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Principles of Chemistry I

by: Houston Kovacek

Principles of Chemistry I CHM 211

Marketplace > Marshall University > Chemistry > CHM 211 > Principles of Chemistry I
Houston Kovacek
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This 0 page Class Notes was uploaded by Houston Kovacek on Sunday November 1, 2015. The Class Notes belongs to CHM 211 at Marshall University taught by Staff in Fall. Since its upload, it has received 15 views. For similar materials see /class/233291/chm-211-marshall-university in Chemistry at Marshall University.

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Date Created: 11/01/15
In general the pre x iso yields a group with the formula CH32CHCH2n where n is a positive integer Thus isopropyl has n 0 and isobutyl has n l We ll encounter the other type unique names in later sections Chapter 21 Nuclear Chemistry The study of nuclear chemistry and physics is important for several reasons Controversies about nuclear weapons power and waste surround us The more information you have the better you will be able to make decisions regarding this matter The use of radioactive materials is here with us to stay The goal of the elimination of nuclear weapons is laudable but unlikely to actually occur in the foreseeable lture The elimination of nuclear power in the US is more likely but that won t occur for decades at the earliest It is also unlikely that nuclear power will be eliminated in all countries in our lifetimes Finally nuclear medicine is still the best way to diagnose andor cure many conditions from thyroid problems to cancer Again while drug therapies may one day supplant nuclear medicine that day is still far off Even if tomorrow all of the above uses of things nuclear ended much of the waste generated in the past will be around for thousands to billions of years As a society we have to make decisions about what to do with that waste 211 Radioactivity We begin with a short review from CHM 211 Go back and review Chapter 2 sections 1 3 Nuclei are composed of positively charged protons and uncharged neutrons These particles are collectively called nucleons Nuclei are expressed by the symbol E1 3y where A is the mass number Z is the atomic number and Sy is the atomic symbol Since the atomic symbol indirectly conveys the atomic number the latter is frequently le off and I will usually do that eg 12C 14N etc Some nuclei are inherently unstable and decompose spontaneously through the emission of electromagnetic energy andor particles These isotopes are radioactive and are called radioisotopes There are three types of naturally occurring radiation OLparticle Bparticle and y ray emission By naturally occurring we mean that these emissions occur in naturally occurring isotopes As we shall soon see there are other types of nuclear processes that occur in arti cial isotopes OLparticle emission is the ejection of a helium nucleus 4He or helium4 from the nucleus of another isotope 222Rn gt 218Po 4He The reaction written above is called a nuclear reaction It is similar in some ways to a normal chemical reaction but differs in some important ways First the species shown are atomic nuclei not atoms Second when balancing you must make sure to balance both the atomic numbers and mass numbers In the example the mass numbers add to 222 on each side of the equation and the atomic numbers add to 86 i particle emission is the ejection of an electron fie from a nucleus Bparticles are physically identical to the electrons in orbitals outside the nucleus Designation as a Bparticle is important because orbital electrons can play a role in nuclear processes vide infra They are sometimes designated as 706 A sample reaction that we ll discuss more later is 14c gt 14N 3e How does the nucleus eject an electron when a er all there are no electrons in a nucleus The nucleus does contain neutrons and they are neutral particles that weigh slightly more than protons When a neutron decomposes it breaks up into a proton and an electron But how does the negatively charged electron escape from the positively charged nucleus The combined masses of the proton and electron total to less than that of the neutron and the mass difference is converted to pure energy which is then used to eject the electron from the nucleus We will discuss this later in the chapter y ray emission 87 is the release of pure energy from a nucleus It usually occurs with other types of emission and is not typically put in the nuclear equation There are two other types of nuclear reactions These occur exclusively in elements that are prepared synthetically manmade elements Positron emission occurs when the nucleus ejects a particle with the same mass as the electron but with a positive charge 1e Although redundant the sign in the lower part of the symbol is frequently used to prevent con lsion as to whether a Bparticle was intended or not A typical positron emission reaction is 110 gt 11B e Positron emission results from the decomposition of nuclear protons They are formed when a proton converts to a neutron Positrons are a form of the antimatter you have no doubt heard of through StarTrek When a positron and an electron collide and they eventually will they annihilate This word has a very specific meaning here When two particles annihilate they both cease to exist as matter