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Ch. 21: Nuclear Chemistry. Nuclear chemistry is the study of nuclear reactions and their uses. Radioactivity • When nuclei change spontaneously, emitting energy, they are said to be radioactive. Nucleons Nucleons: particles in the nucleus p + : proton n º : neutron
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Ch. 21: Nuclear Chemistry • Nuclear chemistry is the study of nuclear reactions and their uses. • Radioactivity • • When nuclei change spontaneously, emitting energy, they are said to be radioactive. • Nucleons • Nucleons: particles in the nucleus • p+: proton • nº: neutron • Mass number: the number of p+ + nº.(In other words, the mass number is the total number of nucleons.) • Atomic number: the number of p+ • Neutrons = Mass # − Atomic #
Isotopes • Isotopes: have the same number of p+ and different numbers of nº. • Example: U-235 and U-238 are isotopes of uranium. • Different isotopes of the same element are distinguished by their mass numbers. • Different isotopes have different natural abundances. • Example: U-238 = 99.3% abundance; U-235 = (trace) • Practice Problem: How many p+ and nºare inU-238? • protons = Atomic # = 92 • neutrons = Mass # − Atomic # = 238 – 92 = 146
Nuclear Equations • Most nuclei are stable. • A radionuclide is unstable. It has a radioactive nucleus. • Atoms containing radionuclei are called radioisotopes. • Radionuclides spontaneously emit particles and/or electromagnetic radiation. • Example: Uranium-238 is radioactive. • - It emits alpha particles, a helium-4 nucleus. • • When a nucleus spontaneously decomposes in this manner, we say it has decayed… (radioactive decay). • • In nuclear equations, the total # of nucleons is conserved: • We can represent the uranium-238 decay by the following nuclear equation: • 23892U 23490Th + 42He
Types of Radioactive Decay • There are three types of radiation to consider: • -Radiation is the loss of a 42He from the nucleus. • -Radiation is the loss of a high speed electron, ( 0-1e) from the nucleus. • -Radiation is the loss of high-energy photon, ( 00) from the nucleus. • In nuclear chemistry to ensure conservation of nucleons we write all particles with their atomic and mass numbers. • Atoms can undergo two other types of decay: • Positron emission: A positron is a particle with the same mass as an electron but an opposite sign, (01e). • Electron capture: The nucleus captures an electron from the electron cloud surrounding the nucleus.
Nucleon Decay • Sub-atomic particles can undergo decay as well: • 10n 11p + 0-1e (-emission) • 0-1e+ 01e 200g (positron annihilation) • 11p 10n + 01e (positron emission) • 11p + 0-1e10n(electron capture) • Notice that in each case, the top and bottom #’s are balanced on both sides of the equation. • Practice Problem: What product is formed when radium-226 undergoes alpha decay? • 22688Ra 42He + _______ 22286Rn
Nucleon Decay • Another Practice Problem: What type of particle is emitted when 116C decays into 115B? • 116 C 115B + _______ 01e (a positron)
Nuclear Stability • The proton has high mass and high charge. Therefore the repulsion of 2 protons is large. • In order for the nucleus to remain stable, the strong nuclear force is involved…It is the attractive force between neutrons & protons. • It is much stronger than the electrostatic repulsion between two protons. • As more protons are added (the nucleus gets heavier) the proton-proton repulsion gets larger, therefore more neutrons are required for stability.
Nuclear Stability • Neutron-to-Proton Ratio: • For lighter nuclei, a 1:1 ratio of nº top+is adequate for stability. • This “belt of stability” deviates from a 1:1 neutron to proton ratio for high atomic masses. • At Bi (83 protons) the belt of stability ends and all nuclei greater than 84 p+ are unstable, or radioactive.
Nuclear Stability • Nuclei above the belt of stability undergo -emission. • - An electron is lost and the number of neutrons decreases, the number of protons increases. • Example: 2411Na 2412Mg + 0-1e • (By the way, the source of the electron is… 10n 11p + 0-1e ) • Nuclei below the belt of stability undergo positron emission or electron capture. • - This results in the number of neutrons increasing and the number of protons decreasing. • Example: 3015P 3014Si + 0+1e • (By the way, the source of the positron is… 11p 10n + 0+1e) • Nuclei with atomic numbers greater than 83 usually undergo -emission. • - The number of protons and neutrons decreases (in steps of 2). • Example: 21284Po 20882Pb + 42He
Nuclear Stability Practice Problem: What mode of decay would you expect for C-14 and Xe-118? C-14 nº : p+ = 8/6 ≈ 1.3…Nuclei in this region will probably try to get to a 1:1 ratio by - emmision… 146C 147N + 0-1e Xe-118 nº : p+ = 64/54 ≈ 1.2…Nuclei in this region should have a higher ratio, so it could either capture an electron or emit a positron… 11854Xe + 0-1e 11853I 11854Xe 11853I + 01e
Radioactive Series • A nucleus usually undergoes more than one transition 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. • The radioactive series for U-238 is shown in the graph.
Further Observations on Nuclear Stability • Certain #’s of neutrons and protons are inherently stable… • These “magic numbers” are: • 2, 8, 20, 28, 50, & 82 for protons • 2, 8, 20, 28, 50, 82, & 126 for neutrons • Finally, nuclei with even numbers of protons and neutrons are more stable than nuclei with any odd numbers of nucleons. • Practice Problems: Which is more likely to be stable? • a) Ca-40 b) Ca-41 • a) Pb-208 b) At-210
Synthesis of Nuclids • Stable nuclei can be converted to other nuclei (stable or unstable) by bombarding the stable nuclei with other nuclei or with high speed particles. • Nuclear transmutations are the collision between nuclei. • Example: nuclear transmutations can occur using high velocity -particles… • 147N + 42He178O + 11p • Many of these transmutations require a lot of energy. • The energy to overcome electrostatic forces can be provided by accelerating the charged particles before they react using a particle accelerator, (“atom smashers”).
