C Three isotopes of carbon. What s different? Nuclear Chemistry. Chapter 22 Nuclear Chemistry GCC CHM 152. Chemical Reactions.

Transcription

1 Chaper Nuclear Chemisry GCC CHM 15 Nuclear Chemisry Nuclear (NOT Nucular) chemisry involves changes in he nuclear composiion (proons and neurons) of radioacive aoms. Applicaions of nuclear chemisry: medical diagnosis and reamen C-14 daing nuclear power plans creae new elemens Chemical Reacions Bonds break/form via elecrons Same number and ype of aoms boh sides of rxn (conservaion of mass) Differen isoopes of an elemen undergo same chemical reacions DH kj Raes affeced by emperaure, pressure, concenraion, caalys Nuclear Reacions Change one elemen ino anoher via reacions in he nucleus Balance aomic # and mass # insead of aoms Nuclear reacions involve a specific isoope of an elemen; Differen isoopes may undergo differen nuclear reacions or may no decay a all. DH can be kj (energy change is much larger). Raes only affeced by changing concenraions Aomic noaion for Isoopes Recall noaions used for isoopes: A Z X A = mass number = # neurons + # proons Z = aomic number = # proons 1 6 C 13 6 C 14 6 C Three isoopes of carbon. Wha s differen? Proons and neurons are collecively called nucleons. The nucleus of a specific isoope is called a nuclide..1 Nuclear Reacions Radioacive decay/emission: an unsable aom emis a paricle or energy. Radiaion arises from nuclear reacions: paren nuclide daugher nuclide + radiaion Paren nuclide undergoes decay. Daugher nuclide is formed by he decay. 1 1

2 Radioaciviy Many nuclides are radioacive (hey sponaneously emi radiaion). Ruherford sudied nuclear reacions and discovered hree ypes of radiaion: alpha (), bea (), and gamma (). Alpha radiaion emis an alpha paricle, bea radiaion emis a bea paricle, and gamma radiaion emis gamma rays. Nuclear Paricles Alpha paricle: = 4 He + ; wrien as or Bea paricle (an e - ): or 0-1e 0 Gamma ray: = high energy wave = 0 Posiron paricle (posiive e - ): + or 0 e Neuron: n 0 = Proon: p + = 1 0 n 1 1p +1 4 He Nuclear Reacions Table.1 Peneraing Abiliy of Paricles Chaper Slide 9 Alpha Emission alpha = 4 He + or 4 He or 4 leas peneraing- sopped by aluminum foil (> 10-3 cm), paper, skin mos massive paricles bu leas harmful Heavy radioacive isoopes (radioisoopes) end o emi alpha paricles: 38 9 U Th + 4 He Noe:he mass # = # nucleons and he aomic # = # proons are he same on boh sides Example Alpha Emission Rxns Daugher: mass # by 4, aomic # by Predic producs: 6 88 Ra 3 90 Th

3 Bea Emission, Posiron Emission = elecron: or 0-1 e + = posiron = posiive elecron: or 0 1e elecrons and posirons are high energy paricles ejeced from he nucleus during he reacion more peneraing - sopped by cm of Al E.g Th 34 91Pa + 0-1e emission: a neuron is convered ino a proon so he # of proons increases by 1 for he daugher. posiron emission: a proon is convered o a neuron so he # of proons decreases by 1. Bea Emission, Posiron Emission Bea: mass # says same, aomic # by 1 Posiron: mass # says same, aomic # by 1 Predic producs: Bea emission: 39 U Posiron emission: 40 K Group Quiz # Bea emission: 131 I Posiron emission: 07 Po Gamma Radiaion (Emission) = high energy radiaion wih no mass or charge Mos peneraing radiaion - sopped by 5-11 cm of aluminum or hick layer of concree or lead Ofen accompanies alpha or bea decay reacions measable (m afer mass number) = Highly energeic/unsable nuclide ha emis rays 11m 5 B 11 5 B + 00 Elecron Capure Elecron capure is he opposie of bea emission. A nucleus capures an innershell elecron, convering a p + o a n o : Hg -1e Au Worked example.1, Problems.1,. Nuclear Reacions Wrie balanced equaions for: 1.Alpha emission from curium-4.bea emission from magnesium-8 3.Posiron emission from xenon Elecron capure by polonium-04 Wha paricle is produced by decay of horium-14 o radium-10? A radioisoope decays o give an alpha paricle and Rn-. Idenify he radioisoope. 3 3

