Docstoc

Nuclear Chemistry

Document Sample
Nuclear Chemistry Powered By Docstoc
					Nuclear Chemistry
    Chapter 21
            Nuclear Reactions

Some nuclei are unstable. They are said
to be radioactive and emit particles and
usually high-energy electromagnetic
radiation (gamma rays) at the same time.

Other nuclear reactions involve
bombarding nuclei with particles such as
neutrons, protons, alpha particles or other
nuclei. (particle accelerators)
Atomic number (Z) = number of protons in nucleus
    Mass number (A) = number of protons + number of neutrons
                      = atomic number (Z) + number of neutrons

               Mass Number      A
                                ZX
                                           Element Symbol
              Atomic Number


         proton    neutron     electron      positron        a particle
       1p
       1   or 1H
              1
                     1n
                     0
                               0e
                              -1  or -1b
                                      0      0e
                                            +1  or +1 b
                                                    0       4He
                                                            2    or 4a
                                                                     2

A         1          1            0             0               4

Z         1          0           -1             +1              2


                                                                    23.1
        Balancing Nuclear Equations

1. Conserve mass number (A).
The sum of protons plus neutrons in the products must equal
the sum of protons plus neutrons in the reactants.
            235               138          96
             92 U   + 1n
                      0        55 Cs   +   37 Rb   + 2 1n
                                                       0

                    235 + 1 = 138 + 96 + 2x1

2. Conserve atomic number (Z) or nuclear charge.
The sum of nuclear charges in the products must equal the
sum of nuclear charges in the reactants.
            235               138          96
             92 U   + 1n
                      0        55 Cs   +   37 Rb   + 2 1n
                                                       0

                     92 + 0 = 55 + 37 + 2x0
                                                            23.1
212Podecays by alpha emission. Write the balanced
nuclear equation for the decay of 212Po.

          alpha particle - 4He or 2a
                                  4
                           2


            212Po       4He   + AX
             84         2       Z


        212 = 4 + A         A = 208

        84 = 2 + Z          Z = 82

           212Po      4He   + 208Pb
            84        2        82




                                                23.1
23.1
       Nuclear Stability and Radioactive Decay

Beta decay
     14C
      6
                 14N
                  7
                        +-1 b + n
                          0
                                            Decrease # of neutrons by 1
     40K
     19
                 40Ca
                 20
                         + -1b + n
                            0
                                            Increase # of protons by 1
                           1n
                           0
                                     1p
                                     1
                                          + -1b + n
                                             0


Positron decay
     11C
      6
                 11B
                  5
                       ++1b + n
                         0
                                            Increase # of neutrons by 1
     38
     19K
                 38Ar
                 18
                        ++1b + n
                          0
                                            Decrease # of protons by 1
                           1p
                           1
                                     1n
                                     0
                                          ++1b + n
                                            0



                  n and n have A = 0 and Z = 0
                                                                    23.2
       Nuclear Stability and Radioactive Decay
Electron capture decay
    37Ar
    18
              0
           + -1e       37Cl
                       17
                              +n        Increase # of neutrons by 1
    55Fe
    26
               0
            + -1e       55Mn
                        25
                                +n      Decrease # of protons by 1
                           1p
                           1
                                   0
                                + -1e   1n
                                        0
                                             +n
Alpha decay

                                        Decrease # of neutrons by 2
    212Po           4He   + 208Pb
     84             2        82
                                        Decrease # of protons by 2

Spontaneous fission

    252Cf           2125In + 21n
     98               49      0
                                                                23.2
Example 21.1

Write balanced nuclear equations for the

a. Decay of U-238 to Th-234
b. Decay of Cs-137 to Ba-137
c. Emission of a beta particle by As-78
n/p too large
beta decay

 X


                Y

             n/p too small
     positron decay or electron capture


                                          23.2
                       Nuclear Stability
•   Certain numbers of neutrons and protons are extra stable
    •   n or p = 2, 8, 20, 50, 82 and 126
    •   Like extra stable numbers of electrons in noble gases
        (e- = 2, 10, 18, 36, 54 and 86)
•   Nuclei with even numbers of both protons and neutrons
    are more stable than those with odd numbers of neutron
    and protons
•   All isotopes of the elements with atomic numbers higher
    than 83 are radioactive
•   All isotopes of Tc and Pm are radioactive




