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New chm 152_unit_10_nuclear_chemistry_power_points-su13 Presentation Transcript

  • 1. NUCLEAR CHEMISTRY • • • • 24-1 BINDING ENERGY RADIOACTIVE DECAY RATES OF DECAY Chapter 24 Silberberg
  • 2. Chapter 24 Nuclear Reactions and Their Applications 24-2
  • 3. Nuclear Reactions and Their Applications 24.1 Radioactive Decay and Nuclear Stability 24.2 The Kinetics of Radioactive Decay 24.3 Nuclear Transmutation: Induced Changes in Nuclei 24.4 Effects of Nuclear Radiation on Matter 24.5 Applications of Radioisotopes 24.6 Interconversion of Mass and Energy 24.7 Applications of Fission and Fusion 24-3
  • 4. 24-4
  • 5. Table 24.1 Comparison of Chemical and Nuclear Reactions Chemical Reactions Nuclear Reactions One substance is converted into another, but atoms never change identity. Atoms of one element typically are converted into atoms of another element. Electrons in orbitals are involved as bonds break and form; nuclear particles do not take part. Protons, neutrons, and other nuclear particles are involved; electrons in orbitals take part much less often. Reactions are accompanied by relatively small charges in energy and no measurable changes in mass. Reactions are accompanied by relatively large charges in energy and measurable changes in mass. Reaction rates are influenced by Reaction rates depend on number of temperature, concentration, catalyst, and nuclei, but are not affected by the compound in which an element occurs. temperature, catalysts, or, except on rare occasions, the compound in which an element occurs. 24-5
  • 6. Components of the Nucleus Most of the mass of the atom is concentrated in the dense, tiny nucleus. The nucleus comprises the neutrons and protons, collectively called nucleons. The total number of nucleons in a nucleus gives its mass number. A nuclide is a nucleus with a particular composition. Each isotope of an element has a different nuclide. A particular nuclide is often designated by its mass number; for example, chlorine-35 and chlorine-37. 24-6
  • 7. Notation for Nuclides The relative mass and charge of a particle is described by the notation: A = mass number Z = charge of the particle Example: electron proton neutron 24-7 A Z X 0 e -1 1 p 1 1 n 0
  • 8. Radioactivity Many nuclides are unstable and spontaneously emit radiation, a process termed radioactive decay. - The intensity of the radiation is not affected by temperature, pressure, or other physical and chemical conditions. When a nuclide decays, it emits radiation and usually changes into a nuclide of a different element. There are three natural types of radioactive emission: Alpha particles (α, 4 α , or 4 He ) are identical to helium-4 nuclei. 2 2 Beta particles (β, β-, or 0 -1β ) are high-speed electrons. Gamma rays (γ or 0 γ ) are very high-energy photons. 0 24-8
  • 9. Radioactive Decay • Natural Radioactivity -- decay of natural isotopes. • Three common types of decay: – alpha( ) – beta( ) – gamma( ) 24-9
  • 10. Alpha Radiation • stream of particles that contain 2 protons and 2 neutrons • represented by 24He or 24 • low penetrating ability • 92238U -> 24He + ______ • Stopped by a piece of paper 24-10
  • 11. Beta Radiation • beam of electrons from the nucleus • represented by -10e or -10 • moderate penetrating ability • 01n -> -10e + ______ • 90234Th -> -10e + ______ • Stopped by a piece of Aluminum foil 24-11
  • 12. Gamma Radiation • electromagnetic radiation of high energy and short wavelength. • represented by 00 • 2760Co -> 00 • Stopped by Lead or Concrete Walls 24-12
  • 13. Figure 24.1 How the three types of radioactive emissions behave in an electric field. The positively charged α particles curve toward the negative plate, the negatively charged β particles curve towards the positive plate, and the γ rays are not affected by the electric field. 24-13
