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Principles of Chem II

by: Vesta Rippin

Principles of Chem II CHEM 1212

Vesta Rippin
GPA 3.98

Julia Metzker

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Julia Metzker
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This 10 page Class Notes was uploaded by Vesta Rippin on Monday October 12, 2015. The Class Notes belongs to CHEM 1212 at Georgia College & State University taught by Julia Metzker in Fall. Since its upload, it has received 35 views. For similar materials see /class/221953/chem-1212-georgia-college-state-university in Chemistry at Georgia College & State University.


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Date Created: 10/12/15
CHEM 1212 Principles of Chemistry 11 Chapter 12 Chemical Kinetics 1 39 1 I a r reaction thei r 39 will not necessarily be fast reaction e g the formation of ammonia from nitrogen and hydrogen I the area of chemistry concerned with reaction rates is called chemical kinetics 121 Reaction Rates I consider the reaction 2N02g gt 2NOg 02g I start with a ask of nitrogen dioxide at 300 C and measure the concentration of nitrogen dioxide nitric oxide and oxygen as the nitrogen dioxide decomposes I results summarized in Table 121 and plotted in Figure 121 I chemical kinetic deals with the speed at which changes occur I the speed or rate of a process is de ned as the change in a given quantity over a specific time I the reaction rate of chemical reaction is de ned as the change in concentration of a reactant or product per unit time I rate A t where A is the reactant or product being considered and the square brackets indicated concentration in molL and t is time the indicates change positive or negative I can calculate the average rate of decomposition of nitrogen dioxide as a function of time see Table 122 I can also calculate and instantaneous rate by computing the slope of line tangent to the curve at that point see Figure 121 I the rate of consumption of N02 rate of production of NO 2rate of production of Oz 12 2 Rate Laws An Introduction I chemical reactions are reversible I so far we have only considered the forward reaction 2NOzg gt 2NOg 02g I the reverse reaction can also occur as NO and Oz accumulate they can react to reform N02 2N0g 02g I gt 2N02g I when gaseous 2N0 is placed in an otherwise empty container initially the dominant reaction is 2NOzg gt 2NOg Ozg I in order to measure just the forward reaction rates data are taken just after the reactants are mixed thus the reaction rate will depend only on the reactants I for the decomposition of nitrogen dioxide we can write Rate kNOzn I such an expression which shows how the rate depends on the concentration of reactants is called a rate law RFGCSU Ch 12 Zumdahl 7th Eddoc Page 1 of 5 I the proportionality constant k called the rate constant and n called the order of the reactant must both be determined experimentally I note that the concentrations of the products do not appear in the rate law because the reaction rate is being studied under conditions where the reverse reaction does not contribute to the overall reaction Types of Rate Laws I the rate law we have used to this point expresses rate as a function of concentration I a rate law that express how the rate depends on concentration is technically called the differential rate law simply called the rate law I a second kind of rate law the integrated rate law also will be important in our study of kinetics I the integrated rate law expresses how the concentration depends on time I the rate law are related ie that is once we determine experimentally either type of rate law for a reaction we know the other one I which rate law we choose to determine by experiment depend on what types of data are easiest to collect I a chemist is usually not interested a rate law for its own sake but because of what it reveals about the steps by which a reaction occurs I see text for Rate Laws A Summary 123 Determining the Form of the Rate Law I the first step in understanding how a given chemical reaction occurs is to determine the form of the rate law I need to determine experimentally the power to which each reactant concentration must be raised in the rate law I consider 2N205soln gt 4NOzsoln Ozg I see data in text I Rate kN2051 kN205 the reaction is first order with respect to N205 Method of Initial Rates I the initial rate of a reaction is the instantaneous rate determined just after the reaction begins just after t 0 I the idea is to determine the instantaneous rate before the