Lesson 2. Nuclear Chemistry & Energy Outline 2.1 Nuclear Reactions and Radioactivity 2.2 Nuclear Stability 2.3 Kinetics of Radioactive Decay 2.4 Energetics of Nuclear Reactions 2.5 Fission, Transmutation, Fusion 2.6 Interaction of Radiation and Matter Nuclear Chemistry Nuclear chemistry is a study of nuclear reactions. Nuclear reactions are changes in matter originating in the nucleus of an atom. When a nuclei change spontaneously, it emits radiation and thus said to be radioactive. Radioactive elements are widely used in medicine as diagnostic tools and as a means of treatment especially for cancer. They are also used to help determine the mechanisms of chemical reactions, to trace the movement of atoms in biological systems, and to date important historical artifacts. Nuclear reactions are also used both to generate electricity and to create weapons of massive destruction. This lesson discusses nuclear reactions and its applications. 2.1 Nuclear Reactions and Radioactivity Nuclear Reactions One of the nuclear reactions in the universe is the formation of carbon-14 which is a result of collision of gas molecules in the atmosphere with cosmic rays. Cosmic rays are particles traveling at high speeds. Majority of cosmic rays are atomic nuclei. Roughly, 87% of cosmic rays are hydrogen nuclei and about 12% are helium nuclei. Some cosmic rays emanate from the sun and others originate outside the solar system. The energies of cosmic rays are far higher than chemical energies. Chemical energies are usually expressed in kilojoules/mol (kJ/mol) but that of cosmic rays are expressed in electron volts (eV). One (1) eV is equal to 96.5853 kJ/mol. As cosmic rays enter the atmosphere, it collides with gas molecules. These collisions induce nuclear reactions in the atmosphere. One of this nuclear reactions leads to the formation of the radioactive carbon-14. Carbon-14 is unstable isotope of carbon, it undergoes spontaneous radioactive decay or disintegration, ejecting particles from the nucleus. Carbon-14 is formed when a free neutron is absorbed by nitrogen nucleus resulting to carbon nucleus and proton. Nitrogen nucleus and neutron are the reactants and carbon nucleus and proton are the products. The nuclear reaction is written as: 14 7N + 10n 14 6C + 1 1p Take note that reactants and products of a nuclear reaction are written in a form of atomic notation, AZE, where E is the symbol of the atom, A is the mass number and Z is the atomic number or nuclear charge. Similar notation is used for subatomic particles, neutrons (10n), protons (11p), and electrons (0-1e). Nuclear reactions also obey conservation rules, hence, nuclear reactions are balanced. Balancing nuclear equation is with respect to both mass number and nuclear charge. For the nuclear equation to form carbon-14, the sum of the mass numbers for each side of the equation is 15, and the sum of the nuclear charges on each side of the equation is 7, therefore the nuclear equation is balanced. 13 Radioactivity Radioactive decay or nuclear decay is any process by which an unstable atom or nucleus spontaneously emits subatomic particles. Radiation refers to particles or photons emitted in radioactive decay or nuclear decay. Soon after radioactivity was discovered, physicist Ernest Rutherford demonstrated three types of radiation, namely: alpha rays, beta rays and gamma rays. The different types of radioactive decay are discussed below. ○ Alpha Decay When a nucleus undergoes an alpha decay it ejects an alpha particle (α), which is actually a helium nucleus (42He). During alpha decay, the mass number decreases by 4 and its atomic number decreases by 2. Example 1. a. b. 238 92U 230 90Th 234 4 90Th + 2He 226 4 88Ra + 2He ○ Beta Decay When a nucleus undergoes beta decay it emits a beta particle (0-1β), which is an electron ejected from the nucleus. But how can an electron be ejected from the nucleus where the nucleus does not have electrons? The answer is, a neutron in the nucleus decays into a proton and an electron. The electron is ejected and the proton remains in the nucleus. Detailed studies of the energetics of beta decay also shows that an additional particle with no charge and no mass, is emitted. This particle is called antineutrino (υ). 