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Problem 1Light with a wavelength of 700 nm will appear red. What is the energy (in eV) of one photon of this light?
Solution 1Energy = Planck’s constant × Frequency
E = hfFrequency = ?
f = c / l = (3.0 x 108 ) / (700 x 10-9 ) = ?f = 4.3 x 1014 Hz
E = (6.6 x 10-34 J s) × (4.3 x 1014 s-1)E = 2.8 x10-19 J
E = 2.8 x10-19 J ( 1 eV / 1.6 x10-19 J ) = 1.8 electron Volts
Problem 2The lowest three energy levels of hydrogen have energies of -13.6 eV, -3.39 eV, and -1.51 eV.a) Draw the energy levels. b) If the atom had only these three energy levels, then how many different kinds of photons could it emit?c) What is the highest-energy photon that the atom can emit?
Solution 2a)
Drawing on the chalkboard.b)
3 energy drops are possible.
c)From the highest to the lowest the energy change is:
-1.51 eV – (-13.6 eV) = ? = 12.09 eV
Problem 3Consider uranium-235.
a) How many protons does it have?b) How many neutrons?c) What is the symbol for this isotope?d) What is the charge of its nucleus, in Coulombs?e) What is the approximate mass of the nucleus, in kilograms?
Solution 3a) Every uranium nucleus has 92 protons.b) 235 – 92 = 143 neutrons.c) 235
92U
d) 92 protons ( 1.602 x 10-19 C / 1 proton ) = ?= 1.47 x 10-17 C
e) 235 nucleons ( 1.67 x 10-27 kg / nucleon ) = ?= 3.9 x 10-25 kg
Problem 4The most common isotope of iron (Fe) has a mass of 53.9396 u. The mass of an isolated proton is 1.00782 u. The mass of a neutron is 1.00866 u. What is the binding energy per nucleon of iron? Find this on the binding energy curve.
Solution 4 Part 154
26Fe has 26 protons and 28 neutrons.
Mass of the protons: 26(1.00782) = ?= 26.203320 u
Mass of the neutrons: 28(1.00866) = ?= 28.243480 uTotal mass = ?= 54.4458 u
Mass Defect = ?= 54.4458 u - 53.9396 u = ?
= 0.5062 u
Solution 4 Part 21 u = 931.5 MeV
(0.5062 u)( 931.5 MeV / 1 u ) = ?= 471.5 MeV
Binding energy per nucleon = ?471.5 MeV / 54 = 8.7 MeV / nucleon
Spot this on the curve…
Problem 6How much energy is released by the alpha
decay of Radium 224?Masses:
Radium-224: 224.0202118 uRadon-220: 220.0113938 u
Helium-4: 4.00260325 u
Solution 6Like a ball rolling downhill, the products have kinetic
energy. This energy came from mass! The mass of the products is less than the mass of the
reactants.Dm = mass of reactants - mass of products
Dm = [ 224.0202118 ] – [ 220.0113938 + 4.00260325 ]
Dm = [ 224.0202118 ] – [ 224.013997 ]Dm = 0.006215 u
E = mc2 = ( 0.006215 u )( 931.5 MeV / 1 u ) = ?E = 5.789 MeV
Problem 8
Cobalt-60 will beta decay with a half life of about 5 years. If you have a sample of cobalt-60 that is emitting beta particles at the rate of 40 s-1, then what will be the rate in 15 years?
Solution 8
In 5 years the decay rate will be 20 / s.In 10 years the rate will be 10 / s.In 15 years the rate will be 5 / s.
Problem 9
Nitrogen-17 has a half life of about 4 s. If you start with 24 g of nitrogen-14, how much will be left after 16 s?
Problem 10
The half life of carbon-14 is about 5000 years. A sample of wood has 1/16 the amount of carbon-14 that it had when it died. How old is the wood?
Problem 11
A high-energy neutron slams into a nitrogen-14 nucleus. This results in a carbon-14 nucleus, and what else?
Solution 11
146N + 1
0n 146C + ?
146N + 1
0n 146C + 1
1p 14
6N + 10n 14
6C + 11H
(Any excess energy will go into the KE of the proton)
Problem 12
Use the binding energy curve to determine if it is possible for 236U to split into 92Kr and 141Ba.(The chart is too fuzzy to read accurately, but please try it to see what the task is like.)
Solution 12 (Part 2)
Isotope Binding Energy per nucleon / MeV
uranium-236 7
krypton-92 8.2
barium-141 8
Solution 12 (Part 3)• The binding energy of a nucleus is the binding
energy per nucleon, times the number of nucleons:
• BE of 236U: (7 Mev) (236) = 1652 MeV• BE of 92Kr: (8.2 Mev) (92) = 754 MeV• BE of 141Ba: (8 Mev) (141) = 1128 MeV• The binding energy of the products is 1882
MeV. This is greater than the binding energy of the reactants. So…
Solution 12 (Part 4)• The reaction is possible.• The increase in BE / nucleon means that the
nuclei are moving toward a more stable state (like a ball rolling downhill).
• The increase in BE / nucleon is not like an increase in mass. Mass is like potential energy, so if a reaction decreases the mass, then the reaction can occur…
Can you spot this problem on this graph?
http://www.google.com/imgres?imgurl=http://www4.nau.edu/meteorite/Meteorite/Images/BindingEnergy.jpg&imgrefurl=http://www4.nau.edu/meteorite/Meteorite/Book-GlossaryB.html&h=459&w=567&sz=37&tbnid=2l4x3t2tTYh6pM:&tbnh=108&tbnw=134&prev=/images%3Fq%3Dbinding%2Benergy%2Bcurve&zoom=1&q=binding+energy+curve&hl=en&usg=__yHj4cmQEXQXAHu4tw_7fByoPI3s=&sa=X&ei=CXHdTM--Kse1nweDorCDDw&ved=0CCQQ9QEwBA