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Chem. 116 Prof. T.L. Heise
1
CHE 116: General Chemistry
CHAPTER TWENTY ONE
Copyright © Tyna L. Heise 2002
All Rights Reserved
Chem. 116 Prof. T.L. Heise
2
Nuclear Chemistry
Nuclear Reactions: changes in matter that occur in the nucleus of an atom
- spontaneous changes of nuclei, which emit radiation, are said to be radioactive
Chem. 116 Prof. T.L. Heise
3
Radioactivity
Nucleus - made up of two subatomic particles
Chap. 21.1
PROTONNEUTRON
Both molecules are called nucleons
Chem. 116 Prof. T.L. Heise
4
Radioactivity
All atoms of a given element have the same number of protons, known as atomic number
All atoms of a given element can have different numbers of neutrons, and therefore different mass numbers
- mass number is the number of nucleons in nucleus
- same atomic number, different mass number is an ISOTOPE
Chap. 21.1
Chem. 116 Prof. T.L. Heise
5
Radioactivity
Different isotopes have different abundancies in nature.
Different nuclei also have different stabilities:
- nuclear properties of an atom depend on the number of protons and neutron - nuclei that are radioactive are called radionuclides - atoms containing these nuclei are called radioisotopes
Chap. 21.1
Chem. 116 Prof. T.L. Heise
6
Radioactivity
The vast majority of nuclei found in nature are stable and remain intact indefinately
Radionuclides - unstable and spontaneously emit particles and electromagnetic energy.
- emission of radiation is one way an unstable nuclide can become a stable
nuclide with less energy - when a nuclide spontaneously decomposes, it is called radioactive decay
Chap. 21.1
Chem. 116 Prof. T.L. Heise
7
Radioactivity
Alpha decay ()- emission of the nucleus of a helium atom : 4He2
238U92 ---> 234Th90 + 4He2
** all mass numbers and atomic numbers are similarly balanced in all nuclear equations
Chap. 21.1
Chem. 116 Prof. T.L. Heise
8
Radioactivity
Sample exercise: What element undergoes alpha decay to form lead-208?
Chap. 21.1
Chem. 116 Prof. T.L. Heise
9
Radioactivity
Sample exercise: What element undergoes alpha decay to form lead-208?
X ---> 208Pb82 + 4He2
Chap. 21.1
Chem. 116 Prof. T.L. Heise
10
Radioactivity
Sample exercise: What element undergoes alpha decay to form lead-208?
X ---> 208Pb82 + 4He2
atomic numbers add up to 212
Chap. 21.1
Chem. 116 Prof. T.L. Heise
11
Radioactivity
Sample exercise: What element undergoes alpha decay to form lead-208?
X ---> 208Pb82 + 4He2
atomic numbers add up to 212
mass numbers add up to 84
Chap. 21.1
Chem. 116 Prof. T.L. Heise
12
Radioactivity
Sample exercise: What element undergoes alpha decay to form lead-208?
212X84 ---> 208Pb82 + 4He2
look up atomic number 84 to identify symbol
Chap. 21.1
Chem. 116 Prof. T.L. Heise
13
Radioactivity
Sample exercise: What element undergoes alpha decay to form lead-208?
212Po84 ---> 208Pb82 + 4He2
Chap. 21.1
Chem. 116 Prof. T.L. Heise
14
Radioactivity
Beta decay ()- emission of the nucleus of a high speed electron : 0e-1
131I53 ---> 131Xe54 + 0e-1
** beta emission is equivalent to the conversion of a neutron to a proton, thereby increasing the atomic number by 1
1n0 --> 1p1 + 0e-1
the electron only comes into existence during nuclear reaction, it was NOT there all along
Chap. 21.1
Chem. 116 Prof. T.L. Heise
15
Radioactivity
Gamma radiation ()- emission of the nucleus of a high energy photons : 00
** not shown when writing nuclear equations
Chap. 21.1
Chem. 116 Prof. T.L. Heise
16
Radioactivity
nope
Chap. 21.1
Chem. 116 Prof. T.L. Heise
17
Radioactivity
Positron emission - emission of the nucleus of a high speed positive electron : 0e+1
11C6 ---> 11B5 + 0e+1
** positron emission is equivalent to the conversion of a proton to a neutron, thereby decreasing the atomic number by 1
1p1 --> 1n0 + 0e+1
the positron only comes into existence during nuclear reaction, it was NOT there all along
.
