Nuclear Chemistry


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Nuclear Chemistry

  1. 1. 11 Nuclear Reactions Chapter 20 (loosely) of Brady & Senese 5th Edition Dr. C. Yau Fall 2009
  2. 2. 22 What do you need to know? You are responsible only for the topics on nuclear reactions I present here. Review: The nuclear symbol 14 6 C Atomic number (also, nuclear charge) = # protons (C has charge of +6 in nucleus, due to 6 protons) In nuclear chemistry, this number does not always give us # protons, but it does give us the nuclear charge. (Not all nuclear particles have protons.) Mass number = # protons + # neutrons (due to 6 p and 8 n)
  3. 3. 33 Isotopes Give the number of p and n for each of the following: 1 2 3 1 1 1H H H 1p 1 p 1 p 0 n 1 n 2 n protium deuterium tritium* 99.985% 0.015% zero% These are isotopes of hydrogen. What exactly are isotopes? -Atoms of the same element with different masses. -Atoms with the same #p but different # n *Tritium is the only one that is radioactive. D2O = heavy water (MW=20 amu instead of 18 amu)
  4. 4. 44 Radioactivity is the spontaneous disintegration of a nucleus accompanied by an emission of rays. Radioisotopes are the isotopes that are radioactive. Note that not all isotopes are radioactive. (e.g. Tritium is, but not protium or deuterium.) Alpha Decay (α-decay) is the disintegration of a nucleus accompanied by an emission of α particles. Beta Decay (β-decay) is the disintegration of a nucleus accompanied by an emission of β particles.
  5. 5. 55 Gamma Decay (γ-decay) is emission of γ-rays. γ-rays are high energy rays (no particles involved). Why alpha, beta, gamma? That’s Greek for A, B, C. It turns out that the α particle is the nucleus of the He atom: α particle = β particle is the electron = neutron has the symbol = proton has the symbol = positron is a "positive electron" = ? 4 2 He 0 -1e 1 0 n Explain the numbers! 1 1p 0 +1e γ is not a particle but is given the symbol 0 0 γ 1 1or H
  6. 6. 66 β-decay involves an emission of an electron from the nucleus. Doesn’t that strike you as being a bit odd??? Which nuclear particles are in the nucleus? How can the nucleus eject an ELECTRON? Actually a neutron breaks up into p+ & e- . The proton stays in the nucleus and the e- is emitted. Write the equation for what is happening to the n. 1 1 0 0 1 -1n p + e→ Check to see it's balanced. What is the significance of a proton as a product? The element is changing into a different element!
  7. 7. 77 Example 1 Uranium-238 undergoes α-decay. Write the nuclear equation for its decay. "Uranium-238" means the isotope of uranium with a mass number of 238. We do not need to specify the atomic number as we can easily find it on the periodic table. Example 2 The product of U-238 decay undergoes β-decay. Write the nuclear equation for it. Note that U changed into another element! This is not possible in chemical reactions!
  8. 8. 88 U-238 decay series involve the disintegration of U- 238 and its subsequent products until we end up with a nonradioactive product (Pb-206). Fig. 20.7 p. 828
  9. 9. 99 Example 3 If we begin with polonium-210 and ended with lead-206, what nuclear particle is emitted during the decay? Example 4 The U-238 decay series involve 14 steps. Somewhere along the series there is a step that involves emission of alpha rays and it produces radioactive radon-222. Write the nuclear equation. What do you know about radon? Where have you heard about it? U-238 decay series produces many other radioactive products. Why do we single out Rn to talk about?
  10. 10. 1010 Why do some nuclei decay and not others? Why are some α-emitters and some β-emitters? Red line shows ratio of #n / #p = 1 (#n = #p) As #p increases, positive charge in nucleus is getting too large, and #n increases to keep them apart. Shaded area shows the Band of Stability. Elements above the band are β- emitters which decr #n and incr #p. Elements below the band are α- emitters which decr # p. # n # protons Fig. 20.8 p.831
  11. 11. 11 Review of Half-life and Decay Rate • Radioactive decay is 1st order. • For 1st order, t½ = • Note that the half-life is not affected by the initial concentration of the reactant. • What does half-life (t½ ) mean? • It is the amount of time it takes for the sample to decrease by ½ (by mass or by number of particles). • Example 5: U-238 has a half-life of 4.5x109 yr. Will it be all gone after two half-lives? Before you do any calculations, first check to see whether it can be done easily. ln 2 k
  12. 12. Review of Half-life and Decay Rate (cont’d.) • Example 6: P-32 has a half-life of 14 days. Starting with 10.0 g of P-32, how much is left after 42 days? • Example 7: P-32 has a half-life of 14 days. Starting with 10.0 g of P-32, how much is left after 45 days? 12
  13. 13. 13 You can determine the half-life graphically by plotting concentration vs. time. What is the half-life of I-131? What is its decay constant? What is the unit of the constant?
