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Transcript
Chapter 23: Nuclear Chemistry • Nuclear Changes • Nuclear Equations • 238 U 92 23490Th + 42He Chapter 23: Nuclear Reactions & the Nucleus • Nucleons: particles in the nucleus: – p+: proton – n0: neutron. – Mass number: the number of p+ + n0. • Isotopes: have the same number of p+ and different numbers of n0. • Nuclear Binding Energy – Mass Defect Chapter 23: Energy Versus Mass • Energy and Mass are interchangable. • Law of Conservation of Mass and Law of Conservation of Energy are the same. – Sum of all energy and all mass is a constant. • Nuclear Binding Energy – Mass Defect Balancing Nuclear Equations • In nuclear equations, the total number of nucleons is conserved: 238 U ? + 4 He 92 2 Chapter 23: Radioactivity • Isotopes: have the same number of p+ and different numbers of n0. • Stable • Radioisotopes – Natural – Manmade Radiosotopes: Radioactivity • Definition • The three primary types of radioactivity are: – -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. Radioactivity Nuclear Equations Radioactivity: Types of radioactive Decay • Alpha Particles/Alpha Emission: • 4 He 2 and 42 represent -radiation. – 23892U 23490Th + 42He Radioactivity: Types of radioactive Decay • Beta Particles/Beta Emission: • Nucleons undergoes decay: 1 n 1 p+ + 0 e- (-emission) 0 1 -1 0 e- + ______ C 6 -1 14 Radioactivity: Gamma Emission • Electromagnetic Energy • Spectrum • Can be emitted with Particles Additional Types of Nuclear Decay: Decay Equations Positron Emission 1 p+ 1 n + 0 e+ (positron or +0 0 1 emission) • A positron is a particle with the same mass as an electron but a positive charge. Additional Types of Nuclear Decay: Decay Equations ELECTRON CAPTURE + + 0 e- 1 n (electron capture) p 1 -1 0 1 7 0 e- ______ Be + 4 -1 Radioactive Series • A nucleus usually undergoes more than one transition/decay 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. 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 Chapter 23: Nuclear Reactions: Nuclear Transmutation Target + Particle Products 14N + 4 17O + 1p. • The above reaction is written in shorthand notation: 14N(,p)17O. Nuclear Transmutations Using Charged Particles Chapter 22: Nuclear ReactionsNuclear Fission • Consider a neutron bombarding a 235U nucleus: Nuclear Fission • During fission, the incoming neutron must move slowly because it is absorbed by the 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 releases 3.5 10-11 J per 235U nucleus. • For every 235U fission 2.4 neutrons are produced. • Each neutron produced can cause the fission of another 235U nucleus. Nuclear Fission Chapter 22: Nuclear Reactions-Nuclear Fusion • Light nuclei can fuse to form heavier nuclei. • . 21H + 31H 42He + 10n • Fusion products are not usually radioactive, so fusion is a good energy source. • High energies are achieved by high temperatures: the reactions are thermonuclear. Chapter 23: Rates of Radioactive Decay: Half-Life (t1/2) • Definition • Multiple Half-Lives Chapter 23: Rates of Radioactive Decay: Half-Life (t1/2) • Definition • Multiple Half-Lives Chapter 23: Rates of Radioactive Decay: Half-Life (t1/2) • Twenty Micrograms of a radioisotope exists at time zero. t1/2 = 12 hours. How much of this radioisotope remains after 12 hours, 24 hours, 36 hours, 48 hours? Rates of Radioactive Decay Rates of Radioactive Decay Rates of Radioactive Decay: Half-Life (t1/2) • Each isotope has a characteristic half-life. • Half-lives are not affected by temperature, pressure or chemical composition. • Natural radioisotopes tend to have longer halflives than synthetic radioisotopes. Rates of Radioactive Decay: Half-Life (t1/2) • 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 carbon/radioactive dating. Calculation of Half-Life • Radioactive decay is a first order process: Rate = kN • 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 Nt ln kt N0 Calculation of Half-Life Ln [A]= (-kt) + ln Aº / At Half-Life Calculations • First Order Rate Constant is Related to Half Life k 0.693 t1 2 Rates of Radioactive Decay Carbon Dating • For us to detect 14C the object must be less than 50,000 years old. • The half-life of 14C is 5,730 years. • We assume the ratio of 12C to 14C has been constant over time. • It undergoes decay to 14N via -emission: 14 C 14 N + 0 e 6 7 -1 Problem • A wooden artifact from a Chinese temple has a 14C activity of 24.9 counts per minute (CPM) as compared with an activity of 32.5 CPM for a standard of zero age. From the half-life of 14C decay, 5715 years, determine the age of the artifact. Problem 21.37 th (9 Edition) • t= 3.3232 (t1/2) log Nº / Nt • (t1/2) = 5715 years • Nº = 32.5 cpm • Nt = 24.9 cpm • time/age = 2.20E3 years Example 23.1, Page 969 • Balance the following nuclear equations and identify X • 212Po 208 Pb + X • 137Cs 137Ba + X Detection of Radioactivity • Matter is ionized by