Notes: Part 1 - Nuclear Chemistry NUCLEAR REACTIONS: NUCLEAR FISSION: NUCLEAR FUSION: NUCLIDES: -most nuclides have even # of protons and neutrons the neutron-to-proton ratio determines the stability of the nucleus: -for low atomic # s: -above atomic #20: nuclei whose neutron-to-proton ratio is unstable undergo radioactive decay by emitting 1 or more particles and/or electromagnetic rays: Type Symbol Identity Mass (amu) Charge Penetration Alpha Beta Gamma Proton Neutron NUCLEAR EQUATIONS: Example 1: Radon-226 transmutates by alpha decay. Write the nuclear equation that represents this process. Example 2: Write the nuclear equation for the beta-decay of boron-12. Example 3: Write the nuclear equation representing gamma radiation given off by the unstable radionuclide cobalt-226.
NOTES: Part 2 - Nuclear Fission & Fusion FISSION: *some elements fission spontaneously * some elements can be induced to undergo fission when bombarded with other particles (e.g. neutrons) FUSION: *the sun is a tremendous fusion reaction; the major fusion reaction in the sun is thought to be: *both fission & fusion release large amounts of energy (fusion more than fission) The Atomic Bomb (FISSION) -when the nucleus of U-235 splits, 2 isotopes are formed, plus neutrons are emitted -these neutrons collide with other U-235 atoms, causing them to undergo fission; they release neutrons, and so on The Result CHAIN REACTION!! -there is a minimum mass of fissionable material that must be used to sustain a chain reaction: CRITICAL MASS! -1 type of bomb: Nuclear Reactors (FISSION) *use subcritical masses of fissionable material CORE: *control rods: absorb neutrons *pull rods out of core: fission increases *push rods back into the core: fission decreases **Safety feature: if power is lost, rods will automatically fall into the core and shut the reaction down. TO GENERATE ELECTRICITY: 1) 2) 3) 4) PROS OF NUCLEAR ENERGY: CONS OF NUCLEAR ENERGY:
Notes: Part 3 Rates of Decay and Half-Life Radionuclides have different stabilities and decay at different rates. Intro to Half-Life: Half-life (t 1/2): Half-Life Examples 1. How much of the original sample remains after 1 half-life? 2. How much of the original sample remains after 2 half-lives? 3. Approximately how many half-lives does it take for 12.5% of the radioisotope to remain? 4. Compared to the energy liberated per gram during a typical chemical reaction, the amount of energy liberated per gram during the decay of a radioisotope is: 5. Based on the decay curve of thorium-234: a. What is the shape of this curve? b. What is the half-life of Thorium-234? c. What percent of the isotope remains after 60 days? d. How many grams of a 250 g sample of thorium- 234 would remain after 40 days had passed? 6. Carbon-14 emits beta radiation and decays with a half-life of 5730 years. Assume you start with a mass of 2.00 x 10-12 g of carbon-14 a. How long is 3 half-lives? b. How many grams of the isotope remain at the end of 3 half-lives? 7. A sample of thorium-234 has a half-life of 24.1 days. Will all the thorium undergo radioactive decay in 48.2 days? Explain.
Integrated rate equation: A = the amt. of decaying sample remaining at some time, t A o= the amt. of sample present at the beginning K = rate constant; different for each radionuclide t = time --- OR --- N = # of disintegrations per unit of time; relative activity N o= original activity Half-life = the amount of time required for half of the original sample to decay Example: Cobalt-60 decays with the emission of beta particles and gamma rays, with a helf-life of 5.27 years. How much of a 3.42 g of cobalt-60 remains after 30.0 years?
Uses of Radionuclides Radiocarbon dating: the ages of specimens of organic origin can be estimated by measuring the amount of cabon-14 in a sample. Example: A piece of wood taken from a cave dwelling in New Mexico is found to have a carbon-14 activity (per gram of carbon) only 0.636 times that of wood today. Estimate the age of the wood. (The half-life of carbon-14 is 5730 years.) ***NOTE: Objects older than 50,000 years have too little activity to be dated accurately using carbon dating; instead the following methods are used: 1. Potassium-40 decays to argon-40: half-life = 1.3 x 10 9 years 2. Uranium-238 decays to lead-206: half-life = 4.51 x 10 9 years Example: A sample of uranium ore is found to contain 4.64 mg of uranium-238 and 1.22 mg of lead-206. Estimate the age of the ore.
