AP Chemistry. CHAPTER 21- Nuclear Chemistry Radioactivity

Transcription

1 AP Chmistry CHAPTER - Nuclar Chmistry. Radioactivity Whn nucli chang spontanously, mitting nrgy, thy ar said to b radioactiv. Nuclar chmistry is th study of nuclar ractions and thir uss. Nuclons ar particls in th nuclus: p + : proton n : nutron Atomic numbr is th numbr of p +. Mass numbr is th numbr of p + + n. Th mass numbr is th total numbr of nuclons in th nuclus. Isotops hav th sam numbr of p + but diffrnt numbrs of n. Diffrnt isotops of th sam lmnt ar distinguishd by thir mass numbrs. Diffrnt isotops hav diffrnt natural abundancs. A radionuclid is a radioactiv nuclus. Atoms containing ths nucli ar calld radioisotops. Nuclar Equations Most nucli ar stabl. Radionuclids ar unstabl and spontanously mit particls and/or lctromagntic radiation. Exampl: Uranium-38 is radioactiv. It mits alpha (α) particls. Ths ar hlium- particls. Whn a nuclus spontanously dcomposs in this mannr, w say it has dcayd (radioactiv dcay). In nuclar quations, th total numbr of nuclons is consrvd. W can rprsnt th uranium-38 dcay by th following nuclar quation: U! 9Th+ H Th total numbr of protons and nutrons bfor a nuclar raction must b th sam as th total numbr of nuclons aftr th raction. Typs of Radioactiv Dcay Thr ar thr typs of radiation which w will considr: α-radiation is th loss of (alpha particls) from th nuclus. H β-radiation is th loss of (an lctron) from th nuclus. Ths high-spd - ar calld bta particls. - γ-radiation is th loss of gamma rays (high-nrgy photons) from th nuclus. Nuclons can undrgo two othr typs of dcay: positron mission A positron is a particl with th sam mass as an lctron but with th opposit sign. lctron captur Th nuclus capturs an lctron from th lctron cloud surrounding th nuclus. Rprsntations: In nuclar chmistry, to nsur th consrvation of nuclons w writ all particls with thir atomic and mass numbrs: H and Nuclons can undrgo dcay:! rprsnt α-radiation. (β mission) n! p+ - (positron annihilation) + # "! (positron or β + -mission) p! n+ (lctron captur) p+ "! n

2 . Pattrns of Nuclar Stability Nutron-to-Proton Ratio Th proton has high mass and high charg. Thrfor, th proton-proton rpulsion is larg. In th nuclus th protons ar vry clos to ach othr. Th cohsiv forcs in th nuclus ar calld strong nuclar forcs. Nutrons ar involvd with th strong nuclar forc. As mor protons ar addd (th nuclus gts havir) th proton-proton rpulsion gts largr. Thrfor, th havir th nuclus, th mor ar nutrons rquird for stability. Th blt of stability is th portion of a graph of (numbr of protons) vs. (numbr of nutrons) that contains all stabl nucli. All nucli with 8 or mor protons ar radioactiv. Nucli abov th blt of stability undrgo β-mission. An is - lost; th numbr of nutrons dcrass and th numbr of protons incrass. Nucli blow th blt of stability undrgo β + -mission or lctron captur. This rsults in th numbr of nutrons incrasing and th numbr of protons dcrasing. Nucli with atomic numbrs gratr than 83 usually undrgo R-mission. Th numbr of protons and nutrons dcrass (in stps of ). Radioactiv Sris A nuclus usually undrgos mor than on transition on its path to stability. Th sris of nuclar ractions on this path compris th radioactiv sris, or th nuclar disintgration sris. Furthr Obsrvations Magic numbrs ar, 8,, 8, 5, or 8 protons or, 8,, 8, 5, 8, or 6 nutrons. Nucli with a magic numbr of nuclons ar mor stabl than nucli without th magic numbr of nuclons. Th magic numbrs corrspond to filld, closd-shll nuclon configurations. Nucli with vn numbrs of protons and nutrons ar mor stabl than nucli with any odd numbrs of nuclons. Th shll modl of th nuclus rationalizs ths obsrvations. Th shll modl of th nuclus is similar to th shll modl for th atom. Pairs of protons and nutrons in th nuclus ar analogous to pairs of lctrons in th atom..3 Nuclar Transmutations Nuclar transmutations ar nuclar ractions rsulting from th collisions btwn nucli or btwn a nuclus and a nutron. Using Chargd Particls For xampl, nuclar transmutations can occur using high-vlocity α-particls: N + α 7 O + H In short-hand notation th prcding raction is writtn as N(α,p) 7 O. Acclrating Chargd Particls To ovrcom lctrostatic forcs, chargd particls nd to b acclratd bfor thy ract. Particl acclrators (atom smashrs, cyclotrons, synchnotrons) ar usd to acclrat particls using strong magntic and lctrostatic filds. A cyclotron consists of D-shapd lctrods (ds) with a larg circular magnt abov and blow th chambr. Particls ntr th vacuum chambr and ar acclratd as th ds ar altrnatly mad positiv and ngativ. Th magnts abov and blow th ds kp th particls moving in a spiral path. Whn th particls ar moving at a sufficint vlocity thy ar allowd to scap and strik th targt. Th circumfrnc of th ring at th Frmi National Acclrator Laboratory in Chicago is 6.3 km.

