Nuclear Chemistry Nuclear Chemistry Nuclear Chemistry Nuclear Reactions vs. Normal Chemical Reactions Nuclear reactions involve the nucleusNuclear reactions
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Nuclear Reactions vs. Normal Chemical ReactionsNuclear reactions involve the nucleusThe nucleus opens, and protons and neutrons are rearrangedThe opening of the nucleus releases a tremendous amount of energy that holds the nucleus together called binding energyNormal Chemical Reactions involve electrons, not protons and neutrons
The NucleusRemember that the nucleus is comprised of the two nucleons, protons and neutrons.The nucleons are bound together by the strong force.The number of protons is the atomic number.The number of protons and neutrons together is effectively the mass of the atom.
IsotopesAtoms of a given element with: same #protons but different # neutronsNot all atoms of the same element have the same mass due to different numbers of neutrons in those atoms.
There are three naturally occurring isotopes of uranium:Uranium-234Uranium-235Uranium-238
H H H
RadioactivityIsotopes of certain unstable elements that spontaneously emit particles and energy from the nucleus.So, some nuclides of an element are unstable, or radioactive.
We refer to these as radionuclides.
There are several ways radionuclides can decay into a different nuclide.
Marie Curie a Pioneer of RadioactivityLived in France1898 discovered the elements polonium and radium.Winner of 1903 Nobel Prize for Physics with Henri Becquerel and her husband, Pierre Curie. Winner of the sole 1911 Nobel Prize for Chemistry.
Types ofRadioactive Decay
Alpha Decay:Loss of an -particle (a helium nucleus)helium nuclei two protons and two neutrons charge +2e can travel a few inches through aircan be stopped by a sheet of paper, clothing.
Beta Decay:Loss of a -particle (a high energy electron)Beta particles : electrons ejected from the nucleus when neutrons decay ( n -> p+ + - )
Beta particles have the same charge and mass as "normal" electrons.
Can be stopped by aluminum foil or a block of wood.
Gamma Emission:Loss of a -ray (high-energy radiation that almost always accompanies the loss of a nuclear particle)Gamma radiation : electromagnetic energy that is released.Gamma rays are electromagnetic waves.They have no mass.Gamma radiation has no charge. Most Penetrating, can be stopped by 1m thick concrete or a several cm thick sheet of lead.
Positron Emission:Loss of a positron (a particle that has the same mass as but opposite charge than an electron)
Electron Capture (K-Capture)Addition of an electron to a proton in the nucleusAs a result, a proton is transformed into a neutron.
Types of Radiation Alpha () a positively charged helium isotope - we usually ignore the charge because it involves electrons, not protons and neutronsBeta () an electronGamma () pure energy; called a ray rather than a particle
Other Nuclear Particles Neutron Positron a positive electronProton usually referred to as hydrogen-1Any other elemental isotope
Examples of Radioactive DecayAlpha DecayPo Pb + He
Beta Decay p n + en p + e C N + eGamma DecayNi Ni + g(excited nucleus)
Balancing Nuclear ReactionsIn the reactants (starting materials on the left side of an equation) and products (final products on the right side of an equation)
Atomic numbers must balanceandMass numbers must balance
Use a particle or isotope to fill in the missing protons and neutrons
Nuclear ReactionsAlpha emissionNote that mass number (A) goes down by 4 and atomic number (Z) goes down by 2.Nucleons (nuclear particles protons and neutrons) are rearranged but conserved
Nuclear ReactionsBeta emissionNote that mass number (A) is unchanged and atomic number (Z) goes up by 1.
Other Types of Nuclear ReactionsPositron (0+1b): a positive electronElectron capture: the capture of an electron
Learning CheckWhat radioactive isotope is produced in the following bombardment of boron?10B + 4He ? + 1n 5 2 0
Write Nuclear Equations!Write the nuclear equation for the beta emitter Co-60.
Part IINuclear Stability Half-Life
Nuclear StabilityFor smaller nuclei (Z 20) stable nuclei have a neutron-to-proton ratio close to 1:1.Depends on the neutron to proton ratio.Neutrons play a key role stabilizing the nucleus.
Therefore, the ratio of neutrons to protons is an important factor.