4 1 s order Decay of a Radionuclide half-life, 1/, is he ime required for he number of radioacive nuclei in a sample o drop o half is iniial value. Figure. 1/ is consan Table.: Half-lives of some radioisoopes Chaper Slide 0 Nuclear decay is 1 s order process N ln k N 0 N = number of aoms of a nuclide (same as A) N = 1/ N o a = 1/ : ln k 1 1 N ln N0 1 Nuclear Kineics Calcs N ln N0 1 If you inges a sample conaining Iodine- 131, how much ime is required for he isoope o fall o 5.0 % of is original aciviy? The half-life for I-131 is 8.05 days. The half-life of 98Au is.7 days. If you begin wih 5.6 mg of his gold isoope, wha mass remains afer 9.5 days? Ex. Calc. Fracion remaining Gold-18 undergoes bea decay o give mercury-18 wih a half-life of.7 days. Wha fracion of gold-18 is lef afer 14 days? N 14 days ln N0.7 days N N 0 e Fracion remaining = Percenage remaining =.76% Group Quiz #3 Pluonium-39 decays by alpha emission and has a half-life of.406 x 10 4 yr. 1) Wrie he balanced decay reacion. ) Wha percenage of a 39 Pu sample remains afer 1000 years? Phosphorus-3 (used in leukemia herapy) has a half-life of 14.6 days. Wha percen of a sample remains afer 35.0 days? 4 4

5 .4 Nuclear Sabiliy Sable isoopes refer o isoopes whose half-lives can be measured. Unsable isoopes decay oo fas o be measured. Every elemen up o Bi (excep Tc) has 1 or more sable isoopes. Elemens > Bi are all radioacive. The heavier elemens (93 and higher) are synheic and radioacive. Elemens wih molar mass in parenheses on he periodic able are radioacive. Why is heir molar mass no very precise? The Band of Sabiliy plos he sable aoms Neuron/proon raio helps predic sabiliy. Sponaneous Decay Band of sabiliy (and he higher island of sabiliy ) For Z = 1-0, N = Z (N/Z = 1) For Z > 0, N > Z (N/Z < 1.6) Nuclear Sabiliy Paerns 1. Nuclei conaining, 8, 0, 8, 50, 8, or 16 proons or neurons are generally more sable han nuclei ha do no possess hese magic numbers.. As Z, more neurons are needed o help bind he nucleus ogeher, so here is a high neuron:proon raio. 3. Nuclei of elemens wih > 83 proons are unsable due o he large # of nucleons. 4. Nuclei wih boh even numbers of proons and neurons are generally more sable han hose wih odd # s. 64 nonradioacive Nuclei # proons # neurons # sable nuclei Even Even 156 Even Odd 53 Odd Even 51 Odd Odd 4 Sponaneous Nuclear Decay Figure.5 Nuclei wih higher neuron/proon raios end o emi bea paricles. Nuclei wih lower neuron/proon raios end o favor posiron emission, elecron capure, or alpha emission. Types of Decay Processes If Z > 83, emission ges rid of excess mass, wih a sligh increase in N/Z. If N/Z is oo high, emission convers a neuron o a proon plus an elecron, decreasing N/Z. If N/Z is oo low, + emission convers a proon o a neuron and a posiron; OR elecron capure resuls in a proon capuring an inner elecron and becoming a neuron, increasing N/Z. 5 5

