Transcript Chemistry and Chemical Reactivity 1 6th Edition John C. Kotz Paul M. Treichel Gabriela C.

Chemistry and Chemical Reactivity 1
6th Edition
John C. Kotz
Paul M. Treichel
Gabriela C. Weaver
CHAPTER 23
Nuclear Chemistry
Lectures written by John Kotz
©2006
2006
Brooks/Cole
Thomson
©
Brooks/Cole
- Thomson
Nuclear Chemistry
2
Pictures of human
heart before and after
stress using gamma
rays from radioactive
Tc-99m
© 2006 Brooks/Cole - Thomson
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•
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Why do you care?
PET scans
Nuclear Power
Space travel
Smoke Detectors (Am-241)
Ionizing Radiation and X-rays
Neutron Activation
Exposure (pilots, nuclear accidents, Radon)
Carbon Dating
Nuclear Weapons
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Nuclear Radiation
• The Process of emitting energy in the form
of waves or particles.
• Comes from the Nucleus of the Atom
–
–
–
–
The Neutrons
Instability – Binding Energy
E=mc2
Non-conservation of Mass
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ATOMIC COMPOSITION
• Protons
– positive electrical charge
– mass = 1.672623 x 10-24 g
– relative mass = 1.007 atomic mass units (amu)
• Electrons
– negative electrical charge
– relative mass = 0.0005 amu
• Neutrons
– no electrical charge
– mass = 1.675523 x 10-24 g
– relative mass = 1.009 amu
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Isotopes
• Atoms of the same element (same Z) but
different mass number (A).
• Boron-10 (10B) has 5 p and 5 n: 105B
• Boron-11 (11B) has 5 p and 6 n: 115B
11B
10B
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Radioactivity
• One of the pieces of evidence for the
fact that atoms are made of smaller
particles came from the work of
Marie Curie (1876-1934).
• She discovered radioactivity,
the spontaneous disintegration of
some elements into smaller pieces.
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Types of Radiation
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Penetrating Ability
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Nuclear Reactions
• Alpha emission
Note that mass number (A) goes down by 4 and atomic
number (Z) goes down by 2.
Nucleons are rearranged but conserved
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Nuclear Reactions
• Beta emission
Note that mass number (A) is unchanged and atomic
number (Z) goes up by 1.
How does this happen?
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Other Types of Nuclear Reactions
Positron (0+1b): a positive electron
207
207
K-capture: the capture of an electron from the first or
K shell
An electron and proton combine to form a neutron.
0 e + 1 p --> 1 n
-1
1
0
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Radioactive Decay Series
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Stability
of Nuclei
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• Heaviest naturally
occurring nonradioactive isotope is
209Bi
with 83 protons
and 126 neutrons
• There are 83 x 126 =
10,458 possible isotopes.
Why so few actually
exist?
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Stability of Nuclei
• Up to Z = 20 (Ca), n = p (except for 73Li, 115B, 199F)
• Beyond Ca, n > p (A > 2 Z)
• Above Bi all isotopes are radioactive. Fission leads to
smaller particles, the heavier the nucleus the greater the
rate.
• Above Ca: elements of EVEN Z have more isotopes and
most stable isotope has EVEN N.
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Stability of Nuclei
Z N Even
Even 157
Odd
50
Odd
52
5
• Suggests some PAIRING of NUCLEONS
• Something inside the nucleus gives each atom a
probability of radioactive decay
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Band of Stability
and Radioactive
Decay
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a emission reduces Z
243
95Am
--> 42a + 23993Np
b emission increases Z
60
27Co
--> 0-1b + 6028Ni
Isotopes with low n/p
ratio, below band of
stability decay, decay by
positron emission or
electron capture
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Binding Energy, Eb
Eb is the energy required to separate the
nucleus of an atom into protons and
neutrons.
Use E=mc2
Find the mass of the isotope.
Sum the masses of the nucleons.
For m, use the DIFFERENCE between those
masses.
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Calculate Binding Energy
2 H ---> 1 p + 1 n
For deuterium, 21H:
1
1
0
Mass of 21H = 2.01410 g/mol
Mass of proton = 1.007825 g/mol
Mass of neutron = 1.008665 g/mol
∆m = 0.00239 g/mol = 2.39x10-6 kg/mol
c = 3x108 m/sec
From Einstein’s equation:
Eb = (∆m)c2 = 2.15 x 1011 J/mol
How much binding energy is there per nuclear particle?
Eb per nucleon = Eb/2 nucleons
= 1.08 x 108 kJ/mol nucleons
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Half-Life
• HALF-LIFE is the time it takes for 1/2 a sample to
disappear.
• The rate of a nuclear transformation depends only on the
“reactant” concentration. It does not depend on any factors
outside the nucleus.
• Half-life is a property that can be used to identify an
element.
• Half-life cannot predict the likelihood a single atom will
decay
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Half-Life
Decay of 20.0 mg of 15O. What remains after 3 half-lives?
After 5 half-lives?
