Nuclear Reactions Chapter 20 loosely of Brady

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Nuclear ReactionsChapter 20 (loosely)of Brady
Senese 5th Edition

• Dr. C. Yau
• Fall 2009

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What do you need to know?
You are responsible only for the topics on
nuclear reactions I present here. Review The
nuclear symbol
Atomic number (also, nuclear charge)
protons (C has charge of 6 in nucleus, due to 6
protons) In nuclear chemistry, this number does
not always give us protons, but it does give us
the nuclear charge. (Not all nuclear particles
have protons.)
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Isotopes
Give the number of p and n for each of the
following
1p 1 p 1 p 0 n 1 n 2 n protium deuterium tritium
99.985 0.015 zero
These are isotopes of hydrogen. What exactly are
isotopes? -Atoms of the same element with
different masses. -Atoms with the same p
but different n Tritium is the only one that
is radioactive. D2O heavy water (MW20 amu
instead of 18 amu)
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Radioactivity is the spontaneous disintegration
of a nucleus accompanied by an emission of
rays. Radioisotopes are the isotopes that are
radioactive. Note that not all isotopes are
radioactive. (e.g. Tritium is, but not protium or
deuterium.) Alpha Decay (?-decay) is the
disintegration of a nucleus accompanied by an
emission of ? particles. Beta Decay (?-decay) is
the disintegration of a nucleus accompanied by an
emission of ? particles.
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• Gamma Decay (?-decay) is emission of ?-rays.
• ?-rays are high energy rays (no particles
  involved).
• Why alpha, beta, gamma?
• Thats Greek for A, B, C.
• It turns out that the ? particle is the nucleus
  of the He atom
• ? particle
• particle is the electron
• neutron has the symbol
• proton has the symbol
• positron is a "positive electron" ?

? is not a particle but is given the symbol
Explain the numbers!
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?-decay involves an emission of an electron from
the nucleus. Doesnt that strike you as being a
bit odd??? Which nuclear particles are in the
nucleus? How can the nucleus eject an
ELECTRON? Actually a neutron breaks up into p
e-. The proton stays in the nucleus and the e-
is emitted. Write the equation for what is
happening to the n.
Check to see it's balanced.
What is the significance of a proton as a
product? The element is changing into a different
element!
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Example 1 Uranium-238 undergoes ?-decay. Write
the nuclear equation for its decay. "Uranium-238"
means the isotope of uranium with a mass number
of 238. We do not need to specify the atomic
number as we can easily find it on the periodic
table. Example 2 The product of U-238 decay
undergoes ?-decay. Write the nuclear equation
for it.
Note that U changed into another element! This is
not possible in chemical reactions!
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U-238 decay series involve the disintegration of
U-238 and its subsequent products until we end up
with a nonradioactive product (Pb-206).
Fig. 20.7 p. 828
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Example 3 If we begin with polonium-210 and ended
with lead-206, what nuclear particle is emitted
during the decay? Example 4 The U-238 decay
series involve 14 steps. Somewhere along the
series there is a step that involves emission of
alpha rays and it produces radioactive radon-222.
Write the nuclear equation.
What do you know about radon? Where have you
heard about it? U-238 decay series produces many
other radioactive products. Why do we single out
Rn to talk about?
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Why do some nuclei decay and not others? Why are
some ?-emitters and some ?-emitters? Red line
shows ratio of n / p 1 (n p) As p
increases, positive charge in nucleus is getting
too large, and n increases to keep them
apart. Shaded area shows the Band of
Stability. Elements above the band are
?-emitters which decr n and incr p. Elements
below the band are ?-emitters which decr p.
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Review of Half-life and Decay Rate

• Radioactive decay is 1st order.
• For 1st order, t½
• Note that the half-life is not affected by the
  initial concentration of the reactant.
• What does half-life (t½) mean?
• It is the amount of time it takes for the sample
  to decrease by ½ (by mass or by number of
  particles).
• Example 5 U-238 has a half-life of 4.5x109 yr.
  Will it be all gone after two half-lives? Before
  you do any calculations, first check to see
  whether it can be done easily.

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Review of Half-life and Decay Rate (contd.)

• Example 6 P-32 has a half-life of 14 days.
  Starting with 10.0 g of P-32, how much is left
  after 42 days?
• Example 7 P-32 has a half-life of 14 days.
  Starting with 10.0 g of P-32, how much is left
  after 45 days?

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• You can determine the half-life graphically by
  plotting concentration vs. time.
• What is the half-life of I-131?
• What is its decay constant?
• What is the unit of the constant?

