1 / 108

Chapter 21: Nuclear Chemistry

Chapter 21: Nuclear Chemistry. Chemistry: The Molecular Nature of Matter, 6E Jespersen/Brady/Hyslop. +. +. How are atoms formed? . Big Bang—Intense heat ~10 9 K Cooled quickly to 10 6 K— T of stars e – , p , n formed and joined into nuclei—atoms Mostly H and He (as in our sun)

herman-ayers
Télécharger la présentation

Chapter 21: Nuclear Chemistry

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Chapter 21: Nuclear Chemistry Chemistry: The Molecular Nature of Matter, 6E Jespersen/Brady/Hyslop

  2. + + How are atoms formed? • Big Bang—Intense heat ~109 K • Cooled quickly to 106 K—T of stars • e–, p, n formed and joined into nuclei—atoms • Mostly H and He (as in our sun) • Rest of elements formed by nuclear reactions • Fusion—two nuclei come together to form another heavier nucleus • Fission—one heavier nucleus splits into lighter nuclei • Various other types of reactions

  3. Nuclear Shorthand • Nucleons • Subatomic particles found in the nucleus • Protons (p) • Neutrons (n) • Nuclide • Specific nucleus with given atomic number (Z) • Atomic Number (Z) • Number of protons in nucleus • Determines chemical properties of nuclide • Z = p • Mass Number (A)—mass of nuclide • A = n + p

  4. Shorthand for Writing Nuclides • Where X = atomic symbol • e.g. • In the neutral atom: e– = p = Z • Isotopes • Nuclides with same Z (same number of p), but different A (different n)

  5. Radioactivity • Radioactive isotopes • Isotopes with unstable atomic nuclei • Emit high energy streams of particles or electromagnetic radiation • Radionuclides • Another name for radioactive isotopes • Undergo nuclear reactions • Uses • Dating of rocks and ancient artifacts • Diagnosis and treatment of disease • Source of energy

  6. Mass Not Always Constant • Mass of particle not constant under all circumstances • It depends on velocity of particle relative to observer • As approaches speed of light, mass increases • When v goes to zero • Particle has no velocity relative to observer • v/c 0 • Denominator  1 • and m = mo m = mass of particle v = velocity of particle m = rest mass c = speed of light

  7. Why don’t we observe mass change? • In lab and ordinary life, velocity of particle is small • Only see mass vary with speed as velocity approaches speed of light, c • As v c, (v/c)  0 and m  ∞ • In lab, m = mo within experimental error • Difference in mass too small to measure directly • Scientists began to see relationship between mass and total energy • Analogous to potential and kinetic energies

  8. Law of Conservation of Mass and Energy • Mass and energy can neither be created nor destroyed, but can be converted from one to the other. • Sum of all energy in universe and all mass (expressed in energy equivalents) in universe is constant • Einstein Equation • E = (mo)c2 • Wherec = 2.9979 × 108 m/s

  9. Mass Defect • Rest mass of nuclide is always less than sum of masses of all individual nucleons (neutrons and protons) in that same nuclide • Mass is lost upon binding of neutrons and protons into nucleus • When nucleons come together, loss of mass translates into release of enormous amount of energy by Einstein's relation Energy released = – Nuclear Binding Energy • Nuclear Binding Energy • Amount of energy must put in to break apart nucleus

  10. What is Mass Loss? For given isotope of given Z and A or

  11. Ex.1 Binding Energy Calculation What is the binding energy of 7Li3+ nucleus? Step 1. Determine mass loss or mass defect A. Determine mass of nucleus mass of 7Li3+ = m(7Li isotope) – 3me = 7.016003 u – 3(0.0005485 u) = 7.0143573 u B. Determine mass of nucleons mass of nucleons = 3 mp + 4 mn = 3(1.007276470 u) + 4(1.008664904 u) =7.056489026 u

  12. Ex. 1 Binding Energy Calculation (cont.) C. m = mnucleus – mnucleons = 7.0143573 u – 7.056489026u = –0.0421317 u = mass lost by nucleons when they form nucleus Step 2. Determine energy liberated by this change in mass E = (mo)c2 E = – 6.287817 × 10–12 J/atom

  13. Ex. 1 Binding Energy Calculation (cont.) E = –6.287817 × 1012 J/atom × 6.0221367 x 1023 atoms/mole E = –3.78655 × 1012 J/mole = –3.78655 × 109 kJ/mole Compare this to: • 104 – 105 J/mol (102 – 103 kJ/mol) for chemical reactions • Nuclear ~ 1 – 10 million times larger than chemical reactions!!