Nothing except energy remains as a y ray This is in part why Scotty became so excited whenever he thought the engines would lose containment m gt fe 5n le fie gt 87 Electron capture happens when the nucleus removes an electron from an inner shell Remember s orbitals have no node at the nucleus Electron capture converts a proton into a neutron 81Rb fie gt81Kr ie lH gt 5n 212 Patterns of Nuclear Stability As you know most of the mass of an atom is located in the nucleus Its density is incomprehensibly large 2 X 1014 gcm3 Compare this with the most dense element osmium which has a density of about 22 gcm3 A rough comparison is a grain of sand weighing 800000 pounds The dilTerence in densities between observed densities and nuclear densities is the volume swept by electrons Since electrons weigh next to nothing large volumes are added to the nucleus with virtually no mass contributed Except for hydrogen all nuclei have multiple protons compressed into a small space So why doesn t the nucleus y apart The answer is something called the strong nuclear force It operates over very short distances about one nuclear diameter and is very power Jl In small nuclei the force operates over the entire nucleus but in very large nuclei its Jll force can t completely overcome the electrostatic repulsions of protons on opposite sides of the nucleus Here the strong nuclear force and electrostatic attraction are comparable and hence the nucleus is unstable Occasionally a nucleon will decompose to create a more stable nucleus Later in this chapter we will calculate the energy of the attraction The principal method of predicting nuclear stability is its neutrontoproton ratio Low nucleon nuclei should have ratios equal to or very close to one This ratio gradually increases until for the heaviest stable nuclei it is about 151 The neutrontoproton ratio is probably not the cause of nuclear instability rather it is a symptom General nuclear stability rules 1 All nuclei containing 84 or more protons are radioactive Beginning with polonium the nuclei of all elements are radioactive 2 All isotopes of technetium 43 and promethium 61 are radioactive 3 In Chapter 8 you learned the octet rule which speci ed an electron count for stable ions and bound atoms A similar rule operates on nuclei For either or both protons and neutrons a total of 2 8 20 28 50 82 or 126 yields a closed shell system These numbers are called magic numbers 4 Nuclei with an even number of both protons and neutrons are more stable than an evenodd combination which in turn are more stable than oddodd combinations For the first 20 elements there are 9 eveneven 10 evenodd and l oddodd nitrogen combinations At first glance this might seem to suggest that evenodd is more stable but remember an eveneven combination is not possible for elements with an odd number of protons Thus of the first twenty elements nineteen have even numbers of neutrons in their most stable isotope the only exception is beryllium On page 835 is a graph of all stable nuclei where their proton total is graphed against their neutron total As you can see they form a band It is called the beltofstabili At the far right side of the graph the neutrontoproton ratio reaches 151 as described earlier The reason for the extra neutrons is that they act as spacers between the protons Their presence helps keep the protons from getting too close and thus reduces electrostatic repulsion As long as the nucleus is small this works well Nuclei outside the beltofstability decay to produce nuclei within it For example if the neutrontoproton ratio is too high the nucleus will convert a neutron into a proton It will do this by a Bparticle emission 6n gt fie lH 14C gt 14N fie If the neutrontoproton ratio is too low the nucleus will convert a proton into a neutron This can occur by either positron emission or electron capture It is interesting to note that since 3 gt 1n 3e 37Ar fie gt 37C1 these two paths never occur in naturally occurring isotopes that all naturally occurring radioactive isotopes have too many neutrons Why might that be Nuclei beyond element 83 frequently decay by ocparticle emission yet this will not have a large change in the neutrontoproton ratio So why does this occur Earlier we said that in large nuclei the competition between the strong nuclear force and electrostatic repulsion was the origin of nuclear instability in large nuclei ocparticle emission allows a nucleus to rapidly reduce the number of nucleons it possesses The radioactive decay series displayed on p 837 Figure 214 shows how ocparticle emission takes uranium from being radioactive to stable lead in a series of fairly large steps None of the other emission methods allows the loss of nucleons only their interconversion 213 Nuclear Transmutations Nuclear transmutation is the induced conversion of one nucleus into a different one The word induced is important because spontaneous conversions result from normal radioactive decay All of the synthetic manmade elements result from nuclear transmutation reactions In a nuclear transmutation the nucleus of one atom is red into the nucleus of a second atom at a high velocity The high speed is required to overcome electrostatic repulsion of the nuclei Sometimes neutrons are used in place of other nuclei as the projectile Read the rest of the section on your own 214 Rates of Radioactive Decay While the