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Nuclear Chemistry Lectures

Nuclear Chemistry Lectures

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  • 2. The Good and Bad
  • 3. Marie Curie Einstein
  • 4. Big Bang ~ approx.14 Billion years subatomic particles leptons hadron electron neutrino muon tau baryon meson 3 quarks quark (larger) (larger) + antiquark protons neutrons First 10th billion of a second- Lepton + quarks and 4 fundamental forces •Strong nuclear force •Weak nuclear force •Electromagnetism •Gravity (weak force) Note: 1 cm3 of lead weighs 11 g. 1 cm3 of pure atomic nuclei would weigh > 100, 000, 000 metric tons!
  • 5. Quarks •A quark is a fast-moving point of energy. There are several varieties of quarks. •Each proton and each neutron contains three quarks. •Protons and neutrons are composed of two types: up quarks and down quarks. • Each up quark has a charge of +2/3. Each down quark has a charge of -1/3. • The sum of the charges of quarks that make up a nuclear particle determines its electrical charge. • Protons contain two up quarks and one down quark. +2/3 +2/3 -1/3 = +1 • Neutrons contain one up quark and two down quarks +2/3 -1/3 -1/3 = 0 • The nucleus is held together by the quot;strong nuclear force”.The strong force counteracts the tendency of the positively charged protons to repel one another. It also holds together the quarks that make up the protons and neutrons. •“Quarks are more like quot;dancing points of energy.quot; -nuclear physicists
  • 6. 18 Quarks. 6 Flavors - up, down, strange, charmed, top and bottom. (3 colors-red, blue and green. Antiquarks-complementary colors -cyan, magenta and yellow). Matter accounted for by neutrons &protons~ 10% Matter unaccounted for -dark matter (missing matter) ~90% (but gravity is felt in universe). 20% of dark matter may be hot dark matter (neutrinos). 80% of the dark matter may be cold dark matter-no detectable radiation (WIMP weakly interacting massive particle)
  • 7. DISCOVERY OF RADIOCHEMISTRY Nuclear medicine saves lives. Nuclear chemistry in labs improve agriculture Nuclear power provides energy Timeline of discoveries in nuclear chemistry Roentgen -1895 discovered X-ray during the Cathode tube experiment - his gift to medicine. Becquerel-1895 discovered radioactivity when photographic film was exposed to Uranium. Thomson -1897 discovered the mass/charge ratio of electrons during the Cathode ray deflection expt Marie and Pierre Curie discovered radioactive Po and Ra-1898. ~25 elements have been found to be radioactive. Einstein 1905- E=mc2 at the age of 26. Goldstein-1907 discovered the presence and mass of protons with perforated Cathode tubes. Milliken-1909 discovered the charge of electrons during the oil drop experiment Rutherford -1910 discovered particles during his famous gold leaf experiment. Paul Dirac-1928 predicted the existence of positrons
  • 8. RADIOACTIVE DECAY The nuclei of some unstable isotopes undergo a change by releasing energy and particles collectively known as radiation Spontaneous nuclear reactions: Radioactive Decay (Becquerel). 4 He 1) Emission of -particles: 2 238 U 234 Th + 42He e.g. 92 90 In air, -particles travel several cm. In Al, -particles travel 10-3mm.
  • 9. Emission of -particles 0 2) Emission of -particles: –1e = electrons. 131 I 131 Xe + 0–1e e.g. 53 54 -particle emission converts a neutron to a proton: 1n 1 + 1–1e 1p 0 In air, -particles travel 10m. In Al, -particles travel 0.5mm.
  • 10. Emission of -rays 0 3) Emission of -rays: 0 -ray emission changes neither atomic number nor mass. In Al, -particles travel 5-10 cm.
  • 11. Emission of positrons Emission of positrons ( +-particles): 4) 0e 1 11 C 11 B + 01e e.g. 6 5 Positron emission converts a proton to a neutron: 1p 1 + 01e 0n 1 Positrons have a short lifetime because they recombine with electrons and annihilate: 0e + 0–1e 2 00 1
  • 12. Electron Capture 5) Electron Capture: an electron from the orbitals surrounding the nucleus can be captured: 81 Rb + 0–1e 81 e.g. 36Kr 37 Electron capture converts a proton to a neutron: 1p + 0–1e 10n 1 ______________________________________
  • 13. Fill in the blanks 239 Pu -> 42He + ? • 94 234 Pr -> 23492U + ? • 91 18 F -> 188O + ? • 9 192 Ir + ? -> 19276Os • 77
  • 14. RADIOACTIVE DECAY Radioactive Decay Rates: First Order Decay Rate = -dN/dt = kN where: k is a constant, N is the number of decaying nuclei. By rearranging and integrating: N t dN'/N' = -kdt' No 0 ln[N(t)/N0] = -kt where No is the number of decaying nuclei at t=0. N(t) = N0e-kt Because the rate is proportional to the number of nuclei, this is called a first order process.
