This preview has intentionally blurred parts. Sign up to view the full document

View Full Document

Unformatted Document Excerpt

21 Chapter Nuclear Chemistry Nuclear chemistry involves changes in the nuclear composition of atoms that are radioactive. Nuclear Chemistry Many applications Source of energy Medical diagnosis and treatment Carbon dating 1 2 21.1 Radioactivity Why do the elements occur in widely different amounts in the universe? H - 91% Radioactivity 90 elements exist naturally on Earth 81 of these have at least one stable isotope Po-U have no stable isotopes (radioactive) PoThe heavier elements (Np - ) are synthetic (Np and radioactive. (H-Bi, except for 43Tc and 61Pm) (H- 3 4 Radioactivity Not all isotopes are equally stable -- the radioactive ones undergo nuclear decay to form other elements. Radioactivity When discussing nuclear reactions, we are interested in specific nucleons (particles in the nucleus) -- protons and neutrons -- which provide the majority of the mass of the nucleus. 5 6 1 1 Radioactivity Z = atomic number = # protons N = neutron number = # neutrons A = Z + N = mass number = # nucleons nuclide = nucleus of an isotope symbol: AZE Only 264 of the 1700 known nuclides are stable; the others decompose spontaneously at some characteristic rate, emitting some type of radiation. 7 Radioactivity Review How many protons and neutrons are in the following nuclides? 14 C 6 18O Uranium-235 Uranium- 8 Nuclear Equations Radiation arises from nuclear reactions: parent nuclide daughter nuclide + radiation Nuclear Equations Types of emission particles Alpha Particles: 42He or 42 Beta Particles: 0-1e or 0-1 0 e or 0 Positrons: +1 +1 Gamma Rays: (no mass or charge) To balance, two conditions must be met: 1 Conserve mass number(A), the #of nucleons 2 Conserve nuclear charge (Z) If we know two of the nuclear particles, we can use these rules to identify the third particle. 9 10 Nuclear Reactions Nuclear decomposition of Radium Emission of alpha particles: 226 Ra 222 Rn + 4 He 88 86 2 Used in radiation therapy A E Z Nuclear Reactions Group Work: What is the product of alpha particle emission by Th-232? Th232 Th 90 ? + 4 He 2 Conserve A: 226 = 222 + 4 Conserve Z: 88 = 86 + 2 Loss of 2 protons and 2 neutrons 11 12 2 2 Nuclear Reactions Emission of beta particles (electrons): 131 I 131 Xe + 0 e 53 54 -1 Used to diagnose thyroid disorders Conserve A: 131 = 131 + 0 Conserve Z: 53 = 54 + -1 Conversion of a neutron to a proton Nuclear Reactions Group Work: What is the product of beta emission of Co-60? Co60 Co ? + 0-1e 27 13 14 Nuclear Reactions Emission of beta+ particles (positrons): 18 F 18 O + 0 e+ 9 8 +1 Conserve A: 18 = 18 + 0 Conserve Z: 9 = 8 + 1 Nuclear Reactions Group Work: What is the product of the positron emission of Mg-23? Mg23 Mg ? + 0+1e+ 12 Conversion of proton to a neutron 15 16 Nuclear Reactions Emission of gamma rays: 224 Ra* 224 Ra + 88 88 Conserve A: 224 = 224 + 0 Conserve Z: 88 = 88 + 0 Accompanies nuclear transformations which leave excited (high energy) nuclei; no nucleon change is associated with gamma rays since A = 0 and Z = 0 for rays Nuclear Reactions Emission Reactions Emission Animation 17 18 3 3 Group Work Identify the type of radiation emitted when carbon-14 decays to nitrogen-14. carbonnitrogenWrite a balanced equation. Types of Radiation Three types of radiation are commonly detected: alpha, beta or gamma 19 20 Types of Radiation Alpha = 42He2+ or 42He or 42 largest, least penetrating can be stopped by aluminum foil > 10-3 cm, paper, skin least harmful most massive 21 Types of Radiation Beta = 0-1ehigh energy electrons (e-) or positrons (e+) more penetrating stopped by 0.05 - 0.1 cm of aluminum travel 10 ft through air commonly emitted by TV sets electron: 0-1 or 0-1e or epositron: 0+1 or 0+1e or e+ 22 Types of Radiation Gamma = energy with no mass or charge Most penetrating radiation Stopped by 5 - 11 cm of aluminum or thick layer