Download Nuclear Chemistry

Chemistry – Unit 4
Chapter 25
Nuclear Chemistry
Mass Defect
• Difference between the mass of an
atom and the mass of its individual
particles.
4.00260 amu
4.03298 amu
Nuclear Binding Energy
• Energy released when a nucleus is
formed from nucleons.
• High binding energy = stable nucleus.
E =
2
mc
E: energy (J)
m: mass defect (kg)
c: speed of light
(3.00×108 m/s)
Nuclear Binding Energy
Unstable nuclides are radioactive and
undergo radioactive decay.
Types of Radiation
• Alpha particle ()
– helium nucleus
• Beta particle (-)
– electron
• Positron (+)
– positron
• Gamma ()
– high-energy photon
4
2
He
2+
0
-1
e
1-
0
1
e
1+
0
paper
lead
concrete
Nuclear Decay
• Alpha Emission
238
92
U
parent
nuclide
Th  He
234
90
daughter
nuclide
4
2
alpha
particle
Numbers must balance!!
Nuclear Decay
• Beta Emission
131
53
I
131
54
Xe  e
0
-1
electron
• Positron Emission
38
19
K  Ar  e
38
18
0
1
positron
Nuclear Decay
• Electron Capture
106
47
Ag  e 
0
-1
• Gamma Emission
106
46
Pd
electron
– Usually follows other types of decay.
• Transmutation
– One element becomes another.
IQ# 1
1.Balance the following equations:
237
93
Np H 
4
2
233
Pa
91
212
0
212
Po

e

Bi
84
83
1
Nuclear Decay
• Why nuclides decay…
– need stable ratio of neutrons to protons
238
92
U
234
90
I
131
54
131
53
38
19
106
47
Th  He
4
2
Xe  e
0
-1
K  Ar  e
38
18
Ag  e 
0
-1
0
1
106
46
Pd
Band of Stability
and Radioactive
Decay
Half-life
• Half-life (t½)
– Time required for half the atoms of a
radioactive nuclide to decay.
– Shorter half-life = less stable.
Half-life
mf  m ( )
1 n
i 2
mf: final mass
mi: initial mass
n: # of half-lives
Half-life
• Fluorine-21 has a half-life of 5.0 seconds. If
you start with 25 g of fluorine-21, how many
grams would remain after 60.0 s?
GIVEN:
WORK:
t½ = 5.0 s
mf = mi (½)n
mi = 25 g
mf = (25 g)(0.5)12
mf = ?
mf = 0.0061 g
total time = 60.0 s
n = 60.0s ÷ 5.0s
=12
Example: How much of a 500. g sample of Uranium-235
would be left after five half-lives?
Mi = 500 g
(n
n=5
Mf = ?
= # of half-lives)
Fraction left after 5 halflives =
500g (0.5)5
Mf =
15.6g
Example: A 16.00 mg sample of Radon-222 decays to 0.250
mg after 24 hours. Determine the half-life.
16→ 8 → 4 → 2 → 1 → 0.5 → 0.25 = 6 half lives
24 h
 4 h  4.0 h
6
Example: The half-life of molybdenum-99 is 67 hours.
How much of a 1.000 mg sample is left after 335
hours?
Mi = 1.000 mg
Half-life = 67 h
Rxn time = 335 h
Mf = ?
n = 335 / 67 = 5
(1.000)0.5  0.03125 mg
5
Mf = 0.031 mg
Learning Check!
The half life of I-123 is 13 hr. How
much of a 64 mg sample of I-123 is left
after 39 hours?
Half Life and Radioactivity Lab
• Work in groups of 2 at your table.
• Each cup has 1 penny in it which will be shaken and then
GENTLY emptied on the table.
• For the first trial, shake the penny out 100 times on
the table. Record the number of times that it came up
heads.
• For the next trail, you will shake out the penny the
number of times that it landed on heads in the last
round.
• The same procedure will follow until no more of the
pennies have landed on “heads” (tails = decayed).
• Record all data in the lab book following the example on
page 809.
• Answer questions 1-4 and be sure to follow the
graphing rules (R74 and in the “Math Review” handout
from the beginning of the year).
Graphing the Results
Important !!
• Graph directly on lab book
• Title every graph and label each axis
• Graph at least 2/3 page
• Use a ruler
• Circle all data points
• Use a best-fit line (no “connect the dots”!)
5) Find the average half-life (in # of trials) of
your sample by interpolating your curve at
exactly 50, 25, and 12.5 flips)
Fission
• splitting a nucleus into two or more
smaller nuclei
• 1 g of 235U =
3 tons of coal
235
92
U
Fission
• chain reaction - self-propagating
reaction
• critical mass mass required
to sustain a
chain reaction
Fusion
• combining of two nuclei to form one
nucleus of larger mass
• thermonuclear reaction – requires temp
of 40,000,000 K to sustain
• 1 g of fusion fuel =
20 tons of coal
• occurs naturally in
stars
2
1
H H
3
1
Fission vs. Fusion
F
I
S
S
I
O
N
• 235U is limited
• danger of
meltdown
• toxic waste
• thermal pollution
F
U
S
I
O
N
• fuel is abundant
• no danger of
meltdown
• no toxic waste
• not yet sustainable
Nuclear Power
• Fission Reactors
Cooling
Tower
Nuclear Power
• Fission Reactors
Nuclear Power
• Fusion Reactors (not yet sustainable)
Nuclear Power
• Fusion Reactors (not yet sustainable)
National Spherical
Torus Experiment
Tokamak Fusion Test Reactor
Princeton University
Synthetic Elements
• Transuranium Elements
– elements with atomic #s above 92
– synthetically produced in nuclear reactors
and accelerators
– most decay very rapidly
238
92
U  He 
4
2
242
94
Pu
Radioactive Dating
• half-life measurements of radioactive
elements are used to determine the age
of an object
• decay rate indicates amount of
radioactive material
• EX: 14C - up to 40,000 years
238U and 40K - over 300,000 years
Nuclear Medicine
• Radioisotope Tracers
– absorbed by specific organs and used to
diagnose diseases
• Radiation Treatment
– larger doses are used
to kill cancerous cells
in targeted organs
– internal or external
radiation source
Radiation treatment using
-rays from cobalt-60.
Nuclear Weapons
• Atomic Bomb
– chemical explosion is used to form a
critical mass of 235U or 239Pu
– fission develops into an uncontrolled
chain reaction
• Hydrogen Bomb
– chemical explosion  fission  fusion
– fusion increases the fission rate
– more powerful than the atomic bomb
Others
• Food Irradiation
–  radiation is used to kill bacteria
• Radioactive Tracers
– explore chemical pathways
– trace water flow
– study plant growth, photosynthesis
• Consumer Products
– ionizing smoke detectors - 241Am