Neutron-Proton RatiosAs nuclei get larger, it takes a greater number of neutrons to stabilize the nucleus.
Band of StabilityWhat happens to an unstable nucleus?They will undergo decay
The type of decay depends on the reason for the instability
Stable NucleiNuclei above this belt have too many neutrons.
They tend to decay by emitting beta particles.
Stable NucleiNuclei below the belt have too many protons.
They tend to become more stable by positron emission or electron capture.
Stable NucleiThere are no stable nuclei with an atomic number greater than 83.
These nuclei tend to decay by alpha emission.
Radioactive SeriesLarge radioactive nuclei cannot stabilize by undergoing only one nuclear transformation.They undergo a series of decays until they form a stable nuclide (often a nuclide of lead).
Measuring RadioactivityOne can use a device like this Geiger counter to measure the amount of activity present in a radioactive sample.The ionizing radiation creates ions, which conduct a current that is detected by the instrument.
Half-LifeHALF-LIFE is the time that it takes for 1/2 a sample to decompose.
The rate of a nuclear transformation depends only on the reactant concentration.
Decay of 20.0 mg of 15O. What remains after 3 half-lives? After 5 half-lives?
Kinetics of Radioactive DecayFor each duration (half-life), one half of the substance decomposes.For example: Ra-234 has a half-life of 3.6 days If you start with 50 grams of Ra-234
After 3.6 days > 25 gramsAfter 7.2 days > 12.5 gramsAfter 10.8 days > 6.25 grams
Half Life CalculationAE = Ao 0.5t/t1/2 AE = amount of substance left Ao = original amount of substance t = elapsed time t1/2 = half-life of the substance If you are given 157 grams of 14C, how much of 14C would be left after 2000 years? (The half-life of 14C is 5730 years.) AE = 157 0.5(2000/5730) Amount of 14C left after 2000 years would be 123 grams.
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?
Kinetics of Radioactive DecayNuclear transmutation is a first-order process.The kinetics of such a process, you will recall, obey this equation:
Kinetics of Radioactive DecayThe half-life of such a process is:Comparing the amount of a radioactive nuclide present at a given point in time with the amount normally present, one can find the age of an object.
Kinetics of Radioactive DecayA wooden object from an archeological site is subjected to radiocarbon dating. The activity of the sample that is due to 14C is measured to be 11.6 disintegrations per second. The activity of a carbon sample of equal mass from fresh wood is 15.2 disintegrations per second. The half-life of 14C is 5715 yr. What is the age of the archeological sample?
Kinetics of Radioactive DecayFirst we need to determine the rate constant, k, for the process.
Kinetics of Radioactive DecayNow we can determine t:
Energy in Nuclear ReactionsThere is a tremendous amount of energy stored in nuclei.
Einsteins famous equation, E = mc2, relates directly to the calculation of this energy.
Energy in Nuclear ReactionsFor example, the mass change for the decay of 1 mol of uranium-238 is 0.0046 g.The change in energy, E, is thenE = (m) c2E = (4.6 106 kg)(3.00 108 m/s)2E = 4.1 1011 J
Nuclear FissionFission is the splitting of atomsThese are usually very large, so that they are not as stableFission chain has three general steps:1. Initiation. Reaction of a single atom starts the chain (e.g., 235U + neutron)2. Propagation. 236U fission releases neutrons that initiate other fissions3. ___________ .
Nuclear FissionHow does one tap all that energy?Nuclear fission is the type of reaction carried out in nuclear reactors.
Nuclear FissionBombardment of the radioactive nuclide with a neutron starts the process.Neutrons released in the transmutation strike other nuclei, causing their decay and the production of more neutrons.
Nuclear FissionThis process continues in what we call a nuclear chain reaction.
Nuclear Fission & POWERCurrently about 103 nuclear power plants in the U.S. and about 435 worldwide.17% of the worlds energy comes from nuclear.
Nuclear FissionIf there are not enough radioactive nuclides in the path of the ejected neutrons, the chain reaction will die out.
Nuclear FissionTherefore, there must be a certain minimum amount of fissionable material present for the chain reaction to be sustained: Critical Mass.
Nuclear ReactorsIn nuclear reactors the heat generated by the reaction is used to produce steam that turns a turbine connected to a generator.Diagram of a nuclear power plant