nuclear chemistry

Document Sample
nuclear chemistry Powered By Docstoc
					                               Nuclear Chemistry

Reactions and properties of nucleus


         Mass number
      (number of nucleons)                n
          Atomic number
        (number of protons)
                                          p   M     nuclide : nuclear species
                                                    Isotopes : nuclides of same chemical species



Nuclear equation


                                                 
                          ni                            ni
                          pi M                          pi M

                              Pi = Pj       ni = nj
                                Nuclear Chemistry
Selected elementary particles


                   0    -        1           1               0
                  -1
                     e           0
                                   n         1
                                               p  +
                                                             1
                                                                 e+
              1                      0   -                            1
              1
                p  +        +
                                 -1
                                    e                                 0
                                                                          n
(atomic) mass unit : 1/12 the mass of a single atom of 12C
                     same as “dalton”
             1u(amu) = 1 dalton = 1.66 x 10-27 Kg
                               Nuclear Chemistry
 Einstein mass-energy relationship

                                     1                    0
                                                            e-
         1

        0
          n                          1
                                       p +         +
                                                         -1
        balanced nuclear equation, but Dm = -8.4 x 10-4 u


Change in mass corresponds to change in energy, and vice versa.

             i.e. DE = Dm C2 = - 1.25x 10-13J = 0.782 MeV              1 eV = 1.60 x 10-19J
                                                                   Energy obtained when e was accelerated
                                                                   by 1V potential difference.

 cf. In chemical reaction, Dm ~ 0

      Ex. CH4의 연소열 DH= -890KJ                      Dm = -9.9 x 10-9g


  In nuclear reaction
                        238                  234              4         Dm = -0.0046 g
                        92 U                       Th     +
                                             90               2 He
                                     Nuclear Chemistry
 Nuclear binding energy

                        mass of nucleus < mass of nucleons

                                  Dm = mass defect

        i.e. DE = Dm C2 : nuclear binding energy


                   nuclear binding energy
                                                  stability of nucleus
                       # of nucleons

Nuclear decay process : spontaneous
                            i.e. DG < 0      ~ Dm < 0

   1. Beta decay        Atomic number : +1, mass number : unchanged
                                                                                   Neutrino


                   14

                                                                          
                                             14
                   6    C                     7   N     +
                                                             0
                                                             -1 e-   +



                                                            electron with energy
                                            Nuclear Chemistry
2. Positron emission          Atomic number : -1, mass number : unchanged

                        11

                                                                                     
                                                      11
                         6    C                       5    B     +
                                                                       0
                                                                       1   e+   +



3. electron capture           When positron emission cannot occur


                                           e-
                       231
                                                                                         
                                        0                            231
                       92 U
                                    +
                                        -1
                                                                     91    Pa   +




 4. Alpha decay              Atomic number : -2, mass number : -4


                              238                          234                  4
                               92 U                              Th         +
                                                           90                   2   He

   5. others           Neutron emission, proton emission, fission


                                    All toward more stable nucleus.
                                     Nuclear Chemistry
Detecting radioactivity

  1. Photographic plates(emulsion)      darkening is proportional to radiation



  2. Scintillation counter     Emits light when absorbs beta particle or gamma ray : phosphorescence
                               can count number of light pulses.



  3. Geiger counter      Beta particle produces positive ions and electrons in a tube filled with gas.
                         Then electric current is generated.
                                    Nuclear Chemistry
Kinetics of radioactive decay

                     is first order rate kinetics
                                        Nt
                         then,     ln      = - kt           Nt = No e-kt
                                        No
                                                    ln 2              0.693
                                   half life t1/2 =              =
                                                     k                  k

                                           t1/2            Nt
                                  t=
                                          0.693
                                                    ln
                                                           No
                                                                     dN
          Measured radioactivity (A) is the decay rate      = -           = kN
                                                                     dt
                                                    t1/2             At
                                           t=
                                                    0.693
                                                                ln
                                                                     Ao

  Radioactivity unit : becquerel (Bq) – 1 radioactive disintegration per second
                       curie (Ci) – activity of 1g of radium is 1 Ci              1 Ci = 3.7 x 1010 Bq
                                    Nuclear Chemistry
Radioactive dating

        If we know half-life of a radioactive nuclide           t1/2        At
                                                        t=
                                                              0.693
                                                                       ln
                                                                            Ao

                            We should know Ao to find out the age of the sample.

1. for the age of uranium containing rock

    Assumption : Pb in the rock came only from uranium decay.

        Then Nt(Pb) = No(U) – Nt(U) = Nt(U) (ekt – 1)        Where Nt = No e-kt


                                             Nt(Pb)
                                   t = ln               + 1
                                             Nt(U)
                                          Nuclear Chemistry
2. Radiocarbon dating : 14C decay
                       14             1                          14
    In nature                                                                    1
                            N         0n                              C
                                  +
                       7                                          6
                                                                           +
                                                                                 1   H
                                Produces 14C all the time and therefore

      Assumption : 14C/12C ratio is constant during last 50,000 years.
      This is the ratio in CO2 in the atm and thus is the raio in the living organisms !