rates of radioactive decay vary widely all radioactive nuclei decay by rstorder processes It would be a good thing for you to review rstorder reactions in Chapter 14 now pp 537 7 539 Because most people nd it easier to discuss decomposition rates in terms of halflives time rather than rate constants inverse time the former are usually used in discussions about nuclear stability Halflives range from ca 103918 s 9B to ca 1017 yr 96Zr NB The universe is only about 1010 years old a span of 43 orders of magnitude although more typical values are in the range of hundredths of a second to hundreds of years One valuable consequence of firstorder decay is the use of radioactive materials to determine the age of objects called radioactive dating The most common forms of radioactive dating are carbonl4 and uranium238 dating In the upper atmosphere cosmic rays destroy some nuclei and produce free neutrons which then can react with atmospheric nitrogen Please note that these are not chemical reactions so 14N 111 a14c 1H the nitrogen nucleus in the question will almost assuredly be part of a nitrogen molecule A er conversion to carbonl4 the resulting chemical fragment will go on to react chemically The carbonl4 that results from this collision is unstable and reverts back to nitrogenl4 with a half life of 5730 years Carbonl4 dating works by assuming a few things over the past 50000 14C gt 14N fie years 1 the amount of nitrogen in the atmosphere has been constant 2 solar output has been relatively constant and 3 the rotation of the Earth ensures that the atmosphere mixes well every few years There is no argument about the first point and the third point has been demonstrated as true Researchers have developed a method of dealing with variations in 2 Regular cycles in the Sun s output eXist and are smoothed out over time Variations in the carbonl4 produced are averaged away as it progresses into the lower atmosphere where it becomes use Jl for dating purposes Carbonl4 atoms are eventually converted into 14CO2 which is breathed in by plants and incorporated into their structures These plants are eventually eaten by animals which in turn are eaten by others Thus 14C makes it into every living creature including you That s right a very small percentage of the carbon in your body is radioactive and decaying as you read this Because of the assumptions made above that amount remains constant during the entire lifecycle of an organism This is important because it means that the ratio of 12C to 14C stays constant until an organism dies On death no new carbonl4 is ingested and the amount present in the organism begins to decline Since the initial ratio and the decay rate are known the ratio in a sample will yield the age of an organism when it died vide infra The upper limit on age determination by this method is about 50000 years Similarly the decay of uranium238 and potassium40 give the ages of very old geological samples The former has a halflife of 45 billion years and the ratio of uranium238 to lead206 is used to determine the age of rocks If a rock contains lead206 as its only lead isotope it is assumed that all of the lead in the rock comes from uranium238 decay The presence of lead 208 in addition to the lead206 suggests another source for the lead Calculations 0693 mg t 2 N0 Since these are rstorder processes k kt where N is the number of radioactive nuclei and rate kN EX 217 I m going to work this problem a little differently from the book Use whichever method seems best for you You will need Inamp kt N0 k 0693 0693 154X1010y tiZ 45 x109 yr39l we can substitute moles for atoms since both are counting terms molpb0257 mg g mol 125X10396molpb 10 mg K206g lg lmol mol total 100 mg 420 X 10396 mol U 103 mg 238g U molUinit 125 X 10396 mole 420 X 10396 molU 545 X 10396 molU 420 X 10396 molU 545 X 10396 molU ln 154X1010y1 1I t 17 X 109 yr only two SF because oftyz 215 Detection of Radioactivity The most common instrument used to measure radioactivity is the Geiger counter The device is an insulated metal tube with a metal rod in the center attached to one end of the tube A window that allows radiation to enter the tube covers the other end of the tube The tube and 10 rod are hooked to opposite ends of a battery and a detector The tube is also lled with argon gas because it is inert When an OL or Bparticle strikes the rod or walls a signal is sent to the detector When a y ray enters it ionizes one of the argon atoms which then strikes the tube walls Read about scintillation counters and radiotracers on your own 216 Energy Changes in Nuclear Reactions Earlier in CHM 211 you learned the Law of Conservation of Mass p 36 It said that the sum of the masses of the reactants in a chemical reaction equaled the sum of the masses of the products It turns out that this is not technically speaking true The burning of one mole of methane CH4 80 g for one mole of methane results in a 10 ug reduction in mass This amount is far too small to be measured NB Weighing a 10 ug sample is comparatively easy Measuring a 10 ug change in a 100 g sample is essentially impossible This tiny change is extraordinarily important as we shall soon see In nuclear reactions mass converts to energy and viceversa according to Einstein s famous equation E mc2 In fact all nuclear reactions result in measurable mass changes The ssion of one pound of uranium235 