Transcript More Nuclear Chemistry Abundance Binding Energy Fission

More Nuclear Chemistry
Abundance
Binding Energy
Fission
Isotopic Signatures
Nuclear Binding Energy Two Ways
Nuclear Rxn Energy from BE
Molar BE and Element Abundance
(Group Work Problem 1)
238U
- 206Pb Dating
238
92 U
206
82 Pb
+ 8 42 He + 6 0-1e-

t1/2 = 4.468 x 109 years

Assume all 206Pb in a sample is
due to 238U decay

206Pb/238U
mass ratio gives age
238U
- 206Pb Dating
(Problem 2 in Group Work)
238
92 U
206
82 Pb
Mass 238U originally ?
Mass 238U now = 10.67mg

Mass 206Pb originally = 0 mg
Mass 206Pb now = 2.81 mg
age = n*length of one half life


+ 8 42 He + 6 0-1e-
So, we must find n (how many half lives of 238U have passed)
(N/No) = (1/2)n


or
log10(N/No) = n[log10(1/2)]
We know N = 10.67 mg
How can we find No?
or
ln(N/No)=n[ln(1/2)]
238U
- 206Pb Dating
(Problem 2 in Group Work)
238
92 U
Mass 238U originally = 13.92mg
Mass 238U now = 10.67mg

206
82 Pb
+
4
2 He
+ 6 0-1e-
Mass 206Pb originally = 0 mg
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
Nuclear Fission
(Consider This 3.53/Problem 3 in Group Work)
Time and Temp Since the Big Bang
(Group Work Problem 6)
Isotopic Signatures
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Deuterium Occurs Naturally in Water
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1H
: 2H ratio in H2O varies slightly with location
ratio is an isotopic signature and can be used as a tracer
Mass Spectrometer
(See Group Work Problem 4)
Molecule ionized by e- beam
Molecule
enters
Ion abundance
Ion abundance
Mass/charge ratio
Isotopic Signatures in Tracing
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H2O
plants
animals
: 2H in H2O affects 1H : 2H in organism’s
H-compounds, so you can trace where an
organism has come from by looking at the
1H : 2H of its H-compounds.
1H
Problem in Conservation Biology:
How to protect monarch butterfly populations?
Correlate natal sites in North America
with wintering sites in Mexico.
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
focus conservation efforts on most
important sites
Experiment

Rear monarchs in

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
lab on milkweeds gown with known
2H concentration water
field on milkweeds naturally watered
by rainfall
Use mass spectrometry to analyze
wings for 2H:1H ratio
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…
Field Study Results
Isotopic signature in
wings depends on
natal water 2H:1H ratio
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different colors =
different 2H:1H ratios
Natal Origins of Monarch Butterflies
Found Wintering in Mexico
Isotope signature studies of wings
suggest that few butterflies found in
Mexico originate in Canada
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