Nuclear Chemistry http://www.skanschools.org/webpages/rallen/ • The study of the structure of atomic nuclei and the changes they undergo. Nuclear Chemistry Chemical Reactions Nuclear Reactions Occur when bonds are broken Occur when nuclei emit particles and/or rays Atoms remain unchanged, though, they may be rearranged Atoms often converted into atoms of another element Involve ONLY valence electrons May involve protons, neutrons and electrons Associated with small energy changes Associated with large energy changes Reaction rate influenced by T, particle size, concentration and catalysts Reaction rate is NOT normally influenced by T, P and catalysts How do Chemical and nuclear reactions compare? • 1895-1898 • Roentgen found that invisible rays were emitted when electrons bombarded the surface of certain materials • Becquerel accidentally discovered that phosphorescent uranium salts produced spontaneous emissions that darkened photographic plates Discovery of Radioactivity • Marie Curie • Isolated the components (uranium atoms) emitting rays • Radioactivity process by which particles give off rays • Radiation the penetrating rays and particles emitted by a radioactive source • Identified 2 new elements, Polonium and Radium on the basis of their radioactivity Discovery of Radioactivity • Marie Curie’s discovery contradicted Dalton’s theory of invisible atoms • Remember isotopes are atoms of the same element that have different numbers of neutrons Discovery of Radioactivity • isotopes of atoms with unstable nuclei • Too many or too few neutrons Radioisotopes • These unstable nuclei lose energy by emitting radiation to attain more stable atomic configurations • Spontaneous process Radioactive Decay • The Neutron: Proton ratio • The nucleus contains all the positively charged protons that should repel each other, BUT neutrons act as a “glue” to hold them together • Neutrons exert a force of attraction known as the strong nuclear force (it is much stronger than the force of gravity) Patterns of Nuclear Stability • Stable nuclei (up to atomic number 20) contain about equal number of protons and neutrons • Above atomic number 20, more neutrons are required to keep the particles held together (so the ratio of neutrons to protons goes above 1:1) • The optimum ratio of neutrons to protons is called the “belt of stability”…outside of this range, atoms are radioactive. • Notice how the belt ends at 83 protons • Above element 83 (bismuth), all nuclei are radioactive…there are so many protons in the nucleus that no number of neutrons can hold them together due to the strong positive-positive charge repulsion • The conversion of an atom of one element into an atom of another element (radioactive decay is one way that this occurs) Transmutation • Alpha Emission • nucleus emits an alpha (α) particle • An alpha particle is a particle composed of 2 protons and 2 neutrons • In • So it is the same as a Helium nucleus Types of Radioactive Decay • Alpha particles are large, slow moving, and easy to stop • They cannot penetrate through a single sheet of paper/clothing • Example Types of Radioactive Decay • Alpha Decay • Example What product is formed when Ra-226 undergoes α decay? Types of Radioactive Decay • Beta Emission- nucleus emits a beta (β-) particle • A beta particle is a high speed electron!!! • In • Since it is the same as an electron, it could also be Types of Radioactive Decay • Beta particles are almost massless, negatively charged, fast moving, and about 100x more penetrating than α particles • They cannot penetrate more than 3 mm of aluminum/wood • Beta particles are formed in the nucleus when a neutron is converted into a proton and an electron…then the electron is emitted from the nucleus Neutron proton + beta Types of Radioactive Decay • Example Atomic # increases by 1 Remember n e + p (then e is emitted) Types of Radioactive Decay • Positron Emission-nucleus emits a beta (β+) particle • There are 2 kinds of beta particles • β- which is called a beta particle • β+ which is called a positron • Positrons are almost massless, positively charged particles Types of Radioactive Decay • Positron particles are formed in the nucleus when a proton is converted into a neutron and a positron…then the positron is emitted from the nucleus • Positrons are short lived because when they leave the nucleus, nucleus they quickly collide with electrons in the electron cloud to produce gamma rays Types of Radioactive Decay • Therefore, positrons don’t really penetrate matter because they don’t get a chance • Example Atomic # decreases by 1 Remember p: n + e (then e is emitted) Types of Radioactive Decay • Nucleus “absorbs” an electron • During electron capture, an inner shell electron is pulled into the nucleus where it combines with a proton to form a neutron • So you lose an electron and proton Electron Capture • An X-Ray is emitted as an electron moves from a higher energy level to a lower energy level to fill the vacancy left by the “captured” electron • Example Electron written as a reactant Electron Capture Atomic # decreases by 1 No change in mass • Nucleus emits high energy photons (energy!!!!) • Since it is just an emission of energy, there is no change in mass or atomic number • Gamma emission accompanies almost every other radioactive emission • Gamma emission is the most penetrating radiation…10,000x more penetrating than alpha