William L MastertonCecile N. Hurleyhttp://academic.cengage.com/chemistry/masterton

Edward J. Neth • University of Connecticut

Chapter 19Nuclear Reactions

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

C126 C14

6

Nuclear Equations

• Must always balance with respect to nuclear mass and charge

• 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

HCnN 11

146

10

147

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

or α

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

ThHeU 23490

42

23892

He42

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

PaeTh 23491

01

23490

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

AreK 4018

01

4019

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

KreRb 8236

01

8237

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

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

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

eSiAl

AlnAl01

2814

2813

2813

10

2713

eSiP

nPHeAl01

3014

3015

10

3015

42

2713

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

SrnSr 8538

10

8438

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

2

1

693.0

ln

tk

ktX

X

kXrate

o

• 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

• t1/2 is the half life

• X is the amount of sample at time t

• X0 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 1010 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

• 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

HCnN 11

146

01

147

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 14th 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

• 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

mcE 2

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

mkg

JXm

kg

JXE

kg

J

s

m

s

mkgJ

ms

mXE

s

mXc

1016

2

2

2

2

2

216

8

1000.91000.9

11;11

1000.9

1000.3

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 1010 kJ/g X 0.03437g = 3.09 X 109

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 109 kJ/mol Li-6 X 1 mol Li-6/6 mol nucleons =

5.15 X 108 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• UO2 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 D2O (2H2O) as a moderator

• The use of D2O 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

• UF6

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.