Chem 112 Nuclear Chemistry and Kinetics
Chem 112 Nuclear Chemistry and Kinetics Chem 112
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Popular in Chemistry
This 9 page Study Guide was uploaded by Rebecca Plummer on Tuesday January 26, 2016. The Study Guide belongs to Chem 112 at Pennsylvania State University taught by Dr. Raymond Shaak in Spring 2016. Since its upload, it has received 118 views. For similar materials see General Chemistry in Chemistry at Pennsylvania State University.
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Date Created: 01/26/16
Rebecca Plummer Course Chem 112 Professor Dr Raymond Schaak Exam 1 Study Guide Nuclear Chemistry Nuclear Chemistry deals with the reactions inside the nucleus of an atom a Each element has a different mass because of the number of protons neutrons and electrons present 0 Mass amu of p 1007316 0 Mass amu of no 1008701 0 Mass amu of el39 0000549 0 Nuclear Decay Also known as Radioactive Decay the nucleus of an unstable atom oses energy by emitting radiation predictable based on the neutron proton ratio of the mass number 0 5 Types of Nuclear Decay 1 Alpha Decay The alpha particle is the product of the reaction The alpha particle is He4 Helium with a mass of 4 amu 20092U 42He 19690Th SubtracTIfrom 200 to obtain the new atomic mass Subtract 2 from 92 to obtain the new mass number 2 neutrons and 2 protons are discharged from the nucleus of a very heavy radioactive atom to form a new atom with a different mass number 2 Beta Emission Really fast electrons are ejected from the nucleus The beta particle is a product of the reaction Because electrons basically have no mass the atomic mass number stays the same 15053 15054Xe 01e Converting neutrons to protons so add one proton to the atomic number but the mass does not change Use this decay method when there are a high number of protons and neutrons and there are MORE NEUTRONS THAN PROTONS Used when isotopes have a high n0 p1 ratio proton de cient 3 Gamma Emission Gammaray emission is the product of the reaction Protons and neutrons in the nucleus only rearrange themselves inside the nucleus to become more stable Atomic mass and the Atomic Number DO NOT change 0w is the symbol for Gamma Emission This Decay process denotes energy loss from the reorganization of nucleons 4 Positron Emission The beta particle is a product of the reaction Atomic mass number stays the same Atomic number decreases by 1 137N 136C 01e Conve i g a proton to a neutron so subtract one proton for Atomic Number Use this method when there is a smaller atomic mass and the number of PROTONS IS GREATER THAN THE NUMBER OF NEUTRONS Used when the isotope s n0 p1 ratio is low neutron de cient 5 Electron Capture 1 Half Life 0 A halflife is used to describe how long it takes for half of a radioactive sample to decay 0 An equation can be used to nd the halflife I t12 k nuclear decay constant a Belt of Stability O O O Occurs when an electron in the electron cloud is captured and pulled into the nucleus The electron is a reactant of the reaction Looks similar to Positron Emission 11555CS 01e 11554Xe This method converts a proton to a neutron proportional to the rate of decay the higher the k value the faster the decay rate the shorter the halflife 0 Different nuclei have radioactively decay at different speeds so unstable radioisotopes can be found in nature Measures the stability of a nucleus Stable nuclei fall on the belt Any isotope with an atomic number greater than 84 is considered unstable and must go through alpha decay to reduce the atomic mass and atomic number O 0 Therefore it is decreasing the number of protons and neutrons in the nucleus Any isotope not on the belt of stability can go through decay processes to become stable enough fall on the belt 1 Energetics and Nuclear Reactions 0 O O O 0 All spontaneous nuclear reactions are exothermic E mc2 changes in mass is related to energy 1Joule 1kgm2 Binding Energy energy required to break down the nucleus into nucleons protons and neutrons Mass Defect the difference in masses between a nucleus and its nucleons Ex What is the change in energy in Jnucleon in the following reaction 2 11p 2 lon 42He Mass of protm00728 amu Mass of neutron 100867 amu Mass of He4 400150 amu 1First you must nd the mass defect subtract the mass of He 4 from the sum of the masses of the protons and neutrons in the equation Am 2100728amu2100867amu 400150amu Am 00304 amu 2 Because the mass must be in kilograms for energy we must then convert amu to kg Am 00304amu X 1g6022E23amux 1kg1000g Am 5048E29 kg 3 Find the energy in JHe4 nucleus AE 5048E29kgx30E8m2 AE 4543E12 JHe4 nucleus 4 Because the question asked forJnucleon we must divide the Jnucleus by 4 because there are 4 nucleons in a He4 nucleus 1 nucleon1atomic mass unit AE 4543E12 JHe4 nucleus4 nucleons AE 114E12 Jnucleon Nuclear Chain Reactions 0 0 Fusion and ssion are both exothermic reactions When most of the neutrons are lost during a reaction it is called subcritical mass This is a small reaction and nothing big really happens When there is a constant rate of ssion it has a critical mass This is considered a medium reaction and examples are nuclear reactors used in power plants When there is an accelerating rate of ssion it has a supercritical mass This is a very large reaction and an example is a bomb explosion Kinetics 1 First and foremost KINETICS ARE NOT RELATED TO AE o Collision Theory 0 0 Not every collision results in a chemical reaction because not all elements react with every element Some collisions result in atoms just bouncing off each other and proceeding on their own paths The rate of a collision is proportional to of effective collisionstime Rate can be in uenced by 0 Temperature Pressure Volume Surface area Concentration of molecules Catalysts 0 Physical state of the reactants solid liquid or gas 0 Kinetic Molecular Theory of Gases O 0 Activation Energy Ea reactants to products When the energy is less than the activation energy the molecules cannot react with anything Activation energy is just what the reaction needs to get started If the energy is less than what the activation energy requires then nothing will happen and the molecules