1 chapter 23 nuclear chemistry. 2 the nature of radioactivity

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CHAPTER 23CHAPTER 23

NUCLEAR CHEMISTRYNUCLEAR CHEMISTRY

NUCLEAR CHEMISTRYNUCLEAR CHEMISTRY

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THE NATURE OF RADIOACTIVITYTHE NATURE OF RADIOACTIVITY

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Relative Penetration abilitiesRelative Penetration abilities

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NUCLEAR REACTIONSNUCLEAR REACTIONS

• Nuclear reactions have a charge and mass Nuclear reactions have a charge and mass balance, and produce one or more new balance, and produce one or more new elements by a nuclear change. elements by a nuclear change.

• The charge balance is performed using The charge balance is performed using the atomic numbers, Z, (subscripts) on the the atomic numbers, Z, (subscripts) on the symbol of the element or nuclear species. symbol of the element or nuclear species.

• The mass balance is performed using the The mass balance is performed using the mass numbers, A, (superscripts) on the mass numbers, A, (superscripts) on the symbol of the element or nuclear species. symbol of the element or nuclear species.

55NUCLEAR REACTIONSNUCLEAR REACTIONS

210210PoPo -----> -----> xx

XX + + 206206PbPb Find the x's.Find the x's.

8484 xx 8282

Reactions InvolvingReactions Involving and and Particles Particles

decay, occurs when the nucleus is too decay, occurs when the nucleus is too massive.massive.

239239Np ----->Np ----->

256256Lr ----->Lr ----->


• , emission, ( nuclear electron) occurs when the n/p , emission, ( nuclear electron) occurs when the n/p ratio is too high; converts a neutron into a proton plus ratio is too high; converts a neutron into a proton plus an electron.an electron.

11 n n ---> ---> 11

p p + + 00 e e

0 1 -1 0 1 -1

218218 Po Po ---> ---> 218 218

At At + + 00 ee

84 85 -1 84 85 -1

Try Try 214214

Bi Bi ---> --->

83 83

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Uranium-238 Decay


Positron Emission Positron Emission and and

Electron Capture (K Capture)Electron Capture (K Capture)• These processes reduce the atomic number by one, by These processes reduce the atomic number by one, by

converting a proton into a neutron. converting a proton into a neutron.

• These processes occur when the p/n ratio is too large. These processes occur when the p/n ratio is too large.

• The particle produced is called a positron, and has the The particle produced is called a positron, and has the same mass as an electron, but has a positive charge. same mass as an electron, but has a positive charge.


• Positron and electrons are anti matter and matter. Positron and electrons are anti matter and matter. When they meet, they are annihilated and a photon When they meet, they are annihilated and a photon of light energy is emitted.of light energy is emitted.

11 p p ---> ---> 11

n n + + 00 ee

1 0 +11 0 +1

• In the electron capture process, a K (1s) electron is In the electron capture process, a K (1s) electron is captured by a nuclear proton.captured by a nuclear proton.

11 pp + + 00

e e -----> -----> 11 nn

1 -1 01 -1 0


Try the following both ways:Try the following both ways:

3838 K K ---> --->

19 19

3939 Ca Ca --->--->

20 20

1111

Beta emission

Electron capture

38 0 3819 1 18K + Ar

38 0 3819 1 18K + e Ar

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Beta emission

electron capture

38 0 3820 1 19 Ca + K

38 0 3820 1 19Ca + e K

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STABILITY OF ATOMIC NUCLEISTABILITY OF ATOMIC NUCLEI

• Region of stability and modes of decay. Region of stability and modes of decay.

• Notice that no elements above bismuth have Notice that no elements above bismuth have stable isotopes (too massive). stable isotopes (too massive).

• Lower atomic numbers have equal numbers of Lower atomic numbers have equal numbers of protons and neutrons. protons and neutrons.

• As the atomic number increases, so does the As the atomic number increases, so does the n/p ratio for stable isotopes. n/p ratio for stable isotopes.

• Even numbers of protons and neutrons are Even numbers of protons and neutrons are more stable than odd numbers.more stable than odd numbers.

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1 2 31 1 1

hydrogen deutrium tritium

H H H

1515STABILITY OF ATOMIC NUCLEISTABILITY OF ATOMIC NUCLEI

Binding EnergyBinding Energy• When protons and neutrons come together to form When protons and neutrons come together to form

a nucleus, the mass decreases. a nucleus, the mass decreases.

• This mass decrease is changed into energy to hold This mass decrease is changed into energy to hold the nucleus together. the nucleus together.

E = (E = (m)cm)c22

• The binding energy per mole of nucleonsThe binding energy per mole of nucleons

• The fusion vs fission split occurs at Fe-56, the most The fusion vs fission split occurs at Fe-56, the most stable nucleus. stable nucleus.