1 0n 1 0 1p + -1β +υ Since the proton remains in the nucleus, beta decay results to an increase in atomic number by 1. Example 2. a. 146C b. 23491Pa 14 0 7N + -1β + υ 234 0 92U + -1β + υ ○ Gamma Decay Gamma decay is the emission of high energy photons and tends to accompany other types of decay. When an alpha and beta particles leave the nucleus, some energy levels in the nucleus are no longer occupied, making the nucleus in an excited state. To return the nucleus to its ground state, it emits a photon. This photon takes the form of a very high energy gamma radiation (00γ). A photon is a quantum of electromagnetic radiation. Gamma decay accompanies the beta decay of most nuclei. Example 3. a. 14 6C 14 7N + 0-1β + υ + 00γ ○ Electron Capture In electron capture, the nucleus captures an electron from the first (n=1) shell or energy level in the atom. Because the first energy level is called the “K shell”, the electron capture is referred to as K capture. The result of electron capture is that a proton in the nucleus is converted to a neutron and an additional particle is emitted. This particle is called neutrino (ν). 1 1p + 0-1e 1 0n + ν The electron capture causes the nuclear charge to decrease by one. Example 4. a. b. 26 13Al 81 37Rb + 0-1e + 0-1e 26 12Mg + ν 81 36Kr + ν 14 ○ Positron Emission A positron is a positively charged electron (01β). Positron and electron are identical in mass and spin but opposite in charge. In positron decay, a proton decays into a neutron and a positron. 1 1p 1 0n + 01β + ν Positron decay results to a decrease in nuclear charge by 1. Example 5. a. 158O b. 116C 15 7N 11 5B + 01β + ν + 01 β + ν Clicker Exercise 1 1. Complete the following equations and identify the mode of decay: a. 4019K ___ + 0-1β + υ 40 b. 4019K + ___ 18Ar + ν 232 4 c. ____ 92U + 2He d. 4019K ___ + 01β + ν 2. Complete the following nuclear equations: 9 6 4 a. 4Be + ____ 3Li + 2He 24 4 b. ____ + 10n 11Na + 2He 40 1 c. 4020Ca + ____ 19K + 1H 243 d. 24195Am + 42He 97 Bk + ____ 2.2 Nuclear Stability The stability of a particular nucleus depends on a variety of factors, and no single rule can predict whether a particular nucleus is radioactive and how it will decay. Nuclear stability can be visualized through the patterns in the chart of the nuclides (Figure 2.1). The chart of nuclides is a plot of the number of protons versus the number of neutrons of all known nuclei. The blue colored dots in the chart of nuclides are stable nuclides. All stable nuclides fall in a central region and this area with blue color is often called the band of stability. The red colored dots are unstable nuclides and this area with red color is referred to as the sea of instability. At low atomic numbers, the stable nuclides lie along the line with N and Z approximately equal. But, beginning with Z = 20, the band of stability begins to deviate from the line. Eventually, when Z>83 all nuclides are unstable. Nuclides below or to the right of the band of stability tend to emit beta particles to gain stability. While, nuclides above or to the left of the band of stability undergo positron emission or electron capture to increase stability. Heavier nuclei tend to emit alpha particles, decaying successively, and often with beta particle emissions as well, until a stable nucleus is formed. 15 Figure 2.1 Chart of nuclides (Source: Brown,WH & Holme, TA. (2011). Chemistry for Engineering Students. 2nd edition) 2.3 Kinetics of Radioactive Decay Each radioactive decay produces a high energy particle or photon, which allows us to count the decays in a given time period. The rate at which a sample decays or disintegrate is called the activity of the sample. For a sample of N nuclei, the rate of disintegration is ΔN/Δt, where N is the number of nuclei and t is time. The SI unit of nuclear activity is Becquerel (Bq), defined as one disintegration per second. Another unit of nuclear activity is Curie (Ci) defined as the number of disintegrations per second in 1 gram of radium-226. One Ci is equal to 3.7 x 1010 Bq. Kinetic Equation of Radioactive Decay The rate at which a sample decay or disintegrate is a first order reaction. The activity decreases exponentially with time (t) as shown in Figure 2.2. Since the activity is proportional to the number of nuclei present, N also decreases exponentially. The equation for the curve in Figure 2.2 is: Nt = N0 e-kt where: No is the initial number of nuclei or initial activity Nt is the number of nuclei at time t or activity at time t k is the decay constant t is time Taking the natural logarithm of each side of the equation and rearranging the equation Nt = N0 e-kt ln Nt = ln N0 – kt ln Nt - ln N0 = - kt (ln Nt - ln N0 = - kt) * -1 ln N0 – ln Nt = kt ln N0/ Nt = kt (kinetic equation for radioactive decay) 16 Figure 2.2 Radioactive decay following first order kinetics (Source: Brown,WH & Holme, TA. (2011). Chemistry for Engineering Students. 2nd edition) Half-life (t1/2) Half- life (t1/2) is the time it takes for N0 to decay or disintegrate to one half its original value (Nt = ½ * N0). From the kinetic equation: ln N0/ Nt = kt Substitute Nt with ½* N0 @ t1/2 in the kinetic equation ln N0 / (1/2 *N0) = k t1/2 ln N0 / (N0/2) = k t1/2 ln 2 = k t1/2 t1/2 = ln 2 / k t1/2 = 0.693/k Example 6. The half-life of carbon-14 used in radiocarbon dating is 5730 years. What is the decay constant for carbon- 14? Given: Required: t1/2 = 5730 years k Solution: Solve for the decay constant (k) using the formula of t1/2: t1/2 = 0.693/k t1/2 = 0.693/k k = 0.693/ t1/2 k = 0.693/ 5730 yr k = 1.21 x 10-4 yr-1 Radiocarbon Dating The kinetic equation of radioactive decay is useful in determining the age of artifacts. Determining the age of the artifacts is known as radiocarbon dating. Why carbon is used in dating the artifacts? As mentioned in section 2.1, carbon-14 is constantly formed through the interaction of cosmic rays and the atmosphere. This carbon-14 is incorporated into plants and animals, although carbon-14 decays, it is constantly replenished in living organisms, so that the 14C/12C ratio in living organisms remains relatively constant over time. Once a plant or 17 animal dies, the decay of 14C continues and the ratio of 14C/12C in the remains of plants and animals decreases as time passes. So, when a plant or animal dies, the 14C/12C ratio becomes the initial point for decay because that is when the 14C/12C ratio starts to decrease due to decay of 14C. If the current value of 14C/12C ratio is known, then, by using the kinetic equation for radioactive decay, the age of the artifacts can be determined. Example 7. A piece of cloth is discovered in a burial pit in the southwestern United States. A tiny sample of the cloth is burned to CO2, and the 14C/12C ratio is 0.250 times the ratio in today’s atmosphere. How old is the cloth? Given: N0/N = 1/0.250 k = 1.21 x 10-4 yr-1 (from example 6) t Required: Solution: Solve for the age of cloth or time (t) using the kinetic equation: ln N0/Nt = kt ln N0/Nt = kt t = (ln N0/Nt) / k t = ln (1/0.250) / 1.21 x 10-4 yr-1 t = 1.386 / 1.21 x 10-4 yr-1 t = 11,454 yr Example 8. 137 Cs has a half- life of 30.2 years. How many years will it take for 100 g sample to decay to 0.01 g? Given: Nt = 0.01 g N0 = 100 g t1/2 = 30.2 yr Required: t Solution: a. Solve for the decay constant (k) using t1/2: t1/2 = 0.693/k t1/2 = 0.693/k k = 0.693 / t1/2 k = 0.693 / 30.2 yr k = 0.023 yr-1 b. Solve for time (t) using the kinetic equation: ln N0/Nt = kt ln N0/Nt = kt t = ln (N0/Nt) / k t = ln (100 g/0.01 g) / 0.023 yr-1 t = 9.21 / 0.023 yr-1 t = 400 yr Example 9. Suppose an archeologist found a carbonized wheat grain in a fire pit at a dig site. Measurement of the decay rate showed 0.070 Bq/g of carbon. How long ago was the wheat harvested? Given: Required: Nt = 0.070 Bq/g of carbon k = 1.21 x 10-4 yr-1 (from example 6) t 18 Solution: Solve for time (t) using the kinetic equation: ln N0/Nt = kt (Take note that the activity of 14C in the atmosphere (and in all living things) today is 0.255 Bq/g of total carbon, so No = 0.255 Bq/g of carbon) ln N0/Nt = kt t = ln (N0/Nt) / k t = ln (0.255 /0.070) / 1.21 x 10-4 yr-1 t = 1.29 / 1.21 x 10-4 yr-1 t = 10,684 yr Clicker Exercise 2 1. What is the half- life of a radioisotope if it decays to 12.5% of its radioactivity in 12 years? 2. A wooden artifact is burned and found to contain 21g of carbon. The disintegrations/min. What is the age of the artifact? 