Chap. 21.1
Chem. 116 Prof. T.L. Heise
18
Radioactivity
Electron capture - capture by the nucleus of a high speed electron : 0e-1
81Rb37 + 0e-1 --> 81Kr36
** electron capture is equivalent to the conversion of a proton to a neutron, thereby decreasing the atomic number by 1
1p1 + 0e-1 --> 1n0
.
Chap. 21.1
Chem. 116 Prof. T.L. Heise
19
Radioactivity
along.
Chap. 21.1
Chem. 116 Prof. T.L. Heise
20
Radioactivity
Write a balanced nuclear equation for the reaction in which oxygen-15 undergoes positron emission.
Chap. 21.1
Chem. 116 Prof. T.L. Heise
21
Radioactivity
Write a balanced nuclear equation for the reaction in which oxygen-15 undergoes positron emission.
15O8 --> 0e+1 + X
Chap. 21.1
Chem. 116 Prof. T.L. Heise
22
Radioactivity
Write a balanced nuclear equation for the reaction in which oxygen-15 undergoes positron emission.
15O8 --> 0e+1 + 15X7
Chap. 21.1
Chem. 116 Prof. T.L. Heise
23
Radioactivity
Write a balanced nuclear equation for the reaction in which oxygen-15 undergoes positron emission.
15O8 --> 0e+1 + 15N7
Chap. 21.1
Chem. 116 Prof. T.L. Heise
24
Patterns of Nuclear stability
The stability of a particular nucleus depends on a variety of factors, and no single rule allows us to predict whether a particular nucleus is radioactive and how it might decay, however empirical observations can be made
- neutron to proton ratio is most important
Chap. 21.2
Chem. 116 Prof. T.L. Heise
25
Patterns of Nuclear stability
neutron to proton ratio
- the more protons packed into the nucleus, the more neutrons needed to bind the nucleus together
stable nuclei with low atomic numbers have approximately equal numbers of neutrons and protons
Chap. 21.2
Chem. 116 Prof. T.L. Heise
26
Patterns of Nuclear stability
neutron to proton ratio
- the more protons packed into the nucleus, the more neutrons needed to bind the nucleus together
nuclei with higher atomic numbers, the number of neutrons exceeds the number of protons because the number of neutrons necessary to create a stable nucleus increases more rapidly than the number of protons
Chap. 21.2
Chem. 116 Prof. T.L. Heise
27
Patterns of Nuclear stability
The belt of stability ends at 83
- above the belt can lower their ratio by emitting a
beta - below the belt can increase their ratio by either positron emission or electron capture - nuclei with atomic numbers above
84 tend to undergo alpha emission
Chap. 21.2
Chem. 116 Prof. T.L. Heise
28
Patterns of Nuclear stability
Sample exercise: Predict the mode of decay of
(a) plutonium-239
Chap. 21.2
Chem. 116 Prof. T.L. Heise
29
Patterns of Nuclear stability
Sample exercise: Predict the mode of decay of
(a) plutonium-239
atomic number of 94, alpha emission
Chap. 21.2
Chem. 116 Prof. T.L. Heise
30
Patterns of Nuclear stability
Sample exercise: Predict the mode of decay of
(a) indium-120
Chap. 21.2
Chem. 116 Prof. T.L. Heise
31
Patterns of Nuclear stability
Sample exercise: Predict the mode of decay of
(a) indium-120
atomic number of 49, neutrons are 71, above the belt of stability; beta emission
Chap. 21.2
Chem. 116 Prof. T.L. Heise
32
Patterns of Nuclear stability
Keep in mind that the previous slides describe guidelines to follow, and not all nuclei abide by the guidelines given.
