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Chemistry – Unit 4 Chapter 25 Nuclear Chemistry Mass Defect • Difference between the mass of an atom and the mass of its individual particles. 4.00260 amu 4.03298 amu Nuclear Binding Energy • Energy released when a nucleus is formed from nucleons. • High binding energy = stable nucleus. E = 2 mc E: energy (J) m: mass defect (kg) c: speed of light (3.00×108 m/s) Nuclear Binding Energy Unstable nuclides are radioactive and undergo radioactive decay. Types of Radiation • Alpha particle () – helium nucleus • Beta particle (-) – electron • Positron (+) – positron • Gamma () – high-energy photon 4 2 He 2+ 0 -1 e 1- 0 1 e 1+ 0 paper lead concrete Nuclear Decay • Alpha Emission 238 92 U parent nuclide Th He 234 90 daughter nuclide 4 2 alpha particle Numbers must balance!! Nuclear Decay • Beta Emission 131 53 I 131 54 Xe e 0 -1 electron • Positron Emission 38 19 K Ar e 38 18 0 1 positron Nuclear Decay • Electron Capture 106 47 Ag e 0 -1 • Gamma Emission 106 46 Pd electron – Usually follows other types of decay. • Transmutation – One element becomes another. IQ# 1 1.Balance the following equations: 237 93 Np H 4 2 233 Pa 91 212 0 212 Po e Bi 84 83 1 Nuclear Decay • Why nuclides decay… – need stable ratio of neutrons to protons 238 92 U 234 90 I 131 54 131 53 38 19 106 47 Th He 4 2 Xe e 0 -1 K Ar e 38 18 Ag e 0 -1 0 1 106 46 Pd Band of Stability and Radioactive Decay Half-life • Half-life (t½) – Time required for half the atoms of a radioactive nuclide to decay. – Shorter half-life = less stable. Half-life mf m ( ) 1 n i 2 mf: final mass mi: initial mass n: # of half-lives Half-life • Fluorine-21 has a half-life of 5.0 seconds. If you start with 25 g of fluorine-21, how many grams would remain after 60.0 s? GIVEN: WORK: t½ = 5.0 s mf = mi (½)n mi = 25 g mf = (25 g)(0.5)12 mf = ? mf = 0.0061 g total time = 60.0 s n = 60.0s ÷ 5.0s =12 Example: How much of a 500. g sample of Uranium-235 would be left after five half-lives? Mi = 500 g (n n=5 Mf = ? = # of half-lives) Fraction left after 5 halflives = 500g (0.5)5 Mf = 15.6g Example: A 16.00 mg sample of Radon-222 decays to 0.250 mg after 24 hours. Determine the half-life. 16→ 8 → 4 → 2 → 1 → 0.5 → 0.25 = 6 half lives 24 h 4 h 4.0 h 6 Example: The half-life of molybdenum-99 is 67 hours. How much of a 1.000 mg sample is left after 335 hours? Mi = 1.000 mg Half-life = 67 h Rxn time = 335 h Mf = ? n = 335 / 67 = 5 (1.000)0.5 0.03125 mg 5 Mf = 0.031 mg Learning Check! The half life of I-123 is 13 hr. How much of a 64 mg sample of I-123 is left after 39 hours? Half Life and Radioactivity Lab • Work in groups of 2 at your table. • Each cup has 1 penny in it which will be shaken and then GENTLY emptied on the table. • For the first trial, shake the penny out 100 times on the table. Record the number of times that it came up heads. • For the next trail, you will shake out the penny the number of times that it landed on heads in the last round. • The same procedure will follow until no more of the pennies have landed on “heads” (tails = decayed). • Record all data in the lab book following the example on page 809. • Answer questions 1-4 and be sure to follow the graphing rules (R74 and in the “Math Review” handout from the beginning of the year). Graphing the Results Important !! • Graph directly on lab book • Title every graph and label each axis • Graph at least 2/3 page • Use a ruler • Circle all data points • Use a best-fit line (no “connect the dots”!) 5) Find the average half-life (in # of trials) of your sample by interpolating your curve at exactly 50, 25, and 12.5 flips) Fission • splitting a nucleus into two or more smaller nuclei • 1 g of 235U = 3 tons of coal 235 92 U Fission • chain reaction - self-propagating reaction • critical mass mass required to sustain a chain reaction Fusion • combining of two nuclei to form one nucleus of larger mass • thermonuclear reaction – requires temp of 40,000,000 K to sustain • 1 g of fusion fuel = 20 tons of coal • occurs naturally in stars 2 1 H H 3 1 Fission vs. Fusion F I S S I O N • 235U is limited • danger of meltdown • toxic waste • thermal pollution F U S I O N • fuel is abundant • no danger of meltdown • no toxic waste • not yet sustainable Nuclear Power • Fission Reactors Cooling Tower Nuclear Power • Fission Reactors Nuclear Power • Fusion Reactors (not yet sustainable) Nuclear Power • Fusion Reactors (not yet sustainable) National Spherical Torus Experiment Tokamak Fusion Test Reactor Princeton University Synthetic Elements • Transuranium Elements – elements with atomic #s above 92 – synthetically produced in nuclear reactors and accelerators – most decay very rapidly 238 92 U He 4 2 242 94 Pu Radioactive Dating • half-life measurements of radioactive elements are used to determine the age of an object • decay rate indicates amount of radioactive material • EX: 14C - up to 40,000 years 238U and 40K - over 300,000 years Nuclear Medicine • Radioisotope Tracers – absorbed by specific organs and used to diagnose diseases • Radiation Treatment – larger doses are used to kill cancerous cells in targeted organs – internal or external radiation source Radiation treatment using -rays from cobalt-60. Nuclear Weapons • Atomic Bomb – chemical explosion is used to form a critical mass of 235U or 239Pu – fission develops into an uncontrolled chain reaction • Hydrogen Bomb – chemical explosion fission fusion – fusion increases the fission rate – more powerful than the atomic bomb Others • Food Irradiation – radiation is used to kill bacteria • Radioactive Tracers – explore chemical pathways – trace water flow – study plant growth, photosynthesis • Consumer Products – ionizing smoke detectors - 241Am