14 C activity of the sample is 105 2.4 Energetics of Nuclear Reactions Nuclear technology is used in nuclear weapons and power plants because it can release tremendous amount of energy. But, where this energy comes from? Binding Energy Binding energy is the energy released by the nucleus. The binding energy of a nucleus is the energy that would be released if the nucleus were formed from a collection of free nucleons. The greater the binding energy, the more stable the nucleus. The calculation of the binding energy is based on the Einstein’s famous equation, which describes the interconversion of mass and energy: E = mc2 where: E is energy m is mass c is the speed of light equal to 2.998 x 108 m/s The mass that is converted to energy is the mass defect (Δm). Mass defect is the mass difference between calculated and observed masses of the atom. Example 10. Calculate the binding energy of helium atom with an observed experimental mass of 4.002603 amu. Given: Required: Experimental mass of He = 4.002603 amu Binding energy (Eb) Solution: Helium atom has 2 protons and 2 neutrons Masses of subatomic particles: p = 1.007825 amu n = 1.008665 amu 19 a. Solve for the calculated mass of helium atom (42He) Mass of helium atom = mass of protons + mass of neutrons Mass of helium atom = 2(1.007825) + 2(1.008665) Mass of helium atom = 4.032980 amu b. Solve for the mass defect (Δm) Δm = calculated mass – experimental mass Δm = 4.032980 amu – 4.002603 amu Δm = 0.030377 amu c. Convert the mass defect to binding energy using Einstein’s equation: E = mc2 Eb = Δm * c2 Eb = 0.030377 amu (2.998 x 108 m/s)2 Eb = 2.7303 x 1015 amu m2/s2 Convert amu to kg: 1 amu = 1.66054 x 10-27 kg Eb = 2.7303 x 1015 amu m2/s2 * 1.66054 x 10-27 kg/amu Eb = 4.5335 x 10-12 kg m2 s-2 Convert kg m2 s-2 to Joules: 1 J = 1 kg m2 s-2 Eb = 4.5335 x 10-12 kg m2 s-2 * 1 J/ 1 kg m2 s-2 Eb = 4.5335 x 10-12 J The binding energy for 1 mole of He: Eb = 4.5335 x 10-12 J * 6.02214 x 1023 mol-1 Eb = 2.7301 x 1012 J/mol (6.02214 x 1023 is Avogadro’s No.) This binding energy of one mole of Helium is equivalent to the energy required to drive an average automobile about 30 times around the Earth at the equator. This is much more energy that can be derived chemically than by just burning a comparable amount of any conceivable fuel. Energy from Nuclear Reactions In power generation and nuclear weapons, the fission reactions of nuclides release tremendous amount of energy. Fission reaction is splitting of heavy nucleus into two smaller ones. Fission reaction may be spontaneous which means the large nucleus simply breaks into smaller nucleus or induced by neutron bombardment. The fission of 235U, for example, is the heart of nuclear power industry. How much energy is released from the fission of 235U? The calculation of energy in this case also uses the Einstein’s equation (E = mc2). The mass that is converted to energy is the mass difference between the fissile nuclide and the resulting products. Example 11. Calculate the energy released by a nucleus of uranium-235 if it splits into a barium-141 nucleus and a krypton-92 nucleus according to the following equation: 235 92U Given: Required: + 10n 236 92U* 141 56Ba + 9236Kr + 3 10n Particles in the nuclear equation are: Uranium-235, Barium-141, Krypton-91, and neutron amount of energy released by fission of Uranium-235 20 Solution: The masses of the particles involved in the nuclear reaction are: Particle Mass (amu) Uranium-235 235.0439231 Barium-141 141.9144064 Krypton-92 91.9261528 Neutron 1.0086649 a. Calculate the mass of reactants Mass of reactants = mass of uranium-235 + mass of neutron Mass of reactants = 235.0439231 amu + 1.0086649 amu Mass of reactants = 236.0525880 amu b. Calculate the mass of products Mass of products = mass of barium-141 + mass of krypton-92 + mass of neutron Mass of products = 141.9144064 amu + 91.9261528 amu + 3 * 1.0086649 amu Mass of products = 235.8665539 amu c. Calculate the mass defect Δm = mass products – mass reactants Δm = 235.8665539 amu – 236.0525880 amu Δm = - 0.1860341 amu d. Convert mass defect to energy E = Δm c2 E = - 0.1860341 amu * * (2.998 x 108 m/s)2 E = - 1.672075 x 1016 amu m2 s-2 * 1.66053886 x 10-27 kg/amu E = - 