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Chapter 19 Nuclear Reactions

Chapter 19 Nuclear Reactions. Outline. 1. Radioactivity 2. Rate of Radioactive Decay 3. Mass-Energy Relations 4. Nuclear Fission 5. Nuclear Fusion. Nuclear Reactions vs. Chemical Reactions. In a chemical reaction Only the outer electron configuration of atoms and molecules changes

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Chapter 19 Nuclear Reactions

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  1. Chapter 19Nuclear Reactions

  2. Outline 1. Radioactivity 2. Rate of Radioactive Decay 3. Mass-Energy Relations 4. Nuclear Fission 5. Nuclear Fusion

  3. Nuclear Reactions vs. Chemical Reactions • In a chemical reaction • Only the outer electron configuration of atoms and molecules changes • There is no change to the nucleus • In a nuclear reaction • Mass numbers may change • Atomic numbers may change • One element may be converted to another

  4. Nuclear Symbols • Recall that a nuclear symbol begins with the element symbol • Mass number is at the top left • Protons + neutrons • Atomic number is at the bottom left • Number of protons = number of electrons

  5. Nuclear Equations • Must always balance with respect to nuclear mass and charge • Notice • Total mass on the left is 15 and the total mass on the right is 15 • Total charge on the left is 7 and total charge on the right is 7

  6. Radioactivity • Radioactive nuclei spontaneously decompose (decay) with the evolution of energy • Radioactivity may be • Natural; there are a few nuclei that are by nature radioactive • Induced; many nuclei can be made radioactive by bombarding them with other particles

  7. Five Modes of Radioactive Decay • We will consider five modes of radioactive decay • Alpha (α) particle emission • Beta (β) particle emission • Gamma (γ) radiation emission • Positron emission • K-electron capture

  8. Alpha Particle Emission • An alpha particle is a helium nucleus • Mass is 4, charge is +2, atomic number 2 • Symbol is or α • When a nucleus emits an alpha particle, its mass decreases by 4 and its atomic number decreases by 2

  9. Beta Particle Emission • Beta particles are high speed electrons • Mass is zero, charge is -1 • Mass number does not change • Effectively the conversion of a neutron into a proton with the emission of an electron • Atomic number increases by 1

  10. Gamma Radiation Emission • Gamma rays are photons • Mass number is zero • Charge is zero • No change in atomic number or mass number

  11. Positron Emission • Positrons are anti-electrons • Mass 0 • Charge +1 • No change in mass number • Effectively a conversion of a proton into a neutron • Atomic number decreases by 1

  12. K-electron Capture • Innermost electron (n=1) falls into the nucleus • Effect is the same as for positron emission • No change in the mass number • Atomic number decreases by 1

  13. Example 19.1

  14. Induced Radioactivity - Bombardment • More than 1,500 isotopes have been prepared in the laboratory • Stable nuclei are bombarded with • Neutrons • Charged particles (electron, positron, alpha) • Other nuclei • The result is a radioactive nucleus

  15. Examples of Bombardment Reactions • Aluminum-27 is converted to radioactive aluminum-28 by neutron bombardment, which decays by beta emission • Aluminum-27 is converted to phosphorus-30 by alpha particle bombardment; P-30 decays by positron emission

  16. Transuranium Elements • Elements beyond uranium are synthetic, having been prepared by bombardment reactions • Most nuclei produced have very short half-lives • In some cases, only the decay products are observed • As of October, 2006 the heaviest element reported is Element 118, Uuo-294

  17. Table 19.1

  18. Applications of Isotopes • Medicine • Some isotopes find use in medical diagnostics and treatment • Cancer treatment • Iodine-131 for thyroid cancer • Cobalt-60 for treatment of malignant cells • Diagnostics • PET, positron emission tomography: carbon-11 • Radioactive labeling

  19. Table 19.2 – Medical Uses of Radioisotopes

  20. Cobalt-60 Therapy

  21. Chemical Applications • Neutron activation analysis • Sample bombarded by neutrons, inducing radioactivity • Isotopes normally decay by gamma emission • Activation of strontium in bones of fossils can indicate something about the diet, since plants contain more strontium than animals

  22. Commercial Applications • Smoke detectors • Americium-241 • Radioactive source ionizes air, which completes a circuit; smoke particles open the circuit and trip the alarm

  23. Figure 19.1 – Smoke Detector

  24. Food Irradiation • Gamma radiation treatment • Kills insects, larvae and parasites • Food that is irradiated has a longer shelf life and can be rid of parasites such as trichina in pork

  25. Figure 19.1 – Irradiated Strawberries

  26. Rate of Radioactive Decay • Radioactive decay is a first-order process • The equations for first-order reactions from Chapter 11 apply to radioactive decay • k is the first-order rate constant • t1/2 is the half life • X is the amount of sample at time t • X0 is the amount of sample at t=0

