CN-Chap 22

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1 Chemistry Matter and Its Changes 4/e Brady Senese Chapter : Nuclear Reactions and Their Role in Chemistry Reference: Chapter 8 in Chemistry 6/e by Steven S. Zumdahl & Susan A. Zumdahl Nucleus Small size radius of a typical nucleus: ~ radius of a typical atom: ~ cm cm Very large density density of the nucleus ~.6 4 g/cm 3 nucleus material ~.5 9 tons Huge energy that holds it together nuclear process energy

2 nucleons (Composed of quarks) atomic number (Z) # of mass number (A) (# of protons) + (# of neutrons) isotopes atomic # but mass # nuclide A Z X 3. Mass and energy are conserved in all of their forms 4

3 Isotopes with unstable atomic nuclei are and are called radionuclides. Radioactive isotopes undergo nuclear reactions as they emit high-energy particles and/or electromagnetic radiation. By the early 9 s physicist realized that the mass of a particle could not always be treated as constant. 5 The mass (m) of a particle depends on its velocity (v) relative to an observer mo m = ( v / c) c = speed of light = 3. 8 m/s v = speed of the particle in m/s m o = particle s rest mass (mass at v = ) The rest mass of the particle can be measured in the lab when the sample is at rest (from our viewpoint). 6

4 The Law of Conservation of Mass-Energy: The sum of all the energy in the universe and the sum of all the mass (expressed as an equivalent in energy) is a constant. The Einstein equation relates the change in rest mass to an energy change. If E is negative, mass is lost from the system. Because c is so large, an enormous amount of energy accompanies even a small change in rest mass. 7 Consider the combustion of methane: o CH 4( g) + O ( g) CO ( g) + H O( l) H = 89 kj Using the Einstein equation, the loss of 89 kj of energy corresponds to the conversion of 9.89 ng of mass into energy. This is about 7 % of the mass of mol of CH 4 and mol of O. Such tiny changes are not detectable by laboratory balances. Thus, the Einstein equation has no practical application in chemistry. 8

5 . The energy required to break a nucleus into separate nucleons is called the nuclear binding energy 9 The mass of an atomic nucleus is always a little smaller than the sum of the rest masses of each of its nucleons (protons and neutrons). The mass difference is called the. The mass difference represents the amount of mass converted into energy, and released by the system, as the nucleons gathered to form the nucleus. This energy is also called the because this amount of energy would need to be added to break the nucleus apart. The higher the binding energy, the stable the nucleus.

6 Consider the case of one He-4 nucleus: rest massof an isolated proton = u rest massof an isolated neutron = u rest massof one The massdefect is then m o = u Using the Einstein equation For theformation of mol 4.54 E = nucleus 4 He nucleus = u = ( u) + ( u) u E = u u = 4.54 J 4 He: -7 kg (3. 3 J 6. nuclei =.73 mol 8 J mol m s ) Consider a hypothetical process: Mass of (8 Mass of n + 8 n + 8 H 6 8 =.6784 O nucleus= O H) = 8( m =.69 E = (.366 E per E per nucleon for -5 O nucleus = g g g) + 8(.676 g/nucleus =.366g/mol kg/mol)(3. = ev = - m/s) J/nucleus MeV/nucleus O = 7.98 MeV/nucleon J 8 = g) J/mol

7 The energy from forming only 4 g of He ( mol) could keep a 6 W light bulb burning for over 44 years. The formation of a nucleus from its nucleons is called nuclear ( ). - No practical method for sustaining nuclear fusion currently exists on earth. The binding energy per nucleon has a maximum (8.79 MeV) at 56 Fe, meaning it is the most stable of all nuclei. As the mass number increases, the binding energy per nucleon begins to drop. One expects, and finds, that some of the heavier isotopes might breakup and form more stable isotopes. Nuclear ( ) is the spontaneous breaking apart of a nucleus to form isotopes of intermediate mass number. 3.3 Radioactivity is an emission of particles and/or electromagnetic radiation by unstable atomic nuclei 4

8 Except for H, all atomic nuclei have more than one proton, each bearing a positive charge. The force of attraction that keeps the protons, which repel, together is called the. The nuclear strong force overcomes the force of repulsion between protons and binds both the protons and neutrons together in the nucleus. Neutrons help dilute the electrostatic repulsions by intermingling with the protons in the nucleus. Nuclei with too few neutrons and lots of protons are often unstable and occur among radionuclides.. 5 Radionuclidies undergo radioactive decay ( ), meaning they eject small nuclear fragments and sometimes high energy electromagnetic radiation as well. This is called ( ). - About 5 of the approximately 35 naturally occurring isotopes are radioactive. Radioactive decay A & Z, C 4 6 7N + 4 6

Types of Radioactive Decay a-particle emission : 38 9 4 34 9 U He + Th The α particles bear a charge of ( 4 He), which is omitted from its symbol.