Certain nuclei can not gain stability by a single emission. Elements like this have a series of emissions called a disintegration series.
Chap. 21.2
Chem. 116 Prof. T.L. Heise
33
Patterns of Nuclear stability
Uranium-238 is an excellent example of
a nuclei which has a disintegration series
Chap. 21.2
Chem. 116 Prof. T.L. Heise
34
Patterns of Nuclear stability
Two other observations have proven useful in the determination of stable nuclei
Nuclei with 2, 8, 20, 28, 50, or 82 protons OR 2, 8, 20, 28, 50 or 82 neutrons are generally more stable. These numbers have been called the magic numbers
Nuclei with even numbers of both protons and neutrons are generally more stable than those with odd numbers of nucleons
Chap. 21.2
Chem. 116 Prof. T.L. Heise
35
Patterns of Nuclear stability
Sample exercise: Which of the following nuclei would you expect to exhibit a special stability:
118Sn50, 210At85, 208Pb82
Chap. 21.2
Chem. 116 Prof. T.L. Heise
36
Patterns of Nuclear stability
Sample exercise: Which of the following nuclei would you expect to exhibit a special stability:
118Sn50208Pb82
Chap. 21.2
Chem. 116 Prof. T.L. Heise
37
Nuclear Transmutations
Another way a nucleus can change identity is to be struck by a neutron or by another nucleus. Nuclear reactions that have been induced this way are called Nuclear (Artificial) Transmutations
Nuclear Transmutations are listed in the following order: target nucleus + bombarding particle --> ejected particle + product nucleus
14N7 + 4He2 --> 1H1 + 17O8
14N7 (, p) 17O8
Chap. 21.3
Chem. 116 Prof. T.L. Heise
38
Nuclear Transmutations
Charged particles must be moving very fast in order to overcome the electrostatic repulsion between them and the target nucleus.
- the higher the nuclear charge on either the projectile or the target, the faster the
particle must be going
- Strong magnetic and electric fields are used to accelerate the particles.
Chap. 21.3
Chem. 116 Prof. T.L. Heise
39
Nuclear Transmutations
Particle Accelerators
Chap. 21.3
Chem. 116 Prof. T.L. Heise
40
Nuclear Transmutations
Particle Accelerators
Chap. 21.3
Chem. 116 Prof. T.L. Heise
41
Nuclear Transmutations
Most synthetic isotopes in quantity in medicine and scientific research are made using neutrons as projectiles
- neutrons are neutral so there is no nuclear repulsion to overcome
- no need to be accelerated
Chap. 21.3
Chem. 116 Prof. T.L. Heise
42
Rates of Radioactive Decay
Different nuclei undergo radioactive decay at different rates.
Radioactive decay is a first order kinetic process
- characteristic half life
- independent of initial concentration
- unaffected by external forces such as temperature, pressure, or state of chemical combination
- radioactive atoms cannot be rendered harmless by a chemical reaction or by any other practical treatment
Chap. 21.4
Chem. 116 Prof. T.L. Heise
43
Rates of Radioactive Decay
Sample exercise: Carbon-11, used in medical imaging, has a half life of 20.4 min. The carbon-11 nuclides are formed and then incorporated into a desired compound. The resulting sample is injected into the patient, and the image is obtained. The entire process takes five half lives. What percentage of original carbon remains at this time?
Chap. 21.4
Chem. 116 Prof. T.L. Heise
44
Rates of Radioactive Decay
Sample exercise: Carbon-11, used in medical imaging, has a half life of 20.4 min. The carbon-11 nuclides are formed and then incorporated into a desired compound. The resulting sample is injected into the patient, and the image is obtained. The entire process takes five half lives. What percentage of original carbon remains at this time?
