nuch_chem2008 - Nuclear Chemistry Roy A. Lacey SUNY Stony...

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Unformatted text preview: Nuclear Chemistry Roy A. Lacey SUNY Stony Brook 1 Why Study Nuclear Processes Nuclear Reactions in the sun provide the energy required to sustain life on Earth 2 Why Study Nuclear Processes Nuclear Reactions play a crucial role in medicine Brain Lungs 3 Why Study Nuclear Processes Nuclear Reactions play a crucial role in national security 4 Why Study Nuclear Processes Nuclear waste issues play an important role in debates on the environment. 5 Biological Effects of Radiation Radon Rn is thought to cause lung cancer Radon exposure accounts for more than half of the average annual exposure to ionizing radiation. Rn is a noble gas typically inhaled The -particles produced The half-life of is 3.82 days. have a high RBE It decays as follows: 222 Rn 218 Po + 4 He 86 84 2 21482Pb + 42He The 218Po gets trapped in the lungs where it continually produces -particles. The EPA recommends 222Rn levels in homes to be kept below 4 pCi per liter of air. 218 Po 84 6 Why Study Nuclear Processes Nuclear reactions studies provide fundamental understanding of our universe 7 Why Study Nuclear Processes Nuclear reactions studies provide fundamental understanding of our universe 8 Why Study Nuclear Processes Women have played an important role in nuclear science Two Nobel Prizes 9 The structure of matter 10 The Structure of matter 11 Atomic Structure Nucleons: p+: proton n0: neutron. Mass number: the number of p+ + n0. Atomic number: the number of p+. Isotopes: have the same number of p+ and different numbers of n0. 12 Nuclear Reactions Induced Reactions Spontaneous Reactions (Radioactivity) 13 Radioactivity Radioactivity: There are four common types of radiation: Radiation is the loss of 42He from the nucleus, Radiation is the loss of an electron from the nucleus, Radiation is the loss of high-energy photon from the nucleus. n Radiation is the loss of neutron from the nucleus 14 Radioactivity Types of radioactive Decay 15 Radioactivity 16 Radioactivity Types of radioactive Decay Nucleons can undergo decay: 1 n 1 p+ + 0 e- (-emission) 0 1 -1 0 e- + 0 e+ 20 (positron annihilation) -1 1 0 1 p+ 1 n + 0 e+ (positron or +-emission) 0 0 1 1 p+ + 0 e- 1 n (electron capture) 1 -1 0 A positron is a particle with the same mass as an electron but a positive charge. In nuclear reactions, the total number of nucleons is conserved: 238 U 234 Th + 4 He 92 90 2 17 Quiz 18 Quiz 19 Quiz 20 Patterns of Nuclear Stability Neutron-to-Proton Ratio In the nucleus the protons are very close to each other. proton-proton repulsion is large. The strong nuclear force binds the nucleus. Neutrons are involved with the strong nuclear force. As more protons are added to a nucleus, the proton-proton repulsion gets larger. 21 Patterns of Nuclear Stability The heavier the nucleus, the more neutrons are required for stability. The belt of stability deviates from a 1:1 neutron to proton ratio for high atomic mass. At Bi (83 protons) the belt of stability ends and all nuclei are unstable. Nuclei above the belt of stability undergo -emission. An electron is lost and the number of neutrons decreases, the number of protons increases. 22 Patterns of Nuclear Stability Neutron-to-Proton Ratio Nuclei below the belt of stability undergo +-emission or electron capture. Increase # neutrons Decrease # protons Nuclei with Z > 83 usually undergo emission. 4 2 23 Patterns of Nuclear Stability Radioactive Series A radioactive nucleus usually undergoes several transitions on its path to stability. The series of nuclear reactions that accompany this path is the radioactive series. Nuclei resulting from radioactive decay are called daughter nuclei. 