  • 14. Modes of Radioactive Decay • Alpha (α) decay involves the loss of an α particle from the nucleus. – For each α particle emitted, A decreases by 4 and Z decreases by 2 in the daughter nuclide. – This is the most common form of decay for a heavy, unstable nucleus. 228 222 4 Ra → 86 Rn + 2 α 88 • β- decay involves the ejection of a β- particle from the nucleus. – A neutron is converted to a proton, which remains in the nucleus, and a β- particle is expelled: 1 n → 1 p + 0 β 0 1 -1 – A remains the same in the daughter nuclide but Z increases by 1 63 63 0 unit. Ni → 29 Cu + -1 β 28 24-14
  • 15. • Positron (β+) emission is the emission of a β+ particle from the nucleus. – The positron is the antiparticle of the electron. – A proton in the nucleus is converted into a neutron, and a positron is emitted: 1 p → 1n + 0β 1 0 1 – A remains the same in the daughter nuclide but Z increases by 1 unit. 11 11 0 C → 5B + 1 β 6 • Electron capture occurs when the nucleus interacts with an electron in a low atomic energy level. – A proton is transformed into a neutron: 1 p 1 0 + -1 e → 0 n 1 – The effect on A and Z is the same as for positron emission. 24-15
  • 16. • Gamma (γ) emission involves the radiation of highenergy γ photons. – Gamma emission usually occurs together with other forms of radioactive decay. – Several γ photons of different energies can be emitted from an excited nucleus as it returns to the ground state. – γ emission results in no change in either A or Z since γ rays have no mass or charge. 24-16
  • 17. Table 24.2 Modes of Radioactive Decay* 24-17
  • 18. Table 24.2 Modes of Radioactive Decay* 24-18
  • 19. Sample Problem 24.1 Writing Equations for Nuclear Reactions PROBLEM: Write balanced equations for the following nuclear reactions: (a) Naturally occurring thorium-232 undergoes α decay. (b) Zirconium-86 undergoes electron capture. PLAN: We first write a skeleton equation that includes the mass numbers, atomic numbers, and symbols of all the particles on the correct sides of the equation, showing the unknown A product particle as Z X. Then, because the total of mass numbers and the total of charges on the left side and the right side of the equation must be equal, we solve for A and Z, and use Z to determine the identity of X from the periodic table. 24-19
  • 20. Sample Problem 24.1 SOLUTION: (a) Writing the skeleton equation, with the α particle as a product: For A, 232 = A + 4, so A= 228. 232 A 4 Th → Z X + 2 α 90 For Z, 90 = Z + 2, so Z = 88. The daughter nuclide produced in this reaction is Ra. 232 90 Th → 228 88 Ra 4 +2α (b) Writing the skeleton equation, with the captured electron as a reactant: 86 0 A For A, 86+ 0 = A so A= 86. Zr + -1e → Z X 40 For Z, 40 -1 = Z so Z = 39. Element X is yttrium, symbol Y. 86 0 86 Zr + -1 e → 39Y 40 24-20
  • 21. Examples • + • 15 30P -> • 238U 92 • 24-21 13 27Al 239U 92 + -> 2 4He 14 -> 30Si 1n 0 -1 0 1n + ____ + ______ -> ______ 0e + ______
  • 22. 24-22
  • 23. Figure 24.3 The 238U decay series. A parent nuclide may undergo a series of decay steps before a stable daughter nuclide is formed. 24-23
  • 24. Detection and Measurement of Radioactivity An ionization counter detects radioactive emissions as they ionize a gas. Ionization produces free electrons and gaseous cations, which are attracted to electrodes and produce an electric current. A scintillation counter detects radioactive emissions by their ability to excite atoms and cause them to emit light. Radioactive particles strike a light-emitting substance, which emits photons. The photons strike a cathode and produce an electric current. 24-24
  • 25. Figure 24.4 24-25 Detection of radioactivity by an ionization counter.
  • 26. Figure 24.5 24-26 A scinatillation “cocktail” in tubes to be placed in the counter.