9 Types of Radioactive Decay a-particle emission : U He + Th The α particles bear a charge of ( 4 He), which is omitted from its symbol. α-emitting radionuclides are common near the end of the periodic table. α particles are the most massive of any commonly emitted by radionuclides. After traveling only a few centimeters in air, an α particle will lose kinetic energy, pick up electrons, and become a neutral He atom. α particles cannot penetrate the skin, but can cause cancer in the soft tissues of the lungs or intestinal track if emitters are carried in air or on particles of food. 7 b-particle emission : H He tritium helium-3 β particle antineutrino Both the β particle and antineutrino are created when a neutron inside a nucleus decays to a proton. e v Emission of a beta particle from a tritium nucleus. The net change can be represented as: n 8

10 α particles are all emitted with the same discrete energy from a given radionuclide. β particles are emitted with a continuous spectrum of energies (with a characteristic upper limit) for a given radionuclide, which appears to violate the law of conservation energy. The ( ) was proposed to account for the missing energy. It is almost massless, has no charge, and was given the symbol. β particles are very small, so are less likely to collide with the molecules of anything it travels through. They can travel up to 3 cm in dry air. Only the highest energy β particles can penetrate the skin. v 9 g-ray emission: U He + Th + γ g radiation consists of high energy photons and often accompanies either α or β radiation. γ radiation is extremely penetrating and can only effectively be blocked by very dense materials like Pb. Emission of gamma radiation involves transition between energy levels within the nucleus. X-rays, like γ rays, consist of high-energy electromagnetic radiation, but their energies are usually less than those of γ radiation. (typically kev) X-rays are emitted by some synthetic radionuclides, but are usually generated by special machines using a highenergy electron beam focused onto a metal target.

positron emission: 54 54 7 Co 6 Fe + e + v + β particle

annihilation radiation photons 5 kev each http://en.

org/wiki/annihilation neutron emission: 87 86 36 Kr 36Kr +

11 positron emission: Co 6 Fe + e + v + β particle neutrino (matter) (antimatter) - (annihilation) e + e γ annihilation radiation photons 5 kev each neutron emission: Kr 36Kr + electron capture: (inner-orbital electron is captured by the nucleus) 5 3 V + e Net change: + 5 Ti + X rays + v n Example.

12 Radioactive disintegration series A radioactive nucleus reaches a stable state by a series of steps series of decay U 6 8 Pb 3.4 Stable isotopes fall within the band of stability on a plot based on numbers of protons and neutrons 4

13 nuclides 79 (with respective to radioactive decay) All nuclides with Z to radioactive decay. are unstable with respective Light nuclides are stable when neutron/proton. Heavy nuclides are stable when neutron/proton > (which increases with Z). nuclides These magic numbers are, 8,, 8, 5, 8, & 6. 5 Isotopes with Z > 83 tend to be emitters. Their nuclei have too many protons. The most efficient way to lose protons is by the loss of an alpha particle. Isotopes occurring above and to the left of the band of stability tend to be emitters. The n/p ratio is apparently too high. A nucleus that undergoes β decay loses a neutron and gains a proton which reduces the ratio. n e + p v + : neutron = proton F Ne e v 6

Isotopes lying below and to the right of the band are positron emitters. These nuclei have too few neutrons to be stable. Positron emission increases the n/p ratio.

14 Isotopes lying below and to the right of the band are positron emitters. These nuclei have too few neutrons to be stable. Positron emission increases the n/p ratio. : 7 9 F 8 O + e + 7 neutron = proton ν 7 Nature favors even # for protons and neutrons. Odd-even rule: When the numbers of neutrons and protons in a nucleus are both even, the isotope is far more likely to be stable than when both numbers are odd. The odd-even rule is related to the spins of nucleons. Of the 64 stable isotopes, only 5 have odd numbers of both protons and neutrons. 57 have even numbers of both protons and neutrons. Another rule for nuclear stability is based on magic numbers of nucleons. Magic numbers supports the hypothesis that a nucleus has a shell structure with energy levels like those for electrons. 8