                                                              23.2
 Nuclear binding energy (BE) is the energy required to break
 up a nucleus into its component protons and neutrons.
                     BE + 19F
                           9        91p + 101n
                                     1      0

     Δ E = (Δ m)c2
       BE = 9 x (p mass) + 10 x (n mass) – 19F mass

BE (amu) = 9 x 1.007825 + 10 x 1.008665 – 18.9984

       BE = 0.1587 amu          1 amu = 1.49 x 10-10 J
       BE = 2.37 x 10-11J

                                binding energy
binding energy per nucleon =
                              number of nucleons
                             2.37 x 10-11 J
                           =                = 1.25 x 10-12 J
                             19 nucleons
                                                               23.2
Nuclear binding energy per nucleon vs Mass number




    nuclear binding energy
                             nuclear stability
           nucleon
                                                 23.2
Ex 21.2 The atomic mass of I-127 is
126.9004 amu. Calculate the nuclear
binding energy and the BE per nucleon.

c = 3.00 x 108m/s
1 kg = 6.02 x 1026 amu
1 J = 1 kg m2/s2
         Natural Radioactivity

Nuclei outside the belt of stability or
with more than 83 protons are
unstable and decay naturally.

This decay obeys 1st order kinetics
and therefore has a constant half-life.
Kinetics of Radioactive Decay
      N         daughter
             DN
    rate = -          rate = lN
             Dt
             DN
           -    = lN
             Dt
N = N0exp(-lt)      lnN = lnN0 - lt
N = the number of atoms at time t

N0 = the number of atoms at time t = 0

l is the decay constant

                ln2
            l =
                 t½
                                    23.3
              Kinetics of Radioactive Decay


      [N] = [N]0exp(-lt)               ln[N] = ln[N]0 - lt




                              ln [N]
[N]




                                                             23.3
EH Assignment
Unit 23                              Mininum score
Sec 1 Properties of Radiation        90
Sec 2 Balancing Nuclear Reactions    90
Sec 3 Predicting Nuclear Stability   90
Sec 4 Radiation Decay Kinetics       90
Sec 5 Radiation Measurement          90
Sec 6 Nuclear Binding Energy         90


      Final Exam – June 17
Radiocarbon Dating
      14N     1
            + 0n         14C   + 1H
       7                  6      1

      14C
       6
                   14N
                    7
                         + -1b + n
                            0
                                      t½ = 5730 years


  The C-14 isotope is produced in the upper
  atmosphere N-14 through the action of cosmic
  rays. All living organisms contain C-14, but
  when the organism dies the amount of C-14
  decreases.

  Paper from the Dead Sea Scrolls was found to
  have only 0.795 times as much C-14 as that of a
  living plant. What is the age of the scroll?


                                                        23.3
               Age of Rocks
  K-40 decays to Ar-40 with a half life of
  1.2 x 109 years.
Uranium-238 Dating
       238U
        92
                  206Pb
                   82     + 8 2 a + 6-1 b
                              4       0
                                            t½ = 4.51 x 109 years




                                                                    23.3
     Nuclear Transmutations

In 1919 Rutherford bombarded N with
alpha particles and produced O-17. This
was the first manmade nuclear reaction.

Why not just mix N and He?

Example 21.3

Neutrons are particularly useful in
producing synthetic isotopes since they
have no charge.
                   Nuclear Transmutation




                                 14N
                                  7      + 2a
                                           4    17O
                                                 8
                                                      + 1p
                                                        1



                                 27
                                 13 Al   + 2a
                                           4    30P
                                                15
                                                        1
                                                      + 0n


                                 14N
                                  7
                                           1
                                         + 1p   11C
                                                 6
                                                      + 4a
                                                        2




Cyclotron Particle Accelerator

                                                             23.4
Nuclear Transmutation




                        23.4
A new type of nuclear reaction – fission-
was discovered in 1938. The experiments
were carried out in Germany by Hahn and
Strassman and interpreted by Lisa
Meitner.

U-235 and Pu-239 will undergo fission
when bombarded with neutrons.