  14. MeV (Energy Unit) • Nuclear scientists find it convenient to use a different Energy unit: MeV (per atom) • Electron volt (eV) • Energy required to move e across energy potential of 1 V • 1 eV = 1.602 × 10–19 J • M(mega) = 1 × 106 • So 1 MeV = 1 × 106 eV • 1 MeV = 1.602 × 10–13 J

  15. Ex. 1 Binding Energy Calculation in MeV • For Ex. 1. Converting Eto MeV gives • Often wish to express binding energy per nucleon so we can compare to other nuclei • For Li3+ with 3 1p and 4 n this would be

  16. Ex. 2 Calculate E Released • The overall reaction in the sun responsible for the energy it radiates is • How much energy is released by this reaction in kJ/mole of He? m (1H) = 1.00782 u m (4He) = 4.00260 u m (0+) = 0.00054858 u

  17. Ex. 2 Calculate E Released (cont.) • m = mproducts – mreactants • m = m(4He) +2m(e+) – 4m(1H) • m = 4.00260 u + 2(0.00054858 u)– 4(1.00782 u) • m = –0.02758 u • [We will convert u to kg, kg m2/s2 to J, and atoms to moles in the following calculation] E = –2.479 × 1012 J/mol = –8.268 × 109 kJ/mol

  18. Your Turn! Determine the binding energy, in kJ/mol and MeV/atom, for an isotope that has a mass defect of –0.025861 u. A. –2.3243 × 109 kJ/mol; 24.092 MeV/atom B. –3.8595 × 10–12 kJ/mol; 24.092 MeV/atom C. –7.7529 kJ/mol; 8.03620 × 10–8 MeV/atom D. –2.3243 × 109 kJ/mol; 4.1508 × 10–2 MeV/atom

  19. Your Turn! - Solution

  20. Binding Energies per Nucleon • Divide binding energy EB by mass number, EB/A • Get binding energy per nucleon

  21. Implications of Curve • Most EB /A in range of 6 – 9 MeV (per nucleon) • Large binding energy EB /A means stable nucleus • Maximum at A = 56 • 56Fe largest known EB /A • Most thermodynamically stable • Nuclear mass number (A) and overall charge are conserved in nuclear reactions • Lighter elements undergo fusion to form more stable nuclei

  22. Implications of Curve Fusion • Researchers are currently working to get fusion to occur in lab • Heavier elements undergo fission to form more stable elements Fission • Reactions currently used in bombs and power plants (238U and 239Pu) • As stars burn out, they form elements in center of periodic table around 56Fe

  23. Radioactivity • Spontaneous emission of high energy particles from unstable nuclei • Spontaneous emission of fundamental particle or light • Nuclei falls apart without any external stimuli • Discovered by Becquerel (1896) • Extensively studied by Marie Curie and her husband Pierre (1898  early 1920's) • Initially worked with Becquerel

  24. Fun Facts • Marie and Pierre Curie discovered polonium and radium • Nobel Prize in Physics 1903 • For discovery of Radioactivity • Becquerel, Marie and Pierre Curie—all three shared • Nobel Prize in Chemistry 1911 • For discovery of Radium and its properties • Marie Curie only • Marie Curie - first person to receive two Nobel Prizes and in different fields

  25. Discovery of Radioactivity • Initially able to observe three types of decay • Labeled them , ,  rays (after first three letters of Greek alphabet) • If they pass through an electric field, very different behavior

  26. Discovery of Radioactivity •  rays attracted to negative pole so its positively charged •  rays attracted to positive pole so its negatively charged •  rays not attracted to eitherso its not charged   

  27. Nuclear Equations • Used to symbolize decay of nucleus e.g. 238U234Th + parentdaughter • Produce new nuclei so need separate rules to balance Balancing Nuclear Equations • Sum of mass numbers (A, top) must be same on each side of arrow • Sum of atomic numbers (Z, bottom) must be same on each side of arrow 92 90

  28. Types of Spontaneous Emission • Alpha()Emission He nucleus 2 n + 2 p A = 4 and Z = 2 • Daughter nuclei has: Adecreases by 4 A = – 4 Zdecreases by 2 Z = – 2 • Very common mode of decay if Z > 83 (large radioactive nuclides) • Most massive particle • e.g.

  29. Balancing Nuclear Equations • The sum of the mass numbers (A; the superscripts) on each side of the arrow must be the same • The sum of the atomic numbers (Z; the subscripts; nuclear charge) on each side of the arrow must be the same • e.g. A: 234 = 230 + 4 Z: 92 = 90 + 2

  30. Emission of electrons Mass number A = 0 and charge Z = –1 But How? No electronsin nucleus! If nucleus neutron rich — nuclide is too heavy 2. Beta (– or e–) Emission

  31. Charge conserved, but not mass mE Ejectede–has very high KE + emits Antineutrino variable energy particle Accounts for extra E generated e.g. 2. Beta (– or e–) Emission

  32. 3. Gamma () Emission • Emission of high energy photons • Often accompanies  or  emission • Occurs when daughter nucleus of some process is left in excited state • Use * to denote excited state • Nuclei have energy levels analogous to those of e– in atoms • Spacing of nuclear E levels much larger •  light emitted as -rays e.g.