Worksheet 1: Nuclear Equations (1 pg) 1. Bombardment of aluminum-27 by alpha particles produces phosphorous-30 and one other particle. Write the nuclear equation for this reaction and identify the other particle. 2. Plutonium-239 can be produced by bombarding uranium-238 with alpha particles. How many neutrons will be produced as a by product of each reaction? Write the nuclear equation for this reaction? 3. When bombarded with neutrons, cobalt-59 is converted to cobalt-60. What is the nuclear equation for this reaction? 4. One method for producing plutonium-238 is by bombarding uranium-238 with deuterium (hydrogen-2), which produced neptunium-238 and 2 neutrons. The unstable neptunium then decays to form plutonium-238. Write the nuclear equations for this two-step reaction. What other particle is produced in the second reaction? 5. Neutron bombardment of plutonium-239 yields americium-240 and another particle. Write the nuclear equation and identify the other particle produced. 6. Alpha-particle bombardment of plutonium-239 produced a neutron and another radionuclide. Write the nuclear equation for this reaction and identify the radionuclide. 7. One possible result of the impact of a neutron on a uranium-235 nucleus is the splitting of the uranium into tellurium-137, zirconium-97 and two other particles. Write the nuclear equation for this reaction and identify the two other particles. 8. When bombarded with neutrons, lithium-6 produced an alpha particle and an isotope of hydrogen. Write the nuclear equation for this reaction. What isotope of hydrogen is produced? 9. With what particle would you bombard sulfur-32 to produce hydrogen-1 and phosphorus-32? Write the appropriate nuclear equation? 10. With what particle would you bombard bismuth-209 to produce astatine-211 and 2 neutrons? Express this reaction in the form of a nuclear equation.
Wkst 2: Half-Life Problem Set #1 (1 pg) 1. The half-life of cesium-137 is 30.2 years. If the initial mass of a sample of cesium-137 is 1.00 kg, how much will remain after 151 years? 2. Given that the half-life of carbon-14 is 5730 years, consider a sample of fossilized wood that, when alive, would have contained 24 g of carbon-14. It now contains 1.5 g of carbon. How old is the sample? 3. A 64-g sample of germanium-66 is left undisturbed for 12.5 hours. At the end of that period, only 2.0 g remain. What is the half-life of this material? 4. With a half-life of 28.8 years, how long will it take for 1 g of strontium-90 to decay to 125 mg? 5. Cobalt-60 has a half-life of 5.3 years. If a pellet that has been in storage for 26.5 years contains 14.5 g of cobalt- 60, how much of this radioisotope was present when the pellet was put into storage? 6. A 1.000-kg block of phosphorus-32, which has a half-life of 14.3 days, is stored for 100.1 days. At the end of this period, how much phosphorus-32 remains? 7. A sample of air from a basement is collected to test for the presence of radon-222, which has a half-life of 3.8 days. However, delays prevent the sample from being tested until 7.6 days have passed. Measurements indicate the presence of 6.5 g of radon-222. How much radon-222 was present in the sample when it was initially collected? 8. A 0.500 M solution of iodine-131, which has a half-life of 8.0 days, is prepared. After 40. days, how much iodine will remain in 1.0 L if solution? Express the result in moles. 9. The half-life of sodium-25 is 1.0 minute. Starting with 1 kg if this isotope, how much will remain after half an hour? 10. What is the half-life of polonium-214 if, after 820. seconds, a 1.0-g sample decays to 31.25 mg?
