There are two kinds of electrochemical cells - voltaic and electrolytic. Know the difference. I'll do my best here to get you to understand the difference. Show
voltaic
electrolytic
A Very Simple Voltaic CellSo the Daniell Cell that was mentioned in section 7.01 is the classic electrochemical cell used as an example of an electrochemical reaction. Here are those reactions again.
Now we just need to split those reactions into two separate containers that will each contain a ½-reaction. We usually show this with two beakers - each with the appropriate solutions and electrode. The two containers are then linked via a salt bridge which is there to complete the circuit and maintain electroneutrality. Then you hook up your wiring to the electrodes and you've got an electrochemical cell. The wire can go to a voltmeter and show the voltage or you could hook it up to other things - maybe a light bulb if you have enough voltage and current. I've shown this cell below with the wire short circuiting the thing... the current flows directly unimpeded from the anode to the cathode here. A bit of a waste of energy but it DOES allow you to see how we "intercept" the electron transfer between copper(II) ions and zinc metal to get electric current. I am also following a convention in that I'm putting the anode on the left side and the cathode on the right. Make sure you know the definitions of the two electrode here:
 the electrode where oxidation takes place (an ox)⊖ if voltaic ⊕ if electrolytic
the electrode where reduction take place (red cat)⊕ if voltaic ⊖ if electrolytic Because oxidation is always at the anode, electrons will always leave the anode through the external circuit (wire). And in a likewise manner... because the cathode is always where reduction occurs, electrons will always enter the cathode from the external circuit (wire).
The Daniell cell happens to be a voltaic cell with a standard potential of +1.10 volts. Let's head to the next section to see how that works. Shorthand Cell NotationIt gets a bit tedious drawing that picture of a cell up above. Once we have the basics of how an electrochemical cell works all we really need is the two half reactions that we are going to use to build it. This is where shorthand cell notation comes in. We still show dividing lines between the cell parts, but we do so in a nice inline manner so that we can easily depict a full cell on a single line. The Daniell cell above can be shown in shorthand notation like this.
Zn(s) Zn2+(aq) Cu2+(aq) Cu(s) Starting on the left and "reading" the cell notation across you start with the anode itself, Zn(s)... then the vertical line is a boundary for a phase change - here is means you are crossing over into the solution that the anode is in. Then you list all the necessary reactants and products to identify the half reaction. We only have to put zinc(II) down here because the half reaction is very simple - Zn oxidizes into Zn2+. Next up in the reading is double vertical bars which is shorthand for going into and then out of the salt bridge. You do not have to include what is in the salt bridge... just a double vertical bar. Now we are in the cathode solution where copper(II) ions are the "active ingredient" and that is all that is listed. Another vertical bar and we arrive at the cathode which in this case is copper metal. That is it! From start to finish and you can depict ANY cell this way. So to summarize in a very general way you have this.
anode anodic solution cathodic solution cathode Always Remember!
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If you form a compound made of only carbon and hydrogen, then you have effectively made a hydrocarbon. Hydrocarbons are really at the heart of organic chemistry - they definitely are when thinking about nomenclature of organic compounds. The alkanes happen to be the most basic of all hydrocarbons. They are made up of only C–C single bonds and C–H single bonds. Alkanes are known as saturated hydrocarbons in that all the bonds are single bonds. This also means that the basic generic formula for all alkanes is CnH2n+2. Each carbon will have 4 single bonds and each hydrogen will have one single bond. The figure below shows the local structure for all C's and H's in an alkane. Now lets build a series of alkanes with a simple rule: all the carbons in the structure must be in a line with no branch points. This means that all the carbons are bonded to only two other carbons and the carbons on the ends will only be attached to one carbon. These are known as straight-chain alkanes or normal alkanes. Realize that the word "straight" really means in a continuous line and not really straight like a meter stick or yardage lines on a football field. Just think of links in an actual chain and how the chain can be arranged in various shapes depending on how long it is. Below are three structures for n-pentane (n is the abbreviation for "normal"). Each structure has 5 carbons in line with each other. What is important is the 5-carbon chain in each (shown in green) is continuous with no branching. The longer generic formula for this type of n-alkane is CH3–(CH2)n–CH3. The ends (CH3–) are called methyl groups and the links (CH2) are called methylene groups. This is the base name for which all organic compounds are named. IsomersThere can literally be hundreds of ways to write different structures for the same formula alkane once there are many carbons. There are only 2 possible different structures for butane, C4H10. Hexane, C6H14 has 5 different structures. Nonane, C9H20, has 35 different structures. And dodecane, C12H26, has 355 different structures. Different structures with the same empirical formulas are called isomers. There are different types of isomers - for now, we are only thinking about structural isomers (aka constitutional isomers). The same set of atoms in a formula are arranged differently in space. Structural isomers are the easiest to recognize. See below the structures for C2H6O which can be dimethly ether or ethanol.