100 50
Chap. 21.4
Chem. 116 Prof. T.L. Heise
45
Rates of Radioactive Decay
Sample exercise: Carbon-11, used in medical imaging, has a half life of 20.4 min. The carbon-11 nuclides are formed and then incorporated into a desired compound. The resulting sample is injected into the patient, and the image is obtained. The entire process takes five half lives. What percentage of original carbon remains at this time?
100 50 25
Chap. 21.4
Chem. 116 Prof. T.L. Heise
46
Rates of Radioactive Decay
Sample exercise: Carbon-11, used in medical imaging, has a half life of 20.4 min. The carbon-11 nuclides are formed and then incorporated into a desired compound. The resulting sample is injected into the patient, and the image is obtained. The entire process takes five half lives. What percentage of original carbon remains at this time?
100 50 25 12.5
Chap. 21.4
Chem. 116 Prof. T.L. Heise
47
Rates of Radioactive Decay
Sample exercise: Carbon-11, used in medical imaging, has a half life of 20.4 min. The carbon-11 nuclides are formed and then incorporated into a desired compound. The resulting sample is injected into the patient, and the image is obtained. The entire process takes five half lives. What percentage of original carbon remains at this time?
100 50 25 12.5 6.25
Chap. 21.4
Chem. 116 Prof. T.L. Heise
48
Rates of Radioactive Decay
Sample exercise: Carbon-11, used in medical imaging, has a half life of 20.4 min. The carbon-11 nuclides are formed and then incorporated into a desired compound. The resulting sample is injected into the patient, and the image is obtained. The entire process takes five half lives. What percentage of original carbon remains at this time?
100 50 25 12.5 6.25 3.125
Chap. 21.4
Chem. 116 Prof. T.L. Heise
49
Rates of Radioactive Decay
Due to the constancy of half lives, they can be used as a molecular clock to determine the ages of different objects
Chap. 21.4
Chem. 116 Prof. T.L. Heise
50
Rates of Radioactive Decay
Shroud of Turin - face
Chap. 21.4
Chem. 116 Prof. T.L. Heise
51
Rates of Radioactive Decay
Shroud of Turin - hands
Chap. 21.4
Chem. 116 Prof. T.L. Heise
52
Rates of Radioactive Decay
Calculation based on Half-livesRate = kNthe first order rate constant is called a decay
constantThe rate at which a sample decays is called its
activity, units are disintegrations/secln(Nt/No) = -kt
k = 0.693/t1/2
Chap. 21.4
Chem. 116 Prof. T.L. Heise
53
Rates of Radioactive Decay
Sample exercise: A wooden object from an archeological site is subjected to radiocarbon dating. The activity of the sample due to carbon-14 is measured to be 11.6 disintegration per second. The activity of a carbon sample of equal mass from fresh wood is 15.2 disintegrations per second. The half-life of 14C is 5,715 yr. What is the age of the archeological sample?
Chap. 21.4
Chem. 116 Prof. T.L. Heise
54
Rates of Radioactive Decay
Sample exercise: A wooden object from an archeological site is subjected to radiocarbon dating. The activity of a carbon sample of equal mass from fresh wood is 15.2 disintegrations per second. The half-life of 14C is 5,715 yr. What is the age of the archeological sample?
k = 0.693/t1/2
Chap. 21.4
Chem. 116 Prof. T.L. Heise
55
Rates of Radioactive Decay
Sample exercise: A wooden object from an archeological site is subjected to radiocarbon dating. The activity of a carbon sample of equal mass from fresh wood is 15.2 disintegrations per second. The half-life of 14C is 5,715 yr. What is the age of the archeological sample?
k = 0.693/t1/2
k = 0.693/5,715 yr
Chap. 21.4
Chem. 116 Prof. T.L. Heise
56
Rates of Radioactive Decay
Sample exercise: A wooden object from an archeological site is subjected to radiocarbon dating. The activity of a carbon sample of equal mass from fresh wood is 15.2 disintegrations per second. The half-life of 14C is 5,715 yr. What is the age of the archeological sample?
