Chapter 21 Lecture- Nuclear Chemistry

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Chapter 21 lecture for AP Chemistry on Nuclear Chemistry


<ul><li>1.Chapter 21 Nuclear Chemistry </li></ul> <p>2. Nuclear ChemistryThe nucleus of an atomcontains protons (+1charge)andneutrons (no charge) 3. The nucleus is held togetherby 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. Protons and neutrons experiencethe strong nuclear force if closeenough Because protons repel each other the nucleus needs a certain proton to neutron ratio for stability 5. An unstable atom decays byemitting radiationThese unstable atoms are radioactive. Carbon-12 is stable. Carbon-14 isradioactive. Carbon-14 is a radioisotope. There are many naturally occurringradioisotopes and some that are human-made. 6. Radioisotopes decay intostable isotopes of a DIFFERENT element In nuclear reactions, the makeup of thenucleus changes. Sometimes the number of protons willchange. If the number of protons changes, theelement has changed. 7. The three most common forms of radioactive decay are alpha(), beta() and gamma() 8. 25.1 Alpha particles are the least penetrating.Gamma rays are the most penetrating. 9. To balance nuclear equations you must know the symbol forthe 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. The stuff that radiates An alpha particle is a helium nucleus ithas a +2 charge and a mass of 4amu. 11. A beta particle is an electron which is formed when aneutron becomes a proton 12. A gamma ray is a high energyelectromagnetic wave. A gamma ray has no mass or charge so it is not in a nuclear equation. 13. particle Alpha Beta Gamma Mass 4amu00 Charge +2-1 0 Effect RadioisotopeRadioisotope Radioisotopeloses two converts a neutron loses energyprotons and two to a proton &amp;neutronsejects an electron The elementThe ELEMENT The ELEMENTdoes notchanges changeschange What it isHelium electron High energy nucleus electromagnetic radiation Stop itPaper/skin1cm/metal foil Lead/concrete damage High ionization Medium Lowestionization ionization 14. Damage from nuclear radiation is due to ionization of living tissue. Nuclear radiation is called ionizingradiation because it produces ions fromneutral 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. Nuclear FissionFission separates heavy elements into two lighter elements. Uranium-235 Barium-141 + Krypton-92 +3neutronsA huge amount of energy is produced. Used by humans as an energy source The fission bomb was used in WWII 16. Nuclear fusionFusion combines two light nuclei into oneheavier element.Produces even more energy than fission. Occurs in the sun. Requires extremely high temperatures and pressures. 17. FUSION = to put together FISSION = to break apart 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. 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. 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. Types of Radioactive Decay 22. Alpha Decay:Loss of an -particle (a helium nucleus) 4 2 He238 234 4 92 U90 U + 2 He 23. Beta Decay:Loss of a -particle (a high energy electron) 0 0 1 or 1 e131 1310 53 I 54 Xe+ 1 e 24. Positron Emission:Loss of a positron (a particle that has the same mass as but opposite charge than an electron)01 e 11 11 06 C 5 B + 1 e 25. Gamma Emission:Loss of a -ray (high-energy radiation that almost always accompanies the loss of a nuclear particle)00 26. Electron Capture (K-Capture)Addition of an electron to a proton in the nucleus As a result, a proton is transformed into aneutron.10 11 p + 1 e 0 n 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. Neutron-Proton Ratios Neutrons play a key role stabilizing the nucleus. Therefore, the ratio of neutrons to protons is an important factor. 29. Neutron-Proton RatiosFor smaller nuclei (Z 20) stable nuclei have a neutron-to-proton ratio close to 1:1. 30. Neutron-Proton RatiosAs nuclei get larger, it takes a greater number of neutrons to stabilize the nucleus. 31. Stable NucleiThe shaded region in the figure shows what nuclides would be stable, the so- called belt of stability. 32. Stable Nuclei Nuclei above this belt have too many neutrons. They tend to decay by emitting beta particles. 33. Stable Nuclei Nuclei below the belt have too many protons. They tend to become more stable by positron emission or electron capture. 34. Stable Nuclei There are no stable nuclei with an atomic number greater than 83. These nuclei tend to decay by alpha emission. 35. Radioactive Series Large radioactivenuclei cannot stabilizeby undergoing onlyone nucleartransformation. They undergo a seriesof decays until theyform a stable nuclide(often a nuclide oflead). 36. Some TrendsNuclei 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. Some TrendsNuclei with an even number of protons and neutrons tend to be more stable than nuclides that have odd numbers of these nucleons. 38. Nuclear TransformationsNuclear transformations can be induced by accelerating a particle and colliding it with the nuclide. 39. Particle Accelerators These particle accelerators are enormous, having circular tracks with radii that are miles long. 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= ktN0 ln[A]t ln[A]0 = -kt for 1st order kinetics 41. Kinetics of Radioactive Decay The half-life of such a process is: Not given, but can0.693be derived from= t1/2 previous equationk 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. First order processes (including nuclear decay) always show a constant half-life 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. Kinetics of Radioactive Decay A wooden object from an archeological siteis subjected to radiocarbon dating. Theactivity of the sample that is due to 14C ismeasured to be 11.6 disintegrations persecond. The activity of a carbon sample ofequal mass from fresh wood is 15.2disintegrations per second. The half-life of14C is 5715 yr. What is the age of the archeological sample? 45. Kinetics of Radioactive Decay First we need to determine the rate constant, k, for the process.0.693= t1/2k0.693= 5715 yrk 0.693=k5715 yr1.21 104 yr1 = k 46. Kinetics of Radioactive Decay Now we can determine t:Nt ln= ktN0 11.6ln= (1.21 104 yr1) t 15.2 ln 0.763 = (1.21 104 yr1) t 6310 yr = t 47. Energy in Nuclear Reactions There is a tremendous amount of energy stored in nuclei. Einsteins famous equation, E = mc2, relates directly to the calculation of this energy. 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. Energy in Nuclear ReactionsFor example, the mass change for the decay of 1 mol of uranium-238 is 0.0046 g. The change in energy, E, is then E = (m) c2 E = (4.6 106 kg)(3.00 108 m/s)2 E = 4.1 1011 J 50. Nuclear Fission How does one tap all that energy? Nuclear fission is the type of reaction carried out in nuclear reactors. 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. Nuclear Fission This process continues in what we call a nuclear chain reaction. 53. Nuclear Fission If there are not enough radioactive nuclides in the path of the ejected neutrons, the chain reaction will die out. 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. Fission reactor 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. Nuclear Reactors The reaction is kept incheck by the use ofcontrol rods. These block the paths ofsome neutrons, keepingthe system from reachinga dangerous supercriticalmass. 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. 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. </p>