  • 27. Units of Radioactivity The SI unit of radioactivity is the becquerel (Bq), defined as one disintegration per second (d/s). The curie (Ci) is a more commonly used unit: 1 Ci = 3.70x1010 d/s 24-27
  • 28. Rate of Radioactive Decay Radioactive nuclei decay at a characteristic rate, regardless of the chemical substance in which they occur. The rate of radioactive decay (A) (also called the activity) is proportional to the number of nuclei present. A = kN Radioactive decay follows first-order kinetics, and the rate constant k is called the decay constant. The larger the value of k, the higher the activity of the substance. 24-28
  • 29. Half-Life of Radioactive Decay The half-life of a nuclide is the time taken for half the nuclei in a sample to decay. - The number of nuclei remaining is halved after each half-life. - The mass of the parent nuclide decreases while the mass of the daughter nuclide increases - Activity is halved with each succeeding half-life. ln Nt N0 24-29 = -kt or Nt = N0e-kt and ln Nt = kt N0
  • 30. Figure 24.6 Decrease in the number of 14C nuclei over time. t1/2 = 24-30 ln 2 k
  • 31. Rates of Decay • Radioactive decay is a first order process and the same equations apply as for any first order reaction. • ln(N/No) = -kt • and • t1/2 = 0.693/k • or • k = 0.693/t1/2 24-31
  • 32. Table 24.5 Decay Constants (k) and Half-Lives (t1/2) of Beryllium Isotopes Nuclide 7 4 Be 8 4 Be 9 4 Be 10 4 Be 11 4 Be 24-32 k t1/2 1.30x10-2/day 53.3 days 1.0x1016/s 6.7x10-17 s Stable 4.3x10-7/yr 1.6x106 yr 5.02x10-2/s 13.8 s
  • 33. Sample Problem 24.4 Finding the Number of Radioactive Nuclei PROBLEM: Strontium-90 is a radioactive by-product of nuclear reactors that behaves biologically like calcium, the element above it in Group 2A(2). When 90Sr is ingested by mammals, it is found in their milk and eventually in the bones of those drinking the milk. If a sample of 90Sr has an activity of 1.2x1012 d/s, what are the activity and the fraction of nuclei that have decayed after 59 yr (t1/2 of 90Sr = 29 yr)? PLAN: The fraction of nuclei that have decayed is the change in the number of nuclei, expressed as a fraction of the starting number. The activity of the sample (A) is proportional to the number of nuclei (N), and we are given A0. We can find At from the integrated form of the first-order rate equation, in which t is 59 yr. We need the value of k, which we can calculate from the given t1/2. 24-33
  • 34. Sample Problem 24.4 SOLUTION: ln 2 ln 2 0.693 t1/2 = so k = = 0.024 yr-1 k t1/2 = 29 yr Nt At ln = ln = kt or lnA0 – lnAt = kt N0 A0 so lnAt = -kt + lnA0 = -(0.024 yr-1 x 59 yr) + ln(1.2x1012 d/s) = -1.4 + 27.81 lnAt = 26.4 At = e26.4 = 2.9x1011 d/s A 0 – At 1.2x1012 d/s – 2.9x1011 d/s fraction decayed = = A0 1.2x1012 d/s 24-34 = 0.76
  • 35. 24-35
  • 36. Radioisotopic Dating • Radioisotopes can be used to determine the ages of certain objects. • Radiocarbon dating measures the relative amounts of 14C and 12C in materials of biological origin. – The ratio of 14C/12C remains the same for all living organisms. – Once the organism dies, the amount of 14C starts to decrease as it decays to form 14N. – Since 14C decays at a predictable rate, measuring the amount present indicates the time that has passed since the organism died. • 24-36 40K/40Ar ratios can be used to determine the age of certain rocks.