particles • Blocked by lead/concrete (thick) Gamma Radiation Radioactive Source α β- γ (+) (-) neutral + - Beta Gamma Alpha Radiation in an electric or magnetic field • Isotopes- atoms of the same element with different number of neutrons • Isotopes are named by giving the name of the element followed by the sum of the neutrons and protons • Example: Carbon Atom • 6 protons, 6 neutrons Carbon-12 (stable) • 6 protons, 7 neutrons Carbon-13 (stable) • 6 protons, 8 neutrons Carbon-14 (radioactive) Radioactive Decay & Half Life • when the nucleus of a radioactive isotope gives up its extra energy • Can take the form of alpha particles, beta particles or gamma rays • Radioactive decay- process of emitting the radiation Ionizing Radiation • Decay Chain-when several decays are required to make a nuclei stable • Scientists have studied decay chains extensively • Know how many decays it will take for each radioactive isotope to become stable and how much energy will be released with each decay Decay Chain • Scientists have not been able to determine exactly when a single radioactive atom will decay • Half Life- the time required for ½ of a radioisotope’s nuclei to decay into products • The half life can vary substantially from one isotope to another Half Life For any radioisotopes # of Half Lives % Remaining 0 100% 1 50% 2 25% 3 12.5% 4 6.25% 5 3.125% 6 1.5625% Half-Life Half-Life • Strontium-90 has a half life of 29 years. What does this mean? • Suppose you have 10.0 g of strontium-90 • 1) Can use a table format: # of Half Lives Time (years) Amount (g) 0 0 10 1 29 5 2 58 2.5 3 87 1.25 4 116 0.625 Half-Life Example Or • 2) Can use a calculation Amount remaining = ½ t/T x initial amount Where: t = time elapsed , T =half-life (t/T= # of half life periods) Half-Life • Example: • If gallium-68 has a half life of 68.3 minutes, how much of a 160.0 mg sample is left after • 1 half-life? 80 mg (1/2 of 160) • 2 half-lives? 40 mg (1/2 of 80) • 3 half- lives? 20 mg (1/2 of 40) Half-Life • Example: • Cobalt-6-, with a half-life of 5 years, is used in cancer radiation treatments. If a hospital purchases a supply of 30.0 g, how much would be left after 15 years 5 yrs 5 yrs 5 yrs 30 15 7.5 3.75 g OR X = ½ t/T X= ½ 15/5 x 30 X = ½ 3 x 30 Half-Life X = 3.75 g • It is possible, under the right conditions, for us to transform one element into another • This type of transmutation is done by “slamming” a particle into a nucleus • This process causes the nucleus to change and as a result the identity of the atom is changed Artificial Nuclear Transmutations • Example: • Alpha particles can be used to transform one nucleus into another • Since the nucleus of the alpha particle is positive and the nucleus of the atom being bombarded is also positive, the particles will naturally repel each other • In order to overcome this repulsion, the reaction must occur at very high speeds (speed of light). • These speeds are reached using a particle accelerator Artificial Nuclear Transmutations • Nuclear Fission- the splitting of a nucleus into smaller nuclei • A large nuclei captures a slow moving neutron • Resulting neutrons can then be used to split other nuclei- chain reaction Fission and Fusion Equation: Fission • Man made not common form of natural decay • Releases a very large amount of energy • Produces energy for nuclear power and drives the explosion for nuclear weapons Fission • The total nuclear mass of the products is less than the total nuclear mass of the reactants • How can that be? Mass Defect Some of the mass is converted into energy We can calculate the amount of energy using Einstein’s equation: E= mc2 Energy Fission Mass Speed of Light (3.8 x 108 m/s) • Einstein’s theories on relativity became a convenient target for Nazi propaganda. • In 1931, the Nazi’s enlisted other physicists to denounce Einstein and his theories as "Jewish physics." • Passed a law barring Jews from holding any official position, including teaching at universities. • Einstein also learned that his name was on a list of assassination targets • 1932moved to US, taught at Princeton • 1939wrote a letter to FDR telling him of the possibility of a Nazi bomb • He was not invited to work on the Manhattan Project Albert Einstein Mass Defect Amu= atomic mass unit Reactants: 235.043924 amu + 1.008665 amu 236.052589 amu Fission Total mass of REACTANTS Reactants Mass Defect Products: 140.914363 amu Krypton 91.926270 amu Barium + 3.025995 amu 235.866628 amu Fission 3 neutrons Total Mass of PRODUCTS Products • Can be controlled by absorbing the neutrons released form the reaction • Where can controlled fission? Nuclear Power Plant Controlled Fission • Fission continues until nuclei are split • Where can uncontrolled fission reactions be used? Nuclear Weapons Uncontrolled Fission • Weapons • WWII: The US Army Air Force received received orders to drop these weapons anytime after August 3, 1945. On August 6, “Little Boy” fell on Hiroshima . Three days later, “Fat Man” destroyed Nagasaki. Nuclear Bombs Little Boy Hiroshima-0815 Size 10 ft Long Weight 8.900 lbs (132 lbs > 90% U-235) Height of Blast 1900 ft Yield 15-16 TNT Casualities 100,000 immediate deaths 200,00 total deaths Fat Man- Nagasaki- 1102 August 9, 1945 Size 10.5 ft long, 5 ft diameter Weight 10.300 lbs (12 lbs. Pu 239 of which 2 lbs underwent fission Height of blast 1650 ft Yield 22 TNT Casualties 70, 000 immediate deaths 