initial concentration of reactants have changed significantly I results are then compared to see how the initial rate depends on the initial concentration I the overall reaction order is the sum of the order of each of the reactants RFGCSU Ch 12 Zumdahl 7th Eddoc Page 2 of 5 124 The Integrated Rate Law I so far the rate laws have expressed the rate as a function of the reactant concentrations I also useful to be able to express the reactant concentrations as a function of time given the dilTerential rate law for the reaction F irstOrder Rate Laws I for a reaction of the form aA gt products where the kinetics are first order in A the rate law is Rate A t kA and the integrated rate law is lnA kt lnAo I the integrated rate law shows how the concentration of A depends on time if the initial concentration of A and the rate constant k are known the concentration of A at any time can be calculated I the integrated rate law equation is of the y mx b where a plot of y lnA versus X t is a straight line with slope m k and intercept b lnA0 H alflife of a F irstOrder Reaction I the time required for a reactant to reach half its original concentration is called the halflife of a reaction and is designated by the symbol t1 2 I see Figure 125 39 12 SecondOrder Rate Laws I for a general reaction involving a single reactant that is aA gt products that is second order in A the rate law is Rate A t kA2 and the integrated rate law is l lnA kt llnA0 I a plot of l A versus t will produce a straight line with a slope equal to k I the integrated rate law show how A depends on time and can be used to calculate A at any time I provided k and A0 are known 39 12 I it important to recognize the difference between the halflife for a firstorder reaction and the halflife for a secondorder reaction for a secondorder reaction tlz depends on both k and A0 for a firstorder reaction tlz depends only on k I for a firstorder reaction t1 2 is constant for a secondorder reaction t1 2 each successive half life is double the preceding one Z eraOrder Rate Laws I most reaction involving a single reactant show either firstorder or secondorder kinetics I however sometimes such a reaction can be a zeroorder reaction I the rate law for a zeroorder reaction is Rate kA0 kl k RFGCSU Ch 12 Zumdahl 7th Eddoc Page 3 of 5 I for a zeroorder reaction the reaction the rate is constant I the integrated rate law for a zeroorder reaction is A kt1z A0 I an A02k I zeroorder reaction are most often encountered when a substance such as a metal surface or an enzyme is required for the reaction to occur I see Figure 128 Integrated Rate Laws for Reactions with More Th an One Reactant I so far have considered the integrated rate laws for simple reactions with only one reactant I special techniques are required to deal with more complicated reactions I you can read the rest of this section 125 Rate Laws A Summary I see Table 126 126 Reaction Mechanisms I most chemical reactions occur by a series of step called the reaction mechanism I to understand a reaction we must know its mechanism and one of the main purposes for studying kinetics is to learn as much as possible about the steps involved in a reaction I consider NOzg COg gt NOg COzg I the rate law is known from experiment to be Rate kNOz2 I the balanced equation for a reaction tells us the reactants the products and the stoichiometry but gives no direct information about the reaction mechanism I the mechanism is thought to be I NOzg NOzg gt NO3g NOg with rate constant k1 I N03g COg gt NOzg COzg with rate constant k2 I in this mechanism N03 is an intermediate a species that is neither a reactant mor a product but that is formed and consumed during the reaction sequence I each of the two reactions is called an elementary step a reaction whose rate law can be written from its molecularity I molecularity is defined as the number of species that must collide to produce the reaction indicated by that step I have 39 39 39 39 39 39 39 and tel 39 reactions I define a reaction mechanism it is series of elementary steps that must satisfy two requirements I 1 The sum of the elementary steps must give the overall balanced equation for the reaction I 2 The mechanism must agree with the experimentally determined rate law I see Table 127 RFGCSU Ch 12 Zumdahl 7th Eddoc Page 4 of 5 CHEM 1212 Principles of Chemistry 11 Chapter 18 The Nucleus A Chemist s View 211 Nuclear Stability and Radioactive Decay nuclear stability is the central topic of this chapter nuclear stability can be considered from both a kinetic and a