Learn the n-alkanes first. All those isomers will have uniquely different names - not to mention, different physical and chemical properties as well. For now, just learn the straight-chain alkanes. Below is a table of the names and formulas of common alkanes. You should memorize and know the first table of the first 10 alkanes. The other table is just there for reference purposes to let you see the systematic nature of the naming.
(simple) Alkenes C CA simple alkene is simply an alkane that has a carbon-carbon double bond (C=C) in any one of the carbon chain positions. In order to accommodate for this you have to remove two hydrogens (you eliminate them to use organic reaction lingo). This means that all simple alkenes have the generic formula of CnH2n. Notice there is no "+2" on the H's. Naming wise, you just replace the "-ane" suffix of the alkane with a "-ene" suffix for the analogous alkene. Propane becomes propene, pentane becomes pentene, etc. You get the idea. (simple) Alkynes C CNow to just finish off the other simple type of hydrocarbon, lets make a simple alkyne now. An alkyne is an alkane that has a carbon-carbon triple bond (C C) in any one of the carbon chain positions. In order to accommodate for this you have to remove four hydrogens (once again, you eliminate them). This means that all simple alkynes have the generic formula of CnH2n-2. Notice that is a "–2" on the H's. Naming wise, you just replace the "-ane" suffix of the alkane with a "-yne" suffix for the analogous alkene. Propane becomes propyne, pentane becomes pentyne, etc.
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I think we all know what food is. We eat food. We need food to stay alive. Food is our fuel. We also, thanks to the way we are engineered, enjoy food. We often find great pleasure in eating/tasting food. And talk about variety - there is a wide range of foods from all sides. We have straight up food identified by a specific country or region: french cuisine, asian fusion, italian, indian,... even mash-ups like Tex-Mex. There are far too many to even try to list. But if we dig in and look, we see that there is a common set of chemical compounds in all of these. The goal of this chapter is to point out the chemistry of food. Maybe you'll finish with a deeper appreciation of just what is all that on a nutritional content label. I do understand that only viewing food as a bunch of chemicals is incredibly short-sighted. But, you need to keep in mind that it is important to know what those basic components are and how your body uses them. I'll try not to get too preachy and just get you the facts. That ingredient list on a package of munchies can be scary - especially if you are eating heavily processed foods. We will certainly NOT cover all the myriad items on an ingredient label - we might hit a few though. More important is the nutritional content because that is more about your fuel and ultimately, your health.
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Really? You're going to read the Colophon? Why is it a colophon? Well, I can't think of a better word. This IS a purely online electronic eBook I've written and manage and this is the page that tells you a bit about how I put all this together. So you want to know some details? Great! Let's start... First of all I code this entire website by hand. Yes, all the basic html and coding of the pages is done my me (Dr. McCord) in a text editor. What editor is it? Well it is either BBEdit or Atom - I use both of them. So this is not Wordpress or Drupal - just straight html. Ok, I DO use a CSS framework and I seem to be partial to Bootstrap. Right now I'm using Bootstrap 4, but I'm already using 5-beta on my regular class websites. I'll switch over to Bootstrap 5 later this year. Yes, I've tried many other frameworks and some of them are really kinda cool - but alas, my brain is still thinking in Bootstrap terms - I've used it ever since about 2011 when I was authoring the gchem website - which, BTW is still on Bootstrap 3. Mobile first design. At least this is what I was thinking. Bootstrap helps a lot with this. I DO look at my pages on various devices and try to make sure it looks good on all screens - especially mobile. I'll admit that I am Apple-centric and do all my work on their products. But occasionally I see my site on a Windows or Android machine/device and things do look pretty there as well. I do NOT code for different systems - I depend on the W3 standards and assume browsers will all comply eventually. I also don't really do any cutting edge new stuff, so that keeps things simple. PHP... yes, certainly. I got into PHP back when there were really no other options for server side programming. PHP seemed fairly easy and I really like the way it coexists with the html on the page. I don't do a lot of real heavy lifting with PHP - just stuff that helps with reusable html code (headers, footers, and tables). I did try to learn Laravel when it came on the scene - it is cool and I'd be using that except for the fact that my chemistry server is not run by me at all and it doesn't have