k = 0.693/t1/2
k = 0.693/5,715 yr
k = 1.21 x 10-4 yr-1
Chap. 21.4
Chem. 116 Prof. T.L. Heise
57
Rates of Radioactive Decay
Sample exercise: A wooden object from an archeological site is subjected to radiocarbon dating. The activity of a carbon sample of equal mass from fresh wood is 15.2 disintegrations per second. The half-life of 14C is 5,715 yr. What is the age of the archeological sample?
k = 0.693/t1/2 t = (-1/k)ln(Nt/No)
k = 0.693/5,715 yr
k = 1.21 x 10-4 yr-1
Chap. 21.4
Chem. 116 Prof. T.L. Heise
58
Rates of Radioactive Decay
Sample exercise: A wooden object from an archeological site is subjected to radiocarbon dating. The activity of a carbon sample of equal mass from fresh wood is 15.2 disintegrations per second. The half-life of 14C is 5,715 yr. What is the age of the archeological sample?
k = 0.693/t1/2 t = (-1/k)ln(Nt/No)
k = 0.693/5,715 yr t = (-1/1.21x10-4)ln(11.6/15.2)
k = 1.21 x 10-4 yr-1 t = (-8264)(-0.2702)
Chap. 21.4
Chem. 116 Prof. T.L. Heise
59
Rates of Radioactive Decay
Sample exercise: A wooden object from an archeological site is subjected to radiocarbon dating. The activity of a carbon sample of equal mass from fresh wood is 15.2 disintegrations per second. The half-life of 14C is 5,715 yr. What is the age of the archeological sample?
k = 0.693/t1/2 t = (-1/k)ln(Nt/No)
k = 0.693/5,715 yr t = (-1/1.21x10-4)ln(11.6/15.2)
k = 1.21 x 10-4 yr-1 t = (-8264)(-0.2702)
t = 2233 yr
Chap. 21.4
Chem. 116 Prof. T.L. Heise
60
Rates of Radioactive Decay
Sample exercise: A sample to be used for medical imaging is labeled with 18F, which has a half-life of 110 minutes. What percentage of the original activity in the sample remains after 300 minutes?
Chap. 21.4
Chem. 116 Prof. T.L. Heise
61
Rates of Radioactive Decay
Sample exercise: A sample to be used for medical imaging is labeled with 18F, which has a half-life of 110 minutes. What percentage of the original activity in the sample remains after 300 minutes?
k = 0.693/t1/2
Chap. 21.4
Chem. 116 Prof. T.L. Heise
62
Rates of Radioactive Decay
Sample exercise: A sample to be used for medical imaging is labeled with 18F, which has a half-life of 110 minutes. What percentage of the original activity in the sample remains after 300 minutes?
k = 0.693/t1/2
k = 0.693/110 min.
Chap. 21.4
Chem. 116 Prof. T.L. Heise
63
Rates of Radioactive Decay
Sample exercise: A sample to be used for medical imaging is labeled with 18F, which has a half-life of 110 minutes. What percentage of the original activity in the sample remains after 300 minutes?
k = 0.693/t1/2
k = 0.693/110 min.
k = 0.0063 min.-1
Chap. 21.4
Chem. 116 Prof. T.L. Heise
64
Rates of Radioactive Decay
Sample exercise: A sample to be used for medical imaging is labeled with 18F, which has a half-life of 110 minutes. What percentage of the original activity in the sample remains after 300 minutes?
k = 0.693/t1/2 ln(Nt/No) = -kt
k = 0.693/110min
k = 0.0063 min-1
Chap. 21.4
Chem. 116 Prof. T.L. Heise
65
Rates of Radioactive Decay
Sample exercise: A sample to be used for medical imaging is labeled with 18F, which has a half-life of 110 minutes. What percentage of the original activity in the sample remains after 300 minutes?