thermodynamic point of View thermodynamic stability refers to the potential energy of a particular nucleus as compared with the sum of the potential energies of its component protons and neutrons kinetic stability refers to the probability that a nucleus will undergo decomposition to form a different nucleus a process called radioactive decay ZX rst a discussion of the symbols used and some terminology A Z is the atomic number A is the mass number X is the element isotopes have different values of A but the same value of Z the term isotopes refers to a group of nuclides with the same atomic number each individual atom is properly called a nuclide not an isotope many nuclei are radioactive example carbon 14 146C gt147N 0le RFGC SU 01e represents an electron which is called a beta particle in nuclear terminology of the approximately 2000 known nuclides only 279 are stable with respect to radioactive decay I Sn has the largest number of stable isotopes 10 see Figure 181 S0111 I e important observations 1 all nuclides with 84 or more protons are unstable with respect to radioactive deca 2 light nuclide are stable when the neutronproton ratio is 1 however heavier element the neutronproton ratio required for stability is greater than 1 and increases with the number of protons 3 certain combinations of protons and neutrons seem to confer special stability for example nuclides with even numbers of protons and neutrons are more often stable than those with odd numbers as shown by the data in Table 181 4 certain speci c numbers of protons or neutrons produce especially stable nuclides the magic numbers are 2 8 20 28 50 82 and 156 this behavior parallels that for atoms in which certain numbers of electrons 2 10 18 36 54 and 86 produce special chemical stability the noble gases Ch 18 Zumdahl 7th Eddoc Page 1 of 7 Types of Radioactive Decay I unstable nucleus decomposes to give daughter nucleus V Particle Emission I leads to a decrease in the atomic mass of a nucleus by four units and a decrease in the atomic number by two units 238 234 92U Dgt 90Th 4zHe 3 Particle Emission I there are no electrons in nuclei the emision of a El particle from a nucleus results from the transformation of a neutron into a proton and an electron 1011 Dgt11H 016 I the emission of an electron leaves the mass of the nucleus unchanged but increases its atomic number by l 146C Dgt147NJr 01 e Positron Emission I a positron is a particle with the same mass as an electron but with a positive charge I symbol is 01 e I an example 3919K Dgt3918Ar01e I the emission of a positron can be considered to result from the conversion of a proton to a neutron 11HDgt10n01e I positrons exist only for a very short time witli109 s they combine with an electron and are converted to highenergy radiation called gamma radiation shorter wavelength and higher frequency than Xrays RFGCSU Ch 18 Zumdahl 7th Eddoc Page 2 of 7 Electron Capture I one of the inner electrons of an atom such as a ls electron enters the nucleus electron capture 8137 Rb 01 e Dgt 8135 KI Ray Emission I the new nucleus formed in a radioactive decay process may be in an excited state in other words its constituent neutrons and protons may not have their most stable arrangement excited nuclei lose energy I see Figure 182 212 The Kinetics of Radioactive Decay I the instant at which any given radioactive nucleus will decay is ingerently unpredictable but each nucleus of the same kind has the same probability of decaying in a certain time interval I the rate of decay of a sample of a radioactive nuclide is proportional to the number of nuclei N in the sample decay rate N or decay rate kN I this is the equation for a rstorder rate law where k is rst order rate costant the decay constant decay rate Nt kN I the integrated form of this equation is In N kt 1n NO or In NNo kt I where N0 is the initial number of nuclei at time t0 and N is the number that remain at time t RFGCSU Ch 18 Zumdahl 7th Eddoc Page 3 of 7 H alfLife I the halflife t1 2 of a radioactive sample is de ned as the time required for the number of nuclides to reach half the original value No2 m 1n NoNk ln 2k 0693k I see Figure 183 I see Figure 184 I the halflives of radioactive nuclides vary over a tremendous range for example 14450Nd has a halflife of5 D 1015 years whil 21484Po has a halflife of 5 D 10394 second I see Table 183 213 Nuclear Transformations I in 1919 Lord Rutherford observed the first nuclear transformation the change of one element into another I he found that by bombarding 147N with alpha particles the