  • 37. Figure 24.7 Ages of several objects determined by radiocarbon dating. t = 1 ln A0 At k 24-37
  • 38. Sample Problem 24.5 Applying Radiocarbon Dating PROBLEM: The charred bones of a sloth in a cave in Chile represent the earliest evidence of human presence at the southern tip of South America. A sample of the bone has a specific activity of 5.22 disintegrations per minute per gram of carbon (d/min·g). If 12C/14C ratio for living organisms results in a specific activity of 15.3 d/min·g, how old are the bones (t1/2 of 14C = 5730 yr)? PLAN: We calculate k from the given half-life. Then use the first-order rate equation to find the age of the bones, using the given activities of the bones and of a living organism. SOLUTION: k= ln 2 t1/2 24-38 = 0.693 5730 yr = 1.21x10-4 yr-1
  • 39. Sample Problem 24.5 t = 1 ln A0 At k = 1 ln 15.3 d/min·g 1.21x10-4 yr-1 5.22 d/min·g = 8.89x103 yr The bones are about 8900 years old. 24-39
  • 40. Examples 24-40 • Determine the amount of a 100.0g sample of cobalt-60 remaining after 15.0 years. The half-life of cobalt-60 is 5.27 years. • Estimate the age of an artifact whose carbon-14 activity of 55% that of living wood. Half-Life = 5.73 x 103 yrs. • Given 100g of an isotope having a halflife of 2.0 minutes. Find the weight remaining after 10.0 minutes.
  • 41. 24-41
  • 42. 24-42
  • 43. 24-43
  • 44. Nuclear Transmutation Nuclear transmutation is the induced conversion of the nucleus of one element into the nucleus of another. This is achieved by high-energy bombardment of nuclei in a particle accelerator. 14 7N 4 1 + 2α → 1 p + 17 8O Nuclear transmutation reactions can be described using a specific short-hand notation: reactant nucleus (particle in, particle(s) out) product nucleus The above reaction can be written as: 14N (α, p) 17O. 24-44
  • 45. Figure 24.8 Schematic diagram of a linear accelerator. The linear accelerator uses a series of tubes with alternating voltage. A particle is accelerated from one tube to the next by repulsion. 24-45
  • 46. Figure 24.9 24-46 Schematic diagram of a cyclotron accelerator.
  • 47. Table 24.6 Formation of some Transuranium Nuclides* Reaction Half-life of Product 239 94 Pu + 2 0n → 1 239 94 Pu + 4 2α 241 95 Am + 4 2α 242 96 Cm + 4 2 253 99 Es 253 99 Am * 24-47 + + 4 2 18 8 → → α → α → O → 241 95 Am + -1 β 0 242 96 + Cm 243 97 Bk 245 98 Cf 256 101Md 256 101 Lr 1 0n 1 + 2 0n + + 432 yr 163 days 4.5 h 1 0n 45 min 1 0 76 min n 1 + 50 n 28 s Like chemical reactions, nuclear reactions may occur in several steps.
  • 48. Effects of Nuclear Radiation on Matter Radioactive emissions collide with surrounding matter, dislodging electrons and causing ionization. Each such event produces a cation and a free electron. The number of cation-electron pairs is directly related to the energy of the incoming ionizing radiation. Ionizing radiation has a destructive effect on living tissue. The danger of a particular radionuclide depends on - the type of radiation, - its half-life, and - its biological behavior. 24-48
  • 49. Units of Radiation The gray is the SI unit for energy absorption. 1 Gy = 1 J absorbed per kg of body tissue. The rad is more widely used: 1 rad = 0.01 J/kg or 0.01 Gy. The rem is the unit of radiation dosage equivalent to a given amount of tissue damage in a human. no. of rems = no. of rads x RBE The RBE is the relative biological effectiveness factor. The rem allows us to assess actual tissue damage by taking into account the strength of the radiation, the exposure time, and the type of tissue. 24-49
  • 50. Figure 24.10 Penetrating power of radioactive emissions. The effect of radiation on living tissue depends on both the penetrating power and the ionizing ability of the radiation. Penetrating power is inversely related to the mass, charge, and energy of the emission. 24-50
  • 51. Molecular Interactions with Radiation The interaction of ionizing radiation with molecules causes the loss of an electron from a bond or a lone pair. This results in the formation of free radicals, molecular or atomic species with one or more unpaired electrons. Free radicals are unstable and extremely reactive. Double bonds in membrane lipids are very susceptible to attack by free-radicals: 24-51
  • 52. Sources of Ionizing Radiation • There are several natural sources of background radiation. • Cosmic radiation increases with altitude. • Radon is a radioactive product of uranium and thorium decay. – Rn contributes to 15% of annual lung cancer deaths. • Radioactive 40K is present in water and various food sources. • Radioactive 14C occurs in atmospheric CO2. 24-52
  • 53. Figure 24.11 24-53 US radon distribution.