ca 2 moles results in well under a one gram loss yet results in an energy output equal to burning 3 million pounds of coal An important question is why do substantive mass changes occur in nuclear reactions First once physicists could accurately determine the weights of subatomic particles it quickly noted that the mass of all nuclei were smaller than the sums of the masses of their constituent nucleons EX 19F measured mass 189984 amu calculated mass 9l00728 amu 10l00866 amu 191521 amu Am 191521 amu 7189984 amu 01537 amu Am is called the mass defect and in this case represents just under one percent of the calculated expected mass The energy associated with this mass defect is calculated by 1kg 103 g AE Amc2 01537 amu 300 x108 ms 2 6022x1023amu 230 x 103911 J per nucleus This value is called the nuclear binding energy and appears very small but now we convert it to a mole of uorine atoms and we nd 3911 23 39 AE W i 138 X1010 kJmol nucleus mol 1000 J This is 14 billion kilojoules released when a mole ca 19 g of uorine nuclei are broken up into constituent nucleons Compare this with a typical chemical reaction burning one mole of methane 16 g 64 g of oxygen yields only 2 k To put this in lrther perspective the Grand Coulee Dam is the largest hydroelectric dam in the US and would have to have to run full out 6500 megawatts for over 30 minutes to put one mole of uorine nuclei together from its constituent nuclei Another value may put this in better perspective the number of one pound boxes of corn akes necessary to produce the same amount of energy 10 boxes lcal 102 box 19 million boxes of corn akes mol 418kJ K110 Cal 16oz We can now answer a question raised on p 4 concerning why the nucleus remains intact The nucleus is held together by the expenditure of a tremendous amount of energy When a nucleus fragments some of that energy is released in reality the breakup to individual nucleons 12 is incomplete and clusters of nucleons ie the nuclei of other atoms form As you might guess even a fraction of this energy can have enormous practical consequences An interesting aside comes from dividing the nuclear binding energy by the number of nucleons The result is plotted for nuclei on p 849 It turns out that iron56 is the most stable nucleus and this too has important consequences Nuclei lighter than iron56 will tend to lse together while those with higher masses will tend to split apart Stars are made mostly of hydrogen until they are quite old Our sun has been burning for about 5 billion years and is still about 75 hydrogen and 25 helium The burning that you observe is the tremendous gravity of a star creating the conditions necessary for lsion to naturally occur As lighter nuclei fuse together to form more stable heavier nuclei energy is released The highest mass nucleus that can form in a star is iron56 So where do cobalt and the next 65 or so elements come from At the end ofits life a star may explode a supernova When this happens the energy of the explosion causes some nuclei to lse together into the heavier nuclei When these nuclei are projected into space they are frozen into a more or less eternal existence 217 Nuclear Fission Nuclear ssion is the process in which an external particle causes a nucleus to cleave into smaller parts Of particular importance is when slow neutrons strike uranium235 and plutonium239 nuclei In this situation the neutrons cleave nuclei releasing more than one neutron 235U 1n gt137Te 97Zr 21n or 235U1n gt142Ba91Kr 31n 13 This is important because a runaway reaction acceleration can occur If most of the neutrons escape the sample such that fewer than one per reaction strike another uranium nucleus the reaction will eventually stop If average number of neutrons captured totals more than one by even a tiny amount the reaction will accelerate uncontrollably and an explosion will result The minimum mass of fissionable material required to maintain a chain reaction is called its critical mass The bomb described on p 851 is actually quite primitive although it has been speculated that if terrorists were to build a bomb this is the design they would employ It is the design of the Little Boy bomb dropped on Hiroshima in 1945 and is the more dangerous of the nuclear weapons designs because if the two parts accidentally come into contact with each other boom The method used in more sophisticated weapons is a shape charge In it a chemical explosive surrounds a subcritical mass of uranium or plutonium When the explosive detonates it liquefies the metal and forces it inward implosion This increases the density of the metal and at some point it becomes so dense that neutron capture becomes sufficiently efficient that it detonates It is safer than the other design because the explosive requires an electrical impulse to explode and when the weapons are on the ground they are disconnected from a power source Richard Rhodes The Making of the Atomic Bomb presents a wonderful history of nuclear physics and the Manhattan Project 218 Nuclear Fusion Currently nearly all nuclear events occurring on Earth that are of practical significance relate to nuclear fission As noted earlier the Sun operates by nuclear lsion This is the process by which two nuclei are forced to combine into a single nucleus When this occurs tremendous amounts of energy are released It has the advantage of using nonradioactive