24 Patterns of Nuclear Stability Radioactive Series For 238U, the first decay is to 234Th (-decay). The 234Th undergoes -emission to 234Pa and 234U. 234U undergoes decay (several times) to 230Th, 226Ra, 222Rn, 218Po, and 214Pb. 214Pb undergoes -emission (twice) via 214Bi to 214Po which undergoes -decay to 210Pb. The 210Pb undergoes emission to 210Bi and 210Po which decays () to the stable 206Pb. 25 Patterns of Nuclear Stability Further Observations Nuclei with even numbers of protons and neutrons are more stable than nuclei with any odd numbers of nucleons. Nuclei with 2, 8, 20, 28, 50, or 82 protons or 2, 8, 20, 28, 50, 82, or 126 neutrons are found to be quite stable These "magic numbers" correspond to filled, closedshell nucleon configurations. 26 Patterns of Nuclear Stability Nuclear Shell Model The shell model of the nucleus is similar to the shell model for the atom. Pairs of protons and neutrons in the nucleus is analogous to pairs of electrons in the atom. The shell model of the nucleus rationalizes many experimental observations. 27 examples 28 Origin of the elements The universe was created about 15 billion years ago in a single violent event known as the Big Bang. All the space, time, energy, and matter that constitute today's universe originated in the Big Bang. The early universe was extremely small, dense, and hot. For the first fraction of a second, only energy existed. Expansion and Subsequent formation Light elements 29 Origin of the elements The young universe did not have a perfectly even distribution of energy and particles. These irregularities allowed forces to start to collect and concentrate matter. Concentrations of matter formed into clouds, then condensed into stars and the collections of stars we call galaxies. (Diameter = 300 light years 1ly = 10 trillion Km) 30 Origin of the elements Concentrations of matter formed into clouds, then condensed into stars and the collections of stars we call galaxies. Nuclear Reactions !! 31 Origin of the elements Formation of Heavy Elements ! We are all remnants of Supernovae Explosions 32 Induced Nuclear Reactions Nuclear transmutations are initiated via collisions between nuclei. 14N + 4 17O + 1p. 14N(,p)17O. To overcome electrostatic forces, charged particles need to be accelerated. 33 Nuclear Transmutations Using Particle Accelerators Linear Accelerators Cyclotrons Synchrotrons Colliders 34 Rates of Radioactive Decay Each isotope has a characteristic half-life. 90Sr has a half-life of 28.8 yr. If 10 g of sample is present at t = 0, then 5.0 g is present after 28.8 years, 2.5 g after 57.6 years, etc. 90Sr decays as follows 90 Sr 90 Y + 0 e 38 39 -1 Half-lives are not affected by temperature, pressure or chemical composition. Natural radioisotopes tend to have longer half-lives than synthetic radioisotopes. 35 Rates of Radioactive Decay Half-lives can range from fractions of a second to millions of years. Naturally occurring radioisotopes can be used to determine how old a sample is. This process is termed radioactive dating. 36 Rates of Radioactive Decay 37 Rates of Radioactive Decay Carbon Dating Carbon-14 is commonly used to determine the age of organic compounds. 14 C 14 N + 0 e (-emission) 6 7 -1 The half-life of 14C is 5,730 years. To detect 14C the object must be less than 50,000 years old. It is assumed that the ratio of 12C to 14C has been constant over time. 38 Rates of Radioactive Decay Calculations Based on Half-Life Radioactive decay is a first order process: Rate = kN In radioactive decay the constant, k, is called the decay constant. The rate of decay is called activity (disintegrations per unit time). If N0 is the initial number of nuclei and Nt is the number of nuclei at time t, then ln Nt = - kt N0 39 Rates of Radioactive Decay Half-Life ln Nt = - kt N0 Half-life is the time taken for Nt = N0, we obtain k= 0.693 t1 2 40 Rates of Radioactive Decay 41 Detecting Radiation Matter is ionized by radiation. The ionized gas results in a current. The current pulse generated when the radiation enters is amplified and counted. A Geiger counter determines the amount of ionization by detecting an electric current. 