of concrete or lead Lead is commonly used to enclose radioactive materials because radiation does not penetrate readily In the 1950s, it was common to build thick concrete bomb shelters Types of Radiation Other particles: proton (p+ or 11p or 11H) neutron (n or 10n) neutrino (00) and antineutrino (00), which have no mass or charge and accompany emission of beta particles; these are generally ignored by chemists 23 24 4 4 21.2 Patterns of Nuclear Stability The stable nuclides occur in a narrow band of N/Z values -- an "island of stability". For Z = 1-20, N = Z 1(N/Z = 1) Eg. Ca-40, O-16, C-12 Eg. CaOC- For Z > 20, N > Z (N/Z < 1.6) Eg. Zr-90, Hg-200 Eg. ZrHg25 26 Radioactivity The stable isotopes form a zig-zag pattern zigwithin the island of stability. Even Z & even N: most stable Even N & even Z for 60% of the stable nuclides (157) Even N or even Z for most of the rest (102) Only 5 stable nuclides have both odd N and odd Z, one of which is nitrogen-14, 147N nitrogen27 Radioactivity Relative abundance of some nuclei B-10 F-19 Zn-64 Zn19% 20% 100% 49% Zn-66 28% ZnZn-68 ZnB-11 80% 28 Group Work Which of each pair of nuclides is most likely to be radioactive? radioactive? 16 O and 17 O 8 8 16 O and 18 O 8 8 29 Si and 28 Si 14 14 14 C and 12 C 6 6 Radioactivity Magic Numbers: Nuclei with Z or N values Numbers: equal to 2, 8, 20, 28, 50, 82, 126 have been found to be particularly stable Nuclides with these numbers of protons and neutrons are especially stable (note that these are all even numbers). Super Stable Nuclides: 42He, 168O, 4020Ca 29 30 5 5 Spontaneous Nuclear Decay Reactions Unstable nuclides decay spontaneously to try to reach a more stable N/Z ratio, while emitting radiation at a characteristic rate. If N is too large (N/Z>1): beta particle emission is likely If Z is too large (N/Z<1): positron emission is likely We can predict nuclear decay reactions 31 Spontaneous Decay N to P P to N 32 What type of decay? N/Z = 8/6 = 1.33 Stable N/Z for light element is 1.00 decay gives smaller N/Z 14 C 14 N + 0 e6 7 -1 14 N N/Z = 7/7 = 1.00 7 14 C 6 33 34 What type of decay? N/Z = 6/7 = 0.86 (too small) Stable N/Z for light element is 1.00 Increase ratio by positron emission or electron capture 13 N 13 C + 0 e+ 7 6 +1 13 C N/Z = 7/6 = 1.17 6 Can't get closer than this; nuclides prefer to be greater than 1 rather than less than 1 35 What type of decay? Sometimes uses electron capture instead. Can't tell which will occur; the result is the same. 0 e- + 22 Na 22 Ne -1 11 10 13 N 7 36 6 6 What type of decay? Heavy elements decay by a combination of and decay. See the uranium series (Figure 21.4). Ultimately a heavy radioactive element will decay through a series of radioactive elements until it gets to a stable isotope of lead or bismuth. Four such series of decays are known: U Th Ac Np 37 Uranium Series Primary mode of decay is alpha, but a few beta decays are also needed 38 Spontaneous Decay Group Work Predict the type of decay and a possible product for the following radioactive nuclides. 23290Th 2312Mg 167N N to P P to N 39 40 Other Reactions Nuclear Bombardment: nuclei collide with Bombardment: high energy (accelerated) particles, possibly followed by decay Fission: nucleus splits into two lighter Fission: nuclei and neutrons Fusion: two light nuclei combine into a Fusion: heavier nucleus 21.3 Nuclear Transmutations Bombardment reactions: Used to create new elements December 1994 in Darmstadt, Germany: 64 Ni + 209 Bi 272 ? + 1 n 28 83 111 0 Conserve A: 64 + 209 = 273 = 272 + 1 Conserve Z: 28 + 83 = 111 = 111 + 0 These reactions usually emit one or more particles, such as neutrons. This new nuclide survives for only 0.002 seconds 41 42 7 7 Bombardment Reactions Reaction in a nuclear reactor 238 U +1 n 239 Np + 0 e92 0 93 -1 Synthesis of transuranium elements: 238 U + 4 He 239 Pu + 31 n 92 2 94 0 238 U + 12 C 246 Cf +41 n 92 6 98 0 238 U + 14 N 247 Es + 51 n 92 7 99 0 Bombardment Reactions In September 1982, two new elements were formed by bombardment with heavy nuclides: 58 Fe + 206 Pb 265 Hn + 1 n Hassium 26 82 108 0 58 Fe + 209 Bi 266 Mt +1 n Meitnerium 26 83 109 0 Prepared only a few atoms of each 43 44 Group Work What is the product of the following reaction? 