15 .5 Transmutation is the change of one isotope into another 9 Transmutation ( ) can be caused by radioactive decay or by the bombardment of nuclei with highenergy particles such as: Alpha particle (from natural emitters) Neutrons from atomic reactors Protons made by stripping electrons from hydrogen Beta particles (electrons) are not normally used because they are repelled by the electrons around the target nuclei. 3

16 In 99, Lord Rutherford observed the first artificial transmutation: He + 7N 9F* 8O + In 934, Irene Curie Joliot and Frédéric Joliot (935 Nobel Laureate in Chemistry) produced the first artificial radioactive isotope: 4 Al + He P + n P α particles can be produced from the bombardment of lithium-7 with protons: 3 5 Si p + 3Li 4Be* + 4 e He 3 A given compound nucleus can be made in a variety of ways. For example, aluminum-7 can be made by any of the following routes: 4 He + p + H Na Mg Mg Al* Al* Al* The decay mode of the compound nucleus depends on its nuclear energy

17 Over the years, many other transmutation have been achieved, mostly using particle accelerators. By using neutron and positive-ion bombardment, scientists have been able to extend the period table (Np, Z = 93) U+ n U Np Z = 93 ~ & 4 (transuranium elements) 6 8 Ni + Pb Ds* Ds e n 33.6 How is radiation measured? 34

18 (Geiger-Müller counter, or Geiger counter): (radioactivity) High - energy particle + Ar ( g) Ar ( g ) + e (scintillation counter): (radioactivity) ( ZnS) (film dosimeter): 35 (activity): (disintegrations) ; If a sample of matter is highly radioactive, many atoms are undergoing decay per unit of time. A small number of nuclei decaying at a rapid rate can produce the same activity as a larger number of atoms decaying at a slower rate. : curie (Ci) = 3.7 disintegrations/s = the activity of. g of Ra-6 µci = 37, disintegrations/s becquerel (Bq) = disintegration/s 36

19 The rate of decay is proportional to the number of nuclides. This represents a first-order process. N activity = = t ln( N N kn ) = kt k: decay constant half - life t / = Example. 37 The energy equivalent to the quantity of radiation absorbed by some material is defined in units of absorbed dose or simply dose. The SI unit of absorbed dose is the gray (Gy) Gy = J of energy absorbed per kg of material The Radiation Absorbed Dose (rad) is an older unit rad =. Gy / : () (activity, i.e. number of disintegrations per second) () (a or b or g) (3) ( ) 38

20 A different unit is used for measuring the biological effects of radiation: H = DQN H: dose equivalent, SI unit = sievert (Sv) D: absorbed dose Q: a factor that takes into account the biologically significant properties for α radiation, Q, for β or γ radiation, Q N: any other factor bearing on the net effect Even small absorbed doses can be biologically harmful. The radiation can generate very reactive species with unpaired electrons called. Once the radicals are generated, they can start a chain of undesirable chemical events. 39 The rem ( ) is an older unit for dose equivalent: rem =. Sv Röntgen (R) = the quantity of X-ray or γ-ray radiation delivered to.93 g of air, such that the ions produced in the air carry C of charge. rem (röntgen equivalent man) = a dose of any radiation that has the same effect of R mrem = -3 rem A typical X-ray involves about.7 rem 4

21 A whole body dose of 5 rem (.5 Sv) induces noticeable changes in blood. A set of symptoms called radiation sickness develops at about rem, and becomes severe at about rems. Symptoms include: nausea, vomiting, a drop in white blood count, diarrhea, dehydration, prostration, hemorrhaging, and loss of hair If everyone in a large group received 4 rem, half would die in 6 days. 6 rem would kill everyone within a week. 4 Natural and man made sources of radiation contribute to a combined background radiation total of about 36 mrem per person per year. About 8% of the background radiation is from natural sources and the remaining 8% from medical sources (much in the form of X-rays). Radiation sources should be avoided because the intensity of radiation diminishes as /(distance from a source). 4

22 .7 Radionuclides have many medical and analytical applications 43 Dating by Radioactivity -4 (radiocarbon dating, or carbon-4 dating) Originated in the 94s by Willard Libby (Nobel laureate, 96) C e N t / = 573 years C-4 N N+ n 6C+ 4 C-4 C-4 H CO 4 C/ 6 6C 4 ( ) / C C

23 4 C/ 6 6C 4 / / C C : C/ 6 6C 4 C -4 ~3 3-4 ( ~ ) CO ~ 3 4 g / C C Medical Applications of Radioactivity Radiotracers provides sensitive and noninvasive methods for: - learning about biological systems - detection of disease - monitoring the action and effectiveness of drugs - early detection of pregnancy -. etc. : I-3 ( Na 3 I) Tl- Tc-99m 46

