William L Masterton Cecile N. Hurley http://academic.cengage.com/chemistry/masterton

Chapter 19 Nuclear Reactions

Edward J. Neth • University of Connecticut

Outline 1. Radioactivity 2. Rate of Radioactive Decay 3. Mass-Energy Relations 4. Nuclear Fission 5. Nuclear Fusion

Nuclear Reactions vs. Chemical Reactions • • In a chemical reaction • Only the outer electron configuration of atoms and molecules changes • There is no change to the nucleus In a nuclear reaction • Mass numbers may change • Atomic numbers may change • One element may be converted to another

Nuclear Symbols • Recall that a nuclear symbol begins with the element symbol • Mass number is at the top left • Protons + neutrons • Atomic number is at the bottom left • Number of protons = number of electrons 12

C

6 14 6

C

Nuclear Equations • Must always balance with respect to nuclear mass and charge 14

N

7  0 1

n

 14 6

C

 1 1

H

• Notice • Total mass on the left is 15 and the total mass on the right is 15 • Total charge on the left is 7 and total charge on the right is 7

Radioactivity • • Radioactive nuclei spontaneously decompose (decay) with the evolution of energy Radioactivity may be • Natural; there are a few nuclei that are by nature radioactive • Induced; many nuclei can be made radioactive by bombarding them with other particles

Five Modes of Radioactive Decay • We will consider five modes of radioactive decay • • • Alpha ( α) particle emission Beta ( β) particle emission Gamma ( γ) radiation emission • • Positron emission K-electron capture

Alpha Particle Emission • An alpha particle is a helium nucleus • Mass is 4, charge is +2, atomic number 2 • Symbol is 4 2

He

or α • When a nucleus emits an alpha particle, its mass decreases by 4 and its atomic number decreases by 2 238

U

92  2 4

He

 234 90

Th

Beta Particle Emission • Beta particles are high speed electrons • Mass is zero, charge is -1 • Mass number does not change • Effectively the conversion of a neutron into a proton with the emission of an electron • Atomic number increases by 1 234

Th

90   0 1

e

 234

Pa

91

Gamma Radiation Emission • • • • Gamma rays are photons Mass number is zero Charge is zero No change in atomic number or mass number

Positron Emission • • Positrons are anti-electrons • Mass 0 • Charge +1 No change in mass number • • Effectively a conversion of a proton into a neutron Atomic number decreases by 1 40

K

19   0 1

e

 40

Ar

18

K-electron Capture • • Innermost electron (n=1) falls into the nucleus Effect is the same as for positron emission • No change in the mass number • Atomic number decreases by 1 82

Rb

 37  0 1

e

 82 36

Kr

Example 19.1

Induced Radioactivity - Bombardment • • • More than 1,500 isotopes have been prepared in the laboratory Stable nuclei are bombarded with • Neutrons • • Charged particles (electron, positron, alpha) Other nuclei The result is a radioactive nucleus

Examples of Bombardment Reactions 27 13

Al

 0 1

n

 28 13

Al

28

Al

13  28 14

Si

  0 1

e

• Aluminum-27 is converted to radioactive aluminum-28 by neutron bombardment, which decays by beta emission 27

Al

13  2 4

He

 30 15

P

 0 1

n

30

P

15  30 14

Si

  0 1

e

• Aluminum-27 is converted to phosphorus-30 by alpha particle bombardment; P 30 decays by positron emission

Transuranium Elements • • Elements beyond uranium are synthetic, having been prepared by bombardment reactions Most nuclei produced have very short half-lives • In some cases, only the decay products are observed • As of October, 2006 the heaviest element reported is Element 118, Uuo-294

Table 19.1

Applications of Isotopes • Medicine • Some isotopes find use in medical diagnostics and treatment • • Cancer treatment • • Iodine-131 for thyroid cancer Cobalt-60 for treatment of malignant cells Diagnostics • • PET, positron emission tomography: carbon-11 Radioactive labeling

Table 19.2 – Medical Uses of Radioisotopes

Cobalt-60 Therapy

Chemical Applications • Neutron activation analysis • Sample bombarded by neutrons, inducing radioactivity • • Isotopes normally decay by gamma emission Activation of strontium in bones of fossils can indicate something about the diet, since plants contain more strontium than animals 84 38

Sr

 0 1

n

 85 38

Sr

Commercial Applications • Smoke detectors • • Americium-241 Radioactive source ionizes air, which completes a circuit; smoke particles open the circuit and trip the alarm

Figure 19.1 – Smoke Detector

Food Irradiation • Gamma radiation treatment • • Kills insects, larvae and parasites Food that is irradiated has a longer shelf life and can be rid of parasites such as trichina in pork

Figure 19.1 – Irradiated Strawberries

Rate of Radioactive Decay

rate

ln

X o X



kX



kt k

 0 .

