Chapter 18 - Nuclear Chemistry

Report
INTRODUCTORY CHEMISTRY
Concepts & Connections
Fifth Edition by Charles H. Corwin
Chapter
18
Nuclear
Chemistry
Christopher G. Hamaker, Illinois State University, Normal IL
© 2008, Prentice Hall
Introduction
• As the Earth’s supply of fossil fuels is used up,
nations are increasingly turning to nuclear power.
• Currently, approximately 20% of the world’s
electricity needs are met using nuclear power.
• However, disposal of radioactive waste and the
threat of accidents are major concerns with the use
of nuclear power.
• In this chapter, we will learn about nuclear
chemistry.
Chapter 18
2
Natural Radioactivity
• There are three types of radioactivity:
– alpha particles, beta particles, and gamma rays
• Alpha particles (a) are identical to helium nuclei,
containing 2 protons and 2 neutrons.
• Beta particles (b) are identical to electrons.
• Gamma rays (g) are high-energy photons.
Chapter 18
3
Charge of Radiation Types
• Alpha particles have a +2 charge, and beta
particles have a –1 charge. Both are deflected by
an electric field.
• Gamma rays are electromagnetic radiation and
have no charge, so they are not deflected.
Chapter 18
4
Behavior of Radiation
• Since alpha particles have the largest mass, they
are the slowest-moving type of radiation.
• Gamma rays move at the speed of light since they
are electromagnetic radiation.
Chapter 18
5
Atomic Notation
• Recall, we learned atomic notation in Chapter 5.
• A nuclide is the nucleus of a specific atom.
• The radioactive nuclide strontium-90 has 90
protons and neutrons. The atomic number is 38.
90
38
Sr
Chapter 18
6
Nuclear Reactions
• A nuclear reaction involves a high-energy change
in an atomic nucleus.
• For example, a uranium-238 nucleus changes into
a thorium-231 nucleus by releasing a helium-4
particle and a large amount of energy.
235
92
U
231
90
Th + 24 He
• In a balanced nuclear reaction, the atomic
numbers and masses for the reactants must equal
those of the products.
Chapter 18
7
Balancing Nuclear Reactions
1. The total of the atomic numbers (subscripts) on
the left side of the equation must equal the sum
of the atomic numbers on the right side.
2. The total of the atomic masses (superscripts) on
the left side of the equation must equal the sum
of the atomic masses on the right side.
3. After completing the equation by writing all the
nuclear particles in an atomic notation, a
coefficient may be necessary to balance the
reaction.
Chapter 18
8
Common Nuclear Particles
• Here is a listing of the common nuclear particles
used to balance nuclear reactions.
Chapter 18
9
Alpha (a) Emission
• Radioactive nuclides can decay by giving off an
alpha particle.
• Radium-226 decays by alpha emission.
226
88
Ra
A
Z
X
+ 24 He
• First, balance the number of protons: 88 = Z + 2,
so Z = 86 (Rn).
• Second, balance the number of protons plus
neutrons: 226 = A + 4, so A = 222.
226
88
Ra
222
86
4
2
Rn + He
Chapter 18
10
Beta (β ) Emission
• Some radioactive nuclides decay by beta
emission.
• Radium-228 loses a beta particle to yield
actinium-228.
228
88
Ra
228
89
Ac +
0
-1
e
• Beta decay is essentially the decay of a neutron
into a proton and an electron.
Chapter 18
11
Gamma (γ ) Emission
• Gamma rays often accompany other nuclear decay
reactions.
• For example, uranium-233 decays by releasing
both alpha particles and gamma rays.
233
92
U
229
90
Th + 24 He + 00 g
• Note that a gamma ray has a mass and a charge of
zero, so it has no net effect on the nuclear
reaction.
Chapter 18
12
Positron (β +) Emission
• A positron (b+) has the mass of an electron, but a
+1 charge.
• During positron emission, a proton decays into a
neutron and a positron.
• Sodium-22 decays by positron emission to
neon-22.
22
11
Na
22
10
Ne +
Chapter 18
0
+1
e
13
Electron Capture
• A few large, unstable nuclides decay by electron
capture. A heavy, positively charged nucleus
attracts an electron.
