NUCLEAR CHEMISTRY (Part I) - KFUPM

06-Jul-12ChChapter20NUCLEAR CHEMISTRY(Part I)Dr. Al‐Saadi120.1Nuclear Chemistry Nuclear chemistry is a subfield of chemistry dealing withand nuclear properties.radioactivity,y, nuclear processespp pEvery year on the 6th of August,Hiroshima holds the atomicbomb memorial. The attack bythe USA in 1945 with an atomicbomb was one reason for Japanto surrender. This had endedWorld War IIDr. Al‐Saadi21

06-Jul-1220.1Radioactivity Radioactivity is a spontaneous emission of particles and/orelectromagnetic radiation by chemical elements (mostelements with an atomic number greater than 83). Theseelements are called radioactive. Radioactive decay is the process when unstable nuclei emitparticles or electromagnetic radiation.Dr. Al‐Saadi320.1Atomic Number and Mass NumberThis is also knownas a nuclide.Mass number2311NaAtomic numberElement symbolMass number # of protons # of neutronsAtomic number # of protons42

06-Jul-1220.1Nuclear Transmutation Nuclear transmutation (also known as nuclear transformation)is the process where a specific nucleus is bombarded (is hit) bya neutron, an electron or another nucleus.neutron10n147N146Bombardment stepC 11Halso called proton Are there reactions associated with nuclear processes?Yes, there are. But they are very different from ordinarychemical reactions.Dr. Al‐Saadi5Chemical Reactions vs. NuclearReactionsH2 (g) O2 (g) H2O (l)Dr. Al‐Saadi101n 147 N 146 C 1 H20.163

06-Jul-1220.1Subatomic Particles The symbols of some subatomic particles are as follows: In balancing any nuclear equation, we must balance the totalof all atomic numbers and the total of all mass numbers forthe products and reactantsDr. Al‐Saadi720.1Nuclear Equations Example:Identify X in the following nuclear equation:7833As AZ X 0 1 reactant mass numbers product mass numbers78 A 0A 78 reactant atomic numbers product atomic numbers33 Z ( ‒ 1)Z 34Dr. Al‐Saadi78Therefore, X is Se or 34Se84

06-Jul-1220.2Nuclear Facts Some facts about the nucleus:o It occupies a very small portion of theatom. “the radius of an atom is more than 10,000times the radius of the nucleus”oooIt contains most of the mass of the atombecause it contains the heavy componentsof the atoms, i.e. protons and neutrons.Its density is approximately 2 1014 g/cm3.There is an extremely strong force thatholds the subnuclear particles together.This results with a very high energy storedin the nucleus. “Nuclear stability ”Dr. Al‐Saadi920.2Nuclear Stability Nuclear stability is determined by a balancebetween two opposing forces:1. the Coulombic repulsions between protons.2. the nuclear force “also called short‐rangenuclear attractions” which is very strong andtakes place between proton‐proton, proton‐neutron, and neutron‐neutron.If repulsions attractions, the nucleus is unstable.If attractions repulsions, the nucleus is stable.Dr. Al‐Saadi105

06-Jul-1220.2Neutron‐to‐Proton Ratio n/p is the neutron‐to‐proton ratio.o The closer the value of n/p to 1,1 the more stable thenucleus. This is the case for elements having atomicnumbers Z 20.o As Z increases, the n/p ratio increases.Why?Because at higher atomic numbers, the repulsionbetween protons becomes more significant. Thus, moreneutrons are needed to overcome the strong repulsionbetween the protons.Dr. Al‐Saadi1120.2Belt of Stability Stable nuclei are locatedwithin the region of thebelt of stability. Radioactive nuclei arelocated outside the belt ofstability. As Z increases,, the n/p/pratio increases.Dr. Al‐Saadi126

06-Jul-1220.2Useful Rules for Nuclear StabilityNuclei that contain a magic number of protons and/orneutrons are stable.o Magic numbers are 2, 8, 20, 50, 82 and 126.2. Nuclei with even numbers of both protons and neutronsare more stable than nuclei with odd numbers of protonsand neutrons.1.Dr. Al‐Saadi1320.2Useful Rules for Nuclear StabilityAll isotopes of the elements with atomic numbers higherthan 83 are radioactive.4. All isotopes of technetium (Tc, Z 43) and promethium(Pm, Z 61) are radioactive.3.Dr. Al‐Saadi147