6 Predic if sable or unsable Idenify wheher each nuclide will mos likely be sable or unsable. Sable N/Z < 1.5 (elemens 1-0) and even Z & even N. 13 B 14 C 17 O 8 Si 40 K 48 Ca 3 H 30 P Figure.6 Uranium Series Some nuclei undergo a series of disinegraions called a decay series, before hey form a nonradioacive species Nuclear Fission and Fusion Nuclear fission: nuclei of heavier isoopes are spli o form ligher, more sable ones afer bombardmen by a neuron. This ofen resuls in a chain reacion. Nuclear fusion: nuclei of aoms are joined ogeher or fused. This requires very ho condiions (i.e., he sun). The hydrogen bomb works by fusion. An aomic bomb is used o provide he hea needed. Chain Reacion One sep in a chain reacion produces more neurons han i consumes. Each sep ges faser. Criical mass is he mass required for he chain reacion o become self-susaining. CHAIN REACTION Nuclear Reacor uses Fission Figure.9 91 Kr 35 U 91 Kr 35 U 35 U 14 Ba 35 U 6 6

7 Reacor core We use B or Cd conrol rods o conrol he rae of he fission reacion. They can be raised and lowered as needed. Energy produced is used o hea waer and drive seam urbines Nuclear Reacor Sorage of fission producs is a major challenge. Chernobyl (1986) used no conainmen, so he radiaion leak was worse han from Three Mile Island (1979). Currenly here are more han 110 nuclear power plans in he U.S.; abou 435 worldwide. 17% of he world s energy comes from nuclear sources. Nuclear Fusion Nuclear Fusion is he formaion of heavier nuclei by he joining of ligher ones. Fusion producs are generally no radioacive. Smaller aoms are fused ogeher ino one larger aom: 1 1H + 1H 3 He (i.e., in he sun) Called hermonuclear because hey require exremely high emperaures. Posiive side: H is cheap, pleniful; He is wase which isn oxic Negaive side: Sill have rouble conrolling he reacions. Aomic vs Hydrogen Bombs Aomic weapons uilize he spliing of aoms ( 35 U) and are deonaed hrough fission and is resuling chain reacion. Hiroshima = 13 kons Hydrogen bombs are deonaed hrough fusion. They are 1000 imes more explosive han aomic bombs. Energy is released because of overall loss of mass. Typical = 10 megaons! Grea sie - summary of nuclear weapons: hp://library.hinkques.org/3471/abomb.hml Nuclear Transmuaion Transmuaion: aoms are bombarded by high energy paricles, his mehod is used o creae new elemens. 14 N + 4 He 17 8 O H 7 Preparaion of Tc for medical imaging: 97 4 Mo + 1H 97 43Tc + 1 0n Synhesis of ransuranium elemens: 38 9 U + 4 He 39 94Pu n 38 9 U + 1 6C 46 98Cf n 38 9 U N 47 99Es n Deecion of Radioacive Decay Geiger Couner Geiger or scinillaion couner NaI fluoresces (ligh flashes) when irradiaed; couns nuclear disinegraions aciviy = number of nuclei disinegraing per uni ime; unis = Curie (Ci) 7 7

8 Measuring Radioaciviy aciviy = number of nuclei disinegraing per uni ime unis = Curie (Ci) 1 Ci = 3.7 x disinegraions/sec = amoun for 1 g of 6 Ra Pierre and Marie Curie.10 Archeological Daing 14 C is produced in upper amosphere and diffuses ino lower amosphere where i is aken up by plans and animals. 14 C is incorporaed ino living sysems, bu undergoes radioacive bea decay wih a half-life of 5730 years: 14 6 C 14 7N + 0-1e - The raio of 14 C o 1 C in living organisms is he same as in he amosphere. When he plan or animal dies, he raio decreases and an age can be deermined. Archeological Daing Living issue has 15.3 disinegraions per minue per gram of Carbon. The Shroud of Turin (hough o be he burial shroud of Chris) was measured o have 14. disinegraions per minue per g C. How old is i? ( 1/ 14 C = 5,730 yrs) ln (N/N o ) = (-ln ) ( / 1/ ) = 617 years old Problem.17 Applicaions of Isoopes Medical diagnoses 99 Tc for umors in spleen, liver, brain, hyroid racer pu ino a meabolie ha concenraes in cancerous cells 131 I or 13 I in hyroid Cancer herapy -- desroy cells wih rays 131 I for hyroid cancers 198 Au for lung cancer 3 P for eye umors PET scans Use radioracers o measure amouns absorbed by he body o deermine srucure and funcion of organs and issues. 8 8