42 Detection of Radioactivity Radiotracers Radiotracers are used to follow an element through a chemical reaction. Photosynthesis has been studied using 14C: 614CO2 + 6H2O sunlight chlorophyll 14 C6H12O6 + 6O2 The carbon dioxide is said to be 14C labeled. 43 TA POSITIONS AVAILABLE FOR FALL 2007 CHE 129, 131, 132 Help Rooms, CHE 130 Instructors QUALIFICATIONS: A/A- in CHE 131/132, CHE 141/142, or equivalent Cumulative & previous semester GPA 3.00 Willingness to help students succeed COMMITMENT: Spend 4-hours each week working with students Attend a one-hour weekly staff meeting Write a brief paper on your experience REWARDS: 3 credits for CHE 475 or 476, Undergraduate Teaching Practicum Review for GRE and MCAT, recommendations TO SIGN UP: Complete a UG Teaching Practicum Permission Request Form available in the Chemistry Main Office Place completed form in Dr. Wolfskill's mailbox by Tue, May 1 For more information, contact Dr. Wolfskill, Chemistry, Room 575 Troy.Wolfskill@stonybrook.edu 44 Quiz Nuclides that have neutron-to-proton ratios that are too HIGH (compared with stable nuclides) are expected to undergo: a) alpha decay. b) beta decay. c) positron decay. d) electron capture. e) none of the above. 45 Quiz The half-life of tritium is 12.3 years. If 48.0 mg of tritium is released from a nuclear power plant during the course of an accident, calculate the mass (in mg) of the nuclide that will remain after 5.0 years. ln a) b) c) d) e) 36.2 mg 40 mg 12 mg 30 mg 50 mg Nt = - kt N0 k= 0.693 t1 2 N t = N 0 e - kt = 36.2 mg 46 Energy Changes in Nuclear Reactions Large amounts of energy are produced in nuclear reactions E = mc2 c = 8.99 1016 m2/s2 If a system loses mass it loses energy (exothermic). If a system gains mass it gains energy (endothermic). Small changes in mass cause large changes in energy. energy Mass and energy changed in nuclear reactions are much greater than chemical reactions. 47 Energy Changes in Nuclear Reactions 238 U 92 23490Th + 42He for 1 mol of the masses are 238.0003 g 233.9942 g + 4.015 g. The change in mass during reaction is 233.9942 g + 4.015 g - 238.0003 g = -0.0046 g. The process is exothermic because the system has lost mass. To calculate the energy change per mole of 23892U: E = (mc )2 = c 2 m = 3.00 108 m/s ( 1 kg )2 (- 0.0046 g ) 1000 g = 2 11 kg - m = -4.1 1011 J = -4.1 10 s2 48 Energy Changes in Nuclear Reactions The mass of a nucleus is less than the mass of its nucleons Binding Mass defect: difference in mass between the nucleus and the masses of nucleons. Binding energy is related to the mass defect; E = mc2 49 Energy Changes in Nuclear Reactions Nuclear Binding Energies The smaller the binding energy the more likely a nucleus will decompose. Average binding energy per nucleon increases to a maximum at mass number 50 - 60, and decreases afterwards. Fusion (bringing together nuclei) is exothermic for low mass numbers and fission (splitting of nuclei) is exothermic for high mass numbers. 50 Nuclear Fission Splitting of heavy nuclei is exothermic for large mass numbers. 235U nucleus + n: 51 Nuclear Fission Fission requires slow moving neutrons. For every 235U fission 2.4 neutrons are produced. Each neutron produced can cause the fission of another 235U nucleus. The heavy 235U nucleus can split into many different daughter nuclei, e.g. 1 n + 238 U 142 Ba + 91 Kr + 31 n 0 92 56 36 0 -11 J per 235U nucleus. releases 3.5 10 52 Nuclear Fission Chain Reactions 53 examples 54 Nuclear Fission When enough material is present for a chain reaction, we have critical mass. Below critical mass (subcritical mass) the neutrons escape and no chain reaction occurs. At critical mass, the chain reaction accelerates. Anything over critical mass is called supercritical mass. Critical mass for 235U is about 1 kg. 55 Nuclear Fission Two subcritical wedges of 235U are separated by a gun barrel. Conventional explosives are used to bring the two subcritical masses together to form one supercritical mass. The supercritical mass leads to uncontrolled nuclear fission and a violent explosion. 