97 Mo 42 21.4 Rates of Radioactive Decay Nuclear decay always follows 1st-order kinetics, 1stwhich gives a constant t1/2 over the course of the decay. 1st Order Rate = kN (N is the # radioactive nuclei) Rate is the Activity k is the decay constant 45 46 + 21H ? + 210n 21.4 Rates of Radioactive Decay Integrated Rate Law: ln(Nt/No) = -kt ln(No/Nt) = +kt +kt ln(2/1) = 0.693= kt1/2 Another common form of the integrated rate law: Nt = Noe-kt (just a different form of the 1st order rate law) 47 21.4 Rates of Radioactive Decay t1/2 = 0.693/k Nuclear decay rates are also independent of temperature. Usually we cite t1/2 instead of the rate constant. 48 8 8 Decay curve for 198Au Successive half-lives half(t1/2 ) have the same value. The radiation intensity and the number of radioactive nuclei decrease by a factor of 2 during each halfhalflife. Rates of Decay The rate of decay, as well as the type and energy of the radiation, determines the damage caused by radiation. 49 50 Kinetics Can use kinetics, just like in Chapter 14, to do various calculations. How much nuclide is left after ___ time? How long do we have to store nuclear waste? (Usually 10 t1/2: 2-10 = 0.000977, so 99.9023% has decayed) 51Cr (t 1/2 = 27.8 days) is stored for > 10 months How much time has elapsed if conversion is ___% complete? Half-Lives Gold-198 undergoes beta decay to give Goldmercury-198 with a half-life of 2.7 days. mercuryhalfWhat fraction (or %) of gold-198 is left goldafter 2.7 days? after 5.4 days? after 8.1 days? Use ln(N/No) = -kt and k= 0.693/t1/2 0.693/ 51 52 Half-Lives Gold-198 undergoes beta decay to give Goldmercury-198 with a half-life of 2.7 days. How mercuryhalflong it will take for 95% of Au-198 to decay? Au- Group Work Gold-198 undergoes beta decay to give Goldmercury-198 with a half-life of 2.7 days. mercuryhalfWhat fraction (or percent) of gold-198 is left goldafter 14 days? 53 54 9 9 Archeological Dating Radiocarbon dating uses 14C content 14C is produced by bombardment of 14N with neutrons (in cosmic rays) 14 N 7 Archeological Dating When a plant or animal dies, it no longer incorporates new 14C, and 14C content begins to decrase, causing the 14C decrase, content to become less than that in the atmosphere Count bristle-cone pine tree rings to bristledate the rings; correlate with a measurement of their 14C content (University of Arizona) 55 56 + 10n 146C + 11H 14C is incorporated into living systems, but undergoes radioactive decay with a half-life halfof 5730 years: 14 C 6 147N + 0-1e Archeological Dating Living tissue has an activity of 15.3 disintegrations per minute per gram of Carbon After 1 half-life: Activity = 7.65 dis/min gC halfdis/min After 2 half-lives: Activity= 3.8 dis/min gC halfdis/min Can measure > 0.03 dis/min g 15.3 0.03 in about 9 t1/2 Thus, the effective time range is < 9 x 5730 ~ 50,000 years Geological Dating Geological dating is similar to archeological dating, but uses longer-lived nuclides longerMeasure ratio of 40K to 40Ar in rocks 40 K + 0 e 40 Ar 89% 19 -1 18 40 K 40 Ca + 0 e 11% 19 20 -1 Combined half-life is 1.27 x 109 years halfMeasure ratio of 238U to 206Pb in rocks 238U ... 