  14. 14. 1414 There is another isotope of uranium of interest: U-235. It is also radioactive but more important it is fissionable (or fissile). Nuclear fission is splitting the nucleus into two large particles by bombardment of a high energy particle (such as a neutron). Nuclear fusion is fusing together two small nuclei into a larger one. Both are accompanied by an enormous amount of E! 235 1 94 1 92 0 36 0U + n Kr + ? + 3 n→ 2 3 4 1 1 2H + H He + ?→ ΔE = - 1010 kJ ΔE = - 108 kJ
  15. 15. 1515 Nuclear Fission U-235 does not always split into the same products. However, the products are always radioactivity and given the term “radioactive daughters”. These radioactive products are in the fallout from a nuclear bomb. Sr-90 has half-life of 28.1 yr. Rule of thumb: It takes 10 half-lives for a radioisotope to be considered "gone." Note also the formation of 3 n. Why is this significant? 235 1 90 143 1 92 0 38 54 0U + n Sr + Xe + 3 n→
  16. 16. 16 Fig. 20.14 p.843 Nuclear Chain Reaction
  17. 17. 1717 Nuclear Fission Nuclear fission is what we use in the nuclear reactor at the power plants and in atomic bombs. Not all isotopes are “fissile” (not fissionable). U-238 is radioactive but not fissile. In nature, only 0.7% of naturally occurring uranium is U-235. The rest is U-238 which is not fissile. Nuclear power plants only require about 3% U-235. Atomic bombs require about 97.3% U-235. To take 0.7% U-235 and convert to weapon-grade 97.3% U-235 we make use of the different rate of effusion and applying Graham’s Law: Heavier atoms effuse slower. Allowing the two isotopes to effuse (a long process as the difference in mass is only 3 amu) eventually we can separate U-235 from U-238. This process is called ENRICHMENT.
  18. 18. 1818 What is the deal with plutonium? Bombardment of U-238 with neutrons converts it to Np (neptunium) which quickly decays to Pu-239. Pu-238 is fissile. Pu-239 can be used for making bombs. Reactor grade Pu can be easily converted into a nuclar bomb. Half-life of Pu is 25,000 yrs. If there is a spill, how long will it take for Pu to be totally gone? This is why there is much concern with where and who has Pu. There is another reason for concern over the location of Pu…It is HIGHLY toxic. • It is an alpha emitter. • It is estimated that 1 microgram in the lungs of a humna is enough to induce lung cancer.
  19. 19. 1919 Nuclear Fusion Another example is Complete the equation: • Nuclear fission is what takes place in the sun and the stars. … and used in the H-bomb. • Advantage over nuclear fission? H isotopes are much easier to obtain than uranium, and much more abundant (from water). • Products have much shorter half-lives. H-3 has t ½ = 12. 3 yrs compared to U-235 has t ½ = 7.04x108 yrs • Nuclear wastes of nuclear fusion also have much shorter half-lives. • Disadvantage is the difficulty in harnessing it for use: requires millions of degrees to initiate. 2 3 4 1 1 2H + H He + ?→ 1 0 1 +14 H 2 e + ?→
  20. 20. 20 What is meant by “cold fusion?” • “Cold fusion” is to initiate fusion without the millions of degrees of heat (only possible in the sun). • Some years ago, two scientists made a dramatic announcement to say they have accomplished “cold fusion.” • What happened?
  21. 21. 2121 Where does all the nuclear energy come from? Nuclear Binding Energy If we want to make a helium atom, what do we need? 2 protons and 2 neutrons 2 x 1.0072764669 amu = 2.0145529338 amu 2 x 1.0086649156 amu = 2.0173298312 amu Total = 4.0318827650 amu He nucleus is actually 4.0015061792 amu Where is the missing mass? "mass defect"
  22. 22. 22 Mass Defect = Binding Energy The missing mass is the mass that is converted into energy: E = m c2 This is the energy released when nuclear particles combine, break and recombine. c = speed of light = 3 x 108 m/s (very large #) c2 = 3 x 1016 m2 /s2 (even larger #!) Significance: a small mass is converted to a very large amount of energy!
  23. 23. 23 Anything good about nuclear reactions? • Nuclear energy does not produce “greenhouse” gas (CO2) or acid rain (SO2). • We do not rely on the Middle East to supply us with uranium. • Nuclear medicine: PET scan: Positron Emission Tomography (Nuclear medicine imaging to produce a 3-D image of functional processes in the body) It uses nucleotides of short half-lives, such as C-11 (20 mins); N-13 (10 mins); O-15 (2 mins). Why is a short half-life desirable? Write the nuclear eqn for C-11 emitting a positron. Write the nuclear eqn for the positron meeting with an electron.