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Nuclear Chemistry

Nuclear Chemistry. The nuclei of some unstable isotopes change by releasing energy and particles, collectively known as radiation. Spontaneous nuclear reactions - five kinds: 1) Emission of  - particles : 4 2 He (helium nucleus) e.g. 238 92 U  234 90 Th + 4 2 He

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Nuclear Chemistry

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  1. Nuclear Chemistry The nuclei of some unstable isotopes change by releasing energy and particles, collectively known as radiation Spontaneous nuclear reactions - five kinds: 1) Emission of-particles: 42He (helium nucleus) e.g. 23892U  23490Th + 42He In air,-particlestravel several cm. In Al,-particlestravel 10-3mm.

  2. 2. Emission of -particles: 0–1e (= electron) e.g. 13153I 13154Xe + 0–1e -emission converts a neutron to a proton: 10n 11p + 1–1e In air, -particles travel 10m. In Al, -particles travel 0.5mm. 3. Emission of -rays: 00 -rayemission changes neither atomic number nor mass. In Al,-particles travel 5-10 cm.

  3. 4) Emission of positrons (= anti-electron, or +-particle): 0+1e e.g. 116C 115B + 01e Positron emission converts a proton to a neutron: 11p 10n + 01e Positrons have a short lifetime because they recombine with electrons and annihilate: 01e + 0–1e  2 00

  4. Electron Capture: an electron from the orbitals near the nucleus can be captured: e.g. 8137Rb + 0–1e 8136Kr Electron capture converts a proton to a neutron: 11p + 0–1e 10n

  5. Fill in the blanks 23994Pu 42He + ? 23491Pa 23492U + ? • 11p • 0–1e • 10n • 42He 19277Ir + ? 19276Os 189F 188O + ?

  6. Natural Anthropogenic Radon Radioisotopes in the body Medical X-rays Rocks and Soil Cosmic Rays 200 mrem (55%) 100 Consumer products Nuclear medicine Average annual exposure (mrem) 40 mrem (11%) 39 mrem (11%) 28 mrem (8%) 27 mrem (8%) 50 14 mrem (4%) 11 mrem (3%) Source Sources of Exposure to Radiation

  7. NUCLEAR DECAY KINETICS Because the mechanism is unimolecular, nuclear decay is always a first order process. Decay Rate = -dN/dt = kN where: k is a constant, N is the number of decaying nuclei. Integrated rate law: ln[N(t)/N0] = -kt N(t) = N0e-kt where N0 is the number of radioactive nuclei at t=0.

  8. Half-Life Half-Life: the time required for half of a radioactive sample to decay. N(t1/2) = N0/2 ln(N/N0) = -kt k = 0.693/t1/2; t1/2 = 0.693/k Examples: Isotopet1/2Decay 23892U 4.5x109 yr  23592U 7.1x108 yr  146C 5.7x103 yr 

  9. Strontium-90, which is a fission product of uranium, has a half-life of 28 years. This isotope is a significant environmental concern. What fraction of 90Sr produced today will remain after 100 years?

  10. Radiocarbon Dating Libby (1946) developed method of determining age using 146C. 146C is produced by cosmic radiation. 147N + 10n 146C + 11H 7.5 kg/year (~constant) It decays: 146C 147N + 1-1e t 1/2 = 5.73 x 103years Initially, in live plant C-14 has 14 dpm of C (dpm = disintegrations/min/g) When the plant dies, the C-14 is not replaced and the disintegrations diminish. Ex. The dead sea scrolls have 11 dpm. What is the age of the document?

  11. NUCLEAR STABILITY • Rules: • 1) Up to atomic number 20, n=p is stable. • 2) Above atomic number 20, n>p is stable. • 3) Above atomic number 84, all nuclei are unstable. • Nuclei with 2, 8, 20, 28, 50, or 82 protons, or 2, 8, 20, 28, 50, 82, or 126 neutrons are particularly stable. These are the nuclear equivalent of closed shell configurations (and are called magic numbers). • 5) Even numbers of protons and neutrons are more stable. • # of Stable Nuclei • With This • Configuration:# Protons# Neutrons • 157 Even Even • 52 Even Odd • 50 Odd Even • 5 Odd Odd

  12. NUCLEAR STABILITY An isotope that is off the belt of stability can use four nuclear reactions to get to it:   positron emission electron capture

  13. NUCLEAR STABILITY An isotope with a high n/p ratio is proton deficient. To convert neutrons to protons, it can undergo -decay: 10n 11p + 0–1e e.g. 9740Zr 9741Nb + 0–1e

  14. NUCLEAR STABILITY contd. An isotope with a low n/p ratio is neutron deficient. To convert protons to neutrons, there are two possibilities: • i) Positron emission: • 11p 10n + 01e • e.g. 2011Na 2010Ne + 01e • ii) Electron capture: • 11p + 0–1e 10n • Elements with atomic numbers greater than 84 undergo-decayin order toreduce both the numbers of neutrons and protons: • e.g. 23592U 23190Th + 42He