206Pb t1/2 = 4.5 x 109 years 57 58 Geological Dating Oldest rocks from Earth's Crust have been determined to be 3x109 yrs old. 21.5 Detection of Radioactivity Study nuclear properties by studying the radiation emitted. Detection: film badge for personal exposure Geiger-Muller counter Geigerradiation causes ionization of Ar(g), which gives a pulse of electric current that is sent to a counter detects , , or 59 60 10 10 Detection of Radioactive Decay Geiger Counter 21.6 Energy Changes in Nuclear Reactions Chemical reactions have energy changes of 100-1000 kJ/mol 100Nuclear reactions are of interest because of their large energy output The mass of an atom is less than the separate masses of the component subatomic particles -- this discrepancy is called the mass defect. defect. scintillation counter ZnS or NaI fluoresces (light flash) when irradiated; light is passed through a photomultiplier tube and recorded on a counter 61 62 Mass Defect (m) 19.9924 amu Assume 2010Ne 10p+ + 10n + 10emass of p+ = 1.00728 amu mass of n = 1.00867 amu mass of e- = 0.0005486 amu 20 Ne 10 Mass Defect Mass Defect can be calculated in two ways: Mass of Isotope Mass of (#p+ + #n + #e-) Mass of Nuclide Mass Mass of (#p+ + #n) Both methods will give the same result. sum of particle masses = 20.1650 amu mass defect = 20.1650 amu - 19.9924 amu = 0.1726 amu 63 64 Mass Defect The energy corresponding to the mass defect is called the nuclear binding energy, E. Einstein's equation is used to determine the mass-energy equivalence: massE = mc2 1 amu = 1.66056 x 10-27 kg c = 2.9979 x 108 m/s joule = kg m2/s2 65 Nuclear Binding Energy For 2010Ne E=mc2 E = (0.1726 amu x 1.66056 x 10-27 kg/amu) x kg/amu) (2.9979 x 108 m/s)2 E = 2.576 x 10-11 J for one atom For 1 mole of atoms (6.022 x 1023 atoms): E = 1.551 x 1013 J/mol = 1.551x1010 kJ/mol! 66 11 11 Nuclear Binding Energy Mega Electron Volts (MeV) (MeV) It is common to use MeV as a unit to get convenient numbers for single atoms: 1 MeV = 1.6602 x 10-13 J E = 2.576 x 10-11 J x (1 MeV/1.6602 x 10-13 J) = 160.8 MeV for one Ne-20 atom NeFor Ne, Binding Energy/nucleon Ne, = 160.8 MeV/20 nucleons = 8.04 BE/Nucleon 67 Binding Energy per Nucleon fission fusion 68 Binding Energy per Nucleon Binding Energy The curve is smooth, with spikes for very stable nuclides: 42He, 126C, 168O (N = Z = even) Maximum value at 5626Fe, which is prevalent in Earth's crust. Elements with Z = 20-30 are prevalent in 20the crust, as are 16O, 12C, and 14N No elements heavier than those at the maximum in the curve are present in amounts >1% in the crust 69 70 fission fusion Figure 21.13 in Text Binding Energy Light nuclides undergo fusion or bombardment to convert to other nuclides closer to the maximum value Heavy nuclides undergo fission to give nuclides closer to the maximum value If a nuclear reaction gives products with a higher nuclear binding energy, then energy is released by the reaction. To calculate the energy released, m is products- reactants: E = mc2 products71 Group Work Which of the following nuclides might undergo nuclear fission to form more stable nuclides? 21 Ne 10 127 Te 52 237 Np 93 27 Si 14 40 Ca 20 72 12 12 Energy Change for a Nuclear Reaction What is the energy change for the following reaction? 2H + 3H 4He + 1n 3.01605 amu 2.01410 amu 4He 4.00260 amu neutron 1.00866 amu 2H 73 Energy Change for a Nuclear Reaction What is the energy change for the following reaction? 2H + 3H 4He + 1n m=(4.00260 + 1.00866) - (2.01410 + 3.01605) 3.01605) = 5.01126 amu - 5.03015 amu = -0.01889 amu E = mc2 = -2.82x10-12 J = -1.70x109 kJ/mol! 74 3H Given: 21.7 Nuclear Fission Some nuclides undergo spontaneous fission; others undergo fission when bombarded with another nuclide or with a nucleon 235U is used in nuclear power plants 235 U + 1 n 236 U mixture of products 92 0 92 236 U 92 Kr + 141 Ba + 31 n + 92 36 56 0 236 U 90 Sr + 143 Xe + 31 n + 92 38 54 0 236 U 94 Zr + 140 Ce + 21 n + 92 40 58 0 and others 75 Nuclear Fission Average of 1 n is consumed, but an average of 2.4 n are produced, so more reaction occurs with the new neutrons and the reaction speeds up. This is called a chain reaction. reaction. 76 Chain Reaction One step in a chain reaction produces more neutrons than it consumes. Chain Reaction Successive steps get faster and faster .... How do we control the reaction? Chain reaction 77 Critical Mass 78 13 13 Nuclear Reactor The size and shape of the uranium fuel determines how many neutrons escape and how many react. If we exceed some critical mass, the reaction mass, becomes increasingly faster and results in a nuclear explosion. To control the process, we must remove some neutrons and slow down fast neutrons so they will react. 