140,000 total deaths • The secret to controlling a chain reaction is to control the neutrons • If the neutrons can be controlled, then the energy can be released in a controlled way. That’s what scientists have done with nuclear power plants • United States has about 100 nuclear reactors, producing a little more than 20% of the country’s electricity. Nuclear Power Plants • In France, almost 80% percent of the country’s electricity is generated through nuclear fission • How do nuclear plants make electricity? Nuclear Power Plants Nuclear Power Plant • The fuel for nuclear reactors is held in fuel rods • Fuels rods usually contain mostly non-fissionable U-238 and about 3% fissionable U-235 • A critical mass of fissionable U-235 is present (but not nearly enough for a nuclear explosion) Fuel for Reactors • A chain reaction fission reaction of U-235 occurs in the reactor core • During the fission of U-235 a small amount of mass is converted into a large amount of energy • The energy generated heats water in the reactor core • Heated water in the reactor core (under high pressure so it remains a liquid) transfers heat to a second loop of water • Heated water in the second loop changes to steam • This steam spins the turbines of giant generators that produce electrical energy (electricity) Energy Production • Steam that has moved past the turbines is cooled by water taken from a nearby body of water • Cooled steam condenses to liquid water and re-circulates • So much heat is generated that some must be released into the air through cooling towers Cooling the Reactor • Moderator= something which slows down the high speed neutrons • Water often acts as the moderator in nuclear reactors • A moderator slows down high-speed neutrons making them more readily captured by other nuclei Facilitating the Reaction • Controlling the neutrons in a reactor allows for control of the nuclear chain reaction • Control rods rods made of neutron absorbing material (such as cadmium or boron) • Control rods are inserted between fuel rods to control the reaction • When all the control rods are pushed all the way into the reactor core, all neutrons are absorbed and the chain reaction stops Controlling the Reaction • Nuclear Power Plants have CERTAIN ADVANTAGES: • No fossil fuels are burned (saving fossil-fuel resources for producing plastics and medicine) • No combustion products like carbon dioxide and sulfur dioxide to pollute the air and water Advantages 1. Cost: expensive to build and operate • The electricity that’s generated by nuclear power costs about twice as much as electricity generate through fossil fuel or hydroelectric plants 2. Supply of fissionable U-235 is limited • Of all the naturally occurring uranium, only about 0.75% is U-235. • A vast majority is non-fissionable U-238 • At current usage levels, we’ll be out of naturally occurring U-235 in fewer than 100 years • But there’s a limit to the amount of nuclear fuel available in the earth, just as there’s a limit to the amount of fossil fuels Problems 3. Accidents (safety) 4. Disposal of nuclear wastes Problems Three Mile Island (1979) • A combination of operator error and equipment failure caused a loss of reactor core coolant. • The loss of coolant led to a partial meltdown and the release of a small amount od radioactive gas. • There was no loss of life or injury to plant personnel or the general public Accidents Chernobyl (Ukraine 1986) • Human error, along with poor reactor design and engineering, contributed to a tremendous overheating of the reactor core, causing it to rupture • Two explosions and a fire resulted, blowing apart the core and scattering nuclear material into the atmosphere. A small amount of this material made its way to Europe and Asia • • • • The area around the plant is still uninhabitable The reactor is encased in concrete and must remain that way for 100s of years 100s of people died Many others felt the effects of radiation poisoning • Instances of thyroid cancer, possibly caused by the release of I-13, have risen dramatically in the towns surrounding Chernobyl Accidents • The fission process produces large amounts of radioactive isotopes, and some of the half lives of radioactive isotopes are rather long • Those isotopes are safe after ten half lives • The wastes are basically buried and guarded at the sites. High level wastes pose a much larger problem • They’re temporarily being stored at the site of generation, with the plans to eventually seal the material in glass and then in drums. The material will then be stored underground in Nevada • At any rate, the waste must be kept safe and undisturbed for at least 10,000 years!!! Disposal of Nuclear Watse • The process in which light weight nuclei combine to form a heavier, more stable nucleus • Nuclear fusion produces more energy than fission • Ex. Hydrogen Bombs Fusion • Fusion Reactions power the sun and other stars. • The reaction that occurs in the sun can be represented as follows Fusion 2.0140 amu + 3.01605 amu • Reactants: 5.03005 amu • Products: Total mass of Reactants 4.00260 amu + 1.008665 amu 5.011265 amu Fusion Total mass of Products • Mass Loss 5.03005 amu- 5.011265 amu = .018785 amu Where it went E = mc2 Energy of Stars and H-bombs Fusion • Equation • A fusion reaction with 1 g of the fuel composed of deuterium and tritium will produce the energy equivalent to 8 tons of petroleum Fusion