Nuclear chemistry2218


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Nuclear chemistry2218

  1. 1. Nuclear Reactions Chapter 20 (loosely)of Brady & Senese 5th Edition Dr. C. Yau Fall 2009 1
  2. 2. What do you need to know?You are responsible only for the topics onnuclear reactions I present here.Review: The nuclear symbol Mass number 14 = # protons + # neutrons (due to 6 p and 8 n) 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.) 2
  3. 3. Isotopes Give the number of p and n for each of the following: 1 2 3 1H 1 H 1H 1p 1p 1p 0n 1n 2n 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) 3
  4. 4. Radioactivity is the spontaneous disintegration ofa nucleus accompanied by an emission of rays.Radioisotopes are the isotopes that areradioactive. Note that not all isotopes areradioactive. (e.g. Tritium is, but not protium ordeuterium.)Alpha Decay (α-decay) is the disintegration of anucleus accompanied by an emission of αparticles.Beta Decay (β-decay) is the disintegration of anucleus accompanied by an emission of βparticles. 4
  5. 5. Gamma Decay (γ-decay) is emission of γ-rays.γ-rays are high energy rays (no particlesinvolved). γ is not a particle but is given the symbol 0Why alpha, beta, gamma? 0 γThat’s Greek for A, B, C.It turns out that the α particle is the nucleus ofthe He atom: 4α particle = 2 Heβ particle is the electron = 0 Explain the -1 e numbers! 1 0nneutron has the symbol =proton has the symbol = 1 p or 1 H 1 1 0positron is a "positive electron" = ? +1 e 5
  6. 6. β-decay involves an emission of an electron fromthe 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-. Theproton stays in the nucleus and the e- is emitted.Write the equation for what is happening to the n. 1 0 n → 1p + 1 0 -1 e Check to see its balanced.What is the significance of a proton as a product?The element is changing into a different element! 6
  7. 7. Example 1Uranium-238 undergoes α-decay. Write the nuclearequation for its decay."Uranium-238" means the isotope of uranium with a massnumber of 238. We do not need to specify the atomicnumber as we can easily find it on the periodic table. Note that U changed into another element! This is not possible in chemical reactions!Example 2The product of U-238 decay undergoes β-decay. Write thenuclear equation for it. 7
  8. 8. 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 8
  9. 9. Example 3If we begin with polonium-210 and ended with lead-206,what nuclear particle is emitted during the decay?Example 4The U-238 decay series involve 14 steps. Somewherealong the series there is a step that involves emission ofalpha rays and it produces radioactive radon-222. Writethe 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? 9
  10. 10. Why do some nuclei decay and notothers?Why are some α-emitters and someβ-emitters?Red line shows ratio of #n / #p = 1(#n = #p)As #p increases, positive charge innucleus is getting too large, and #nincreases to keep them apart.Shaded area shows the Band ofStability.Elements above the band are β- #nemitters which decr #n and incr #p.Elements below the band are α-emitters which decr # p. 10 # protons Fig. 20.8 p.831
  11. 11. Review of Half-life and Decay Rate• Radioactive decay is 1st order.• For 1st order, t½ = ln 2 k• 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. 11
  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. 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? 13
  14. 14. There is another isotope of uranium of interest: U-235.It is also radioactive but more important it is fissionable (orfissile).Nuclear fission is splitting the nucleus into two largeparticles by bombardment of a high energy particle (suchas a neutron). 235 92 U + 0n → 1 94 36 1 Kr + ? + 3 0 n ΔE = - 1010 kJNuclear fusion is fusing together two small nuclei into alarger one. 2 1 H + 3 H → 2 He + ? 1 4 ΔE = - 108 kJBoth are accompanied by an enormous amount of E! 14
  15. 15. Nuclear Fission 235 92 U+ n → 1 0 90 38 Sr + 143 54 1 Xe + 3 n 0U-235 does not always split into the same products.However, the products are always radioactivity and giventhe term “radioactive daughters”.These radioactive products are in the fallout from anuclear bomb.Sr-90 has half-life of 28.1 yr. Rule of thumb: It takes 10half-lives for a radioisotope to be considered "gone."Note also the formation of 3 n. Why is this significant? 15
  16. 16. Nuclear ChainReactionFig. 20.14 p.843 16
  17. 17. Nuclear FissionNuclear fission is what we use in the nuclear reactor at the powerplants and in atomic bombs.Not all isotopes are “fissile” (not fissionable). U-238 is radioactivebut not fissile.In nature, only 0.7% of naturally occurring uranium is U-235. Therest 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 wemake use of the different rate of effusion and applying Graham’sLaw: Heavier atoms effuse slower. Allowing the two isotopes toeffuse (a long process as the difference in mass is only 3 amu)eventually we can separate U-235 from U-238. This process is calledENRICHMENT. 17
  18. 18. 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. 18
  19. 19. Nuclear Fusion 2 1 H + 3H → 1 4 2 He + ?Another example isComplete the equation: 4 H → 2 e +? 1 1 0 +1• 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. 19
  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? 20
  21. 21. Where does all the nuclear energy come from?Nuclear Binding EnergyIf we want to make a helium atom, what do weneed?2 protons and 2 neutrons2 x 1.0072764669 amu = 2.0145529338 amu2 x 1.0086649156 amu = 2.0173298312 amuTotal = 4.0318827650 amuHe nucleus is actually 4.0015061792 amuWhere is the missing mass? "mass defect" 21
  22. 22. Mass Defect = Binding EnergyThe missing mass is the mass that is converted into energy: E = m c2This 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! 22
  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 an23 electron.