k = 0.693/t1/2 ln(Nt/No) = -kt
k = 0.693/ 110 min ln(x/100g) = -0.0063(300)
k = 0.0063 min-1
Chap. 21.4
Chem. 116 Prof. T.L. Heise
66
Rates of Radioactive Decay
Sample exercise: A sample to be used for medical imaging is labeled with 18F, which has a half-life of 110 minutes. What percentage of the original activity in the sample remains after 300 minutes?
k = 0.693/t1/2 ln(Nt/No) = -kt
k = 0.693/ 110 min ln(x/100g) = -0.0063(300)
k = 0.0063 min-1 x/100 g = e-1.89
Chap. 21.4
Chem. 116 Prof. T.L. Heise
67
Rates of Radioactive Decay
Sample exercise: A sample to be used for medical imaging is labeled with 18F, which has a half-life of 110 minutes. What percentage of the original activity in the sample remains after 300 minutes?
k = 0.693/t1/2 ln(Nt/No) = -kt
k = 0.693/110 min ln(x/100g) = -0.0063(300)
k = 0.0063 min-1 x/100 g = e-1.89
x/100 g = 0.151
Chap. 21.4
Chem. 116 Prof. T.L. Heise
68
Rates of Radioactive Decay
Sample exercise: A sample to be used for medical imaging is labeled with 18F, which has a half-life of 110 minutes. What percentage of the original activity in the sample remains after 300 minutes?
k = 0.693/t1/2 ln(Nt/No) = -kt
k = 0.693/ 110 min ln(x/100g) = -0.0063(300)
k = 0.0063 min-1 x/100 g = e-1.89
x/100 g = 0.151
x = 15.1 g or 15.1%
Chap. 21.4
Chem. 116 Prof. T.L. Heise
69
Detection of Radiation
A variety of methods have been designed to detect emissions from radioactive substances.Photographic film and plates, the greater the
exposure, the darker the area exposedGeiger counters, uses the conduction of electricity by
ions and electrons produced by radioactive substancesPhosphors glow when as electrons excited by
radiation fall back down to ground stateScintillation counter detects tiny flashes of light from
phosphors
Chap. 21.5
Chem. 116 Prof. T.L. Heise
70
Detection of Radiation
Geiger counters
Chap. 21.5
Chem. 116 Prof. T.L. Heise
71
Detection of Radiation
Radiotracers: a radioactive element that can be traced so easily they are used to follow the pathway a chemical reaction takes
- ability to do this comes from the fact that all isotopes of an element have essentially
identical chemical properties
- the chemicals pathway is revealed by the radioactivity of the radioisotope
Chap. 21.5
Chem. 116 Prof. T.L. Heise
72
Energy Changes
The energies involved in nuclear reactions must be considered using Einstein’s famous equation
E = mc2
This equation states that the mass and energy of an object are proportional, if a system loses mass, it loses energy and vice versa.
The proportionality constant c2 is so large, even small changes in mass cause large changes in energy
Chap. 21.6
Chem. 116 Prof. T.L. Heise
73
Energy Changes
The mass changes and the associated energy changes in nuclear reactions are much greater than those in chemical reactions.
- the mass change in the decay of 1 mole of Uranium-238 is 50,000 times
greater than that for the combustion of one mole of methane.
