Nuclear Chemistry CHEM 1200
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Popular in Chemistry
This 5 page Class Notes was uploaded by Alexi Martin on Friday April 15, 2016. The Class Notes belongs to CHEM 1200 at Rensselaer Polytechnic Institute taught by Dr. Alexander Ma in Spring 2016. Since its upload, it has received 8 views. For similar materials see Chemistry II in Chemistry at Rensselaer Polytechnic Institute.
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Date Created: 04/15/16
Ch 21 Nuclear Chemistry Nuclear Medicine nuclear radiation for tests labelling intake and output nuclear radiation is ionizing tracks have unhealthy tissue Discovery of Radioactivity phosphorescent minerals give off xrays emission of light glow caused by radiation Certain minerals produce energy rays that could penetrate matter 1. Contained Ur 2. All rays produced not exposed to outside energy uronic rays U like xrays not related to the glowing energy produced without energy input Curies electroscope= uronic rays Specific elements other than U such as Ra and Pl Electroscope Metal foils spread out due to like charge repulsion Ionizing radiation tracks electrons off air discharge than electron repulsion Radioactivity Ionize matter from uncharged to charged matter Increase in energy Penetrate matter Phosphorescence of chemicals causes them to glow Release of tiny increasing in energy particles (gamma rays) Ejected from nucleus Types (increasing in ionization power and decreasing in penetration) alpha particles charge of +2 protons with mass 4 amu nucleus of He 2+ can be stopped by 0.01 mm of lead Beta Charge 1 Electrons negligible mass Stop by 1 mm Pb E like Gamma Form of light energy Stopped by 100 mm of lead Electron capture lower level electrons are pulled into nucleus Unstable nuclei emit positrons, + charge e (e+) anti matter Nucleus 1 Small volume, however entire mass of the atom, very dense, strong force holds it together Protons and neutrons together are called nucleons Proton number + atomic number= (Z) Atoms of sure elements have different numbers of neutrons (also known as isotopes) A= Z+neutron Neutrons = AZ Nucleus of isotope (nuclide) 10 % of known are radioactive, they have radioactive nuclides Element mass number> XA such as C13 Mass number= element ^A x\/Z Atomic number Radioactivity Radioactive nuclei spontaneously decay into smaller nuclei that are unstable, nuclide emits particles or energy Parent nucleus undergoing radioactive decay Daughter nuclei new nucleus formed All nuclides of 84 or more atomic number are radioactive Transmutation 1 element changed into atoms of a different element # protons must change Chemical vs. Nuclear chemical = atom gain or share e no charge nuclear structure or nucleus number of protons change, creating a different element Nuclear equations (nuclear processes) symbol of nuclide=nucleus A& Z conserved, can predict daughter nuclei Alpha emission Most ionizing but least penetrating (298,88) Ra> 4,2 He+ Rn (218,86) X> 2 X+ 2,4 He Beta emissions Moves faster, produced from nucleus 1 times more penetrating and ½ as ionizing Loss of beta, A increases by 1 Z=same (234,90) Th> 0,1 e+ 234,91 Pa 1,0 n> 1,1p+ 0,1 (e) Gamma Emission Increase energy photons of electromagnetic radiation A & Z are the same No loss particle, no change in composition Decrease in ionizing and increase in penetrating After nucleus undergoes other types of decay 2 X>X+gamma Positron Emission Charge +1 no mass antielectron Similar to beta in ionizing and penetrating Loose e+ Z+same A decreases by one Proton>neutron Electron capture inner orbital e into nucleus unstable no particle emission, same result as positron e+p+>n A decreases by one Z=same 1,1 p+ 0,1 e>1,0 n Nuclear equation Mass number and atomic number conserved Determine identity of daughter nuclei if we know parent and method of decay Example 1 : in order to see all examples accurately need to look on powerpoint What causes nuclei to decompose Held together by strong force Neutrons stabilize their nucleus N/Z ratio Neutrons to protons ratio important