693

t

1 2 • • Radioactive decay is a first order process The equations for first-order reactions from Chapter 11 apply to radioactive decay • • • • k is the first-order rate constant t 1/2 is the half life X is the amount of sample at time t X 0 is the amount of sample at t=0

Activity • Activity is the rate of decay • • • Number of atoms per unit time A = kN Units of activity • • 1 Becquerel (Bq) = 1 atom/sec 1 Curie (Ci) = 3.700 X 10 10 atoms/sec

Example 19.2

Example 19.2, (Cont’d)

Figure 19.3 – Scintillation Counter

Age of Organic Material • W.F. Libby, University of Chicago, 1950s • Age of organic material related to the decay of carbon-14 • Carbon-14 forms in the upper atmosphere by bombardment of nitrogen-14 by neutrons • • • 14

N

7  0 1

n

 14 6

C

 1 1

H

Carbon-14 incorporates itself into living things • • Steady-state while the organism is alive Once an organism dies, C-14 level falls due to radioactive decay The original rate of decay is 15.3 atoms/min Half-life of C-14 is 5730 yr

Example 19.3

The Shroud of Turin • A sample of 0.1 g of the Shroud of Turin was analyzed for its C-14 content • Evidence showed the flax used to weave the shroud dated from the 14 th century • Could not have been the burial cloth of Christ

Mass-Energy Relations • The energy change accompanying a nuclear reaction can be calculated from the equation 

E



c

2 

m

• Where • Δm = change in mass = mass of products minus mass of reactants • ΔE = change in energy = energy of products – energy of reactants • c is the speed of light

Change in Mass • In any spontaneous nuclear reaction, the products weigh less than the reactants • Therefore, the energy of the products is less than the energy of the reactants • There is a release of energy when the reaction takes place

Units

c

 3 .

00

X

10 8

m s



E

 9 .

00

X

10 16

m

2

s

2  

m

1

J

 1

kg m

2

s

2 ; 1

m

2

s

2 

J

1

kg



E

 9 .

00

X

10 16

J kg



m

 9 .

00

X

10 10

J kg



m

Example 19.4

Example 19.4, (Cont’d)

Nuclear Binding Energy • The nucleus weighs less than the sum of the individual masses of the neutrons and protons • • This is called the mass defect The mass defect leads to the binding energy, which holds the nucleus together

Binding Energy of Lithium-6 • • • • Mass of one mole: 6.01348 g Mass of nucleons: • (3 X 1.00867)+(3 X 1.00728) = 6.04785g

Mass defect: 6.04785 - 6.01348 = 0.03437g/mol ΔE = 9.00 X 10 10 kJ/g X 0.03437g = 3.09 X 10 9 kJ/mol

Example 19.5

Figure 19.4

Nuclear Stability and the Binding Energy • Binding energy per mole of nucleons • Divide the binding energy by the number of nucleons • For Li-6 this is • 3.09 X 10 9 kJ/mol Li-6 X 1 mol Li-6/6 mol nucleons = 5.15 X 10 8 kJ/mol • Release of the binding energy • • Nuclear fission: split large nucleus into smaller ones Nuclear fusion: fuse small nuclei into larger ones

Nuclear Fission • • Discovery, 1938 • • Otto Hahn Lise Meitner World War II • The Manhattan Project – produced the first atomic bomb • First nuclear explosion, July 16, 1945 • • Hiroshima, August 6, 1945 Nagasaki, August 9, 1945

The Fission Process • • Uranium-235 is 0.7% of naturally occurring uranium U-235 undergoes fission • Splits into two unequal fragments • Releases more neutrons than are consumed

The Fission Process (Cont’d) • The first products of nuclear fission are radioactive and decay by beta emission • Note that in the fission process, more neutrons are produced than consumed • • A chain reaction results Energy is released due to the conversion of mass into energy

Chain Reactions • To sustain a chain reaction, the sample of fissile material must be large enough to contain the neutrons that are generated • Samples that are too small will not sustain a chain reaction • The sample that will sustain a chain reaction is called a

critical mass

Nuclear Reactors • About 20% of the electricity generated in the US comes from the fission of U-235 in nuclear reactors • US reactors are called light water reactors • • • UO 2 pellets in a zirconium alloy tube Control rods are used to moderate the reaction • • Can be inserted to absorb neutrons Prevent a runaway chain reaction Tremendous amount of heat is produced, which turns water to steam and turns a turbine to produce electricity • Ordinary water is used both to cool the reaction and to slow the neutrons • Most reactors use ordinary (light) water

Heavy Water Reactors • Canadian reactors (CANDU) • • Use D • 2 O ( 2 H 2 O) as a moderator The use of D 2 O allows the use of natural uranium without enrichment • Enrichment is the process of increasing the U-235 content to a few percent from 0.7% Enrichment is an expensive, technologically demanding process • • Done by gaseous effusion UF 6

Nuclear Energy and History • In the 1970s it was assumed that nuclear reactors would replace fossil fuels (oil, gas, coal) as the major source of electricity • In France, this has indeed happened • In the US, this has not happened • • Accident at Three Mile Island, Chernobyl Disposal of radioactive waste

Figure 19.5 – Pressurized Water Reactor

Nuclear Fusion • • Light isotopes such as hydrogen are unstable with respect toward fusion into heavier isotopes Considerably more energy is released in fusing light nuclei than in splitting heavy nuclei

Example 19.6

Example 19.6 (Cont’d)

Issues with Nuclear Fusion • • As an energy source, nuclear fusion has several advantages over fission • Light isotopes are more abundant than heavy ones • • Greater energy release Non-radioactive products Disadvantages • Large activation energies • High temperatures are difficult to contain

Nuclear Fusion and Stars

Figure 19.6

Key Concepts 1. Write balanced nuclear reactions 2. Relate activity to rate constant and number of atoms 3. Relate activity to age of objects.

4. Relate m and E in a nuclear reaction 5. Calculate binding energies

Key Concepts 1. Draw a diagram for a voltaic cell, labeling the electrodes and diagramming current flow.

2. Use standard potentials to Compare relative strengths of oxidizing and reducing agents.

Calculate E and/or reaction spontaneity.

3. Relate E ° to ΔG° and K.

4. Use the Nernst equation to relate voltage to concentration.

5. Relate mass of product to charge, energy or current in electrolysis reactions.