  • 15. 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: Isotope t1/2 Decay 238 U 4.5x109 yr 92 235 U 7.1x108 yr 92 14 C 5.7x103 yr 6 Example: Plutonium-240, produced in nuclear reactors from U-235, has a half-life of 6.58 x 103 years. What would be the fraction after 100 years?
  • 16. Plutonium-240, produced in nuclear reactors from U-235, has a half-life of 6.58 x 103 years. What would be the fraction after 100 years?
  • 17. Dating Libby-1946 developed method of determining age using 146C. 146C is produced by cosmic radiation. 14 N + 1 n -> 14 C + 1 H 7.5 kg/year 7 0 6 1 It decays 14 C -> 14 N + 1 e t 1/2 =5.73 x 103years 6 7 -1 Initially, in live plant C-14 has 14 disintegrations/min/g of C When the specimen dies, the C-14 is not replaced and the disintegrations diminish. •Ex. The dead sea scrolls had 11 dpm/g (disintegrations/min/g). What is the age of the document?
  • 18. 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. 4) 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
  • 19. Nuclear Belt of Stability:
  • 20. NUCLEAR STABILITY An isotope with a high n/p ratio is proton deficient. To convert neutrons to protons, it can undergo -decay: 1 1 p + 0–1e 0n 1 97 Zr 97 Nb + 0–1e e.g. 40 41
  • 21. 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: 1p 1 n+0 e 1 0 1 20 Na 20 + 01e e.g. 10Ne 11 ii) Electron capture: 1 p+0 e 1n 1 –1 0 Elements with atomic numbers greater than 84 undergo -decay in order to reduce both the numbers of neutrons and protons: 235 U 231 Th + 42He e.g. 92 90
  • 22. Result of emission, emission, and electron capture:
  • 23. Decay of 23892U (arrows show decay steps).
  • 24. NUCLEAR BINDING ENERGY 2 11p + 2 10n 4 2He 1 1p mass is 1.00728 amu 1 0n mass is 1.00867 amu 4 2He 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
  • 25. NUCLEAR BINDING ENERGYcontd. E = (5.047x10-29kg) (3x108m/sec)2 = 4.543x10-12J/42He = 2.736x1012J/mole 42He Binding Energy per nucleon for 4He = (4.54x10-12)/4 = 1.14x10-12J. Binding energy per nucleon: 4 He: 1.14x10-12J 2 56 Fe: 1.41x10-12J 26 238 U: 1.22x10-12J 92 Nuclei with mass greater than 50-60 amu can fall apart exothermically – nuclear fission. Combining nuclei/particles with total mass less than 50-60 can be exothermic – nuclear fusion.
  • 26. The rest masses of proton, neutron, and He nuclei are 1.007276470 amu, 1.008664904 amu, and 4.031882748 amu respectively. Calculate (a) the Binding energy/nucleon (b) the Binding energy/mole of Helium.
  • 27. NUCLEAR CHAIN REACTIONS Fission 235 U + 10n 137 + 9740Zr + 210n 52Te 92 142 Ba + 91 Kr + 31 n 56 36 0 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.
  • 28. 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. 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. 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.
  • 29. Breeder reactors Breeder reactors are a second type of fission nuclear reactor. A breeder reactor produces more fissionable material than it uses. 239 Pu and 23392U are also fissionable nuclei 94 and can be used in fission reactors. 238 U + 10n 239 239 Np + 0–1e 92U 92 93 239 Pu + 0–1e 94 232 Th + 10n 233 Th 233 Pa + 0-1e 90 90 91 233 U + 0-1e 92
  • 30. Basic design of a nuclear power plant
  • 31. NUCLEAR REACTORS Fusion “Chemistry of the stars” The sun contains 73% H, and 26% He. 1H + 11H 2 + 0+1e 1H 1 1H + 21H 3 2He 1 3 He + 32He 4 + 21H 2He 2 3 He + 11H 4 + 01e 2He 2 Initiation of these reactions requires temperatures of 4x107K - not currently obtainable on a stable basis.
  • 32. Sources of average annual exposure to radiation
  • 33. Discovery of Nuclear Fission 1934-Enrico Fermi discovered 4 particles when U- 238 with neutrons. 1938-Otto- Hahn and Fritz (Sweden) discovered 137Ba to be a product of 238U when repeating 56 92 Fermi’s experiment. They wrote to Lisa Meitner 1938-Lisa Meitner who had escaped from Austria to Swede suggested that neutrons were splitting the U! Nuclear Fission. 235 U +1 n -> 236 U-> 141 Ba +92 Kr + 3 1 n 92 0 92 56 36 0 E=-2x1010kJ/mol Frisch who was Meitner’s nephew was working with Neils Bohr at the University of Copenhagen in Denmark. Neils Bohr who was going to US for a physics conference released the news at the conference. Enrico Fermi who had just received the Nobel prize in physics fled from Italy and Mussolini to US 1939. Leo Szilard who discovered that the U-235 fission was a chain reaction persuaded Einstein to write to President Franklin Roosevelt about it in 1939.