Wkst 3: Half- Life Problem Set #2 (2 pgs) 1) Nitrogen-13 decays to carbon-13 with a half-life of 10.0 minutes. Assume that you are given a starting mass of 2.00 grams of nitrogen-13. a) How long are four half-lives? b) How many grams of nitrogen-13 will remain after 42.7 minutes? 2) Maganese-56 has a half-life of 2.60 hours. Assuming you start with a sample of 10.0 grams of manganese-56, how much will remain after 24.3 hours? 3) The mass of cobalt-60 in a sample is found to have decreased from 5.40 grams to 1.20 grams in a period of 10.6 years. From this information, calculate the half-life of cobalt-60. 4) A patient is administered 20.0 mg of iodine-131. How much iodine-131 will remain in the patient s body after exactly 3 weeks (21 days) if the half-life of iodine-131 is 8.00 days? 5) Suppose you have a sample containing 800. grams of a radioactive substance. If after exactly 1 hour only 22.7 grams of the original compound remain, what is the half-life of this isotope?
6) You are an archaeologist and you have discovered the remains of an ancient civilization. In one of the human bones that you find, you determine that of the original 60.6 grams of carbon-14 present in the bone, only 12.2 grams remain. Knowing that the half-life of carbon-14 is about 5730 years, what do you determine is the age of the bone? 7) You are a famous paleontologist and an expert in radioactive-dating techniques. One day, two visitors come to your laboratory and present you with two different fossils. One fossil is a dinosaur footprint, the other a human jawbone. Both were found at the bottom of a deep valley cut by a stream through cliffs of sedimentary rock. Your guests are very exited. Because they found these fossils next to each other near the steam bed, they feel they have found conclusive evidence that humans and dinosaurs lived at the same time. You are asked to date the samples to confirm their claims. a) You first test the human jawbone. You determine that the carbon-14 activity is only 0.224 times that of a human bone today. Estimate the age of the human jawbone. b) You next examine the fossil footprint. You discover that the fresh mud that the dinosaur stepped in had just been covered with a thin layer of volcanic ash. You study the amount of potassium-40 and argon-40 in the ash. You find that the sample contains 2.76 mg of potassium-40 and 0.210 mg of argon-40. The half-life of potassium-40 is 1.3 billion years. What is the age of the fossil footprint? c) Were your visitors conclusions about these fossils ages correct? Explain. d) If they were not, could you explain the fact that they were found together at the bottom of the valley?
Wkst 4: RADIATION REVIEW GUIDE (2 pgs) 1. Nuclear Chemistry: a. What is the difference between regular elements and radioactive elements? b. How are the following two reactions different? i. NaCl Na + + Cl - ii. U Th + He 2. Half-life a. What is half life? b. Why is it beneficial that different substances have different half lives? 3. Fission a. What is fission? b. Draw a picture of U-235 atom going through fission. c. What are the key things that keep fission going? d. Which element was used in the atomic bomb named Little Boy? e. What is Critical mass? f. Why did scientists begin creation of the atomic bomb in the first place? 4. Nuclear reactors a. What is the purpose of a nuclear reactor? (What does it do?) b. What do the control rods do? How do they do the controlling? c. What do you do with the control rods if you want to increase fission? d. What do you do with the control rods if you want to decrease fission?
5. Chernobyl a. Why did people die right away after Chernobyl exploded? b. How did radiation reach 20 other countries? 6. Fusion a. What is fusion? b. Where does fusion commonly take place? c. Why is fusion difficult to start? 7. Write balanced nuclear equations for the following reactions: a. Radium-223 undergoes alpha decay b. Carbon-14 undergoes negative beta decay c. Fission of uranium-235 activated by the bombardment of a neutron to form tellurium-137, another large nucleus and three neutrons. 8. The half-life of cesium-137 is 30.2 years. If the initial mass of a sample of cesium-137 is 1.00 kg, how much will remain after 162 years? 9. Given that the half-life of carbon-14 is 5730 years, consider a sample of fossilized wood that, when alive, would have contained 22.5 g of carbon-14. It now contains 1.5 g of carbon-14. How old is the sample? 10. A sample of air from a basement is collected to test for the presence of radon-222, which has a half-life of 3.8 days. However, delays prevent the sample from being tested until 9.0 days have passed. Measurements indicate the presence of 6.5 micrograms of radon-222. How much radon-222 was present in the sample when it was initially collected?