Nuclear Chemistry Chapter 19 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1
Chemical Processes vs. Nuclear Processes Chemical reactions involve changes in the electronic structure of the atom atoms gain, lose, or share electrons no change in the nuclei occurs Nuclear reactions involve changes in the structure of the nucleus when the number of protons in the nucleus changes, the atom becomes a different element 2 Tro: Chemistry: A Molecular Approach 2
Review 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 Atomic Number A Z X Element Symbol proton or 1 1 H 1 p 1 neutron 1 n 0 electron 0 e -1 or 0 b -1 positron or 0 +1 b 0 e +1 a particle 4 He 2 or 4 a 2 A 1 1 0 0 4 Z 1 0-1 +1 2 3
Important Atomic Symbols Tro: Chemistry: A Molecular Approach 4 4
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 U 1 0 n 92 + Cs + + 2 138 96 55 37 Rb 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 n 0 235 U 1 0 n 92 + Cs + + 2 138 96 55 37 Rb 92 + 0 = 55 + 37 + 2x0 1 n 0 5
212 Po decays by alpha emission. Write the balanced nuclear equation for the decay of 212 Po. alpha particle - 4 He 2 or 4 a 2 212 Po 4 He + A X 84 2 Z 212 = 4 + A A = 208 84 = 2 + Z Z = 82 212 Po 4 He + 208 Pb 84 2 82 6
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Beta decay Nuclear Stability and Radioactive Decay 14 C 14 N + 0 b 6 7-1 40 K 40 Ca + 0 b 19 20-1 Decrease # of neutrons by 1 Increase # of protons by 1 1 n 1 p + 0 b 0 1-1 Positron decay 11 C 11 B + 0 b 6 5 +1 38 K 38 Ar + 0 b 19 18 +1 Increase # of neutrons by 1 Decrease # of protons by 1 1 p 1 n + 0 b 1 0 +1 8
Nuclear Stability and Radioactive Decay Electron capture decay 37 Ar + 0 e 37 Cl 18 17-1 55 Fe + 0 e 55 Mn 26 25-1 Increase number of neutrons by 1 Decrease number of protons by 1 Alpha decay 1 p + 0 e 1 n 1 0-1 212 Po 4 He + 208 Pb 84 2 82 Decrease number of neutrons by 2 Decrease number of protons by 2 Spontaneous fission 252 Cf 2 125 In + 2 1 n 98 49 0 9
Particle Changes Tro: Chemistry: A Molecular Approach 10 10
Chapter 23: The Nucleus: A Chemist s View 1. Balance the following nuclear reactions: 1. 211 Po 207 Pb + 2. 61 Ni (d, 2n) 3. 27 Al + α 30 P + 4. 187 Os + β 5. 235 U + n 135 Xe + + 2 1 n 6. 75 As (α, ) 78 Br 7. 20 Na + +1 β 11
n/p too large beta decay X Y n/p too small positron decay or electron capture 12
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 13
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Nuclear binding energy is the energy required to break up a nucleus into its component protons and neutrons. Nuclear binding energy + 19 F DE = (Dm)c 2 9 1 p + 10 1 n 9 1 0 Dm = 9 x (p mass) + 10 x (n mass) 19 F mass Dm= 9 x 1.007825 + 10 x 1.008665 18.9984 Dm = 0.1587 amu DE = 0.1587 amu x (3.00 x 10 8 m/s) 2 = -1.43 x 10 16 amu m 2 /s 2 Using conversion factors: 1 kg = 6.022 x 10 26 amu 1 J = kg m 2 /s 2 DE = 2.37 x 10-11 J 15
DE = (2.37 x 10-11 J) x (6.022 x 10 23 /mol) DE = -1.43 x 10 13 J/mol DE = -1.43 x 10 10 kj/mol Nuclear binding energy = 1.43 x 10 10 kj/mol binding energy per nucleon = binding energy number of nucleons = 2.37 x 10-11 J 19 nucleons = 1.25 x 10-12 J/nucleon 16
Nuclear binding energy per nucleon vs mass number nuclear binding energy nucleon nuclear stability 17
Kinetics of Radioactive Decay N daughter rate = kn N t ln N0 = -kt N = the number of atoms at time t N 0 = the number of atoms at time t = 0 k is the decay rate constant t ½ = 0.693 k 18
Half-Lives of Various Nuclides Nuclide Half-Life Type of Decay Th 232 1.4 x 10 10 yr alpha U 238 4.5 x 10 9 yr alpha C 14 5730 yr beta Rn 220 55.6 sec alpha Th 219 1.05 x 10 6 sec alpha Tro: Chemistry: A Molecular Approach 19 19
2. The half-life of Co-60 is 5.30 years. How many grams of a 4.567 g sample of Co-60 is left after 16.7 years? 3. Given the following atomic mass units: n = 1.008665 and p = 1.007838. Calculate a) The mass defect per C-12 atom b) The binding energy per C-12 atom c) The binding energy per nucleon for C-12 atom d) The amount of energy released per mole of C-12 4. How much energy is lost or gained when one mole of Polonium- 211 decays by alpha emission? The atomic masses are: Po-211: 210.98665 Pb-207: 206.97590 a: 4.0026036 20
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Radiocarbon Dating 14 N + 1 n 14 C + 1 H 7 0 6 1 14 C 14 N + 0 b + n 6 7-1 t ½ = 5730 years Uranium-238 Dating 238 U 206 92 82 Pb + 8 2 4 a + 6-1 0 b t ½ = 4.51 x 10 9 years 22
Nuclear Transmutation 14 N + 4 a 17 O + 1 p 7 2 8 1 27 Al + 4 a 30 P + 1 n 13 2 15 0 14 N + 1 p 11 C + 4 a 7 1 6 2 23
Nuclear Transmutation 24
Nuclear Fission 235 U + 1 n 90 Sr + 143 Xe + 3 1 n + Energy 92 0 38 54 0 Energy = [mass 235 U + mass n (mass 90 Sr + mass 143 Xe + 3 x mass n )] x c 2 Energy = 3.3 x 10-11 J per 235 U = 2.0 x 10 13 J per mole 235 U Combustion of 1 ton of coal = 5 x 10 7 J 25
Nuclear Fission Representative fission reaction 235 U + 1 n 90 Sr + 143 Xe + 3 1 n + Energy 92 0 38 54 0 26
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. 27
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Schematic Diagram of a Nuclear Reactor refueling U 3 O 8 29
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 30
Nuclear Fusion Fusion Reaction 2 H + 2 H 3 H + 1 H 1 1 1 1 2 H + 3 H 4 He + 1 n 1 1 2 0 6 Li + 2 H 2 4 He 3 1 2 Energy Released 6.3 x 10-13 J 2.8 x 10-12 J 3.6 x 10-12 J solar fusion Tokamak magnetic plasma confinement 31
Thyroid images with 125 I-labeled compound normal enlarged 32
Radioisotopes in Medicine Research production of 99 Mo 98 Mo + 1 n 99 Mo 42 0 42 Commercial production of 99 Mo 235 U + 1 n 99 Mo + other fission products 92 0 42 Bone Scan with 99m Tc 99 Mo 99m Tc + 0 b 42 43-1 t ½ = 66 hours 99m 43 Tc 43 99 Tc + g-ray t ½ = 6 hours 33
Geiger-Müller Counter 34
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 35
Chemistry In Action: Food Irradiation 36