2.7765 x 10-11 kgm2 s-2 * 1 J/ 1 kg m2 s-2 E = - 2.7765 x 10-11 J Converting to kJ/mol E = - 2.7765 x 10-11 J * 1kJ/1000J * 6.023 x 1023 mol-1 E = -1.67 x 1010 kJ/mol Notice that the nuclear equation for the fission of uranium-235 shows that bombardment the uranium-235 with one neutron produces three neutrons. Each neutron product is capable of inducing another fission reaction. So, the three neutron product can induce three fission reactions that will release nine neutrons to induce nine more fissions from which 27 neutrons are obtained, and so on. The neutron induced fission of uranium-235 is extremely rapid, this sequence of reactants can lead to an explosive chain reaction as illustrated in Figure 2.3. . 21 Figure 2.3 A self- propagating nuclear chain reaction initiated by capture of neutron (Source: Brown,WH & Holme, TA. (2011). Chemistry for Engineering Students. 2nd edition) In nuclear power plants, nuclear engineers control the fission reaction to run away explosive chain reaction. This is done by limiting the number and energy of the neutrons available so that energy can be derived safely and used as a heat source. This is done by inserting cadmium rods or other neutron absorbers to the nuclear reactor. The rods absorb neutrons that would propagate fission reactions. 2.5 Fission, Transmutation and Fusion Nuclear Fission Fission is splitting of heavy nucleus into two smaller ones. Not all nuclei can undergo fission, but those that can do are said to be fissionable or fissile. Some fission reactions are spontaneous which means the large nucleus simply breaks into smaller nucleus. Other fission reactions are induced by neutron bombardment. In induced fission, a neutron is absorbed by a fissile nucleus producing a highly unstable intermediate compound nucleus. This compound nucleus breaks into two smaller nuclei, emitting neutrons in the process. Example 12. 235 92U + 10n 236 92U* 141 56Ba + 9236Kr + 3 10n compound nucleus Nuclear Transmutation Transmutation is changing one nucleus to another either by natural decay or in response to some outside intervention like neutron bombardment. Transmutation reactions are used to produce a number of medically useful radioisotopes. Like for example the radioactive 131I is often used to diagnose thyroid problems. The patient is given an injection of a small amount of the radioisotope, and through natural biochemical pathways carry the 131I to the thyroid. 131I undergoes beta decay, so detection of the radiation emanating from the thyroid is used to produce an image of the gland. Example 13. a. 105B + 10n b. 24094Pu + 10n 11 5B* 7 3Li + compound nucleus 241 94Pu* compound nucleus 4 2He 241 95Am + 0-1e 22 Nuclear Fusion Fusion is combining small nuclei to form larger and more stable nuclei. The energy of the sun originates in a fusion reaction where four hydrogen nuclei combined to form helium nucleus: 4 11H Example 14. a. 21H + 21H b. 11H + 32He c. 21H + 31H 4 2He + 2 01β + 2ν + energy 3 2He 4 2He 4 2He + 10n + 0-1e + 10n All the fusion reactions in example 14 generates tremendous amount of energy. Development of nuclear fusion as commercial source of energy is appealing because hydrogen isotopes are available. Deuterium ( 21H) for example is naturally occurring, its supply is practically unlimited. While, tritium (31H) can be produced from 6Li 4 3 through this reaction: 63Li + 10n 2He + 1H Aside from the availability of hydrogen isotopes, fusion does not produce the high level radioactive wastes that fission generates. However, the use of fusion to generate electricity is complicated by a number of factors and these factors present enormous engineering challenges. 