radiation. • Geiger counter determines the amount of ionization by detecting an electric current. • A thin window is penetrated by the radiation and causes the ionization of Ar gas. • The ionized gas carried a charge and so current is produced. • The current pulse generated when the radiation enters is amplified and counted. Detection of Radioactivity Chapter 22: Mass-Energy Conversions in Nuclear Reactions • Einstein showed that mass and energy are proportional: E = mc2 • If a system loses mass it loses energy (exothermic). • If a system gains mass it gains energy (endothermic). E= 2 mc • Since c2 is a large number (8.99 1016 m2/s2) small changes in mass cause large changes in energy. • Mass and energy changed in nuclear reactions are much greater than in nonnuclear/chemical reactions. Chapter 22: Energy Changes in Nuclear Reactions Nuclear Binding Energies • The mass of a nucleus is less than the mass of their nucleons. • Mass defect is the difference in mass between the nucleus and the masses of nucleons. Calculation of Binding Energy • Binding energy is the energy required to separate a nucleus into its nucleons. • ∆E = ∆mc2 c=2.99792458E8 meters/seconds Calculation of Binding Energy MUST CONNECT MASS CHANGES TO ENERGY KINETIC ENERGY=1/2 (MASS VELOCITY)2 • Joule = kg (m2/second2) Problem • Calculate the binding energy per nucleon for the following nuclei. • (a) 126C (nuclear mass = 11.996708 amu) • Proton = 1.0072765 amu • Neutron = 1.0086649 amu • Amu = 1.66053873E-24 grams Problem • Joule = kg (m2/second2) • Calculated mass = [(6) (1.0072765 amu) + (6) (1.0086649 amu)] = • Mass defect = calculated mass – actual mass = Energy Changes in Nuclear Reactions Nuclear Binding Energies Binding Energy: Units • The energy carried by radiation is usually given in electron volts (eV) • 1 eV is the energy an electron receives when accelerated under the influence of 1 volt • It is related to the joule as: • 1 eV = 1.602E-19 Joules Recognition of Radisotopes: Nuclear Stability 14C, 12C, 13C • Look for neutron/protron ratio • Look for magic numbers • Or, Refer to A Belt of Stability Recognition of Radioistopes: Neutron/Proton Ratio • If N/P ratio = 1, probably stable. • If N/P ratio < 1, maybe radioactive – Positron Emission – Electron Capture If N/P ration > 1, maybe radioactive Probably Beta Emitter Recognition of Radioisotopes: Neutron/Proton Ratio • Elements with Atomic Number of 83 or higher are usually radioactive. – Alpha, and Gamma Emitters Recognition of Radioisotopes: Magic Numbers Isotopes with specific numbers of protons or neutrons are more stable than the rest • The magic numbers are: 2, 8, 20, 28, 50, 82, and 126 • Magic numbers do not cancel the need for a favorable neutron:proton ratio 4He 16O 208Pb • Magic numbers supports the hypothesis that a nucleus has a shell structure with energy levels like those for electrons Recognition of Radioisotopes: Band/Belt of Stability • Band/Belt of Stability will be provided. • Most Reliable method • Isotopes with atomic number greater than 83 tend to be alpha emitters • Their nuclei have too many protons. The most efficient way to lose protons is by the loss of an alpha particle. • Isotopes occurring above and to the left of the band of stability tend to be beta emitters • The neutron:proton ration is apparently too high. A nucleus that undergoes beta decay loses a neutron and gains a proton which reduces the ratio. For example, beta decay by fluorine-20 decreases the neutron:proton ratio from 11/9 to 10/10. The band of stability. A 1:1 ratio of neutrons to protons is indicated by the straight line. The band of stability curves away from this. More protons require more neutrons to provide a compensating nuclear strong force and to dilute the electrostatic protonproton repulsions. • Isotopes lying below and to the right of the band are positron emitters • These nuclei have too few neutrons to be stable. • Positron emission increases the neutron:proton ratio. – For example, a fluorine-17 has a neutron:proton ratio of 8/9. Positron emission converts it into oxygen-17, an isotope in the band of stability with a neutron:proton ratio of 9/8. Recognition of Radioisotopes Band/Belt of Stability • Isotopes lying below and to the right of the band are positron emitters • These nuclei have too few neutrons to be stable. • Positron emission increases the neutron:proton ratio. – For example, a fluorine-17 has a neutron:proton ratio of 8/9. Positron emission converts it into oxygen-17, an isotope in the band of stability with a neutron:proton ratio of 9/8. Biological Effect of Radioactivity • The radiation can generate very reactive species with unpaired electrons called free radicals or simply radicals • target + radioactivity free radicals • Once the radicals are generated, they can start a chain of undesirable chemical events Terms Used to Measure Exposure to