Laravel (or the PHP version to support it). So I gave up on Laravel and I just use PHP to make coding in html a little easier. Graphics? Well I generally use Adobe Illustrator for any real graphics work. I then export to svg and try my best to use all svg files. However, there are a lot of svg graphics that I actually hard code. I jump over onto codepen.io and start editing away on many of my svg graphics. So most of my graphics are svgs. Raster graphics? Yes, sometimes you just gotta do it. I generally use Adobe Photoshop for all raster type graphics (jpg, png, and gif). Why gif? Well I do occasionally dabble in an animated gif file to show something that needs movement. However, I do have some looping animations in svg which are cool too. My goal is to have all graphics be mine. However, there are times when you just have to go grab something off of Wikipedia or the web and use that. I consider those placeholders for "my graphic or image" to come later. Fonts? Well, the old me would just ignore fonts and let the browser do what it wants. However, I found that a nice soft rounded sans-serif font is a lovely read and it conveys the more relaxed tone my book is supposedly written in. So I use Nunito which is available as a Google font. I like the look and have been using it now for about 3 years. It is close to Apple's San Francisco font. Before Nunito, I used Saira which is a sans-serif font available as a google font. Mathematical Equations? I've been using MathJax for years now. They do keep making it better and better. I tried for years to get publishers to use MathJax but they just kept doing image based (as in a crappy looking pixelated gif) equations. So I like MathJax. However, I also like my Nunito as well... so you will see me occasionally build out some equations with html, css, svg, and Nunito. Some of those equations I like a lot more than MathJax - but they are a real pain in the ass to build with all the CSS relative positioning and the like. Navigation? When I started on chembook in July of 2019, I decided to really try and write the whole thing in just html with no javascript ajax stuff (which I did on gchem site). The javascript stuff on gchem was and is a bit convoluted and things can break. So here I decided to use all html. The PHP does help on building out the pages on the fly. The chapter and section numbers are passed in the url and the page is built. I made sure that the navigation looks good (enough) on mobile as well. I even added the little "no nav" button to reload the page only showing content and no navigation. This helps if you are printing (to pdf of course) and just want the content. Chemistry Content? Well on that front I try to cover some fundamentals of chemistry up front. My content is fairly common for any starting chemistry course. Some of the more detail oriented aspects of chemistry are purposely left out or they are treated very lightly. Our intro course for non-majors here at UT does carry a QR flag (quantitative reasoning). So there does need to be some actual math and analytical problem solving. I tie in to environmental issues where I can but only in a general sort of way. I do not "deep dive" on any one issue. - paul
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Associate Professor of Instruction email: Dr. McCord is a native of Abilene, Texas. He earned his B.S. in Chemistry from Abilene Christian University (ACU) in 1983. He continued at ACU to earn his M.S. in organic synthesis in 1985, after which he moved to Austin, TX. Dr. McCord recieved his Ph.D from the University of Texas in 1992 in the field of analytical chemistry under the mentorship of Dr. Allen Bard. After a short post-doc experience with Dr. Bard, Dr. McCord became a lecturer (Assistant Professor of Instruction) at UT in 1994. He is currently an Associate Professor of Instruction in the chemistry department. Dr. McCord has taught mostly freshman level courses such as Principles of Chemistry I & II, and Introductory Chemistry (aka: Chemistry in Context) over the years. In addition to those courses, Dr. McCord has also taught Analytical Chemistry and Physical Chemistry. His primary teaching mission has been to educate UT freshman and prepare them for their journey into science related fields. Dr. McCord is a big supporter of lowering the cost of education by providing a no-cost chemistry textbook to UT students. He is currently working on his free eBook "chembook" which is a free book for non-science majors. He is also currently teaching the non-major chemistry classes at the university. Dr. McCord's online chemistry book websites:
https://gchem.cm.utexas.edu is known as the "gchem" site which is a full chemistry textbook for freshman level chemistry for all science majors. This site was authored in calaboration with Dr. David Vanden Bout and Dr. Cynthia LaBrake. It has been active since 2012.
https://mccord.cm.utexas.edu/chembook is Dr. McCord's relatively new site that is specifically written for non-science majors at the university who are getting their required science credit. It has been active from 2019 to the present.