56 Nuclear Bomb Nuclear Fission Nuclear Reactors 57 Nuclear Fission Nuclear Reactors Use fission as a power source. Use a subcritical mass of 235U (enrich 238U with about 3% 235U). Enriched 235UO2 pellets are encased in Zr or stainless steel rods. Control rods are composed of Cd or B, which absorb neutrons. Moderators are inserted to slow down the neutrons. Heat produced in the reactor core is removed by a cooling fluid to a steam generator and the steam drives an electric generator. 58 Nuclear Fusion Light nuclei can fuse to form heavier nuclei. Most reactions in the Sun are fusion. Fusion products are not usually radioactive, so fusion is a good energy source. The hydrogen fuel required for reaction can easily be supplied by seawater. High energies are required to overcome repulsion between nuclei before reaction can occur. High energies are achieved by high temperatures: the reactions are thermonuclear. 59 Nuclear Fusion Fusion of tritium and deuterium requires about 40,000,000K: 2 H + 3 H 4 He + 1 n 1 1 2 0 These temperatures can be achieved in a nuclear bomb or a tokamak. A tokamak is a magnetic bottle: strong magnetic fields contained a high temperature plasma so the plasma does not come into contact with the walls. (No known material can survive the temperatures for fusion.) To date, about 3,000,000 K has been achieved in a tokamak. 60 Biological Effects of Radiation The penetrating power of radiation is a function of mass. Therefore, -radiation (zero mass) penetrates much further than -radiation, which penetrates much further than -radiation. Radiation absorbed by tissue causes excitation (nonionizing radiation) or ionization (ionizing radiation). Ionizing radiation is much more harmful than nonionizing radiation. 61 Biological Effects of Radiation 62 Biological Effects of Radiation Most ionizing radiation interacts with water in tissues to form H2O+. The H2O+ ions react with water to produce H3O+and OH. OH has one unpaired electron. It is called the hydroxy radical. Free radicals generally undergo chain reactions. 63 examples 64 Biological Effects of Radiation Radiation Doses The SI unit for radiation is the becquerel (Bq). 1 Bq is one disintegration per second. The curie (Ci) is 3.7 1010 disintegrations per second. (Rate of decay of 1 g of Ra.) Absorbed radiation is measured in the gray (1 Gy is the absorption of 1 J of energy per kg of tissue) or the radiation absorbed dose (1 rad is the absorption of 10-2 J of radiation per kg of tissue). 65 Biological Effects of Radiation Radiation Doses Since not all forms of radiation have the same effect, we correct for the differences using RBE (relative biological effectiveness, about 1 for - and -radiation and 10 for radiation). rem (roentgen equivalent for man) = rads.RBE SI unit for effective dosage is the Sievert (1Sv = RBE.1Gy = 100 rem). 66 Biological Effects of Radiation Radiation Doses 67 Biological Effects of Radiation Radon The nucleus 22286Rn is a product of 23892U. Radon exposure accounts for more than half the 360 mrem annual exposure to ionizing radiation. Rn is a noble gas so is extremely stable. Therefore, it is inhaled and exhaled without any chemical reactions occurring. The half-life of is 3.82 days. It decays as follows: 222 Rn 218 Po + 4 He 86 84 2 68 Biological Effects of Radiation Radon The -particles produced have a high RBE. Therefore, inhaled Rn is thought to cause lung cancer. The picture is complicated by realizing that 218Po has a short half-life (3.11 min) also: 218 Po 214 Pb + 4 He 84 82 2 The 218Po gets trapped in the lungs where it continually produces -particles. The EPA recommends 222Rn levels in homes to be kept below 4 pCi per liter of air. 69 ...
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This note was uploaded on 06/29/2008 for the course CHE 132 taught by Professor Hanson during the Spring '08 term at SUNY Stony Brook.

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