to stability N/Z increase neutrons to protons via beta decay N/Z decrease protons to neutrons via positron emission or e capture Z=1=20 is stable Z from 20 to 40 stable 1.25 Z from 40 to 80 stable Z greater than 83 unstable Example 2: M g22 Z=12 N=10 10/12<1 more reactions unstable positron emission or electron capture Example 3: Kr 39 49/36=1.36 increase ratio so less reactions b eta emission causes ratio to decrease to 1.29 Magic number N/Z actual number of protons and neutrons affect stability stable even over odd few odd total number of nucleons add to magic number=stable stable when N= 2 8 20 28 50 82 or 126 Decay series Native 1 radioactive nuclei another Daughter also radioactive until stable decay series steps large unstable radioactive nuclei to stable 1. Count beta and alpha decay 2. Z number 4 alpha 3 3. A 2 alpha +1 for beta Example 4: see powerpoint to understand process 8 2 Pb Detecting radioactivity radioactive exposed film Electroscope penetrate flash and ionize air Geiger counter works by counting e when Ar is ionized scintillation counter counts # of flashes per minute Natural Radioactivity Food Small amounts of radioactive materials in air, ground and water Background radiation Rate of Decay Constant rate of change, different for each isotope Radionuclide has a particular length of time constant ½ life Rate of radioactivity not affected by temperature rate=kN t ½=0.693/k Shorter the half life the more nuclei decay every second sample ‘hotter’ Ln Nt/N0= kt= ln rate t/rate 0=ln mt/m0 Example 5: mt=? Ln Nt/N0= kt t1/2=0.693/k k=0.693/t1/2 =0.693/2.864=0.2423 1/yr Nt=1.35e^ (0.2423)5 Nt= 0.402 mg Example 6: t1/2=0.693/k k=0.693/3.8=0.182 1/day lnx/10.24 (37.8 days)=0 .011 g Radiometric Dating Change in the amount of radioactivity of radionuclide measuring, or parent radioactive isotope and its stable daughter (half life and previous equations) Mineral geological dating Radio C dating 3 isotopes C14 half life 5730 years (C13, C12) C14/C12 constant because it is replenished ratio decreases after death, compared to natural ratio to determine death date ( 50,000 years limit) Example 7: 4.5 dis/mingC living 15.3 dis/mingC how old? k=0.693/t1/20.693/5730= 1.209x10^4 1/yr (Ln 4.5/15.3)/1.209x10^4 =1x10^4 yr Example 8: t 1/2ln2/k k0.693/5730=9x10^4 ratex=ratee^kt=20e^1.29x10^4=3 .03 counts/mincG Nonradioactive nucleic charges Nucleus unstable hit by between = 2 smaller nuclei is fission Small nuclei acceleration added together= larger nucleus fusion Fusion releases more E than fission Fission chain reaction reactant is also part of the product fission= neutrons ejected from U before they hit U235 or absorbed by U238 4 need critical mass Fissionable material U235 Pu239 Pu240 U needs to be enriched in U235 Nuclear Power fission=electricity Heat boils water into steam, steam turns turbine into energy No air pollution, more efficient core= fuel rods, subcritical Between fuel rods control rods neutron absorbing B and/or Cd Moderator to slow down ejected neutrons Pressurized light water water coolant and moderator, fuel U E fission E=mc^2 Mass Defect nucleus=energy Difference in mass between separate nucleons and combined nucleus E released binding E 1 Mev1.602x10^13 J 1cimumd=931.51 Mev Increase BE per nucleon the more stable nucleus is Example 9: mass defect= 6(1.00783)+10(1.0366)16.01470=0.118879 amu 0.118879x931.5=110.74 MeV/16=6 .921 MeV/nucleon Example 10: mass defect= 26(1.0078)+30(1.0086)53.9344=0.52844 0.52844x931.5=492.2/56= 8.790 MeV/nucleons Nuclear fusion Lighter nuclei to create more stable heavier nuclei Sun uses this to turn H to He Increase in energy, fission to create fusion (see pp for equation) Artificial transmutation Increase in energy particles smashed together into target nuclei creates new nuclei Can be radiation or charged particles (see pp for equations) Bombardment of 1 nucleus with another= new atoms, also new neutrons Performed in a particle accelerator (linear, cyclotron) Example 11: see powerpoint for equations and answers 5
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