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The metric system is incredibly efficient at allowing one to switch to human friendly units simply by using the appropriate prefix with a given unit. Learn as many as you can, but definitely learn the ones that say to know these for class.
— from big to small —
Make sure you know these for your all your chemistry classes!
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1 L = 1000 mL 1 L = 1000 cm3 1 L = 0.2642 gal 1 gal = 3.785 L 1 gal = 231 in3 1 gal = 128 fl oz 1 gal = 4 qts 1 gal = 8 pints 1 cup = 8 fl oz 1 cup = 1/2 pint 1 shot = 1.5 oz 1 fl oz = 29.57 mL 1 tbsp ≈ 15 mL 1 tsp ≈ 5 mL Page 10
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10 Appendix - Data & Tables Standard Potentials (long scroll) which is a continuous long-form table which is more suitable for screens where you scroll to view it all. Page 12
This is a fairly full listing of polyatomic ions.
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for Some Select Substances Specific (Cs) and Molar (Cm) Heat capacities at constant pressure (1 atm) and 25°C.
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for Some Select Substances Standard melting points with corresponding heats of fusion and boiling points with corresponding heats of vaporization for substances at 1 atm.
source: NIST Chemistry WebBook (https://webbook.nist.gov)
previous back to top next © 2019-2022 · mccord Page 16 I really expect you to be able to count things. There will be questions about counting stuff. One, two, three,... you get the idea, right? Here's the thing, no tricks, just count. Sometimes you have to be careful not to miss things - but counting is pretty easy. Try the following self test:
How many dogs are in this picture? How'd you do? Did you get 3 ? The answer is 3. If you got 3, you're a good counter of dogs. Maybe you can branch out and count some other stuff. Sometimes our counting objects are part of a bigger picture or even part of a group. This complicates things, but it is still fairly easy. Here's you another question with groups...
Carol and Steve invited their favorite 4 couples over for a dinner party. As they all sat around the table someone counted. How many people were sitting around the table? Much tougher question, right? Why though? It is really straight forward. The answer is 10 people. A "couple" is two. 4 couples is 8 people, then you add in the hosting couple of Carol and Steve and you get 10. Straight forward and easy. Did you miss this? If you did, you are going to have a tough time in this class. Don't add more to the problem or less. Go with the obvious answer to the question that was asked. It doesn't "depend" on anything - if there were other factors at play, the question would have to state it. Please tell me you got 10 people. Now let's move on again. Here is another one...
Larry was really hungry. He went out and purchased a dozen donuts to take home and enjoy. He also bought a dozen eggs, a carton of milk, a lotto ticket, and a dozen golf balls. When he got home, his wife - Donna, gave him 5 more golf balls that she found under the couch. Larry put everything out on the kitchen table. How many donuts are on the table?
Think about it... did you arrive at twelve donuts? That is the answer. Do you know what a dozen is? It's 12 of something. In this case, it is 12 donuts. Nevermind the more obvious number that wasn't asked for - the 17 golf balls... there were 12 donuts there ready to be eaten. Remember to always just answer the question with the facts given. Don't add or subtract from the facts and make crazy assumptions. If you thought only 9 donuts were on the table because Larry's friend Bob bumped into him in the store parking lot and took 3 donuts... you would be wrong and you've read far too much into the question that wasn't there. Don't be that guy... don't be "conspiracy guy" about questions in this class. Don't make up scenarios that aren't there and aren't intended to be there. That is it. I hope you count well all semester long. Page 17
go ahead and give it to me So matter. What's up with matter? Let's see... it's all around us and we are constantly interacting with it. We are it as a matter of fact. We chemistry folks like to break matter into specific groupings. It helps us understand it much better. That is a "science-y" thing to do, and were are being science-y in this class. So let's start. First, matter is everything, solid, liquid, gas - those are the 3 common states of matter. You nerdy types will say "what about plasmas?" and I'll say, let's not go there. When was the last time you interacted with a plasma? Let's just acknowledge it and then move on - another class can deal with plasmas.