U-235 makes up 0.7% of natural uranium
and Pu-239 can be produced from the
more abundant U-238.
                          Nuclear Fission




            235U     1
                   + 0n       90Sr   + 143Xe + 31n + Energy
             92               38        54      0

Energy = [mass 235U + mass n – (mass 90Sr + mass 143Xe + 3 x mass n )] x c2

                    Energy = 3.3 x 10-11J per 235U
                             = 2.0 x 1013 J per mole 235U
              Combustion of 1 ton of coal = 5 x 107 J
                                                                       23.5
              Nuclear Fission
       Representative fission reaction
235U     1
       + 0n     90Sr   + 143Xe + 31n + Energy
 92             38        54      0




                                                23.5
Notice that the fission products
are not the normal isotopes and
are always highly radioactive.
This is true of fission bombs and
nuclear reactors.
                        Nuclear Fission
Nuclear chain reaction is a self-sustaining sequence of
nuclear fission reactions.
The minimum mass of fissionable material required to
generate a self-sustaining nuclear chain reaction is the
critical mass.




         Non-critical




                                     Critical
                                                           23.5
The little boy bomb. It used U-235
The fat man bomb. It used Pu-239
Fig. 21.9
Nuclear Fission


                          Schematic
                        diagram of a
                           nuclear
                       fission reactor



           Much of the heat comes
           from decay of fission
           products. Even if
           fission stops, this
           decay does not.


                                    23.5
                        Nuclear Fission

    35,000 tons SO2               Annual Waste Production
   4.5 x 106 tons CO2




                                                   70 ft3
                      3.5 x 106                   vitrified
                       ft3 ash                     waste


1,000 MW coal-fired                1,000 MW nuclear
    power plant                       power plant
                                                              23.5
                                    Nuclear Fission




     Hazards of the
 radioactivities in spent
    fuel compared to
      uranium ore




From “Science, Society and America’s Nuclear Waste,” DOE/RW-0361 TG   23.5
                             Nuclear Fusion

   Fusion Reaction             Energy Released
2     2      3     1
1 H + 1H     1 H + 1H            6.3 x 10-13 J
2H
1
        3
      + 1H      4He
                2     + 1n
                        0
                                 2.8 x 10-12 J
6Li     2
      + 1H     2   4He           3.6 x 10-12 J
3                  2

       High T required.
       Why?

         Tokamak magnetic
              plasma
            confinement


                                                 23.6
Thermonuclear
   bombs




 Mark 17 model
                    Radioisotopes in Medicine
•   1 out of every 3 hospital patients will undergo a nuclear
    medicine procedure
•   24Na,    t½ = 14.8 hr, b emitter, blood-flow tracer
•   131I,   t½ = 14.8 hr, b emitter, thyroid gland activity
•   123I,   t½ = 13.3 hr, g-ray emitter, brain imaging
•   18F,    t½ = 1.8 hr, b+ emitter, positron emission tomography
•   99mTc,    t½ = 6 hr, g-ray emitter, imaging agent



                                              Brain images
                                              with 123I-labeled
                                              compound

                                                                  23.6
Geiger-Müller Counter




                        23.6
Biological Effects of Radiation
              Radiation absorbed dose (rad)
              1 rad = 1 x 10-5 J/g of material
            Roentgen equivalent for man (rem)
             1 rem = 1 rad x Q    Quality Factor
                                    g-ray = 1
                                      b=1
                                     a = 20




                                             23.6
The Standard Model is the name
give to the current theory of
fundamental particles and how they
interact.
There are four fundamental forces in
  nature.

1. Electromagnetic force
2. Gravity
3. Strong nuclear force
4. Weak nuclear force
Electroweak Theory has shown that 1 and 4 are really
different aspects of the same force.
There are 12 fundamental fermion
  particles in nature. (spin =1/2, 3/2,
  5/2, …

6 flavors of quarks and 6 flavors of
   leptons. All the matter that we are
   familiar with are made up of two
   quarks – up and down and one
   lepton – the electron.
The other category of particles are
  the bosons (spin =0, 1, 2,…)

The bosons are force carriers. For
  example, the electromagnetic
  force is caused by an exchange of
  photons.

Each force has one or more force
  carriers.
The strong nuclear force is carried
  by gluons. Quarks come in three
  colors – red, green and blue. This
  is analogous to + and – for the
  electromagnetic force.

Unlike colors attract. This force is
 also called the color force.
Fermilab

Ring is 4 miles around.
Protons and antiprotons
collide.
Fermilab – particle tracks in detector.
Discovery of top quark.
and Ca-45, c. Mo-95, Tc-92 d. Hg-195, Hg-196

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:4
posted:8/9/2012
language:
pages:50