79 Reactor Core We use B or Cd control rods to absorb neutrons. 105B + 10n 73Li + 42He 80 Nuclear Reactor Fuel is not pure 235U. Usually use U3O8 enriched from a natural 0.7% 235U to 2-3% 235U. 2Use a moderator to slow down neutrons: H2O or D2O or graphite. Get an energy output of 200 MeV/atom or 2 x 1010 kJ/mol as the kinetic energy of the products. Use this kinetic energy to heat water (the coolant) to 310-350oC (under pressure to 310prevent boiling). Diagram of Nuclear Reactor 81 82 Diagram of Nuclear Reactor 21.8 Nuclear Fusion Fusion is already a source of energy; this is the process that produces sunlight. Sun: 411H 42He + 20+1+ + 25 MeV energy Need energy to initiate fusion (to overcome inter-nuclear repulsions). In the sun, the intertemperature is about 107 K 83 84 14 14 Nuclear Fusion In a fusion bomb (hydrogen bomb), high temperature and pressure are provided by a fission explosion. Nuclear Fusion To date, it has been possible to carry out controlled fusion, but the input energy still exceeds the output energy. The most likely candidate for a fusion reaction is: 2 H + 3 H 4 He + 1 n 1 1 2 0 85 86 Nuclear Fusion 2 H is deuterium (D), available from water 1 (heavy water) 3 H is tritium (T), available from the 1 bombardment of Li with neutrons. We currently have about a thousand-year thousandsupply of lithium. 21.9 Biological Effects of Radiation Can damage tissue cells Radiation comes continuously from many sources besides nuclear power plants and applications of isotopes Natural radiation sources: granite soil water food air 87 brick concrete cosmic rays (airplane flights) radon in houses 88 Bioligical Effects Damage from radiation depends on: Activity of radioactive substance Type of radiation Length of exposure Source: inside or outside body Biological Effects of Radiation Amount of radiation exposure measured in rem rad = radiation absorbed dose (10-2 J/kg tissue) RBE = relative biological effectiveness RBE = 1 for x-ray, , xRBE = 2.5 for slow neutrons RBE = 10 for , protons, fast neutrons RBE = 20 for heavy ions Water in our bodies is ionized forming free radicals. Free radicals are highly reactive species that can cause unwanted cellular reactions. 89 1 rem = 1 rad x 1 RBE rem = roentgen equivalent for man 90 15 15 Biological Effects of Radiation Single dose of 0-25 rem: no effect 025-100 rem: temporary blood cell 25changes 100-300 rem: radiation sickness; 100decrease in white blood cells 400-600 rem: 50% chance of death 400> 1000 rem: 100% chance of death Biological Effects of Radiation Normal exposure = 360 mrem (0.36 rem) per rem) year, which produces no observable effects 91 92 Biological Effects of Radiation Calculate your radiation exposure: Cosmic radiation at sea level 27 mrem/year Add 1 for every 250 ft elevation 4 for Phoenix Radiation from earth 28 Building materials in houses 4 Radon gas from the ground 200 Radiation from food and water 39 Jet plane travel (9.5 mrem/hr) ___ If you smoke (~1300 mrem/yr) ___ 210Po Biological Effects of Radiation Average medical exposure Add 6 for each chest x-ray xAdd 245 for intestinal x-ray xSmoke detectors Power plants 25-55 mrem/yr 25____ ____ 10 1 Your total exposure this year: _____ mrem in cigarette smoke 93 94 Medical Applications of Isotopes Medical diagnoses (Radiotracers) 99Tc for tumors in spleen, liver, brain, thyroid tracer put into a metabolite that concentrates in cancerous cells 131I or 123I in thyroid Cancer therapy -- destroy cells with rays for thyroid cancers for lung cancer 32P for eye tumors 198Au 95 96 131I 16 16 ... View Full Document

CHM 116 Ch 21-Nuclear Chemistry

End of Preview

Sign up now to access the rest of the document