and inexhaustible 14 hydrogen as a fuel It has one signi cant disadvantage To start a fusion reaction a temperature well in excess of 1 million degrees is needed No known material can withstand those temperatures so a device called a tokarnak is used This is a toroid donut shaped device in which a magnetic eld is generated around the inside of the donut The magnetic elds help heat the plasma a fourth state of matter and keep it from approaching the device walls To date no such device has ever sustained nuclear fusion The best guess I have heard is at least 40 years in the future before any practical device is developed We study nuclear fusion because it olTers the possibility of generating nuclear power with little or no nuclear waste Uncontrolled nuclear fusion has been achieved on Earth in the form of thermonuclear weapons The hydrogen bomb is a fusion weapon that uses as its trigger initiator an old fashioned fission bomb The Making of the Hydrogen Bomb by Richard Rhodes provides an excellent account 219 Biological Effects of Radiation Before we go into detail we need to talk about how radiation damages substances in a general sense All of the particulate radiation encountered here possesses electrical charge Cations may oxidize materials they come into contact and anions may reduce them The charge on these species in some ways limits the damage they do because they cannot penetrate deeply into a substance because there is a sea of electrons and nuclei they must pass through It usually doesn t take long for the other charged particles to slow and nally stop them We ll come back to this shortly The other kind of radiation is pure energy In this chapter the radiation of concern is y rays But cosmic rays Xrays and even UV radiation are related They are dangerous because when 15 they strike a molecule and are absorbed the energy is distributed within the molecule In the case of UV radiation there isn t much energy there and usually nothing signi cant happens But it is possible for that energy to break some chemical bond in the molecule or to ionize it Highenergy radiation is sometimes called ionizing radiation because it knocks electrons off water molecules The resultant cation can then react with other water molecules to generate free radicals Hydroxyl radicals are extremely reactive and will go a er DNA and H20 y ray gt H20 e39 H20 H20 gt H3O 00H other biologically active molecules with gusto Dietary and cosmetic products frequently claim to be able to protect you from these free radicals So how dangerous are the various radiation sources With the exception of medicinal uses of radiation it is always bad for you In this one exception it is good only because the alternative is worse That said our bodies are constantly bombarded with radiation of various sorts on a daily basis and have been since humans first walked the planet The point here is we have natural defenses against radiation The better your body s defense is the less likely you are to get a disease frequently cancer from radiation One type of radiation you can do nothing about is the carbonl4 decaying in your body right now Others you can have some but not total control over For example some areas of the country have higher levels of radon than others You can install blowers to keep it from building up in your basement or move to a place where there is less radon in the soil You can live closer to sea level higher altitudes result in higher exposures to cosmic rays Human made sources include Xrays consumer products and nuclear waste Though unless you live very close to a nuclear power plant this is probably not an issue 16 Of the types of radiation presented in this chapter OLparticles are least dangerous They barely penetrate the skin the top layer of which is not living tissue anyway Thus your clothing and skin provide very adequate protection from sources of alpha particles with one exception vide infra In many places in your body Bparticles can penetrate to your bone but not into it Even so a good distance of air and may be some kind of wrapping will protect most people in most situations from Bparticles The good news is that nearly all y rays pass through you without stopping The bad news is those that do get absorbed are ionizing radiation Very few things will act as insulation to protect you from gamma rays e g 10 of lead You want to avoid y ray sources like the plague I ll say just a few words about radon Its an odorless tasteless colorless nonreactive radioactive gas that occurs naturally just about everywhere It is an OLparticle emitter From 222Rn gt 218Po 4He the previous discussion you might think the stuff was nothing to worry about but it is the number two cause of lung cancer behind cigarette smoking Why is this OLparticle emitter bad My guess is that your lungs are one of the few places your body has living tissue in direct contact with the outside world When an OLparticle hits a lung cell its hitting living tissue whereas on your hand it hits a cell that no longer reproduces Second as your book notes the product of this decay is another radioactive element that can shoot a second OLparticle into lung tissue Again remember your body has natural defense systems to protect itself from this Cancer results at least in part in the mechanism breaking down January 10 2005


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