Radioactivity • Must measure rate of radioactivity production. • Must Also Measure Energy Deposited by Radioactivity. • Must Also Measure Biological Damage. • The curie (Ci), named after Marie Curie, is an older unit equal to the activity of a 1.0 g sample of radium-226 1 1 Ci 3.7 10 disintegra tions s 3.7 10 Bq 10 10 • SI Equivalent Unit is the Becquerel (Bq). • Bq = one disintegration per second • 1 Ci = 3.7E10 Bq • The energy equivalent to the quantity of radiation absorbed by some material is defined in units of absorbed dose or simply dose • The SI unit of absorbed dose is the gray (Gy) • 1 Gy = 1 J of energy absorbed per kg of material • The Radiation Absorbed Dose or rad is an older unit • 1 rad = 0.01 Gy • A different unit is used for measuring the biological effects of radiation because the effects depend on the type and energy of the radiation • The sievert (Sv) is the SI unit of dose equivalent, H • The dose equivalent (in Sv) is a product of the absorbed dose (D) from some radiation source times a factor (Q) that takes into account the biologically significant properties times any other factor (N) bearing on the net effect • The rem is an older unit for dose equivalent: 1 rem = 0.01 Sv • A typical X-ray involves about 0.007 rem – A whole body dose of 25 rem (0.25 Sv) induces noticeable changes in blood – A set of symptoms called radiation sickness develops at about 100 rem, and becomes severe at about 200 rems • Symptoms include: nausea, vomiting, a drop in white blood count, diarrhea, dehydration, prostration, hemorrhaging, and loss of hair – If everyone in a large group received 400 rem, half would die in 60 days – 600 rem would kill everyone within a week • Natural and man made sources of radiation contribute to a combined background radiation total of about 360 mrem per person per year • About 82% of the background radiation is from natural sources and the remaining 18% from medical sources (much in the form of X-rays) • Radiation sources should be avoided because the intensity of radiation diminishes as 1/(distance from a source)2 Problem 23.6-C: Page 1017 • Complete the following nuclear equation and identify X. 1 n Co + 27 0 59 56 25Mn + X Problem 23.14-A, B: Page 1017 • For each pair of isotopes listed, predict which one is less stable. (A) 63Li or 93Li (B) 2311Na or 2511Na Problem 23.34-A: Page 1018 • Write a balanced nuclear equation for the following reaction and identify X. d (deuterium) is 21H (A) 80 Se(d, 34 p)X Problem 23.36: Page 1018 • A long cherished dream of alchemists was to produce gold from cheaper and more abundant elements. This dream was finally realized when 19880Hg was converted into gold by nuclear bombardment. Write a balanced equation for this reaction. Problem 23.86: Page 1020 • A 0.0100 gram sample of a radioactive isotope with a half-life of 1.3E9 years decays at the rate of 2.9E4 dpm. Calculate the molar mass of the isotope. Strategy: 23.86 • Have number of grams. Need number of moles. • Calculate value of k. Since the unit of this rate are DP minute (DPM), convert t1/2 to minutes • Substitute in: Rate = k N – “N” can be number of Nuclei – Calculate “N” – Use Avogardo’s number to convert value of “N” to moles. – 0.0100 grams/calculated number of moles Problem 23.95-A: Page 1021 • In 2006, an ex-KGB agent was murdered in London. Subsequent investigation showed that the cause of death was poisoning with the radioactive isotope 210Po, which was added to his drinks/food. (A) 210Po is prepared by bombarding 209Bi with neutrons. Write the equation for this reaction Problem 23.95-D: Page 1021 • In 2006, an ex-KGB agent was murdered in London. Subsequent investigation showed that the cause of death was poisoning with the radioactive isotope 210Po, which was added to his drinks/food. (D) t1/2 = 138 d and decays by alpha emission. Calculate the energy of an emitted alpha particle. Assume both the parent and daughter nuclei to have zero kinetic energy. The atomic masses are: 210Po (209.9825 amu); 206Pb (205.97444 amu) ; 42α (4.00150 amu) Strategy: 23.86 D • Need mass defect: ∆m = final – initial • Δm = (mass 206Pb + mass α) − mass 210Po • ∆E = (Δm) C2 Calculated value of ∆E represents value of alpha particle since it is only on the reactant side ∆E = 1.04E-12 J Problem 23.95-E: Page 1021 In 2006, an ex-KGB agent was murdered in London. Subsequent investigation showed that the cause of death was poisoning with the radioactive isotope 210Po, which was added to his drinks/food. (E) Ingestion of 1 µg of 210Po could prove fatal. What is the total energy released by this quantity of 210Po? Strategy: 23.86 E • The energy calculated in part (d) is for the emission of one α particle. The total energy released in the decay of 1μg of 210Po is: (1.0E-6 g 210Po) (mole 210Po /209.98285 g) (1 mole α /mole 210Po) (6.022E23 α particles/1 mole α) (1.04E-12 J/1 α particle = 2.98E3 J