  • 54. Table 24.7 Typical Radiation Doses from Natural and Artificial Sources Source of Radiation Average Adult Exposure Natural Cosmic radiation 30-50 mrem/yr Radiation from the ground From clay soil and rocks In wooden houses In brick houses In concrete (cinder block) houses ~25-170 mrem/yr 10-20 mrem/yr 60-70 mrem/yr 60-160 mrem/yr Radiation from the air (mainly radon) Outdoors, average value In wooden houses In brick houses In concrete (cinder block) houses 20 mrem/yr 70 mrem/yr 130 mrem/yr 260 mrem/yr Internal radiation from minerals in tap water and daily intake of food. (40K, 14C, Ra) ~ 40 mrem/yr 24-54
  • 55. Table 24.7 Typical Radiation Doses from Natural and Artificial Sources Source of Radiation Average Adult Exposure Artificial Diagnostic x-ray methods Lung (local) Kidney (local) Dental (does to the skin) 0.04-0.2 rad/film 1.5-3 rad/film ≤ 1 rad/film Locally ≤ 10,000 rad Therapeutic radiation treatment Other Sources Jet flight (4 h) Nuclear testing Nuclear power industry ~1 mrem < 4 mrem/yr < 1 mrem/yr Total average value 100-200 mrem/yr 24-55
  • 56. Figure 24.12 Two models of radiation risk. The linear response model proposes that radiation effects a`ccumulate over time regardless of dose. The S-shaped response model implies there is a threshold 24-56 above which the effects are more significant.
  • 57. Table 24.8 24-57 Acute Effects of a Single Dose on Whole-Body Irradiation
  • 58. Radioactive Tracers • The isotopes of an element exhibit very similar chemical and physical behavior. – A small amount of radioactive isotope mixed with the stable isotope will undergo the same chemical reactions, and can act as a tracer. • Radioactive tracers are used – – – – to study reaction pathways, to track physiological functions, to trace material flow, to identify the components of a substance from a very small sample, and – to diagnose a wide variety of medical conditions. 24-58
  • 59. Figure 24.13 The use of radioisotopes to image the thyroid gland. This 131I scan shows an asymmetric image that is indicative of disease. 24-59 A 99Tc scan of a healthy thyroid.
  • 60. Table 24.9 Some Radioisotopes Used as Medical Tracers Isotope 11C, 18F, 13N, 15O PET studies of brain, heart 60Co, 192Ir Cancer therapy 64Cu Metabolism of copper 59Fe Blood flow, spleen 67Ga Tumor imaging 123I, 131I Thyroid 111In Brain, colon 42K Blood flow 81mKr Lung 99mTc Heart, thyroid, liver, lung, bone 201Tl Heart muscle 90Y 24-60 Body Part or Process Cancer, arthritis
  • 61. Figure 24.14 PET and brain activity. These PET scans show brain activity in a normal person (left) and in a patient with Alzheimer’s disease (right). Red and yellow indicate relatively high activity within a region. 24-61
  • 62. Other Applications of Ionizing Radiation • Radiation therapy – Cancer cells divide more rapidly than normal cells, and are therefore susceptible to radioisotopes that interfere with cell division. • Destruction of microbes – Irradiation of food increases its shelf life by killing microorganisms that cause rotting or spoilage. • Insect control • Power for spacecraft instruments 24-62
  • 63. Figure 24.15 24-63 The increased shelf life of irradiated food.