238U92 --> 234Th90 + 4He2
Chap. 21.6
Chem. 116 Prof. T.L. Heise
74
Energy Changes
238U92 --> 234Th90 + 4He2
mass of nuclei: 238.0003 233.9942 + 4.0015
(amu)
238.0003 = 237.9957
Chap. 21.6
Chem. 116 Prof. T.L. Heise
75
Energy Changes
238U92 --> 234Th90 + 4He2
mass of nuclei: 238.0003 233.9942 + 4.0015
(amu)
238.0003 = 237.9957
0.0046 amu are LOST, so proportional energy is LOST
**Lost energy is exothermic
Chap. 21.6
Chem. 116 Prof. T.L. Heise
76
Energy Changes
238U92 --> 234Th90 + 4He2
mass of nuclei: 238.0003 233.9942 + 4.0015
(amu)
238.0003 = 237.9957
0.0046 amu
If 1 mole of U-238 is considered, amu turns into grams
Chap. 21.6
Chem. 116 Prof. T.L. Heise
77
Energy Changes
238U92 --> 234Th90 + 4He2
mass of nuclei: 238.0003 233.9942 + 4.0015
(g)
238.0003 = 237.9957
0.0046 g
E = mc2
E = 0.0000046 kg(3.00x108m/s)2
E = 4.14x1011 kg m2/s2
E = 4.14x1011 J
Chap. 21.6
Chem. 116 Prof. T.L. Heise
78
Energy Changes
Sample exercise: Positron emission form 11C,11C6 --> 11B5 + 0e1
occurs with release of 2.87x1011 J per mole of 11C. What is the mass change per mole of 11C in this nuclear reaction?
Chap. 21.6
Chem. 116 Prof. T.L. Heise
79
Energy Changes
Sample exercise: Positron emission form 11C,11C6 --> 11B5 + 0e1
occurs with release of 2.87x1011 J per mole of 11C. What is the mass change per mole of 11C in this nuclear reaction?
E = mc2
Chap. 21.6
Chem. 116 Prof. T.L. Heise
80
Energy Changes
Sample exercise: Positron emission form 11C,11C6 --> 11B5 + 0e1
occurs with release of 2.87x1011 J per mole of 11C. What is the mass change per mole of 11C in this nuclear reaction?
E = mc2
2.87x1011 J = m(3.00x108m/s)2
Chap. 21.6
Chem. 116 Prof. T.L. Heise
81
Energy Changes
Sample exercise: Positron emission form 11C,11C6 --> 11B5 + 0e1
occurs with release of 2.87x1011 J per mole of 11C. What is the mass change per mole of 11C in this nuclear reaction?
E = mc2
2.87x1011 J = m(3.00x108m/s)2
2.87x1011 J = m
(3.00x108m/s)2
Chap. 21.6
Chem. 116 Prof. T.L. Heise
82
Energy Changes
Sample exercise: Positron emission form 11C,11C6 --> 11B5 + 0e1
occurs with release of 2.87x1011 J per mole of 11C. What is the mass change per mole of 11C in this nuclear reaction?
E = mc2
2.87x1011 J = m(3.00x108m/s)2
2.87x1011 J = m
(3.00x108m/s)2
3.18x 10-6 kg = m
Chap. 21.6
Chem. 116 Prof. T.L. Heise
83
Energy Changes
Sample exercise: Positron emission form 11C,11C6 --> 11B5 + 0e1
occurs with release of 2.87x1011 J per mole of 11C. What is the mass change per mole of 11C in this nuclear reaction?
E = mc2
2.87x1011 J = m(3.00x108m/s)2
2.87x1011 J = m
(3.00x108m/s)2
3.19x 10-6 kg = m
0.00319 g = m
Chap. 21.6
Chem. 116 Prof. T.L. Heise
84
Energy Changes
Scientists discovered in the 1930’s that the masses of nuclei are always less than the masses of the individual nucleons of which they are composed.The mass difference between a nucleus and its
constituent nucleons is called the mass defectThe origin of the mass defect is readily understood if
we consider that energy is used to break into the nucleons
The larger the binding energy, the more stable the nucleus
Chap. 21.6
Chem. 116 Prof. T.L. Heise
85
Energy Changes
nuclei of intermediate mass numbers are more tightly bound than those with smaller or larger mass numbers
- a larger atom will break up into two intermediates
- 2 or more smaller atoms will fuse into an intermediate
Chap. 21.6
Chem. 116 Prof. T.L. Heise
86
Nuclear Fission
Chap. 21.7
Chem. 116 Prof. T.L. Heise
87
Nuclear Fission
2.4 neutrons produced by every fission of uranium-235.