  33. 4. Positron (+ or e+) Emission • Emission of e+ • Positive electron • Where does + come from? • If nucleus is neutron poor) • Nuclide too light • Balanced for charge, but NOT for mass

  34. 4. Positron (+ or e+) Emission • Product side has much greater mass! • Reaction costs energy • Emission of neutrino • Variable energy particle • Equivalent of antineutrino but in realm of antimatter • e+ emission only occurs if daughternucleus is MUCH more stable than parent

  35. 4. Positron (+or e+) Emission What happens to e+? • Collides with electron to give matter anti-matter annihilation and two high energy -ray photons mE • Annihilation radiation photons • Each with E = 511 keV • What is antimatter? • Particle that has counterpart amongordinary matter, but of opposite charge • High energy light, massless • Detect by characteristic peak in -ray spectrum

  36. 5. Electron Capture (EC) • e– in 1s orbital • Lowest Energy e– • Small probability that e– is near nucleus • e– actually passes through nucleus occasionally • If it does: • Net effect same as e+ emission

  37. Types of Spontaneous Emission 6. NeutronEmission = ( ) • Fairly rare • Occurs in neutron rich nuclides • Does not lead to isotope of different element 7. Proton Emission = ( ) • Very rare

  38. Types of Spontaneous Emission 8. Spontaneous Fission • No stable nuclei with Z > 83 • Several of largest nuclei simply fall apart into smaller fragments • Not just one outcome, usually several different—see distribution

  39. Summary—Common Processes 1. Alpha () Emission • Very common if Z > 83 2. Beta () Emissione– • Common for neutron-rich nuclides—below belt of stability 3. Positron (+) Emission e+ • Common for neutron-poornuclides—above belt of stability 4. Electron Capture (EC) • Occurs in neutron-poor nuclides, especially if Z > 40 5. Gamma () Emission • Occurs inmetastable nuclei (in nuclear excited state)

  40. –2 –2 0 –4 yes +1 –1 –1 0 yes –1 +1 +1 0 yes –1 +1 +1 0 yes 0 0 0 0 no Learning Check • Complete the following table which refers to possible nuclear reactions of a nuclide:

  41. Learning Check Balance each of the following equations

  42. Your Turn! What is the missing species, , in the following nuclear reaction? A. B. C. D.

  43. What Holds Nucleus Together? • Consider nucleus • Neutrons and protons in close proximity • Strong proton-proton repulsions • Neutrons spread protons apart • Neutron to proton ratio increases as Zincreases Strong Forces • Force of attraction between nucleons • Holds nuclei together • Overcomes electrostatic repulsions between protons • Binds protons and neutrons into nucleus

  44. Table of Nuclides • Chart where Rows = different atomic number Columns = different number of neutrons • Symbol entered if element is known • Stable nuclei • Natural abundance entered below symbol • Shaded area • Trend of stable nuclei = Belt of Stability • Z ≈ number of neutrons (for elements 1 to 20) • Unstable nuclei • Give type(s) of radioactive decay (spontaneous) • Outer edges, most of atoms

  45. Table of Nuclides Number of neutrons Atomic number (Z = number of protons)

  46. Table of Nuclides • Shaded area = stable nuclei • Trend of stable nuclei = diagonal line = Belt of Stability • Z ≈ number of neutrons (for elements 1 to 20) • Note: only a small corner of table is shown. (The complete is in Handbook of Chemistry and Physics)

  47. Belt of Stability 1.5n:1p  Stable nuclide, natural  Unstable nuclide, natural  Unstable nuclide, synthetic • Each isotope is a dot • Up to Z = 20 • Ratio n /Z = 1 • As Zincreases, n > Z and • By Z = 82, n/Z ~1.5 • n = number of neutrons • Z = number of protons 1.4n:1p Band of Stability n e– emitters 1.3n:1p 1.2n:1p 1n:1p 1.1n:1p e+ emitters 1n:1p Z = p

  48. How To Predict if Nuclei are Stable 1) Atomic Mass = weighted average of masses of naturally occurring isotopes, i.e. most stable ones 2) Compare atomic mass of element to A (atomic mass number) of given isotope and see if it is more or less • Atomic Mass > A too light to be stable • Atomic Mass < A too heavy to be stable 190.2 Too light, neutron poor Too heavy, neutron rich 126.9 • Final note: • All nuclei with Z > 83 are radioactive

  49. More Patterns of Stability • If we look at stable and unstable nuclei, other patterns emerge • 283 stable nuclides (out of several thousand known nuclides) • If we look at which have even and odd numbers of protons (Z) and neutrons (n); patterns emerge 2H, 6Li, 10B, 14N, 138La

  50. More Patterns of Stability • Clearly NOT random: even must imply greater stability • Not too surprising • Same is true of electrons in molecules • Most molecules have an even number of electrons, as electrons pair up in orbitals • Odd electron molecules, radicals, are very unstable, i.e. very reactive!!

More Related