  • 64. The Interconversion of Mass and Energy The total quantity of mass-energy in the universe is constant. Any reaction that releases or absorbs energy also loses or gains mass. E = mc2 or ΔE = Δmc2 so Δm = ΔE c2 In a chemical reaction, the energy changes in breaking or forming bonds is relatively small, so mass changes are negligible. In a nuclear reaction, the energy changes are enormous and the mass changes are easily measurable. 24-64
  • 65. Nuclear Binding Energy The mass of the nucleus is less than the combined masses of its nucleons. - Mass always decreases when nucleons form a nucleus, and the “lost” mass is released as energy. - Energy is required to break a nucleus into individual nucleons. The nuclear binding energy is the energy required to break 1 mol of nuclei into individual nucleons. - Binding energy is expressed using the electron volt (eV). Nucleus + nuclear binding energy → nucleons 1 amu = 931.5 x 106 eV = 931.5 MeV 24-65
  • 66. Sample Problem 24.6 Calculating the Binding Energy per Nucleon PROBLEM: Iron-56 is an extremely stable nuclide. Compute the binding energy per nucleon for 56Fe and compare it with that for 12C (mass of 56Fe atom = 55.934939 amu; mass of 1H atom = 1.007825 amu; mass of neutron = 1.008665 amu). PLAN: Iron-56 has 26 protons and 20 neutrons. We calculate the mass difference, Δm, when the nucleus forms by subtracting the given mass of one 56Fe atom from the sum of the masses of 26 1H atoms and 30 neutrons. To find the binding energy per nucleon, we multiply Δm by the equivalent in MeV and divide by the number of nucleons. SOLUTION: Calculating the mass difference: Δm = [(26 x mass 1H atom) + (30 x mass neutron)] – mass 56Fe atom = [(26 x 1.007825 amu) + (30 x 1.008665 amu)] - 55.934939 = 0.52856 amu 24-66
  • 67. Sample Problem 24.6 Binding energy per nucleon = 0.52846 amu x 931.5 MeV/amu 56 nucleons = 8.790 MeV/nucleon An 56Fe nucleus would require more energy per nucleon to break up into its nucleons than would 12C, so 56Fe is more stable than 12C. 24-67
  • 68. Figure 24.16 The variation in binding energy per nucleon. The greater the binding energy per nucleon, the more stable the nuclide. 24-68
  • 69. Binding Energy m = the calculated mass - actual mass m = (sum of the masses of all p, n and e) - (actual mass of the atom) • Binding energy = mc2 – where c = speed of light = 3.00x108m/s 24-69
  • 70. Example • Determine the mass deficiency for potassium-39 atoms. The actual mass of a potassium-39 atom is 39.32197amu. (masses: p=1.0073, n=1.0087, e=0.00054858 amu) • Calculate the binding energy for the potassium-39 in J/mole. (c=3.00x108m/s and 1 joule=1 kgm2/s2) 24-70
  • 71. 24-71
  • 72. 24-72
  • 73. Fission or Fusion The binding energy per nucleon peaks at elements with mass number A ≈ 60. - Nuclides become more stable with increasing number up to around 60 nucleons, after which stability decreases. There are two ways nuclides can increase their binding energy per nucleon: A heavier nucleus can split into lighter ones by undergoing fission. Lighter nuclei can combine to form a heavier nucleus in a process called fusion. 24-73
  • 74. Figure 24.17 24-74 Fission of 235U caused by neutron bombardment.
  • 75. Nuclear Fission Nuclear fission involves the splitting of large nuclei into smaller nuclei, using neutron bombardment to start the process. Fission releases energy and generates more high-energy neutrons, which cause further fission to occur. The fission process becomes self-sustaining by a chain reaction. The mass required to achieve this is called the critical mass. The energy from nuclear fission can be harnessed and converted to other forms of energy. 24-75
  • 76. Figure 24.18 24-76 A chain reaction involving fission of 235U.
  • 77. Figure 24.19 An atomic bomb based on 235U. An atomic bomb uses an uncontrolled chain reaction to produce a powerful explosion. 24-77
  • 78. Figure 24.20 A light-water nuclear reactor. The dome-shaped structure is the containment shell for the nuclear reactor. 24-78
  • 79. Figure 24.20 24-79 A light-water nuclear reactor.