Chapter 21 Lecture- Nuclear Chemistry


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Chapter 21 lecture for AP Chemistry on Nuclear Chemistry

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Chapter 21 Lecture- Nuclear Chemistry

  1. 1. Chapter 21 Nuclear Chemistry
  2. 2. Nuclear Chemistry The nucleus of an atom contains protons (+1charge) and neutrons (no charge)
  3. 3. The nucleus is held together by the strong nuclear force The strong nuclear force is the strongest force known Protons and neutrons are very close together They exchange a teeny bit of mass back and forth. When disrupted, the mass is converted to energy according to E=mc2 The mass is tiny. The energy is immense.
  4. 4. Protons and neutrons experience the strong nuclear force if close enough Because protons repel each other the nucleus needs a certain proton to neutron ratio for stability
  5. 5. An unstable atom decays by emitting radiation These unstable atoms are radioactive. Carbon-12 is stable. Carbon-14 is radioactive. Carbon-14 is a radioisotope. There are many naturally occurring radioisotopes and some that are human- made.
  6. 6. Radioisotopes decay into stable isotopes of a DIFFERENT element In nuclear reactions, the makeup of the nucleus changes. Sometimes the number of protons will change. If the number of protons changes, the element has changed.
  7. 7. The three most common forms of radioactive decay are alpha(α), beta(β) and gamma(γ)
  8. 8. 25.1 Alpha particles are the least penetrating. Gamma rays are the most penetrating.
  9. 9. To balance nuclear equations you must know the symbol for the emitted particle You must balance the atomic number (number on bottom) and the atomic mass (number on top) Here 222 + 4 = 226 and 86 + 2 = 88 its balanced
  10. 10. The stuff that radiates An alpha particle is a helium nucleus it has a +2 charge and a mass of 4amu.
  11. 11. A beta particle is an electron which is formed when a neutron becomes a proton
  12. 12. A gamma ray is a high energy electromagnetic wave. A gamma ray has no mass or charge so it is not in a nuclear equation.
  13. 13. particle Alpha α Beta β Gamma γ Mass 4amu 0 0 Charge +2 -1 0 Effect Radioisotope Radioisotope Radioisotope loses two converts a neutron loses energy protons and two to a proton & neutrons ejects an electron The element The ELEMENT The ELEMENT does not changes changes change What it is Helium electron High energy nucleus electromagnetic radiation Stop it Paper/skin 1cm/metal foil Lead/concrete damage High ionization Medium Lowest ionization ionization
  14. 14. Damage from nuclear radiation is due to ionization of living tissue. Nuclear radiation is called ionizing radiation because it produces ions from neutral molecules. Alpha radiation has a low penetration, but it is the most damaging to living tissue because it deposits all its energy along a short path
  15. 15. Nuclear Fission Fission separates heavy elements into two lighter elements. Uranium-235  Barium-141 + Krypton-92 +3neutrons A huge amount of energy is produced. Used by humans as an energy source The fission bomb was used in WWII
  16. 16. Nuclear fusion Fusion combines two light nuclei into one heavier element. Produces even more energy than fission. Occurs in the sun. Requires extremely high temperatures and pressures.
  17. 17. FUSION = to put together FISSION = to break apart
  18. 18. The Nucleus • Remember that the nucleus is comprised of the two nucleons, protons and neutrons. • The number of protons is the atomic number. • The number of protons and neutrons together is effectively the mass of the atom.
  19. 19. Isotopes • Not all atoms of the same element have the same mass due to different numbers of neutrons in those atoms. • There are three naturally occurring isotopes of uranium: Uranium-234 Uranium-235 Uranium-238
  20. 20. Radioactivity • It is not uncommon for some nuclides of an element to be unstable, or radioactive. • We refer to these as radionuclides. • There are several ways radionuclides can decay into a different nuclide.
  21. 21. Types of Radioactive Decay
  22. 22. Alpha Decay: Loss of an α-particle (a helium nucleus) 4 2 He 238 → 234 4 92 U 90 U + 2 He
  23. 23. Beta Decay: Loss of a β-particle (a high energy electron) 0 0 −1 β or −1 e 131 131 0 53 I → 54 Xe + −1 e
  24. 24. Positron Emission: Loss of a positron (a particle that has the same mass as but opposite charge than an electron) 0 1 e 11 11 0 6 C → 5 B + 1 e
  25. 25. Gamma Emission: Loss of a γ-ray (high-energy radiation that almost always accompanies the loss of a nuclear particle) 0 0 γ
  26. 26. Electron Capture (K-Capture) Addition of an electron to a proton in the nucleus As a result, a proton is transformed into a neutron. 1 0 1 1 p + −1 e → 0 n
  27. 27. Neutron-Proton Ratios • Any element with more than one proton (i.e., anything but hydrogen) will have repulsions between the protons in the nucleus. • A strong nuclear force helps keep the nucleus from flying apart.
  28. 28. Neutron-Proton Ratios • Neutrons play a key role stabilizing the nucleus. • Therefore, the ratio of neutrons to protons is an important factor.
  29. 29. Neutron-Proton Ratios For smaller nuclei (Z ≤ 20) stable nuclei have a neutron-to-proton ratio close to 1:1.