3 Using Nutrons Most synthtic isotops usd in mdicin and rsarch ar mad using nutrons as projctils. An xampl is th prparation of cobalt-6 for us in cancr radiation thrapy. Transuranium Elmnts Transuranium lmnts follow uranium in th priodic tabl.. Rats of Radioactiv Dcay Radioactiv dcay is a first-ordr procss. Each isotop has a charactristic half-lif. Half-livs ar not affctd by tmpratur, prssur or chmical composition. Natural radioisotops tnd to hav longr half-livs than synthtic radioisotops. Half-livs rang from fractions of a scond to millions of yars. Naturally occurring radioisotops can b usd to dtrmin th ag of a sampl. This procss is radioactiv dating. Dating Carbon- ( C) is usd to dtrmin th ags of organic compounds bcaus half-livs ar constant. For C to b dtctd, th objct must b lss than 5, yars old. W assum that th ratio of C to C has bn constant ovr tim. Th half-lif of C is 5,75 yars. C undrgos dcay to N via β-dcay: 6 C! 7 N+ - Othr dating mthods ar also usd. Uranium-lad dating has bn usd to stimat th ag of th Earth at approximatly. billion yars. Calculations Basd on Half-lif Radioactiv dcay is a first-ordr procss: Rat = kn If th activity of a sampl at tim = t is N t, and th activity at tim = is N, thn: N t ln =! kt N o Th half-lif of th sampl is givn by:.693 t = k In radioactiv dcay th constant, k, is calld th dcay constant. Th rat of dcay is calld activity (disintgrations pr unit tim). Thr ar svral units usd to xprss activity or radioactivity. Th bcqurl (Bq) is th SI unit of radioactivity. Bq disintgration pr scond (dps). Th Curi (Ci) is an oldr, but still vry widly usd, unit of activity. Ci = 3.7 x disintgrations pr scond.5 Dtction of Radioactivity Mattr is ionizd by radiation. A Gigr countr dtrmins th amount of ionization by dtcting an lctric currnt. A thin window is pntratd by th radiation and causs th ionization of Ar gas. Th ionizd gas carris a charg, so currnt is producd. Th currnt puls gnratd whn th radiation ntrs is amplifid and countd. Othr mthods ar also usd to dtct radioactivity. On common mthod mploys an instrumnt calld a scintillation countr. A substanc calld a phosphor is allowd to intract with radiation. Light is producd whn radiation striks a suitabl phosphor. This light is dtctd and usd to quantify th amount of radiation.