• The electron combines with a proton to produce a
neutron.
• Lead-205 decays by electron capture.
205
82
Pb +
0
-1
e
Chapter 18
205
81
Tl
14
Decay Series
• Some heavy nuclides must go through a series of
decay steps to reach a nuclide that is stable.
• This stepwise disintegration of a radioactive
nuclide until a stable nucleus is reached is called a
radioactive decay series.
• For example, uranium-235 requires 11 decay steps
until it reaches the stable nuclide lead-207.
Chapter 18
15
Uranium-238 Decay Series
• Uranium-238 undergoes
14 decay steps before it
ends as stable lead-206.
• The decay series for
uranium-238 is shown
here.
• The series includes 8
alpha-decays and 6
beta-decays.
Chapter 18
16
Parent and Daughter Nuclides
• The term parent-daughter nuclides describes a
parent nuclide decaying into a resulting daughter
nucleus.
• For example, the first step in the decay series for
U-238 is:
238
92
U
224
90
Th + 24 He
• U-238 is the parent nuclide and Th-234 is the
daughter nuclide.
Chapter 18
17
Activity
• The number of nuclei that disintegrate in a given
period of time is called the activity of the sample.
• A Geiger counter is used to count the activity of
radioactive samples.
• Radiation ionizes gas in a tube,
which allows electrical
conduction
• This causes a clicking to be
heard and the number of
disintegrations to be counted.
Chapter 18
18
Half-Life Concept
• The level of radioactivity for all radioactive
samples decreases over time.
• Radioactive decay shows a systematic progression.
• If we start with a sample that has an activity of
1000 disintegrations per minute (dpm), the level
will drop to 500 dpm after a given amount of time.
After the same amount of time, the activity will
drop to 250 dpm.
• The amount of time for the activity to decrease by
half is the half-life, t½.
Chapter 18
19
Half-Life
• After each half-life, the activity of a radioactive
sample drops to half its previous level.
• A decay curve
shows the
activity of a
radioactive
sample over
time.
Chapter 18
20
Radioactive Waste
• A sample of plutonium-239 waste from a nuclear
reactor has an activity of 20,000 dpm. How many
years will it take for the activity to decrease to
625 dpm?
• The half-live for Pu-239 is 24,000 years.
• It takes 5 half-lives for the activity to drop to 625
dpm.
24,000 y
5 t½ ×
= 120,000 y
1 t½
Chapter 18
21
Half-Life Calculation
• Iodine-131 is used to measure the activity of the
thyroid gland. If 88 mg of I-131 are ingested,
how much remains after 24 days (t½ = 8 days).
• First, find out how many half-lives have passed:
1 t½
24 days ×
= 3t½
8 days
• Next, calculate how much I-131 is left:
1
1
1
88 mg I-131 × × × = 11 mg I-131
2
2
2
Chapter 18
22
Radiocarbon Dating
• A nuclide that is unstable is called a radionuclide.
• Carbon-14 decays by beta emission with a halflife of 5730 years.
14
6
C
14
7
N +
0
-1
e
• The amount of carbon-14 in living organisms
stays constant with an activity of about 15.3 dpm.
After the plant or animal dies, the amount of C-14
decreases.
Chapter 18
23
Radiocarbon Dating, continued
• The age of objects can therefore be determined by
measuring the C-14 activity. This is called
radiocarbon dating.
• The method is considered reliable for items up to
50,000 years old.
Chapter 18
24
Uranium-Lead Dating
• Uranium-238 decays in 14 steps to lead-206. The
half-life for the process is 4.5 billion years.
238
92
U
206
82
4
2
0
He
Pb + 8
+ 6 -1 e
• The age of samples can be determined by
measuring the U-238/Pb-206 ratio.
• A ratio of 1:1 corresponds to an age of about 4.5
billion years.
Chapter 18
25
Agricultural Use of Radionuclides
• Cobalt-60 emits gamma rays when it decays and is
often used in agriculture.
• Gamma radiation is used to
sterilize male insects instead
of killing them with pesticides.