24 (food irradiation) g Co-6 Cs- 37 radiolytic products 47 (Materials testing) 6 g ; (Radioactive tracers) e.g. P uptake with Phosphorus-3) (Pest control) 48

(Medical applications) gamma-ray scan of

8 Nuclear fission is the breakup of a nucleus

25 (Medical applications) gamma-ray scan of healthy lungs, produced by technicium (Tc)-99m Study of Alzheimer s disease with positron emission tomography (PET) scans 49.8 Nuclear fission is the breakup of a nucleus into two fragments of comparable size after capture of a slow neutron 5

26 Neutrons are electrically neutral and so can penetrate an atoms electron cloud and enter the nucleus relatively easily. When they enter U-35 the nucleus breaks apart: 35 9 U + n X + Y + b This is called nuclear fission ( ), for example: U + n 9U * 36Kr + 56Ba Uranium-35 is called a fissile isotope because it is capable of fission after neutron capture. The neutrons produced are called secondary neutrons and can be absorbed by other nuclei, leading to a nuclear ( ). n n 5 Fission Processes A self-sustaining fission process is called a chain reaction. Event Neutrons Causing Fission Result subcritical < reaction stops critical = sustained reaction supercritical > violent explosion 5

27 Binding energy in Binding energy in Binding energy in Energy yield from fission 94 Kr : (8.5 MeV/nucleon) 94 nucleons = 8 MeV 39 Ba : (8.5 MeV/nucleon) 39 nucleons = MeV Total binding energy of products: MeV 35 U : (7.6 MeV/nucleon) 35 nucleons = 8 MeV BE is ~ MeV ( J) energy released by each fission event Energy produced by fission of kg (4.5 mol) 35 U is ~8 3 J -- enough to keep a W lightbulb work for 3 years. 53 Coolant Key Parts of a Fission Reactor Reactor Core: 3% control rods. Containment Shell 35 9 U + moderator and - Uranium has been enriched ( ~.7% 35 U ~4%) and is housed in cylinders. - Moderator surrounds the cylinders to slow down the neutrons so that the uranium fuel can capture them more efficiently. - Control rods, composed of substances that absorb neutrons, are used to regulate the power level of the reactor. 54

Fig. 8.4: A schematic diagram of a nuclear power plant. 55 Radioactive waste from nuclear power plants occur as solids, liquids, and gases The gases are mostly isotopes of krypton and xenon.

28 Fig. 8.4: A schematic diagram of a nuclear power plant. 55 Radioactive waste from nuclear power plants occur as solids, liquids, and gases The gases are mostly isotopes of krypton and xenon. With the exception of xenon-85 (t / =. years), the gases have short half-lives and decay quickly. Iodine-3 must be contained because iodine is concentrated in the human thyroid and it could cause cancer. Cesium-37 and strontium-9 can be absorbed by the body and possibly cause leukemia. Some wastes are so long-lived that the solid waste from reactors must be kept away from humans for many centuries. Perhaps the most intensely studied procedure for making solid radioactive waste secure is to convert them to glasslike or rocklike solids, which could then be safely buried deep within a rock stratum or in a mountain believed to be geologically stable with respect to earthquakes and volcanoes 56

29 Fusion ( ) - Combining two light nuclei to form a heavier, more stable nucleus - Producing large quantities of energy (75% H + 6% He + % other elements) 3 3 H+ H+ He+ He+ H H 3 3 He H H+ He 4 4 e He+ He+ e H 57 ( H ) 4 7 K - high-powered lasers - heating by electric currents At present, many technical problems remain to be solved. 58

圖二. 壓水式輕水反應堆核電站示意圖如圖一所示, 當可裂變的鈾 -235 核吸收一個中子, 核裂變就發生了裂變將鈾 -235 核分為兩個具有動能的較小的核 ( 如鍶 -90 和氙 -143)

圖二. 壓水式輕水反應堆核電站示意圖如圖一所示, 當可裂變的鈾 -235 核吸收一個中子, 核裂變就發生了裂變將鈾 -235 核分為兩個具有動能的較小的核 ( 如鍶 -90 和氙 -143) 福島核危機背後的科學繆謙 ( 香港中文大學化學系 ) 翻譯自 Science behind the Fukushima Nuclear Accident by W. H. Chan, Department of Chemistry, Hong Kong Baptist University, 原英文本已上載於 IYC2011 Forum 引言