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Kinetics of Radioactive Decay
Activity (A) = Disintegrations/time
N is the number of atoms
Decay is first order, and so
ln (A/Ao) = -kt or
ln (A) – ln (Ao) = -kt
The half-life of
radioactive decay is
t1/2 = 0.693/k
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Radiocarbon Dating
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Radioactive C-14 is formed in the upper atmosphere
by nuclear reactions initiated by neutrons in cosmic
radiation
14N + 1 n ---> 14C + 1H
o
The C-14 is oxidized to CO2, which circulates through
the biosphere. There is a constant % of C-14 in the
atmosphere. While a plant is alive, it has the same %
of C-14 in it as the atmosphere.
When a plant dies, the C-14 is not replenished.
But the C-14 continues to decay with t1/2 = 5730 years.
Activity of a sample can be used to date the sample.
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Radiocarbon Dating
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Man-made Eyes to See
Small Things
• Humans needed to find a way to extend their senses, to
gather knowledge about things beyond our physical
constraints.
• Light can be thought of as a piece of information sent
between matter.
• The wavelength/frequency/energy of light determines how it
interacts with matter and also predicts where it came from.
• Certain materials can “see” light that our eyes cannot.
• Using these materials we learn about the elements in space
and on earth.
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Human Limitations
• The molecules in our
eyes only work within
a very specific range
of wavelengths.
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Our Sun- Seen by Ultraviolet Light
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Extending Our Vision
• Common detector materials that interact with
light:
• Sodium Iodide crystal:
• Plastic scintillator:
• Germanium Crystal:
• Silicon:
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Cosmic Rays
• Super fast particles from the
sun and outer space (protons
and ions)---
• Strike the atmosphere and
become pions (positively
charged fundamental particle),
then muons (heavy electrons).
• Built a detector to “see” them
using a plastic scintillator.
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Proton
from sun
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Cosmic Rays
Molecule in
atmosphere
Pion
• Obtainable info:
Muon
– Direction of radiation
– Shielding effects
Neutrino
Light
Atom of
Hydrocarbon
200 muons/m2/second
Photomultiplier
Tube (PMT)
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• Pyramids example
• Depth inside Earth
– Solar activity levels
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Cosmic rays
are the source
of C-14 used
in radiocarbon
dating!
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Terrestrial Radiation
• Uses gamma ray spectroscopy to “see” light that
comes from matter in the ground
• Obtainable info:
– Naturally occurring radioactive isotopes can be identified.
– Composition of isotopes in rocks is compared to rocks
from around the world.
– Background radiation in the air can be measured
– Investigation of radiation in the ground.
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Summary
• Certain materials interact with the light that our eyes
don’t detect.
• Devices made from these materials have lead to the
field of spectroscopy, meaning “seeing light.”
• All modern devices convert a light signal into an
electrical signal.
• The electrical signal is arranged in a way that allows
us to ‘see’ what is going on with our eyes.
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Bubble
Chambers
Alpha, Beta, and Gamma Particles rip through a supercooled gas,
ionizing them, and forming bubbles.
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Artificial Nuclear Reactions
New elements or new isotopes of known
elements are produced by bombarding an
atom with a subatomic particle such as a
proton or neutron -- or even a much
heavier particle such as 4He and 11B.
Radioisotopes used in medicine are
often made by these n,g reactions.
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Neutron Activation
– Shoot neutrons into a substance, stuffing them into a
nucleus to make it unstable. They will then decay in a
special way that we can “see” what is in them.
• Applications:
– Test for the presence of
heavily shielded dangerous
nuclear material.
– Create small amounts of
elements (alchemy)
– Find approximate percent
compositions of elements
in a substance.
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Transuranium Elements
Elements beyond 92 (transuranium) made
starting with an n,g reaction
92U +
239
92U
--->
239
93Np +
0
239 Np
93
--->
239
94Np +
0
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1 n
0
--->
239
+ g
238
92U
-1b
-1b
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Transuranium
Elements &
Glenn
Seaborg
106Sg
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Nuclear Fission
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Nuclear Fission
Fission chain has three general steps:
1. Initiation. Reaction of a single atom
starts the chain (e.g., 235U + neutron)
2. Propagation.
236U
fission releases
neutrons that initiate other fissions
3. Termination.
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Nuclear
Fission & Lise
Meitner
109Mt
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Nuclear Fission & POWER
• Currently about 104 nuclear
power plants in the U.S. and
about 400 worldwide.
• 17% of the world’s energy
comes from nuclear fission.
• What are would be the benefits
and drawbacks to using nuclear
FUSION instead of nuclear
fission?
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Nuclear Medicine: Imaging
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BNCT
Boron Neutron Capture Therapy
•
10B
isotope (not 11B) has the ability to
capture slow neutrons
• In BNCT, tumor cells preferentially
take up a boron compound, and
subsequent irradiation by slow
neutrons kills the cells via the energetic
10B --> 7Li neutron capture reaction
(that produces a photon and an alpha
particle)
•
10B
+ 1n ---> 7Li + 4He + photon
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Food Irradiation
•Food can be irradiated with g rays from
60Co or 137Cs.
•Irradiated milk has a shelf life of 3 mo.
without refrigeration.
•USDA has approved irradiation of meats
and eggs.
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Effects of Radiation
Rem: Quantifies biological tissue damage
Usually use “millirem”
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