     From this, we can deduce Ao of carbon source from fossils or dead ones.


                                                                               t1/2              At
                                  And the time of its death.          t=
                                                                            0.693
                                                                                            ln
                                                                                                 Ao


                    This can change with burning of fossil fuel that increases        12C   in the atm.
                                       Nuclear Chemistry
3. Radiation in biology and medicine

    Effect of radiation : chemical changes in cellular molecules
                          leads to change in cellular function and becomes cancerous
                           Damage is proportional to the quantity and energy of the particle
                                              Somatic damage – burning, cancer
                                              Genetic damage – DNA mutation

    Measurement of radiation damage

                rad ( radiation absorbed dose) : the amount of radiation that deposits 0.01J/Kg

                rem ( roentgen equivalent in man) : effective dosages of radiation received by humans

              1 rad of beta or gamma radiation = 1 rem,                  rem = rad x RBE
              1 rad of alpha radiation = 10 rem
                                                                       RBE : relative biological effectiveness

                  1 Gy(gray) = 100 rad,     1 Sv(sievert) = 100 rem

       Safe level of exposure : ~ 100 mrem/yr. (from nature), ~ 100 mrem/yr. (from human activity)
       LD50 (in 30 days) = 500 rad
                                    Nuclear Chemistry

Radon : gaseous, generated from rocks and soils, especially from granite
        222Rn decay quickly but its products are also radio active and can accumulate in the confined air.


                       222                            218            4
                        86   Rn                        84   Po   +
                                                                     2   He
Biology : biosynthetic route of biomolecules
Medical use of radiation : X-ray – imaging, cancer therapy
                           131I – thyroid cancer

                           PET(positron emission tomography) : diagnostic tool for cancer, neurological disease
                                                                good tool to study brain function
                      11
                                                                                  
                                                11             0
                      6C                        5B                 e-
                                                        +                  +                            T1/2 = 20.3 min.
                                                               1

                     18
                                                                                 
                                               18             0
                     9 F                           O              e-
                                                       +                  +                             T1/2 = 109.8 min.
                                               8              1




                              Normal Brain                  Image of the brain of a 9 year old female
                                                            with a history of seizures poorly
                                                            controlled by medication. PET imaging
                                                            identifies the area (indicated by the
                                                            arrows) of the brain responsible for the
                                                            seizures. Through surgical removal of
                                                            this area of the brain, the patient is
                                                            rendered "seizure-free".
                                Nuclear Chemistry
Nuclear Fission

     Fragmentation of an element by neutron bombardment

            235         1                        94           139
             92 U       0n                       36Kr
                    +                                                        1
                                                              56Ba
                                                          +          +   3
                                                                             0   n
                                     Nuclear Chemistry
Nuclear Fission

     Fragmentation of an element by neutron bombardment

            235         1                            94            139
             92 U       0n                           36Kr
                    +                                                              1
                                                                   56Ba
                                                               +           +   3
                                                                                   0   n
                            Self-propagating chain reaction
              Controllable by absorbing produced neutrons( 112Cd or 10B)

              235U, 233U, 239Pu   can be used.

   Nuclear power reactor

      Fuel : 235U (~3% for “light water” reactor, 0.7~1.5% for “heavy water” reactor)
      Moderator : slows down neutron and transfer heat to a steam generator: water, carbon
      Controller: controls fission reaction rate by capturing neutrons
 Nuclear Fusion

       Thermonuclear reactions : requires very high kinetic energies
                      1            1
                                                                                                       
                                                                    2                 0
                      1   H    +
                                   1   H                            1H
                                                                                  +
                                                                                      1   e+   +

                          2            1
                                                                                                
                                                                        3                                          Hydrogen burning
                          1H           1H
                               +
                                                                        2 He
                                                                                      +

                 3                     3                                    4                   1
                 2 He                      He                                   He             2 1H
                               +                                                           +
                                       2                                    2

                                                Requires 40,000,000K

In stars    Helium burning starts at 108K
                               4                                    8
                              2 2 He                                4   Be
                               8                         4                                     12
                               4   Be           +        2   He                                6   C
            Nucleosynthesis : nuclear transmutation
                  14                            4                                     17                   1
                  7   N            +            2   He                                8   O     +          1   H
                                                             14              17
                                                             7   N(p) 8 O

   Fusion reactor : requires very high temperature and control --- still a dream
                     cf. cold fusion
14장숙제 : 4, 14, 16, 22, 26, 38, 42, 48

제출기한 : 11월 6일

제출장소 : 자연과학동 4109호

				
DOCUMENT INFO