will just bounce off each other when they collide If the energy is more or equal to the activation energy when molecules collide a reaction will occur minimum energy needed to transform 0 Activation Energy 0 O O In an activation energy plot of energy vs reaction pathway the vertex of the hump is where the reactants are changed to products Therefore the left side of the plot is the reactants and the right side is the products If the products have less energy than the reactants the right side hangs lower than the left side the reaction is exothermic The energy from the start of the reaction to the tip of the hump is the Ea activation energy The energy from the initial energy to the nal energy is AE o This picture can help visualize what was just V attitaied ttt39o stated o The bigger the hump the slower the reaction o The smaller the hump the faster the reaction ene rgy 39 39 39 39 39 39 39 39 39 39 39 39 39 39 39 BroadEH reaction coordinate 0 Activation Energy 0 Fraction of molecules when AE is greater than or equal to Ea I f eEaRT o Arrhenius Equation 0 The collision frequency orientation factor and fraction of molecules are combined to form the Arrhenius Equation 0 K AeEaRT K rate constant collision frequency i A Frequency factory orientation factor E39EaRT fraction of molecules 0 To solve this equation you must take the natural log of both sides 3 nk1k2 39EaR 1T1 39 1T2 D You cannot heat a reaction inde nitely because eventually the molecules you are heating up will burn 0 Reaction Rates 0 A reaction rates plot can tell you a lot about a reaction The reactants will be the line that starts with the highest concentrations I The products will initially start with a concentration of zero D If there are more products than reactants the reaction was a decomposition reaction 0 All reaction rates are positive numbers 0 Generic Rate Law 0 GA 33 gt yC 6D 0 The 0 B y and 6 act only as coefficients just as in stoichiometry O The generic rate law can be expressed as I Rate kAXBY D Because the brackets in the equation represent concentrations rate is measured in Ms i XY is the overall order of the reaction K is the rate constant and the units can vary based on the values of X and y 0 K also depends on temperature X AND Y ARE NOT THE SAME AS THE COEFFICIENTS 0 Elementary Steps 0 Elementary Reaction a reaction that happens because of 1 molecular collision 0 There are three elementary reactions Unimolecular I Bimolecular D Termolecular O Unimolecular one reactant I A products D Themf a unimolecular reaction is directly proportional to the concentration of the reactant molecule A i Rate kAX D The units for A is molesLiter Units for Rate is Ms i Therefore the units for k is Us I Overall Order X 1 O Bimolecular 2 Reactants i A B gt products 0 Rate kAXBY 0 Units for A and B a M so together their units are M2 0 Units for k is 1Ms because the units for Rate always have to be Ms Overall Order XY 11 2 D 2A products gt Rate kAX Overall Order 2x 2 Units for A is M2 Units for Rate is Ms 0 Units for k then is 1Ms O Termolecular 3 reactants least common elementary reaction i A B C products gt Rate kAXBYCZ OverallOrderABC1113 Units for A B and C are M but together they are M3 Units for Rate is Ms Units for k is 1Mzs I 2A B gt 0 Rate kA2B A 23 gt 0 Rate kAB2 products gt 0 Rate kA3 products products 393A O The only time x y or z exponents will equal the coef cients is if the reaction is an stated as an elementary reaction 0 Do NOT assume a reaction is an elementary reaction If it does not speci cally say it is an elementary reaction you cannot assume it is one so do not treat it as one 0 Reaction Mechanisms O 0 Reaction Mechanisms a series of elementary steps that together provide the overall reaction If told that each step is a mechanism you can assume that each step is an elementary reaction Often we don t know what each elementary step is because the reactions go so fast that you can t detect what the intermediate is D Intermediate stable separable molecule 0 The intermediate is used to get from one elementary reaction to another 0 Intermediates are not included in the nal balanced overall equation 0 Rate laws do not include intermediates 0 Experimental Method 1 Method of Initial Rates 0 O The reactants have no effect on the rate when there is a change in concentration of a zero order reactant When a rst order reactant concentration is doubled the rate is doubled When a second order reactant concentration is doubled the rate quadruples 22 When a second order reactant concentration is tripled the rate increases by a factor of 9 32 When a third order reactant concentration is doubled the rate increases by a factor of 8 23 The base is the action being done doubling tripling to the concentration D The exponent is the order of the concentration 0 Experimental Method 2 Integrated Rate Laws 0 First Order Reactions 39 nAAo kt D an easier way to think of this equation is a linear function lnA kt lnA0 O the slope is k with the units of Us slope will be negaUve the yintercept is lnA0 lnA will be along the yaXis t time will be along the xaxis Rate kA 0000 D Halflife of First Order Reactions The amount of time it takes for the concentration to be half of the initial concentration The halflife does not depend on concentration in these reactions n12 39ktlZ O t12 0693k All radioactive decay reactions are rst order reactions so the halflife equation is the same for them all 0 Second Order Reactions D 1A kt 1A0 Again this is a linear function but the slope will be positive for second order reactions Slope is k with the units of 1Ms 1A0 is the yintercept 1A is along the yaxis and time is along the xaxis i HalfLife of First Order Reactions Initial concentration is important in these reactions because they depend on the initial concentration A0 0 2Ao kt12 1A0 O Subtract both sides by 1A0 39 1A0 ktlZ O t12 1kA0 0 Zero order Reactions i A kt 0 k is the slope and because it is negative the graph will also have a negative slope References Sanjaya Y Reaction Pro les UCDavis Chemwiki httpchemwikiucdaviseduphysicalchemistrykineticsmodelingreactionki neticsreactionpro les accessed Jan 31 2016
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