  27. Activity • Activity is the rate of decay • Number of atoms per unit time • A = kN • Units of activity • 1 Becquerel (Bq) = 1 atom/sec • 1 Curie (Ci) = 3.700 X 1010 atoms/sec

  28. Example 19.2

  29. Example 19.2, (Cont’d)

  30. Figure 19.3 – Scintillation Counter

  31. Age of Organic Material • W.F. Libby, University of Chicago, 1950s • Age of organic material related to the decay of carbon-14 • Carbon-14 forms in the upper atmosphere by bombardment of nitrogen-14 by neutrons • Carbon-14 incorporates itself into living things • Steady-state while the organism is alive • Once an organism dies, C-14 level falls due to radioactive decay • The original rate of decay is 15.3 atoms/min • Half-life of C-14 is 5730 yr

  32. Example 19.3

  33. The Shroud of Turin • A sample of 0.1 g of the Shroud of Turin was analyzed for its C-14 content • Evidence showed the flax used to weave the shroud dated from the 14th century • Could not have been the burial cloth of Christ

  34. Mass-Energy Relations • The energy change accompanying a nuclear reaction can be calculated from the equation • Where • Δm = change in mass = mass of products minus mass of reactants • ΔE = change in energy = energy of products – energy of reactants • c is the speed of light

  35. Change in Mass • In any spontaneous nuclear reaction, the products weigh less than the reactants • Therefore, the energy of the products is less than the energy of the reactants • There is a release of energy when the reaction takes place

  36. Units

  37. Example 19.4

  38. Example 19.4, (Cont’d)

  39. Nuclear Binding Energy • The nucleus weighs less than the sum of the individual masses of the neutrons and protons • This is called the mass defect • The mass defect leads to the binding energy, which holds the nucleus together

  40. Binding Energy of Lithium-6 • Mass of one mole: 6.01348 g • Mass of nucleons: • (3 X 1.00867)+(3 X 1.00728) = 6.04785g • Mass defect: 6.04785 - 6.01348 = 0.03437g/mol • ΔE = 9.00 X 1010 kJ/g X 0.03437g = 3.09 X 109 kJ/mol

  41. Example 19.5

  42. Figure 19.4

  43. Nuclear Stability and the Binding Energy • Binding energy per mole of nucleons • Divide the binding energy by the number of nucleons • For Li-6 this is • 3.09 X 109 kJ/mol Li-6 X 1 mol Li-6/6 mol nucleons = 5.15 X 108 kJ/mol • Release of the binding energy • Nuclear fission: split large nucleus into smaller ones • Nuclear fusion: fuse small nuclei into larger ones

  44. Nuclear Fission • Discovery, 1938 • Otto Hahn • Lise Meitner • World War II • The Manhattan Project – produced the first atomic bomb • First nuclear explosion, July 16, 1945 • Hiroshima, August 6, 1945 • Nagasaki, August 9, 1945

  45. The Fission Process • Uranium-235 is 0.7% of naturally occurring uranium • U-235 undergoes fission • Splits into two unequal fragments • Releases more neutrons than are consumed

  46. The Fission Process (Cont’d) • The first products of nuclear fission are radioactive and decay by beta emission • Note that in the fission process, more neutrons are produced than consumed • A chain reaction results • Energy is released due to the conversion of mass into energy

  47. Chain Reactions • To sustain a chain reaction, the sample of fissile material must be large enough to contain the neutrons that are generated • Samples that are too small will not sustain a chain reaction • The sample that will sustain a chain reaction is called a critical mass

  48. Nuclear Reactors • About 20% of the electricity generated in the US comes from the fission of U-235 in nuclear reactors • US reactors are called light water reactors • UO2 pellets in a zirconium alloy tube • Control rods are used to moderate the reaction • Can be inserted to absorb neutrons • Prevent a runaway chain reaction • Tremendous amount of heat is produced, which turns water to steam and turns a turbine to produce electricity • Ordinary water is used both to cool the reaction and to slow the neutrons • Most reactors use ordinary (light) water

  49. Heavy Water Reactors • Canadian reactors (CANDU) • Use D2O (2H2O) as a moderator • The use of D2O allows the use of natural uranium without enrichment • Enrichment is the process of increasing the U-235 content to a few percent from 0.7% • Enrichment is an expensive, technologically demanding process • Done by gaseous effusion • UF6

  50. Nuclear Energy and History • In the 1970s it was assumed that nuclear reactors would replace fossil fuels (oil, gas, coal) as the major source of electricity • In France, this has indeed happened • In the US, this has not happened • Accident at Three Mile Island, Chernobyl • Disposal of radioactive waste

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