4 Radiotracrs Photosynthsis has bn studid using C: Th carbon dioxid is said to b C labld. Th prsnc of C in th intrmdiats or products of photosynthsis can b dtrmind. C is dtctd as it movs from carbon dioxid to ultimatly bcom incorporatd into glucos. Thus th path of th carbon atoms may b tracd. Radiotracrs ar usd to follow an lmnt through a chmical raction..6enrgy Changs in Nuclar Ractions Einstinshowd that mass and nrgy ar proportional: E = mc If a systm loss mass, it loss nrgy (xothrmic). If a systm gains mass, it gains nrgy (ndothrmic). Sinc c is a larg numbr, small changs in mass caus larg changs in nrgy. Mass and nrgy changs in nuclar ractions ar much gratr than in chmical ractions. Considr: 9 U! CO + 6HO! Th+ H sunlight chlorophyll C6H For mol of Uranium-38, th masss ar: 38.3 g g +.5 g. Th chang in mass during th raction is: g +.5 g 38.3 g =.6 g Th procss is xothrmic bcaus th systm has lost mass. To calculat th nrgy chang pr mol of Uranium-38, plug mass chang and spd of light (3.x 8 m/s) into E=mc quation! O 6 + 6O Nuclar Binding Enrgis Th mass of a nuclus is lss than th mass of its nuclons. Mass dfct is th diffrnc in mass btwn th nuclus and th masss of its nuclons. Nuclar binding nrgy is th nrgy rquird to sparat a nuclus into its nuclons. Sinc E = mc, th binding nrgy is rlatd to th mass dfct. Th largr th binding nrgy, th mor likly a nuclus is to dcompos. Havy nucli gain stability by splitting into smallr nucli. Thy giv off nrgy if fragmntd into two mid-sizd nucli. This raction is calld fission. Vry light nucli ar combind or fusd togthr to form mor massiv nucli. Enrgy is rlasd from this nuclar fusion..7 Nuclar Fission ) E = ( mc ) = c ( m)! E =!! 8 & kg # (.9979 ( m/s) ('.6 g) $! kg m #E = ".! = ".! s Splitting of havy nucli is xothrmic for larg mass numbrs. % g " J

5 Considr a nutron bombarding a 35 U nuclus. Th havy 35 U nuclus can split in svral diffrnt ways such as: 35 n+ 9 U! 56 Ba+ 36Kr + 3 n For vry 35 U fission an avrag of. nutrons ar producd. Each nutron producd can caus th fission of anothr 35 U nuclus. Th numbr of fissions and th rsulting nrgy incras rapidly. Ractions that multiply this way ar calld chain ractions. Without controls, an xplosion rsults. Considr th fission of a nuclus which rsults in th production of nutrons. Each nutron can caus anothr fission. Evntually, a chain raction forms. A minimum mass of fissionabl matrial is rquird for a chain raction (or nutrons will scap bfor thy can caus anothr fission). This is calld a critical mass. Whn nough matrial is prsnt for a chain raction, w hav a critical mass. If th mass is lowr than th critical mass (subcritical mass) th nutrons scap and a chain raction dos not occur. At th critical mass on nutron from ach fission is ffctiv in causing anothr fission. Any mass ovr th critical mass is calld suprcritical mass. Critical mass for 35 U is about kg. Nuclar Ractors Us fission as a powr sourc. Us a subcritical mass of 35 U ( 38 U is nrichd with about 3% 35 U). Enrichd 35 UO pllts ar ncasd in Zr or stainlss stl rods. Control rods ar composd of Cd or B, which absorb nutrons. Thy hlp to rgulat th flux of nutrons. Modrators ar insrtd to slow down th nutrons to mak thm mor asily capturd. Hat producd in th ractor cor is rmovd by a cooling fluid and carrid to a stam gnrator. Th stam is usd to driv an lctric gnrator. Storag of radioactiv wasts from such ractions is not a simpl problm. Th potntial for nvironmntal contamination by long-livd isotops is a srious considration..8 Nuclar Fusion Light nucli can fus to form havir nucli. Most ractions in th Sun ar fusion ractions. Fusion products ar not usually radioactiv, so fusion is a good nrgy sourc. Also, th hydrogn rquird for th raction can asily b supplid by sawatr. Howvr, high nrgis ar rquird to ovrcom rpulsion btwn nucli bfor th raction can occur. High nrgis ar achivd by high tmpraturs, th ractions ar known as thrmonuclar ractions. Fusion of tritium and dutrium rquirs a tmpratur of about,, K: 3 9 H+ H! H+ n Ths tmpraturs can b achivd in a nuclar bomb, or a tokamak. A tokamak is a magntic bottl; strong magntic filds contain a high-tmpratur plasma, so th plasma dos not com into contact with th walls. (No known matrial can surviv th tmpraturs rquird for fusion.) To dat, tmpraturs of about 3,, K hav bn achivd in a tokamak..9 Biological Effcts of Radiation Ionizing radiation involvs ionization that occurs whn radiation rmovs an lctron from an atom or molcul. This is gnrally mor harmful to biological systms than nonionizing radiation.

