Nuclear Chemistry Part II Sections 25.3-5 Nuclear Chemistry Part II

Radioactive Decay Rates Half-life – time it takes for ½ of the atoms in a sample of radioactive material to decay Amount remaining = (Initial amount)(½)n n = # of half-lives passed Amount remaining = (Initial amount)(½)t/T t is the elapsed time T is the duration of the half-life Practice Problems p. 818-819

Nuclear Reactions & Energy Energy released in chemical reactions is insignificant compared to the energy released in a nuclear reaction. Any time a reaction involves an energy change, there is a mass change as well. (Even though this violates the Law of Conservation of Mass, the relationship between the two is expressed in Einstein’s equation, E =  mc2.) Energy consumed/produced in chemical reactions is so small that the accompanying mass change is considered insignificant. (*not true for nuclear reactions) Energy released from 1 kg of uranium is equivalent to energy released from 4 billion kg of coal. Mass defect – difference in mass between nucleus and individual nucleons (plugged into E =  mc2 to account for huge release of energy) Energy released when an atom’s nucleons bind together. binding energy – energy required to break nuclei into individual nucleons higher the binding energy  stronger the nucleons are held together  more stable nucleus binding energy reaches peak around 60  elements with mass number of 60 are most stable

Fission – heavy atoms fragment into smaller atoms; always accompanied by very large release of energy used to produce energy in power plants chain reaction - neutrons released in one fission reaction used to initiate another, and another, and another . . . An atomic bomb is an uncontrolled chain reaction. Critical mass - fissionable material must contain a sufficient mass in order for a chain reaction to occur subcritical mass – not enough to initiate chain reaction supercritical mass – so much material that the reaction escalates violently (nuclear explosion)

Fusion – two or more light nuclei bond together to form a single more stable nucleus; capable of releasing large amounts of energy occurs on the sun hydrogen atoms fuse to form a helium atom The lowest temperature capable of sustaining a fusion reaction is 40 000 000 K!!!

Applications of Nuclear Rxns Nuclear power plants – generation of electricity Pros: reduces dependence on fossil fuels, no greenhouse gases (no global warming) produced, produces large amount of electricity from little fuel Cons: storage of nuclear waste, possibility of nuclear failure (meltdown), target for terrorist attack, uranium a scarce resource Nuclear medicine - radioisotopes used as radiotracers, PET scans, radiation therapy used to treat cancers Military - atomic bombs, nuclear weapons

What Problems Are Associated with Nuclear Energy? Proliferation Risks Plutonium is a waste product of the fission of uranium and could be used to produce massive numbers of nuclear weapons. Risk of Accident Chernobyl, Three Mile Island Can lead to death at site of explosion, but thousands’ health will be affected by radiation contamination Environmental Degradation The mining, refining & enrichment of uranium all pose environmental hazards. Nuclear Waste The waste products of the fission of uranium can remain radioactive for hundreds of thousands of years. How do we store this waste?