  • 34. Nuclear 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, Washington. • Plutonium can be used for bombs or as a fuel source. However, small amounts of PuO2 dust in air causes lung cancer. Very toxic.
  • 35. NUCLEAR BOMBS CONTD • 1939-(brink of WWII)-Einstein calls attention the Uranium being a new energy source. • Manhattan project established by President Roosevelt builds the A-bomb under Robert Openheimer . – How to sustain fission chain reactions, how to enrich U-235, how to make Pl-239 and how to buil the bomb. • 1942-Enrico and team who were working under the bleachers at the Stagg Baseball field at the Chicago University find the critical mass of U- 235 (4 kg). 15 kg was obtained. • 1945- Test detonation of atom bomb assembled in Los Alamos went off in New Mexico at 5.30 am July 16 • As a response to the Pearl harbor bombing that killed 2,700 in Hawaii, on Aug 6 and 9 - 1945- two Bombs were dropped on Japan ( “Big boy” a U bomb and “Fat man” -which was a plutonium-239 bomb) under Harry Trumen . ~80,000 died instantly in Hiroshima-100,000 later. Japan surrendered and this was the beginning of the end of WWII.
  • 36. Measuring radiation damage-rems • The radiation energy absorbed is measured in a unit called rad. • 1 rad (radiation absorbed dose) is the absorbtion of 0.01 joule of radiation energy per kilogram of tissue • Not all radiation is equally harmful. particles are 10 time more dangerous than or or x. So this is factored in, n=10 for , 5 for low energy neutrons and n=1 for or or x • rem (roentgen equivalent man) = n x rad Chest x-ray is 10 mrem/visit (1 mrem=10-6rem) • Radioactive fallout from testing Sr-90, Cs-137, I-131 causes cancer. Detected in 1950. Nuclear test ban treaty was signed in 1963 advocated by Linus Pauling.
  • 37. Nuclear Power plants • Heart of the power plant is the reactor where fission takes place in the core. • Nuclear fuel is Uranium oxide enriched with 2- 4% U235 formed into glassy pellets. (bombs pure U235, Pu239) • These are housed in steel metal tubes called cladding and are cooled by water or liquid Na. Light water reactor -uses water, Heavy water reactor -uses D2O. • (Chernobyl 1986-high heat of fusion split H2 and O2 from water and these on recombination produced an immense explosion) • Excess neutrons are absorbed (2/3) by Cd rods and Boron-called control rods to prevent chain reactions • Water absorbs energy released during fission and high pressure steam (1000 psi, 285 oC) drives the turbines to produce electricity.
  • 38. Radon detectors 222 Rn -> 21884Po + 42He 86 218 Po -> 214 Pb + 42He 84 82 • Radon gas is found trapped under soil • Polonium can get trapped in tissue and emit alpha particles which could induce lung cancer. • A particle range is only 0.7 mm. But this is greater than the thickness of epithelial cells in the lungs • 4pCi/L air is the action level set by EPA. (PA~1.5-8) 1 Pico curie = 10-12 curies • 1 curie = 3.7 x 1010 dps (dps = • disintegrations per second)
  • 39. Pet Scans Positron emmision topography Elements that are neutron deficient and have short half lives can be used ex C-11, F-18, N-13, O-15 etc. They are prepared before use 1 P -> 1 n + 0 e + 1 0 1 (Proton -> neutron + positron + neutrino) 0e + 0-1e >2 1 Positron + electron annihilate to give The 2 rays are 180 o apart 2 scintillation detectors are placed above and below. By detecting several million annihilation rays within a circular slice around the subject over ~10 min the region of the tissue containing the radioisotope can be imaged with computer signal-averaging techniques
  • 40. 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
  • 41. H-bomb 6 1 3 H + 42He 3Li + 0n -> 1 E=-1.7 kJ/mol/ mol tritium The nucleons combine in a high energy plasma ( 106 ). 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.
  • 42. • The Eberly Family Distinguished Lecture in Science will be held on Thursday, April 18, at 4:00 p.m. in 112 Kern Auditorium. The speaker will be Eric Cornell, a physicist with the National Institute of Standards and Technology and the co-winner of the 2001 Nobel Prize in Physics. Dr. Cornell will present quot;Stone Cold Science: Bose-Einstein Condensation and the Weird World Within a Millionth of a Degree of Absolute Zero.quot;