06-Jul-1220.2Belt of StabilityFor the region above thebelt, the nucleus tends tolower the n/p/p ratio.For the region below thebelt, the nucleus tends toincrease the n/p/p ratio.β‐particle emission:Positron emission:It leads to an increase inthe number of protonsand a decrease in thenumber of neutrons.Example:Electron capture:They lead to a decrease inthe number of protonsand an increase in thenumber of neutrons.Dr. Al‐Saadi1520.2Nuclear Binding Energy The nuclear binding energy is the energy required to breakup a nucleus into its component protons and neutrons. It represents the conversion of mass to energy which occursduring an exothermic nuclear reaction. The measured masses of atoms are always less than thecalculated sum of masses of neutrons, protons andelectrons. This is known as the mass defect.Dr. Al‐Saadi168

06-Jul-1220.2Nuclear Binding EnergyConsider the199F isotope. Its measured mass is 18.9984 amu.Usingg the followingg masses:11 H 1.007825 amu10n 1.008665 amuMass of an199Fatom can be calculated:9 1.007825 amu 10 1.008665 amu 19.15708 amuMass of 9 protonsand 9 electronsMass of 10 neutronsMass defect According to relativity theory, the loss in mass shows up asenergy (heat) that is given off by the nucleus (exothermicprocess).Dr. Al‐Saadi1720.2Nuclear Binding Energy We can calculate the energy released upon formation of the Fisotope in the previous example.Einstein equation: E ( m)c2 E E (products) – E (reactants) m m (products) – m (reactants) m 18.9984 amu – 19.15708 amu – 0.1587 amu m – 0.1587 amu X1.00 kg6.022 1026 amu – 2.635 10-26 kgc : the speed of light (3.00 108 m/s)Dr. Al‐Saadi189

06-Jul-1220.2Nuclear Binding Energy E (– 2.635 10-26 kg)(3.00 108 m/s)2 (– 2.37 10-11 kg·m2/s2) (– 2.37 10-11 J)Exothermic-11 (– 2.37 10 J) (6.022 1023 /mol) – 1.43 1010 kJ/mol Therefore, the nuclear binding energy for 1 mole of F‐19nuclei is 1.43 1010 kJ.Compare this with the enthalpy for the combustion ofmethaneh((890.4 kkJ per molel off CH4).)Nuclear binding energies are extremely large compared to theenergies associated with ordinary chemical reactions.Dr. Al‐Saadi1920.2Nuclear Binding EnergyThe highest binding energies per nucleonare for elements that belong to this region Nuclear binding energy per nucleon nuclear binding energynumber of nucleonsnumber of protons plus neutronsDr. Al‐Saadi2010

06-Jul-1220.2Nuclear Binding Energy Nuclear binding energy per nucleon nuclear binding energynumber of nucleonsFor F-19, the nuclear2.37 10-11 Jbinding energy per nucleon 1.25 10-12 J/nucleon19 nucleonsDr. Al‐Saadi2120.2Nuclear Binding Energy Exercise:Calculate:a) the nuclear binding energy in kJ/mol, andb) the nuclear binding energy in joules per nucleon of 20883 Bi .The exact atomic mass of bismuth is 208.9804 amu.Mass of 83 protons and 83 electrons:Mass of 10 neutrons:Calculated mass :83 1.007825 83.649475 amu125 1.008665 126.083125 amu209.732600 amu m 208.9804 amu 209.732600 amu 0.7522 amu1.00 kg 27 m 0.7522 amu 1.2491 10 kg266.022 10amu Dr. Al‐Saadi2211

06-Jul-1220.2Nuclear Binding Energy E 1.2491 10 27 kg 3.00 108 m/s 2 E 1.12 10 10 kg m2 / s2 1.12 10 10 J E 1.12 10 10 J 6.022 1023 /mol E 6.74 1013 J/mol 6.74 1013 kJ/mol“Exothermic”Nuclear binding energy 6.746 74 1013 kJ/molb) Nuclear binding energy per nucleon.1.12 10 10 J E 5.38 10 13 J/nucleon208 nucleonsDr. Al‐Saadi2320.3Nuclear Radioactivity Radioactivity is a spontaneous emission of particles and/orelectromagnetic radiation by chemical elements (like thosewith an atomic number greater than 83, or those that lieoutside the belt of stability). These elements (or nuclei) arecalled radioactive. Types of radioactive emission:2 γ raysElectromagnetic radiation with veryshort wavelength (0.1 – 104 nm)Dr. Al‐Saadi2412