Rates of Radioactive Decay • • Each isotope has a characteristic half-life…the time required for half of any given quantity of a substance to change (react or decay). • Half-lives are not affected by temperature, pressure or chemical composition. • Natural radioisotopes tend to have longer half-lives than synthetic radioisotopes. • Half-lives range from fractions of a second to millions of years. • Naturally occurring radioisotopes can be used to determine the age of a sample. • This process is radioactive dating.
Carbon-14 Dating • A small amount of 146C is present in the atmosphere along with 126C and 136C. • 146C remains uniform in abundance because, as it decays, it is replenished by cosmic rays striking 147N to produce 146C. • As a result, the ratio of 14CO2 to 12CO2 is constant. • Plants use CO2 in photosynthesis; as along as a plant is alive the CO2 ratio will be the same in the plant as it is in the atmosphere. • When the plant dies the 14CO2 to 12CO2 ratio diminishes as 146C decays. • The half-life is 5715 years, so after 5715 years, the ratio is half the normal ratio. • Wooden relics thought to be less than 50,000 years old are dated by burning a sample to release CO2 and the ratio of CO2 is measured.
Half-Life Calculations • Radioactive decay is a first-order process. • …where N0 is the initial number of nuclei and Nt is the number of nuclei at time t. • In radioactive decay the constant, k, is the decay constant. • The rate of decay is called activity (disintegrations per unit time). • The relationship between k and half-life, t½ is…
Half-Life Calculations • Practice Problem: a) What is the age of a dead tree stump containing 11.6 ppm of 14C compared to a living tree that has 15.2 ppm of 14C? (t1/2 = 5715 years) k = (0.693/5715 yrs) = 1.21 x 10-4/yr ln (11.6/15.2) = −1.21 x 10-4 (t) t = 2233.8 ≈2230 years old b) What percentage of a radioisotope remains after 8 half-lives? (½)8 = 0.39 %
Nuclear Energy • Fusion: Light nuclei can fuse together to form heavier nuclei. • Most (if not all) of the reactions in the Sun are fusion. • Example: 11H + 21H 32He • High energies are required to overcome repulsion between nuclei before these reactions can occur. • High energies are achieved by high temperatures: the reactions are “thermonuclear”. • Fusion of tritium and deuterium requires about 40,000,000K: • 21H + 31H 42He + 10n • These temperatures can be achieved in a nuclear explosion. • An atom bomb generates the heat needed for fusion of a hydrogen bomb.
The Energy of Nuclear Fusion Reactions • The He nucleus contains 2 neutrons and 2 protons for a calculated mass of 4.03190 amu. • The measured mass of this nucleus is 4.0015 amu. • We have 0.0304 amu “missing”. • This “missing matter” is converted to energy according to E = mc2 when hydrogen nuclei collide in the Sun. • This is about 652 million kcal per gram-atom. • 1 kg of hydrogen fused into helium yields about the same amount of energy as burning 20 million kg of coal. • This could be a great source of energy, and it’s relatively safe since the products of fusion are not radioactive, but we can’t sustain the necessary temperatures quite yet.
Nuclear Fission • The splitting of heavy nuclei intolighter products is called nuclearfission. • Just as in fusion, there is a small amount of “missing mass” that gets converted into energy according to E=mc2. • Example: • Notice that the reaction needs neutrons in order to start the process. • Also notice that once the reaction proceeds, 3 neutrons are produced which can further initiate other fission reactions. • This is the basis for nuclear “chain reactions”. • To maintain the chain reaction with a constant rate of fission, a critical mass of U-235 is needed…(about 1 kg.)
Nuclear Fission Power Plants • Nuclear power plants use fissionable substances like U-235 to generate heat. • The heat boils water, which turns a turbine… (fan blades), which turns a generator… (large magnet & coils of wire) which generates electricity. • Fission capture by 235U occurs only with slow neutrons (2200 m/s) and a moderator is needed to slow the neutrons down, usually graphite or water. • Control rods contain cobalt or boron plus other metals and can be used to regulate the neutron capture. • Neutrons escape the reactor. But, the larger the reactor, the less likely it is that neutrons will escape. • This puts a limit on reactor size… (no atomic powered cars or planes. They would be too small with too many escaping neutrons.)
Nuclear Safety Issues • A reactor is not a bomb. There is no critical mass of uranium present. • The worst accident would be the release of radiation through a core melt down or breach of the containment vessel…. (Chernobyl, Russia) • Leaks in the coolant system do develop and some radioactive water may escape the reactor, or gaseous emissions may be released…3-mile Island, USA) • Contrary to popular belief, nuclear power is the safest significant source of energy being used right now. • Think about how many people die in coal mining accidents or in natural gas explosions if you don’t believe me!
Disposal of Radioactive Waste • The fuel rods accumulate impurities and must be disposed of. • The waste is first stored under water at the plant site while radioactivity decreases (usually a few months). • The option that the USA is looking into… • Salt mine disposal (Yucca Mountain site) • *potential problem = pollution of the water table • Another Issue: • Thermal pollution:Cooling water is heated and then returned to the environment. • Environmental water is too hot; no O2 for fish which could result in fish kills. • It turns out that this is less of a problem. The fish actually like the warmer, (but not too warm), water and breed more.