• Calculations of energy in J/mole require the mass Calculations of energy in J/mole require the mass in kilograms.in kilograms.

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RATES OF DISINTEGRATION RATES OF DISINTEGRATION REACTIONSREACTIONS

• The time required for one-half of a pure The time required for one-half of a pure radioactive sample to decay is called the half-radioactive sample to decay is called the half-life, tlife, t1/21/2..

• A short half-life means that the isotope decays A short half-life means that the isotope decays quickly.quickly.

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Half-LifeHalf-LifeHalf-LifeHalf-LifeHALF-LIFE is the time it takes for 1/2 a sample is disappear.HALF-LIFE is the time it takes for 1/2 a sample is disappear.

For 1st order reactions, the concept of HALF-LIFE is especially useful.For 1st order reactions, the concept of HALF-LIFE is especially useful.

1919

• Reaction is 1st Reaction is 1st order order decomposition of decomposition of HH22OO22..

Half-LifeHalf-LifeHalf-LifeHalf-Life

2020


• Reaction after Reaction after 654 min, 1 half-654 min, 1 half-life.life.

• 1/2 of the 1/2 of the reactant reactant remains.remains.

2121


• Reaction after Reaction after 3 half-lives, or 3 half-lives, or 1962 min.1962 min.

• 1/8 of the 1/8 of the reactant reactant remains.remains.

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Half-Lives of Radioactive ElementsHalf-Lives of Radioactive ElementsHalf-Lives of Radioactive ElementsHalf-Lives of Radioactive Elements

Rate of decay of radioactive isotopes given in Rate of decay of radioactive isotopes given in terms of 1/2-life. terms of 1/2-life.

238238U --> U --> 234234Th + HeTh + He 4.5 x 104.5 x 1099 y y1414C --> C --> 1414N + betaN + beta 5730 y5730 y131131I --> I --> 131131Xe + betaXe + beta 8.05 d8.05 d

Element 106 - seaborgiumElement 106 - seaborgium263263Sg Sg 0.9 s0.9 s

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• As before in the kinetics chapter, As before in the kinetics chapter,

tt1/21/2 = ( = (llnn 2) / k. 2) / k.

• Use -dN/dt = A = kN to find the rate at Use -dN/dt = A = kN to find the rate at

one point in timeone point in time..

• Use Use llnn A/A A/Aoo = -kt, or = -kt, or llnn

N/NN/Noo = -kt when the problem involves = -kt when the problem involves

two timestwo times..

RATES OF DISINTEGRATION REACTIONSRATES OF DISINTEGRATION REACTIONS

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Radioactive decay is a first order process. Radioactive decay is a first order process.

Tritium ---> electron + heliumTritium ---> electron + helium

33HH 00-1-1ee 33HeHe

If you have 1.50 mg of tritium, how much If you have 1.50 mg of tritium, how much is left after 49.2 years? tis left after 49.2 years? t1/21/2 = 12.3 years = 12.3 years

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SolutionSolutionln [A] / [A]ln [A] / [A]00 = -kt = -kt[A] = ?[A] = ? [A][A]00 = 1.50 mg = 1.50 mg t = 49.2 yearst = 49.2 years

Need k, so we calc k from: k = 0.693 / tNeed k, so we calc k from: k = 0.693 / t1/21/2

Obtain k = 0.0564 yObtain k = 0.0564 y-1, -1, (from: (from: tt1/21/2 = ( = (llnn 2) / k) 2) / k)

Now ln [A] / [A]Now ln [A] / [A]00 = -kt = - (0.0564 y = -kt = - (0.0564 y -1-1) • (49.2 y) ) • (49.2 y)

ln [A] / [A]ln [A] / [A]00 = - 2.77 = - 2.77

Take antilog: [A] / [A]Take antilog: [A] / [A]00 = e = e-2.77-2.77 = 0.0627 = 0.0627

0.0627 is the fraction remaining !0.0627 is the fraction remaining !

Start with 1.50 mg of tritium, how much is left Start with 1.50 mg of tritium, how much is left after 49.2 years? tafter 49.2 years? t1/21/2 = 12.3 years = 12.3 years


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SolutionSolution

[A] / [A][A] / [A]00 = 0.0627 = 0.0627

0.0627 is the fraction remaining !0.0627 is the fraction remaining !

