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Chapter 19 Nuclear Reactions

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1 Chapter 19 Nuclear Reactions

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

3 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

4 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

5 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

6 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

7 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

8 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

9 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

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

11 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

12 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

13 Example 19.1

14 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

15 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

16 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

17 Table 19.1

18 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

19 Table 19.2 – Medical Uses of Radioisotopes

20 Cobalt-60 Therapy

21 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

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

23 Figure 19.1 – Smoke Detector

24 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

25 Figure 19.1 – Irradiated Strawberries

26 Rate of Radioactive Decay
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

27 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) = X 1010 atoms/sec

28 Example 19.2

29 Example 19.2, (Cont’d)

30 Figure 19.3 – Scintillation Counter

31 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

32 Example 19.3

33 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

34 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

35 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

36 Units

37 Example 19.4

38 Example 19.4, (Cont’d)

39 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

40 Binding Energy of Lithium-6
Mass of one mole: g Mass of nucleons: (3 X )+(3 X ) = g Mass defect: = g/mol ΔE = 9.00 X 1010 kJ/g X g = 3.09 X 109 kJ/mol

41 Example 19.5

42 Figure 19.4

43 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

44 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

45 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

46 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

47 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

48 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

49 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

50 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

51 Figure 19.5 – Pressurized Water Reactor

52 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

53 Example 19.6

54 Example 19.6 (Cont’d)

55 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

56 Nuclear Fusion and Stars

57 Figure 19.6

58 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

59 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.

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