Chapter 23

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Chapter 23

  1. 1. Nuclear Chemistry Chapter 23 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
  2. 2. XA Z Mass Number Atomic Number Element Symbol Atomic number (Z) = number of protons in nucleus Mass number (A) = number of protons + number of neutrons = atomic number (Z) + number of neutrons A Z 1 p1 1 H1or proton 1 n0 neutron 0 e-1 0 β-1or electron 0 e+1 0 β+1or positron 4 He2 4 α2or α particle 1 1 1 0 0 -1 0 +1 4 2 23.1
  3. 3. 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. 1 n0U235 92 + Cs138 55 Rb96 37 1 n0+ + 2 235 + 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. 1 n0U235 92 + Cs138 55 Rb96 37 1 n0+ + 2 92 + 0 = 55 + 37 + 2x0 23.1
  4. 4. 212 Po decays by alpha emission. Write the balanced nuclear equation for the decay of 212 Po. 4 He2 4 α2oralpha particle - 212 Po 4 He + A X84 2 Z 212 = 4 + A A = 208 84 = 2 + Z Z = 82 212 Po 4 He + 208 Pb84 2 82 23.1
  5. 5. 23.1
  6. 6. Nuclear Stability and Radioactive Decay Beta decay 14 C 14 N + 0 β + ν6 7 -1 40 K 40 Ca + 0 β + ν19 20 -1 1 n 1 p + 0 β + ν0 1 -1 Decrease # of neutrons by 1 Increase # of protons by 1 Positron decay 11 C 11 B + 0 β + ν6 5 +1 38 K 38 Ar + 0 β + ν19 18 +1 1 p 1 n + 0 β + ν1 0 +1 Increase # of neutrons by 1 Decrease # of protons by 1 ν and ν have A = 0 and Z = 0 23.2
  7. 7. Electron capture decay Increase # of neutrons by 1 Decrease # of protons by 1 Nuclear Stability and Radioactive Decay 37 Ar + 0 e 37 Cl + ν18 17-1 55 Fe + 0 e 55 Mn + ν26 25-1 1 p + 0 e 1 n + ν1 0-1 Alpha decay Decrease # of neutrons by 2 Decrease # of protons by 2 212 Po 4 He + 208 Pb84 2 82 Spontaneous fission 252 Cf 2125 In + 21 n98 49 0 23.2
  8. 8. n/p too large beta decay X n/p too small positron decay or electron capture Y 23.2
  9. 9. 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
  10. 10. Nuclear binding energy (BE) is the energy required to break up a nucleus into its component protons and neutrons. BE + 19 F 91 p + 101 n9 1 0 BE = 9 x (p mass) + 10 x (n mass) – 19 F mass E = mc2 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 10-11 J binding energy per nucleon = binding energy number of nucleons = 2.37 x 10-11 J 19 nucleons = 1.25 x 10-12 J 23.2
  11. 11. Nuclear binding energy per nucleon vs Mass number nuclear stability 23.2 nuclear binding energy nucleon
  12. 12. Kinetics of Radioactive Decay N daughter rate = - ∆N ∆t rate = λN ∆N ∆t = λN- N = N0exp(-λt) lnN = lnN0 - λt N = the number of atoms at time t N0 = the number of atoms at time t = 0 λ is the decay constant ln2 = t½ λ 23.3
  13. 13. Kinetics of Radioactive Decay [N] = [N]0exp(-λt) ln[N] = ln[N]0 - λt [N] ln[N] 23.3
  14. 14. Radiocarbon Dating 14 N + 1 n 14 C + 1 H7 160 14 C 14 N + 0 β + ν6 7 -1 t½ = 5730 years Uranium-238 Dating 238 U 206 Pb + 8 4 α + 6 0 β92 -182 2 t½ = 4.51 x 109 years 23.3
  15. 15. Nuclear Transmutation Cyclotron Particle Accelerator 14 N + 4 α 17 O + 1 p7 2 8 1 27 Al + 4 α 30 P + 1 n13 2 15 0 14 N + 1 p 11 C + 4 α7 1 6 2 23.4
  16. 16. Nuclear Transmutation 23.4
  17. 17. Nuclear Fission 23.5 235 U + 1 n 90 Sr + 143 Xe + 31 n + Energy92 54380 0 Energy = [mass 235 U + mass n – (mass 90 Sr + mass 143 Xe + 3 x mass n )] x c2 Energy = 3.3 x 10-11 J per 235 U = 2.0 x 1013 J per mole 235 U Combustion of 1 ton of coal = 5 x 107 J
  18. 18. Nuclear Fission 23.5 235 U + 1 n 90 Sr + 143 Xe + 31 n + Energy92 54380 0 Representative fission reaction
  19. 19. Nuclear Fission 23.5 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
  20. 20. Schematic Diagram of a Nuclear Reactor 23.5
  21. 21. Annual Waste Production 23.5 35,000 tons SO2 4.5 x 106 tons CO2 1,000 MW coal-fired power plant 3.5 x 106 ft3 ash 1,000 MW nuclear power plant 70 ft3 vitrified waste Nuclear Fission
  22. 22. 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. 23. Chemistry In Action: Nature’s Own Fission Reactor Natural Uranium 0.7202 % U-235 99.2798% U-238 Measured at Oklo 0.7171 % U-235
  24. 24. 23.6 Nuclear Fusion 2 H + 2 H 3 H + 1 H1 1 1 1 Fusion Reaction Energy Released 2 H + 3 H 4 He + 1 n1 1 2 0 6 Li + 2 H 2 4 He3 1 2 6.3 x 10-13 J 2.8 x 10-12 J 3.6 x 10-12 J Tokamak magnetic plasma confinement
  25. 25. 23.7 Radioisotopes in Medicine • 1 out of every 3 hospital patients will undergo a nuclear medicine procedure • 24 Na, t½ = 14.8 hr, β emitter, blood-flow tracer • 131 I, t½ = 14.8 hr, β emitter, thyroid gland activity • 123 I, t½ = 13.3 hr, γ−ray emitter, brain imaging • 18 F, t½ = 1.8 hr, β+ emitter, positron emission tomography • 99m Tc, t½ = 6 hr, γ−ray emitter, imaging agent Brain images with 123 I-labeled compound
  26. 26. 23.7 Radioisotopes in Medicine 98 Mo + 1 n 99 Mo42 0 42 235 U + 1 n 99 Mo + other fission products92 0 42 99m Tc 99 Tc + γ-ray43 43 99 Mo 99m Tc + 0 β + ν42 43 -1 Research production of 99 Mo Commercial production of 99 Mo t½ = 66 hours t½ = 6 hours Bone Scan with 99m Tc
  27. 27. Geiger-Müller Counter 23.7
  28. 28. 23.8 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 γ-ray = 1 β = 1 α = 20
  29. 29. Chemistry In Action: Food Irradiation Dosage Effect Up to 100 kilorad Inhibits sprouting of potatoes, onions, garlics. Inactivates trichinae in pork. Kills or prevents insects from reproducing in grains, fruits, and vegetables. 100 – 1000 kilorads Delays spoilage of meat poultry and fish. Reduces salmonella. Extends shelf life of some fruit. 1000 to 10,000 kilorads Sterilizes meat, poultry and fish. Kills insects and microorganisms in spices and seasoning.

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