Because [A]Because [A]00 = 1.50 mg, [A] = 0.094 mg = 1.50 mg, [A] = 0.094 mg

But notice that 49.2 y = 4.00 half-livesBut notice that 49.2 y = 4.00 half-lives

1.50 mg ---> 0.750 mg after 11.50 mg ---> 0.750 mg after 1

---> 0.375 mg after 2---> 0.375 mg after 2

---> 0.188 mg after 3---> 0.188 mg after 3

---> 0.094 mg after 4---> 0.094 mg after 4

Half-LifeHalf-LifeHalf-LifeHalf-LifeStart with 1.50 mg of tritium, how much is left Start with 1.50 mg of tritium, how much is left after 49.2 years? tafter 49.2 years? t1/21/2 = 12.3 years = 12.3 years

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Rate of Radioactive DecayRate of Radioactive Decay• The rate of decay or activity, A, is directly proportional The rate of decay or activity, A, is directly proportional

to the number of atoms present: to the number of atoms present:

-dN/dt = A = kN, -dN/dt = A = kN,

where k is the rate constant and N is the number of where k is the rate constant and N is the number of atoms. atoms.

• From this equation, we can see that the rate law is first From this equation, we can see that the rate law is first order. order.

Therefore, Therefore, llnn [A/A [A/Aoo] = -kt, ] = -kt, or or

llnn [N/N [N/Noo] = -kt. ] = -kt.


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Radiochemical DatingRadiochemical Dating

• C-14 dating is used to determine the carbon date of C-14 dating is used to determine the carbon date of substances that were once living. substances that were once living.

• It is based on the assumption that the rate of C-14 in It is based on the assumption that the rate of C-14 in the atmosphere is and has been constant based on the atmosphere is and has been constant based on the conversion of N-14 to C-14 by cosmic neutron the conversion of N-14 to C-14 by cosmic neutron bombardment. bombardment.

• In the equationIn the equation l lnn A/A A/Aoo = -kt, A = -kt, Aoo = 14 = 14 d/mind/min..g, the baseline specific activity. g, the baseline specific activity.

• Other methods involve Pb - U and K - Ar.Other methods involve Pb - U and K - Ar.


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Carbon-14 changes

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ARTIFICIAL ARTIFICIAL TRANSMUTATIONSTRANSMUTATIONS

• The transuranium elements are made by The transuranium elements are made by bombarding target atoms of uranium with bombarding target atoms of uranium with nuclei of other elements. nuclei of other elements.

• The process is also used to produce other The process is also used to produce other isotopes of elements that do not naturally isotopes of elements that do not naturally occur, O-17, I-123, etc.occur, O-17, I-123, etc.

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NUCLEAR FISSIONNUCLEAR FISSION

• Fission is the process of splitting heavy nuclei Fission is the process of splitting heavy nuclei to produce lighter nuclei and energy. to produce lighter nuclei and energy.

• This is the process used in nuclear reactors. This is the process used in nuclear reactors.

• The most common element fissioned is U-235. The most common element fissioned is U-235.

• This isotope must first be converted to U-236 by This isotope must first be converted to U-236 by a slow moving neutrona slow moving neutron

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NUCLEAR FISSIONNUCLEAR FISSION

Radioactive waste products and run-away Radioactive waste products and run-away reactions are a concern with this type of power reactions are a concern with this type of power production.production.

3333

This figure demonstrates the chain reaction phenomenon of fission

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Fission reactorFission reactor

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NUCLEAR FUSIONNUCLEAR FUSION

• Nuclear fusion is the joining together of two Nuclear fusion is the joining together of two light nuclei to produce a heaver nucleus and light nuclei to produce a heaver nucleus and energy. energy.

• The process occurs on the sun and in The process occurs on the sun and in Hydrogen Bombs. Hydrogen Bombs.

• Attempts to use this process to produce Attempts to use this process to produce electrical energy has been unsuccessful to electrical energy has been unsuccessful to date, but progress has been made. date, but progress has been made.

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RADIATION EFFECTS AND UNITS RADIATION EFFECTS AND UNITS OF RADIATIONOF RADIATION

• Rontgen, rad, rem, and curie are common Rontgen, rad, rem, and curie are common

units used. units used.

• Exposure comes from natural sources, Exposure comes from natural sources,

about 65%; medical sources, about 32%; about 65%; medical sources, about 32%;

and artificial sources, about 3%.and artificial sources, about 3%.

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APPLICATIONS OF APPLICATIONS OF RADIOACTIVITYRADIOACTIVITY

• Food irradiation to reduce spoilage and kill Food irradiation to reduce spoilage and kill bacteria, mold, and yeasts.bacteria, mold, and yeasts.

• Radioactive tracers for following molecules.Radioactive tracers for following molecules.

• Radioactive isotopes for biomedical purposed Radioactive isotopes for biomedical purposed including cancer and medical imaging.including cancer and medical imaging.

1 chapter 23 nuclear chemistry. 2 the nature of radioactivity

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