Number of fissions and energy released quickly escalates exponentially is unchecked
In order for a fission chain reaction to occur a minimum mass of material must be present
(critical mass) - with minimum present only one neutron is effective in producing another fission
Chap. 21.7
Chem. 116 Prof. T.L. Heise
88
Nuclear Fission
2.4
Chap. 21.7
Chem. 116 Prof. T.L. Heise
89
Nuclear Fission
To trigger the fission reaction, two subcritical masses are slammed together using chemical explosives.
The two combined masses are supercritical which rapidly leads to an uncontrolled nuclear explosion
Chap. 21.7
Chem. 116 Prof. T.L. Heise
90
Nuclear Fission
Nuclear Reactors: Uranium is enriched to about 3% U-235 and
then used to form UO2 pellets that are encased in zirconium or stainless steel tubes
Rods composed of materials such as cadmium or boron control the fission process by absorbing neutrons
Moderators slow down neutrons so they can be captured more readily by the fuel
Chap. 21.7
Chem. 116 Prof. T.L. Heise
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Nuclear Fission
Nuclear Reactors: A cooling liquid is circulated through the core
to carry off heat generated by the nuclear fission.
Cooling liquid and moderator could be one and the same substance
Steam is used to drive a turbine connected to an electrical generator, however steam must be condensed so additional cooling liquid is required, generally acquired from lake or river
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Nuclear Fission
Nuclear Reactors: Reactor is surrounded by a concrete shell to shield
personnel and nearby residents from radiationReactor must be stopped periodically so that the
fuel can be replaced or reprocessedSpent fuel rods are being kept in storage at
reactor sites20 half-lives are required for their radioactivity to
reach levels acceptable for biological exposure (600 years)
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Nuclear Fission
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Nuclear Fusion
Fusionappealing as an energy source because of
availability of light isotopes and because fusion products are generally not radioactive
not presently used to generate energy because high energies are needed to overcome the repulsion between nuclei
reaction requires temps of about 40,000,000 Kthese temps have only been achieved using a
hydrogen bomb
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Nuclear Fusion
Fusionalso a problem with confining the reaction - no
known structural material can withstand such temps
possibilities? Tokamak
Lasers
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Nuclear Fusion
Tokamak
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Biological Effects
We are continually bombarded with radiation!
When matter absorbs radiation, the energy of radiation can cause either excitation or ionization of the matter
- ionizing radiation is more harmful
When living tissue is irradiated, most of the energy is absorbed by the 70% water by mass of living tissue
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Biological Effects
Ionizing radiationelectrons are removed from water forming
highly reactive H2O+ ions
H2O+ + H2O --> H3O+ + OH
the unstable and highly reactive OH molecule is an example of a free radical due to the unpaired electron, •OH
in tissue, free radicals attack a host of surrounding biomolecules to produce more free radicals
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Biological Effects
Damage depends onactivity and energy of the radiationlength of exposurewhether source is inside or outside the body
Tissue that shows most damagereproduce at rapid ratesbone marrowblood forming tissuelymph nodes
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Biological Effects
Extended Exposure to Low Dosescancerdamage to growth regulation mechanism in cell,
inducing cells to reproduce in an uncontrolled manner
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Biological Effects
Units used to measure radiationbecquerel (Bq) = 1 nuclear disintegration per
secondcurie (Ci) = 3.7 x 1010 disintegrations per
secondgray (Gy) = 1 J absorbed per kilogram of tissuerad (radiation absorbed dose) = 1 x 10-2 J per
kilogram of tissueto correct for differences in strengths of varying
radiation, a multiplication factor is used
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Biological Effects
RadonRn-222 is a product of nuclear disintegration of
U-238being a noble gas, radon is extremely unreactive
and easily escapes the groundradon has a short half life and emits alpha
particles222Rn86 --> 218Po84 + 4He2
polonium is also an alpha emitter
Chap. 21.9