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There is another isotope of uranium of interest
U-235. It is also radioactive but more important
it is fissionable (or fissile). Nuclear fission
is splitting the nucleus into two large particles
by bombardment of a high energy particle (such as
a neutron). Nuclear fusion is fusing together
two small nuclei into a larger one. Both are
accompanied by an enormous amount of E!
?E - 1010 kJ
?E - 108 kJ
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Nuclear Fission U-235 does not always split
into the same products. However, the products are
always radioactivity and given the term
radioactive daughters. These radioactive
products are in the fallout from a nuclear
bomb. Sr-90 has half-life of 28.1 yr. Rule of
thumb It takes 10 half-lives for a radioisotope
to be considered "gone." Note also the formation
of 3 n. Why is this significant?
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Nuclear Chain Reaction

• Fig. 20.14 p.843

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Nuclear Fission Nuclear fission is what we use
in the nuclear reactor at the power plants and in
atomic bombs. Not all isotopes are fissile
(not fissionable). U-238 is radioactive but not
fissile. In nature, only 0.7 of naturally
occurring uranium is U-235. The rest is U-238
which is not fissile. Nuclear power plants only
require about 3 U-235. Atomic bombs require
about 97.3 U-235. To take 0.7 U-235 and
convert to weapon-grade 97.3 U-235 we make use
of the different rate of effusion and applying
Grahams Law Heavier atoms effuse slower.
Allowing the two isotopes to effuse (a long
process as the difference in mass is only 3 amu)
eventually we can separate U-235 from U-238.
This process is called ENRICHMENT.
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• What is the deal with plutonium?
• Bombardment of U-238 with neutrons converts it to
  Np (neptunium) which quickly decays to Pu-239.
• Pu-238 is fissile. Pu-239 can be used for
  making bombs.
• Reactor grade Pu can be easily converted into a
  nuclar bomb.
• Half-life of Pu is 25,000 yrs. If there is a
  spill, how long will it take for Pu to be totally
  gone?
• This is why there is much concern with where and
  who has Pu.
• There is another reason for concern over the
  location of PuIt is HIGHLY toxic.
• It is an alpha emitter.
• It is estimated that 1 microgram in the lungs of
  a humna is enough to induce lung cancer.

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• Nuclear Fusion
• Another example is
• Complete the equation
• Nuclear fission is what takes place in the sun
  and the stars. and used in the H-bomb.
• Advantage over nuclear fission? H isotopes are
  much easier to obtain than uranium, and much
  more abundant (from water).
• Products have much shorter half-lives.
• H-3 has t ½ 12. 3 yrs compared to
• U-235 has t ½ 7.04x108 yrs
• Nuclear wastes of nuclear fusion also have much
  shorter half-lives.
• Disadvantage is the difficulty in harnessing it
  for use requires millions of degrees to initiate.

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What is meant by cold fusion?

• Cold fusion is to initiate fusion without the
  millions of degrees of heat (only possible in the
  sun).
• Some years ago, two scientists made a dramatic
  announcement to say they have accomplished cold
  fusion.
• What happened?

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Where does all the nuclear energy come
from? Nuclear Binding Energy If we want to make a
helium atom, what do we need? 2 protons and 2
neutrons 2 x 1.0072764669 amu 2.0145529338
amu 2 x 1.0086649156 amu 2.0173298312 amu Total
4.0318827650
amu He nucleus is actually 4.0015061792
amu Where is the missing mass? "mass defect"
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Mass Defect Binding Energy

• The missing mass is the mass that is converted
  into energy
• E m c2
• This is the energy released when nuclear
  particles combine, break and recombine.
• c speed of light 3 x 108 m/s (very large )
• c2 3 x 1016 m2/s2 (even larger !)
• Significance a small mass is converted to a very
  large amount of energy!

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Anything good about nuclear reactions?

• Nuclear energy does not produce greenhouse gas
  (CO2) or acid rain (SO2).
• We do not rely on the Middle East to supply us
  with uranium.
• Nuclear medicine
• PET scan Positron Emission Tomography
• (Nuclear medicine imaging to produce a 3-D image
  of functional processes in the body)
• It uses nucleotides of short half-lives, such as
  C-11 (20 mins) N-13 (10 mins) O-15 (2 mins).
  Why is a short half-life desirable?
• Write the nuclear eqn for C-11 emitting a
  positron.
• Write the nuclear eqn for the positron meeting
  with an electron.