2.6 Interaction of Radiation and Matter The effects of radiation on matter are governed by three factors. The first factor is the amount of radiation to which matter is exposed. Higher doses of radiation have more serious effects. The second factor is ionizing power of radiation and the third factor is penetrating power of radiation. The impact of radiation exposure depends on the characteristics of the radiation which are ionizing power and penetrating power. Ionizing and Penetrating Power of Radiation Radiation can be classified as ionizing and non-ionizing; the distinction between ionizing and non-ionizing radiation is based on the energy carried by the photon or particle. If the energy carried by the photon or particle is greater than the ionization energy of typical atoms or molecules, then the radiation will induce ionization of whatever material it encounters. If the photon or particle energies are smaller than the typical ionization energy, then the radiation is non-ionizing. Non-ionizing radiation includes visible light, radio waves, and microwaves. Ionizing radiation includes X-rays, gamma rays, and alpha and beta particles. Ionizing radiation is much more likely to cause damage to any materials that it encounters including living tissues. Looking into ionizing power alone cannot predict the impact of radiation on matter, the penetrating power must also be considered. Penetrating power refers to how far a photon or particle penetrates into a material before its energy is absorbed or dissipated. Alpha particles, for example, have high energies and therefore have high ionizing power. But because alpha particles are relatively large, it does not tend to penetrate deeply into matter. The energy of the alpha particles is dissipated in the skin. Alpha particles originating outside the body usually do not cause serious harm because they do not penetrate sufficiently to reach internal organs. But alpha particles produced inside the body is very dangerous because their energy will be deposited in internal organs. Just like for example inhaling radon gas, radon is a radioactive material that once inhaled, the radon atom undergoes decay in the lungs. The dissipation of energy of the alpha particles produced during radon decay can cause serious tissue damage. Beta particles have lower energy than alpha particles, but because beta particles are smaller than alpha particles, they can penetrate several centimeters into the body. Beta radiation is often more dangerous than 23 alpha radiation because of its penetrating power. Gamma rays can pass entirely through the body, and their interactions with atoms and molecules in vital organs can do great damage. Both ionizing and penetrating power are important in assessing possible health effects of radiation exposure. Measuring Radiation Dose There are different ways to express radiation because of the interplay between ionizing and penetrating power. One is exposure; this measures the number of ions produced in air. Second is absorbed dose; this measures the amount of radiation actually absorbed by a particular material. Third is equivalent dose; this quantify the resulting damage to human tissue. The quality factor (Q) is used in calculating equivalent dose. It varies from a value of 1 for high energy photons (gamma and X-rays) to about 20 for alpha particles. The units used in measuring dose are: Expression of dose a. Exposure US Roentgen (R) 1 R = 2.58 x 104 C/kg of dry air SI b. Absorbed dose Radiation absorbed dose (rad) 1 rad = 1 erg/g (1 erg = 10-2 J) Gray (Gy) 1 Gy = 100 rads c. Equivalent dose Roentgen equivalent man (rem) 1 rem= Q*absorbed dose in rad Sievert (Sv) 1 Sv = 100 rem 1 Sv = Q*absorbed dose in Gy Clicker Exercise 3 1. Compute the binding energy of the 7Li nucleus; the experimentally determined mass of 7Li is 7.016004 amu. 2. How much energy is released in the fission of 1 kg of 235U according to the equation below? The experimentally determine masses of the products are 136.92532 amu (Te-137) and 96.91095 amu (Zr-97). 235 92U + 10n 137 52Te + 9740Zr + 2 10n 3. An average person is exposed to about 360 millirem of background radiation a year from a variety of sources. Two-thirds of that comes from inhalation of 222Ra produced in soil. Given that 222Ra decays by alpha emission, estimate the absorbed dose in a) Joules and b) Grays for a 60 kg person from one-year inhalation of radon. References 1. Brown, WH & Holme, TA. (2011). Chemistry for Engineering Students. 2nd edition, Brooks/Cole, Centage Learning, USA. 2. Gaffney J & Marley N. (2017). General Chemistry for Engineers. 1st edition, Elsevier Publishing. 3. Moore, JW, Stanitski, CL & Jurs, PC. (2005). Chemistry: The Molecular Science. 2nd edition, Brooks/Cole, Thomson learning, Canada. 24