nuclear energy In America, about 20% of electricity is generated using nuclear power (fission reactors). Imagine: a nuclear powered car Fuel = pencil-sized Uranium cylinder Energy = 1000 20-gallon tanks of gasoline Re-fuel every 1000 weeks (about 20 years) fusion reactions An artificial transmutation reaction that involves combining 2 nuclei to make a one Ex. Hydrogen atoms combine to form helium in a star (like the Sun) 1
9/19/15 fusion reactions Advantages: Produces even energy than fission Materials more readily available (and not radioactive) Less Less dangerous (no chain reaction Disadvantages: Requires activation energy to overcome repulsion of nuclei Too video 4.3 benefi t s and risks of nuc lear energy 2
uses of radioactive isotopes : can be used as a tracer (follow path of materials) in a reaction and can be used for dating fossils. : can be used for treating cancer. Radium: can also be used for treating cancer. uses of radioactive isotopes to Lead-206: This ratio is used for dating rocks and geological formations. : Used for detecting and diagnosing thyroid disorders. 3
uses of radioactive isotopes : Used to determine the location of brain tumors. : Used in food preservation. Medical Uses: Must have a short half-life AND be quickly eliminated from the body. benefits of nuclear energy Nuclear reactions release billions of times more energy than a chemical reaction Very efficient (small amount of mass is converted to a large amount of energy). The fuel will last longer than fossil fuels. The carbon dioxide and smoke products of burning fossil fuels are not produced by fission (so fission does not contribute to air pollution, global warming, or acid rain). 4
problems with nuclear energy Exposure to radioactivity causes Nuclear waste has a long half-life and must be stored for long periods of time until it can be safely released into the atmosphere. Possibility of theft of fuels from reactor sites to make nuclear weapons. Nuclear accidents can occur Chernobyl Three Mile Island Japan after 2011 earthquake 5
9/19/15 video 4. 4 half l ife half life The time it takes for of the atoms in a given sample of a radioactive element to decay Radioactive substances decay at a rate that is dependent on temperature, pressure, or concentration (Half life is always constant and changed by environmental factors) 6
calculating half life After 1 half life, or of the radioactive element is still present After 2 half lives, or of the radioactive element is still present After 3 half lives, or of the radioactive element is still present This continues forever; the number will never be zero The half lives are listed on table n 7
half life The the half life of a radioactive isotope, the it is The the half life of a radioactive isotope, the it is half life problems 8
ex. amount remaining If a sample of I-131 has an original mass of 52.0g, what mass will remain after 40 days? Look up half life of I-131 on Table N 8.021 days Determine how many half lives have passed by dividing the total time by the half life days/ days = HL periods Set up t-chart with HL periods on the left and amount on the right ex. fraction remaining What fraction of a sample of I-131 will remain after 40 days? Look up half life of I-131 on Table N 8.021 days Determine how many half lives have passed by dividing the total time by the half life 40 days/8.021 days = 5 HL periods Set up t-chart with HL periods on the left and amount on the right 9
ex. original mass The half-life of Tc-99 is 6 hours. If 10 micrograms are left after 24 hours, how much was in the original sample? Divide the total time by the half life hours/ hours = HL periods Set up your T-chart with number of half life periods on the left and amount on the right. Write 10 micrograms at the BOTTOM of the amount column and double it until you reach the top of the chart. ex. number of half lives How many half life periods will it take for 50 grams of Tc-99 to decay to 6.25 grams? Fill in T-chart, starting with right column (amount). Put 50 g at the top of the chart and divide in half until you reach 6.25 g. Fill in the left side of the chart to determine the number of half life periods. 10
ex. half life What is the half-life of a 500 gram sample of a radioactive element if 125 grams remain after 20 hours? Fill in T-chart, starting with right column (amount). Put 500 g at the top of the chart and divide in half until you reach 125 g. Fill in the left side of the chart to determine the number of half life periods. Divide the total time given in problem by the number of half life periods to determine half life. ex. time elapsed How long will it take for a 400 gram sample of P-32 to decay to 50 grams? Look up the half life on Table N 14.3 days Fill in the T-chart, starting with the amount column. Put 400 grams on top and divide in half until you get to 50 grams. Fill in the left side to determine # of HL periods Multiply the half life by the number of HL periods 11