  30. 30. Neutron-Proton Ratios As nuclei get larger, it takes a greater number of neutrons to stabilize the nucleus.
  31. 31. Stable Nuclei The shaded region in the figure shows what nuclides would be stable, the so- called belt of stability.
  32. 32. Stable Nuclei • Nuclei above this belt have too many neutrons. • They tend to decay by emitting beta particles.
  33. 33. Stable Nuclei • Nuclei below the belt have too many protons. • They tend to become more stable by positron emission or electron capture.
  34. 34. Stable Nuclei • There are no stable nuclei with an atomic number greater than 83. • These nuclei tend to decay by alpha emission.
  35. 35. Radioactive Series • Large radioactive nuclei cannot stabilize by undergoing only one nuclear transformation. • They undergo a series of decays until they form a stable nuclide (often a nuclide of lead).
  36. 36. Some Trends Nuclei with 2, 8, 20, 28, 50, or 82 protons or 2, 8, 20, 28, 50, 82, or 126 neutrons tend to be more stable than nuclides with a different number of nucleons.
  37. 37. Some Trends Nuclei with an even number of protons and neutrons tend to be more stable than nuclides that have odd numbers of these nucleons.
  38. 38. Nuclear Transformations Nuclear transformations can be induced by accelerating a particle and colliding it with the nuclide.
  39. 39. Particle Accelerators These particle accelerators are enormous, having circular tracks with radii that are miles long.
  40. 40. Kinetics of Radioactive Decay • Nuclear transmutation is a first-order process. • The kinetics of such a process, you will recall, obey this equation: Nt ln = kt N0 ln[A]t – ln[A]0 = -kt for 1st order kinetics
  41. 41. Kinetics of Radioactive Decay • The half-life of such a process is: Not given, but can 0.693 be derived from = t1/2 previous equation k with [A]t equal to half of [A]0. • Comparing the amount of a radioactive nuclide present at a given point in time with the amount normally present, one can find the age of an object.
  42. 42. First order processes (including nuclear decay) always show a constant half-life
  43. 43. Measuring Radioactivity • One can use a device like this Geiger counter to measure the amount of activity present in a radioactive sample. • The ionizing radiation creates ions, which conduct a current that is detected by the instrument.
  44. 44. Kinetics of Radioactive Decay A wooden object from an archeological site is subjected to radiocarbon dating. The activity of the sample that is due to 14C is measured to be 11.6 disintegrations per second. The activity of a carbon sample of equal mass from fresh wood is 15.2 disintegrations per second. The half-life of 14C is 5715 yr. What is the age of the archeological sample?
  45. 45. Kinetics of Radioactive Decay First we need to determine the rate constant, k, for the process. 0.693 = t1/2 k 0.693 = 5715 yr k 0.693 =k 5715 yr 1.21 × 10−4 yr−1 = k
  46. 46. Kinetics of Radioactive Decay Now we can determine t: Nt ln = kt N0 11.6 ln = (1.21 × 10−4 yr−1) t 15.2 ln 0.763 = (1.21 × 10−4 yr−1) t 6310 yr = t
  47. 47. Energy in Nuclear Reactions • There is a tremendous amount of energy stored in nuclei. • Einstein’s famous equation, E = mc2, relates directly to the calculation of this energy.
  48. 48. Energy in Nuclear Reactions • In the types of chemical reactions we have encountered previously, the amount of mass converted to energy has been minimal. • However, these energies are many thousands of times greater in nuclear reactions.
  49. 49. Energy in Nuclear Reactions For example, the mass change for the decay of 1 mol of uranium-238 is −0.0046 g. The change in energy, ΔE, is then ΔE = (Δm) c2 ΔE = (−4.6 × 10−6 kg)(3.00 × 108 m/s)2 ΔE = −4.1 × 1011 J
  50. 50. Nuclear Fission • How does one tap all that energy? • Nuclear fission is the type of reaction carried out in nuclear reactors.
  51. 51. Nuclear Fission • Bombardment of the radioactive nuclide with a neutron starts the process. • Neutrons released in the transmutation strike other nuclei, causing their decay and the production of more neutrons.
  52. 52. Nuclear Fission This process continues in what we call a nuclear chain reaction.
  53. 53. Nuclear Fission If there are not enough radioactive nuclides in the path of the ejected neutrons, the chain reaction will die out.
  54. 54. Nuclear Fission Therefore, there must be a certain minimum amount of fissionable material present for the chain reaction to be sustained: Critical Mass.
  55. 55. Fission reactor
  56. 56. Nuclear Reactors In nuclear reactors the heat generated by the reaction is used to produce steam that turns a turbine connected to a generator.
  57. 57. Nuclear Reactors • The reaction is kept in check by the use of control rods. • These block the paths of some neutrons, keeping the system from reaching a dangerous supercritical mass.
  58. 58. Nuclear Fusion • Fusion would be a superior method of generating power.  The good news is that the products of the reaction are not radioactive.  The bad news is that in order to achieve fusion, the material must be in the plasma state at several million kelvins.
  59. 59. Nuclear Fusion • Tokamak apparati like the one shown at the right show promise for carrying out these reactions. • They use magnetic fields to heat the material.