  15. 238U DECAY Cascade of  and  decay reactions Moves diagonally down belt of stability Eventually gets to stable isotope (206Pb)

  16. NUCLEAR BINDING ENERGY 2 11p + 2 10n 42He 11p mass is 1.00728 amu 10n mass is 1.00867 amu 42He mass is 4.00150 amu Mass defect= (2)(1.00728 amu) + (2)(1.00867 amu) – 4.00150 amu = 0.03040 amu = 5.047x10-29 kg Binding energy is the energy required to decompose the nucleus into nucleons (p and n): E = mc2 Probably better to write: E = (m)c2 E = (5.047x10-29kg) (3x108m/sec)2

  17. NUCLEAR BINDING ENERGY contd. E = (5.047x10-29kg) (3x108m/sec)2 = 4.543x10-12J/42He = 2.736x1012J/mole 42He (huge compared to E for chemical reaction) Binding energy per nucleon: 42He: 1.14x10-12J 5626Fe: 1.41x10-12J (largest - most stable nucleus) 23892U: 1.22x10-12J Nuclei with mass greater than ~200 amu can fall apart exothermically – nuclear fission. Combining light nuclei can be exothermic – nuclear fusion.

  18. The rest masses of proton, neutron, and 12C nuclei are: • 11p = 1.007276470 amu • 11n = 1.008664904 amu • 126C = 12 amu (exact) • Practice problem: • Calculate the binding energy/mole of 12C. • Calculate the binding energy/nucleon. • Compare to E for combustion of one mole C.

  19. NUCLEAR CHAIN REACTIONS Fission 23592U + 10n 13752Te + 9740Zr + 210n 14256Ba + 9136Kr + 310n An average of 2.4 neutrons are produced per 235U. Chain reactions: Small: most neutrons are lost, subcritical mass. Medium: constant rate of fission, critical mass, nuclear reactor. Large: increasing rate of fission, supercritical mass, bomb.

  20. CRITICAL MASS

  21. NUCLEAR REACTORS Nuclear reactor fuel is 238U enriched with 3% 235U. This amount of 235U is too small to go supercritical. The fuel is in the form of UO2 pellets encased in Zr or steel rods. Liquid circulating in the reactor core is heated and is used to drive turbines. This liquid needs to be cooled after use, so reactors are generally near lakes and rivers.

  22. NUCLEAR REACTORS Cadmium or boron are used in control rods because these elements absorb neutrons. Moderators are used to slow down the emitted neutrons so that they can be absorbed by adjacent fuel rods.

  23. Nuclear Fission Bombs • Mainly U-235. Fortunately, U-235 is hard to purify • Uranium ore is concentrated and treated with Fluorine to form UF6. This is low boiling and can be evaporated at 56 oC. • 99.3% is non-fissionable U-238. Chemical reactions don’t help separate isotopes. • Gaseous diffusion separates the heavier particles (UF6 with U-235 moves 0.4% faster than U-238) • Repeated diffusion over long barriers or centrifugation concentrates U-235 • Breeder reactors- 238 U + n 239 Pu + 2e. • Under Glenn Seaborg, Plutonium bomb was produced at Hanford, WA. • Plutonium can be used for bombs or as a fuel source. However, small amounts of PuO2 dust in air causeslung cancer. Very toxic.

  24. Breeder Reactors Breeder reactors are a second type of fission nuclear reactor. A breeder reactor produces more fissionable material than it uses. 23994Pu and 23392U are also fissionable nuclei and can be used in fission reactors. 23892U + 10n 23992U 23993Np + 0–1e 23994Pu + 0–1e 23290Th + 10n 23390Th 23391Pa + 0-1e 23392U + 0-1e

  25. NUCLEAR REACTORS • Fusion “Chemistry of the stars” • The sun contains 73% H, and 26% He. • 11H + 11H 21H + 0+1e • 11H + 21H 32He • 32He + 32He 42He + 21H • 32He + 11H 42He + 01e • Initiation of these reactions requires temperatures of 4x107K - not currently obtainable on a stable basis.

  26. Nuclear Fusion Tremendous amounts of energy are generated when light nuclei combine to form heavy nuclei-Sun (plasma ~106 K) Short range binding energies are able to overcome the proton-proton repulsion in the nuclei 211H + 210n 42He E= -2.73 x 1012 J/mol Binding energy = +2.15 x 108 kJ/mol Note: (covalent forces are only are fraction H-H bond E =436 kJ/mol) The huge energy from 4 g of helium could keep a 100 Watt bulb lit for 900 years

  27. H-bomb 63Li + 10n 31H + 42He E= -1.7 kJ/mol/ mol tritium The nucleons combine in a high energy plasma (~106 K). A U-235 or Pu-239 bomb is set off first. A 20-megaton bomb has 300 lbs Li-D as well as a fission/atomic bomb.

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