
-radiation– the loss of an electron from the nucleus,
-radiation – the loss of high-energy photon from the nucleus.
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Radioactivity
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Types of Radioactive Decay
Ensure conservation of nucleons
– Write all particles with their atomic and mass numbers.
Nucleons can undergo decay
epn 10
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01
nep 01
10
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-particle emission
Electron capture
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Types of Radioactive Decay
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Neutron-to-Proton Ratio
The proton has high mass and high charge– proton-proton repulsion is large.
The cohesive forces in the nucleus are called strong nuclear forces. Neutrons are involved with the strong nuclear force.
As more protons are added (the nucleus gets heavier) the proton-proton repulsion gets larger.
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The ‘Belt of Stability’
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Radioactive Series
A nucleus usually undergoes more than one transition on its path to stability.
The series of nuclear reactions that accompany this path is the radioactive series.
Nuclei resulting from radioactive decay are called daughter nuclei.
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An Example Radioactive Decay Series
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Nuclear Transmutations
Nuclear transmutations are the collisions between nuclei.
14N + 4 17O + 1H. The above reaction is written in short-hand
notation: 14N(,p)17O.
To overcome electrostatic forces, charged particles need to be accelerated before they react.
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Nuclear Transmutations
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Radioactive Half-Lives
90Sr has a half-life of 28.8 yr. 90
38Sr 9039Y + 0
-1e Each isotope has a characteristic half-life. Half-lives are not affected by temperature, pressure
or chemical composition. Natural radioisotopes tend to have longer half-lives
than synthetic radioisotopes.
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Rates of Radioactive Decay
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Rates of Radioactive Decay
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Carbon Dating
Carbon-14 is used to determine the ages of organic compounds
– We assume the ratio of 12C to 14C has been constant over time.
For us to detect 14C the object must be less than 50,000 years old.
The half-life of 14C is 5,730 years.
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147
146 NC
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ktNN
ln0
t
Rates of Radioactive Decay
Radioactive decay is a first order process:
Rate = kN
N – the number of radionuclides
k – the first order rate constant
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Detection of Radioactivity
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614CO2 + 6H2O 14C6H12O6 + 6O2sunlightchlorophyll
Radiotracers
Radiotracers are used to follow an element through a chemical reaction.
Photosynthesis has been studied using 14C:– The carbon dioxide is said to be 14C labeled.
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Einstein showed that mass and energy are proportional:
E = mc2
The mass of a nucleus is less than the mass of their nucleons. – the mass defect!
Binding energy is the energy required to separate a nucleus into its nucleons.
Since E = mc2 the binding energy is related to the mass defect.
Energy Changes in Nuclear Reactions
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Nuclear Binding Energies
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Nuclear Fission
Splitting of heavy nuclei is exothermic for large mass numbers.
Consider a neutron bombarding a 235U nucleus:
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A Nuclear Fission Process
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Chain Reactions
The number of fissions and the energy increase rapidly - eventually, a chain reaction forms.
The minimum mass of fissionable material is required for a chain reaction – critical mass.
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The Fission Process
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The Fission Process
For subcritical masses, the neutrons escape and no chain reaction occurs.
At critical mass, the chain reaction accelerates.
Anything over critical mass is called supercritical mass.
Critical mass for 235U is about 1 kg.
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Atomic Bombs
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Nuclear Reactors
Use a subcritical mass of 235U (enrich 238U with about 3% 235U)
Enriched 235UO2 pellets are encased in Zr or stainless steel rods.
Control rods are composed of Cd or B, which absorb neutrons.
Moderators are inserted to slow down the neutrons.
Natural abundance uranium used as a fuel souce.
Enriched 235UO2 pellets are encased in Zr rods.
Heavy water is used as the moderator and the coolant.
Heat produced in the reactor core is removed by a cooling fluid to a large tank of water (producing
steam). Steam drives an electric generator.
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A Schematic Nuclear Reactor
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Nuclear Fusion
Light nuclei can fuse to form heavier nuclei. Most reactions in the Sun are fusion. Fusion products are not usually radioactive, so fusion is a good
energy source. Also, the hydrogen required for reaction can easily be supplied
by seawater. However, high energies are required to overcome repulsion
between nuclei before reaction can occur. High energies are achieved by high temperatures: the reactions
are thermonuclear.
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Fusion of tritium and deuterium requires about 40,000,000K:2
1H + 31H 4
2He + 10n
These temperatures can be achieved in a nuclear bomb or a tokamak.
A tokamak is a magnetic bottle: strong magnetic fields contained a high temperature plasma so the plasma does not come into contact with the walls. (No known material can survive the temperatures for fusion.)
To date, about 3,000,000 K has been achieved in a tokamak.
Nuclear Fusion
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Biological Effects of Radiation
The penetrating power of radiation is a function of mass. -radiation (zero mass) penetrates deeply -radiation penetrates much further than -
radiation Radiation absorbed by tissue causes
excitation (nonionizing radiation) or ionization (ionizing radiation).
Ionizing radiation is much more harmful than nonionizing radiation.
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Biological Effects of Radiation
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Biological Effects of Radiation
Most ionizing radiation interacts with water in tissues to form H2O+.
The H2O+ ions react with water to produce H3O+and OH.
OH has one unpaired electron. It is called the hydroxy radical.
Free radicals generally undergo chain reactions.
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The SI unit for radiation is the becquerel (Bq). 1 Bq is one disintegration per second. The curie (Ci) is 3.7 1010 disintegrations per
second. (Rate of decay of 1 g of Ra.) Absorbed radiation is measured in the gray (1 Gy is
the absorption of 1 J of energy per kg of tissue) or the radiation absorbed dose (1 rad is the absorption of 10-2 J of radiation per kg of tissue).
Radiation Doses
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The Relative Biological Effectiveness
Not all forms of radiation have the same effect, Account for the differences using RBE (relative
biological effectiveness for - and -radiation and 10 for radiation).
rem (roentgen equivalent for man) = rads.RBE SI unit for effective dosage is the Sievert (1Sv =
RBE.1Gy = 100 rem).
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Radiation Doses
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Radon
The nucleus 22286Rn is a product of 238
92U. Radon exposure accounts for more than half the 360
mrem annual exposure to ionizing radiation. Rn is a noble gas so is extremely stable. The half-life of is 3.82 days. It decays as follows:
22286Rn 218
84Po + 42He
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Biological Effects of Radon
The -particles produced have a high RBE. Therefore, inhaled Rn is thought to cause lung cancer. The picture is complicated by realizing that 218Po has a short
half-life (3.11 min) also:218
84Po 21482Pb + 4
2He The 218Po gets trapped in the lungs where it continually
produces -particles. The EPA recommends 222Rn levels in homes to be kept below 4
pCi per liter of air.