nuclide 1780 could be produced 147N4zHe Dgt178011H I 14 years later Irene Curie and her husband Frederick Joliot observed a similar transformation from aluminum to phosphorus 2713 Al 4 HeDgt 3015 F 10 11 I see Figure 185 I see Figure 186 I in the years since 1040 the elements with atomic numbers 93 through 112 called the transuranium elements have been synthesized 214 Detection and Uses of Radioactivity I the most familiar instrument that measures radioactivity is the Geiger Miiller counter I see Figure 187 I another instrument often used is scintillation counter which takes advantage of the fact that certain substances such as zinc sul de give off light when they are struck by highenergy radiation a photocell senses the ashes of light that occur as the radiation strikes and thus measures the number of decay events per unit time Dating by Radioactivity I archeologists geologists and others involved in reconstructing the ancient history of the RFGCSU Ch 18 Zumdahl 7th Eddoc Page 4 of 7 earth rely heavily on radioactivity to provide accurate dates for artifacts and rocks I a method that has been very important for dating ancient articles made form wood or cloth is radiocarbon dating or carbon14 dating a technique originated in the 1940s by Willard Libby an American chemist who received a Nobel prize for his efforts in the field I Carbon14 has a halflife of 5760 yr I carbon is present in all formerly living material I living organism have a steady concentration of 14C taken in from surroundings I when dead the carbon 14 decays I use 23892 U to establish geologic history has long halflife Medical Applications of Radioactivity I determine metabolic pathways using carbon 14 and phosphorus 32 I use other isotopes to image organs or glands I see Figure 188 I see Table 215 215 Thermodynamic Stability of the Nucleus I the thermodynamic stability of a nucleus can be determined by calculation the change in potential energy that would occur if that nucleus were formed from its constituent protons and neutrons I see discussion in text note terms mass defect and binding energy I see Figure 189 216 Nuclear Fission and Nuclear Fusion I see Figure 189 very important implications for the use of nuclear processes as sources of energy I recall energy is released when a process goes from a less stable to a more stable state I the higher a nuclide is on the curve the more stable it is this means that two type of nuclear processes with be exothermic I see Figure 1810 I 1 combining two light nuclei to form a heavier more stable nucleus fusion I 2 splitting a heavy nucleus into two nuclei with smaller mass numbers fission I because of the large binding energies involved in holding the nucleus together both these processes involve energy changes more than a million times larger than those associated with chemical reactions RFGCSU Ch 18 Zumdahl 7th Eddoc Page 5 of 7 Nuclear Fission I nuclear ssion was discovered in the late 193039s when U235 nuclides bombarded with neutrons were observed to split into two lighter elements 10 II 23592 U Dgt14155 Ba 9235 KI 310 II I see Figure 1811 I this reaction releases 21 D 1013 Jmol of 23592 U compare to the combustion of methane where only 8 D 105 Jmol methane are released that s 26 000 000 times as much energy I see Figure 1812 I see Figure 1813 I a fission bomb operates by suddenly combining subcritical masses of fissionable material to form a supercritical mass thereby producing an explosion of incredible I Hiroshima and Nagasaki compare to bombing of Dresden Nuclear Reactors I see Figure 1814 I see Figure 1815 Breeder Reactor I fissionable material is produced while the reactor runs I see reactions in text I breeder reactors used in France France obtains 75 of its electrical energy from nuclear power plants Fusion I large amount of energy are also produced by the fusion of two light nuclei I stars produce their energy through nuclear fusion I our sun which consists of 73 hydrogen 26 helium and 1 other elements gives off vast quantities of energy from the fusion of protons to form helium I see reactions in text I intense research is underway to develop a feasible fusion process because of the readily availability of many light nuclides deuterium 1 H in seawater for example that can serve as fuel infusion reactors I major stumbling block is that high temperatures are required to initiate fusion I must get two protons close to each other electrostatic repulsion must be overcome RFGCSU Ch 18 Zumdahl 7th Eddoc Page 6 of 7


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