• Gamma-irradiation of food is
used to kill microorganisms:
– irradiation of pork to kill the
parasite that causes trichinosis
– irradiation of fruits and
vegetables to increase shelf life
Chapter 18
26
Critical Thinking: Nuclear Medicine
• The term nuclear medicine refers to the use of
radionuclides for medical purposes.
• Iodine-131 is used to measure thyroid activity.
• The gas xenon-133 is used to diagnose respiratory
problems.
• Iron-59 is used to
diagnose anemia.
• Breast cancer can be
treated using the
isotope iridium-159.
Chapter 18
27
Artificial Radioactivity
• A nuclide can be converted into another element
by bombarding it with an atomic particle.
• This process is called transmutation.
• The elements beyond uranium on the periodic
table do not occur naturally and have been made
by transmutation.
• For example, rutherfordium can be prepared from
californium:
249
98
Cf +
12
6
C
Chapter 18
257
104
1
0
Rf + 4 n
28
Nuclear Fission
• Nuclear fission is the process where a heavy
nucleus splits into lighter nuclei.
• Some nuclides are so unstable, they undergo
spontaneous nuclear fission.
252
98
142
56
Cf
Ba +
106
42
1
0
Mo + 4 n
• A few nuclides can be induced to undergo nuclear
fission by a slow-moving neutron.
1
0
235
n + 92
U
141
56
Ba +
92
36
Chapter 18
Kr + 3 01 n + energy
29
Nuclear Chain Reaction
• Notice that one neutron produces three neutrons.
These neutrons can induce additional fission
reactions and produce additional neutrons.
• If the process becomes self-sustaining, it is a
chain reaction.
Chapter 18
30
Nuclear Chain Reaction
• The mass of material required for a chain reaction
is the critical mass.
Chapter 18
31
Critical Thinking: Nuclear Power Plant
• Nuclear energy is an attractive source because of
its potential:
– 1 gram of U-235 produces about 12 million times more
energy than 1 gram of gasoline
• A nuclear power plant produces
energy from a nuclear chain
reaction.
• Fuel rods are separated by control
rods to regulate the rate of fission.
• A liquid coolant is circulated to
absorb heat.
Chapter 18
32
Nuclear Fusion
• Nuclear fusion is the combining of two lighter
nuclei into a heavier nucleus.
• It is more difficult to start a fusion reaction than a
fission reaction, but it releases more energy.
• Nuclear fusion is a cleaner process than fission
because very little radioactive waste is produced.
• The Sun is a giant fusion reactor, operating at
temperatures of millions of degrees Celsius.
Chapter 18
33
Fusion in the Sun (and Other Stars)
• The Sun is about 73% hydrogen, 26% helium, and
1% all other elements.
• Three common fusion reactions that occur in the
Sun are:
1
1
1
1
H+ H
2
1
3
2
2
1
H +
H + 11 H
He + 11 H
3
2
4
2
e + energy
He + energy
He +
Chapter 18
0
+1
0
+1
e + energy
34
Fusion Energy
• There are two fusion reactions being investigated
for use in commercial power generation.
• The first uses deuterium (H-2) as a fuel:
2
1
H + 12 H
4
2
He + energy
• The second involves deuterium and tritium (H-3)
as fuels:
3
1
2
1
H + H
4
2
He +
Chapter 18
1
0
n + energy
35
Chapter Summary
• There are three types of natural radiation:
– alpha particles
– beta particles
– gamma rays
• Gamma rays are electromagnetic radiation.
• Alpha particles are helium nuclei, and beta
particles are electrons.
Chapter 18
36
Chapter Summary, continued
• Radioactive nuclides decay by 4 processes:
–
–
–
–
alpha emission
beta emission
positron emission
electron capture
• The parent nuclide decays to yield the daughter
nuclide.
• If a nuclide decays through the emission of
radiation in more than one step, the overall
process is called a radioactive decay series.
Chapter 18
37
Chapter Summary, continued
• The time required for 50% of the radioactive
nuclei in a sample to decay is constant and is
called the half-life. After each half-life, only 50%
of the radioactive nuclei remain.
• Artificial nuclides are produced by transmutation.
• The splitting of a heavy nucleus into two lighter
nuclei is nuclear fission.
• The combining of two lighter nuclei into one
nucleus is nuclear fusion.
Chapter 18
38

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