6 Radiation absorbd by tissu causs xcitation (nonionizing radiation) or ionization (ionizing radiation). Most ionizing radiation intracts with watr in tissus to form H O +. Th H O + ions ract with watr to produc H 3 O + and OH. OH has on unpaird lctron. It is calld th hydroxy radical. This is an xampl of a fr radical, a substanc with unpaird lctrons. Fr radicals gnrally undrgo chain ractions. Thy ar capabl of causing substantial damag in biological tissus. Th pntrating powr of radiation is a function of th mass of th radiation. Thrfor, γ-radiation (zro mass) pntrats much furthr than β-radiation, which pntrats much furthr than α-radiation. Radiation Doss Absorbd radiation is masurd in: Gray: Gy is th SI unit for absorption of J of nrgy pr kg of tissu. Rad is th radiation absorbd dos. On rad is th absorption of J of radiation pr kg of tissu. On gray is quivalnt to rads. Bcaus not all forms of radiation hav th sam ffct, w corrct for th diffrncs using th RBE (rlativ biological ffctivnss). Th RBE is about for β- and γ-radiation and for α-radiation. A rm (rontgn quivalnt for man) = (rads) x (RBE). Th SI unit for ffctiv dosag is th Sivrt (Sv = Gy = rm). Radon 38 Th nuclus is which is a dcay product of 86 Rn 9 U Radon xposur accounts for mor than half of th 36 mrm annual xposur to ionizing radiation. Rn is a nobl gas; it is xtrmly stabl. Thrfor, it is inhald and xhald without any chmical ractions occurring. Th half-lif of radon- is 3.8 days. It dcays as follows: 86 Rn! 8 8 Po+ H Th α-particls producd hav a high RBE. Thrfor, inhald Rn is thought to b a caus of lung cancr. Th situation is complicatd bcaus 8 8 Po 8 8 Po! 8 AP Chmistry Spring Brak Homwork has a short half-lif (3. min) also: Pb+ H Th Polonium-8 gts trappd in th lungs whr it continually producs α-particls. Th EPA rcommnds Radon- lvls in homs b kpt blow pci pr litr of air. Radon tsting kits ar radily availabl in many aras of th country.. Rad Chaptr & complt pg 93 #,, 6, 9,, 3, 5, 7, 33, 35, 9, 55. Tak th AP Practic xam a. In th allottd tim priod with th appropriat rsourcs, answr th qustions on a sparat papr. Do not writ on th practic xam. b. For th multipl choic qustions, mak a not of how many choics you liminatd bfor you guss at an answr. c. Go back and find som of th answrs that you wr unsur of and rcord thm in a diffrnt color. 3. Study for cumulativ xams. (Ex. Solubility ruls & ion formulas)