More Nuclear Chemistry

1
More Nuclear Chemistry

• Abundance
• Binding Energy
• Fission
• Isotopic Signatures

2
Nuclear Binding Energy Two Ways
3
Nuclear Rxn Energy from BE
4
Molar BE and Element Abundance (Group Work
Problem 1)
5
238U - 206Pb Dating

• t1/2 4.468 x 109 years
• Assume all 206Pb in a sample is due to 238U decay
• 206Pb/238U mass ratio gives age

6
238U - 206Pb Dating (Problem 2 in Group Work)
Mass 238U originally ? Mass 206Pb
originally 0 mg Mass 238U now 10.67mg
Mass 206Pb now 2.81 mg

• age nlength of one half life
• So, we must find n (how many half lives of 238U
  have passed)
• (N/No) (1/2)n or log10(N/No)
  nlog10(1/2) or ln(N/No)nln(1/2)
• We know N 10.67 mg
• How can we find No?

7
238U - 206Pb Dating (Problem 2 in Group Work)
Mass 238U originally 13.92mg Mass 206Pb
originally 0 mg Mass 238U now 10.67mg
Mass 206Pb now 2.81 mg

• How can we find No?
• Mass 238U that produced 2.81mg of 206Pb
  2.81mg(238/206) 3.25mg
• Original mass 238U 3.25 mg 10.67 mg 13.92
  mg
• log10(10.67/13.92) n-0.301 n 0.383 half
  lives
• age n t1/2 (0.393)(4.468 x 109 yrs/half
  life) 1.71 x 109 years

8
Nuclear Fission (Consider This 3.53/Problem 3 in
Group Work)
9
Time and Temp Since the Big Bang(Group Work
Problem 6)
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Isotopic Signatures
11
Deuterium Occurs Naturally in Water
1H 2H ratio in H2O varies slightly with
location ratio is an isotopic signature and can
be used as a tracer
12
Mass Spectrometer(See Group Work Problem 4)
Molecule ionized by e- beam
Molecule enters
Ion abundance
Ion abundance
Mass/charge ratio
13
Isotopic Signatures in Tracing
H2O plants animals
1H 2H in H2O affects 1H 2H in organisms
H-compounds, so you can trace where an organism
has come from by looking at the 1H 2H of its
H-compounds.
14
Problem in Conservation Biology How to protect
monarch butterfly populations?

• Correlate natal sites in North America with
  wintering sites in Mexico.
• focus conservation efforts on most important sites

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Experiment

• Rear monarchs in
• lab on milkweeds gown with known 2H concentration
  water
• field on milkweeds naturally watered by rainfall
• Use mass spectrometry to analyze wings for 2H1H
  ratio

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Results

• Lab monarch study
• isotope signature in wings highly correlates to
  isotope signature of water so isotope signature
  is a good way to trace where the butterflies
  originated
• Field monarch study
• isotope signatures vary depending on site

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Field Study Results
Isotopic signature in wings depends on natal
water 2H1H ratio
different colors different 2H1H ratios
18
Natal Origins of Monarch Butterflies Found
Wintering in Mexico

• Isotope signature studies of wings suggest that
  few butterflies found in Mexico originate in
  Canada