06-Jul-1220.3Nuclear Radioactivity The radioactive decay series is asequence of nuclear reactions(nuclear decay or disintegration)that result in the formation of astable isotope. The scheme of the a radioactivedecay series usually shows the half‐lives of all the nuclear decay stepsinvolved. Each step in the series must bebalanced in terms of protons andneutrons.Dr. Al‐Saadi2520.3Nuclear RadioactivityStep 1:23892U Th 24 α23490Step 1Step 2parentStep 3Dr. Al‐SaadiSt 2Step2:daughter23490Th 23491P Pa 0‐12613

06-Jul-1220.3Kinetics of Radioactive Decay For a particular radioactive nuclide, the rate of decay can beexpressed as:where N is the number ofΔNrate ‒nuclides in a given sampleΔt)(1000 atoms of acertain nuclides990 atomsremainedunchanged1 minuteThe rate of decay will be:(‒10 atomsdecayed1000 – 990 atoms60 s)Dr. Al‐Saadi2720.3Kinetics of Radioactive Decay The rate of decay is directly proportional to the number ofnuclides (N) in a given sample:ΔNrate ‒α NΔtΔN The rate law of a 1 order process kNrate ‒ Δt(())stThus, all radioactive decays are assumed to obey 1st orderkineticskinetics. The integrated 1st order rate law is:lnDr. Al‐SaadiNt ‒ ktN0N0 : The original number of nuclides (at t 0)Nt : The number of remaining nuclides at time tk : The rate constant of radioactive decay of thatgiven nuclide.2814

06-Jul-1220.3Half‐life of Radioactive Decay The half‐life (t1/2) of aradioactive sample is the timerequired for the number ofnuclides to reach half theoriginal value.t1/2 0.693kThe half-life of a 1st order processThus, rate constants ofdifferent radioactive decaysprocesses are different.Dr. Al‐Saadi2920.3Half‐life of Radioactive DecayThe decay of a 10.0‐g sample of strontium‐90 over time.Dr. Al‐Saadi3015

06-Jul-1220.3Half‐life of Radioactive Decayk 0.693t1/2α1.53 10-10 yr-1β0.0288 day-1β0.103 hr-1ααααThe rate constant of a radioactive decay process is independent ofenvironmental conditions, such as pressure and temperature.Dr. Al‐Saadi3120.3Half‐life of Radioactive Decay Example:Technetium‐99 (Tc) is used to form picturesof internal organs in the body, such asbones and the heart. The rate constant ofthe decay of Tc is known to be 1.16 10‐1h‐1. What is the half‐life of this isotope?t1 / 2 Dr. Al‐Saadi0 .693k0.693 5.98 h1.16 10 1 h 13216

06-Jul-1220.3Dating Based on Radioactivity Decay Half‐lives of radioactive isotopes “usually known as atomicclocks” are used to determine the age of several materialssuch as rocks, dead humans and animals, and ancient objects. This measurement is called radioactive dating.Archaeologists routinely use radioactive decays to determine the age ofancient and historical materials and artifacts.Dr. Al‐Saadi3320.3C‐14 Radioactive Decay C‐14 is continuously produced inthe atmosphere by nucleartransmutation of N‐14.147N 01 n 146C 11 H Also, C‐14 continuously decomposesby transmitting β‐ particles.146C 147N 0‐1 Over years, the rates of the above twoprocesses have become equal. Thus, theratio of C‐14/C‐12 in the atmosphere andin living matters remains almost constant.Dr. Al‐Saadi3417

06-Jul-12Dating Based on C‐14 RadioactiveDecay146C 147N 0‐120.3t1/2 5715 yyr When the living matter (human, plant, animal, etc.) dies, C‐14 inside it continue to decay without replenishment, as itdoesn’t breath anymore. Comparing the C‐14/C‐12 ratio of a deadmatter with the ration of a living mattergives information about the age of thatparticular matter.10 gC‐14Dr. Al‐Saadiafter 5717 yr5gC‐142.5 gC‐14after 11432 yrDating Based on C‐14 RadioactiveDecay3520.3 In practice, the decay activity of C‐14 in the old object iscompared with the corresponding one in a living object.The activity is measured in units of (disintegrations persecond).Dr. Al‐Saadi3618