Pure Substances A pure substances is matter that is the same thoughout all the way down to a molecular level. The "down to the molecular level" is very important here. Every single piece of a pure substance has the exact same make up or chemical formula. Pure water is always H2O and never ever anything else. Pure oxygen gas is O2 - you're breathing it right now. And pure quartz is SiO2 (repeat) - I say repeat because that formula is repeated over and over in a solid crystalline lattice. Quartz is the primary component of sand. We chemists/scientists like to work with pure substances because we know exactly what we have and even have an exact formula to describe it.
Mixtures Anytime you combine two or more pure substances, you have a mixture. Most of the matter that we encounter here on earth is some form of mixture. Many mixtures are well defined and have names to go with them. Air is a mixture of gases that make up our atmosphere here on earth. Some mixtures sound like they'd be a pure substance but they are actually mixtures. For example, the metal called bronze is a mixture of primarily copper and a bit of tin, and possibly some other metals too...which illustrates my point to be made: Mixtures can have variable compositions. A little more of this, a little less of that... a dash of the other. Go nuts, mixtures are generally quite variable in their compositions. But remember, we chemists and science guy types can define that composition and make it exact. Mixtures are variable but also definable - remember that.
Homogeneous Mixtures Now we are going to split up the two divisions of matter, mixtures and pure substances. Let's split up mixtures first. If and only if the pure substances you combine mix all the way down to a molecular level, you will have a homogeneous mixture (aka a solution). Any macroscopic sampling (big enough to see) of a homogeneous mixture you will get the same exact composition of substances. The mixture is the "same throughout" - that is homogeneity. This is true all the way down to where you can't see anymore. Your vision peters out around 50 microns (µm). Read my blurb on molecular true homogeneity vs our day to day perceived homogeneity.
Heterogeneous Mixtures Any (perceived) heterogeneous mixture, you can actually see the differences in the mixture. It is obvious that there are different substances in the mixture. An overly obvious heterogeneous mixture is sand in water. The sand is easily seen in a glass of water... heterogeneous. A chocolate chip cookie is heterogeneous. There are the chocolate chips and there is the rest of cookie. A glass of ice tea is heterogeneous because the ice is a different substance than the tea. If all the ice were to melt though, you'd have a homogeneous mixture. That was fun splitting up mixtures, now lets split up pure substances.
Elements If you take any form of matter and just keep breaking it down to smaller and smaller bits, you will eventually reach the molecular level and then you reach single atoms of stuff. Those atoms are elements that make up all the matter of the universe. The elements are the simplest form of matter in that there is only one kind of atom in an element. If you only have iron atoms, you've got the element iron (Fe). There is a real easy check for elements, look at a periodic table. If it is on the periodic table, it's an element. If it is not on the periodic table, it is not an element.
Compounds If you combine, via a chemical bond, two or more different elements - you've got a compound. Compounds are very defined in there elemental ratios as well. Sodium chloride (table salt) is NaCl and will always exist as one atom of sodium to one atom of chlorine in its makeup. It will never vary. Water is a molecule that is always two atoms of hydrogen bonded to one atom of oxygen, H2O. matter mixtures pure substances compounds heterogeneous homogeneous elements
There is a difference in the physical properties of solutions when they are truly homogeneous down to the molecular level versus just being homogeneous to a perceived level. I'm a big believer in "watch you see is watch you get". The classic gotcha! question in chemistry textbooks and I suppose on exams is to ask what category is milk? If you look really close - like zoom way in, maybe get a microscope and look. You'll see that there are differences in the tiny bits. Milk is really a bunch of fat suspended in an aqueous solution. The fat doesn't really dissolve fully at all - but instead of just being a big giant wad of fat (easy to see there), it is broken up into teeny tiny globules of fat and those get emulsified with some proteins to make all the parts completely mix and be homogeneous on a human perceived scale. In order to know that it is really heterogeneous, you have to have equipment to look further - or you've now been told and can carry on the tradition of doing the whole gotcha! thing. I (Dr. McCord) believe you call 'em as you see 'em. We generally make our first assessments based on simple human observation of a sample. If it looks homogeneous, then I'd check that box on the information form. I don't believe in tricking you to think its homogeneous and then turn around and point out that at a 10 µm level, there are little bits that are different and therefore heterogeneous. LATER... after much experimentation and examination... I might conclude the mixture is heterogeneous. But my first look says it's homogeneous for milk, blood, and even paint. There IS a sort of in between state called colloids. They LOOK homogeneous, but they are actually a suspension of tiny-ass particles.... just like the milk and blood. Study colloids on your own. I'm not going to go there for now.
back to top next © 2019-2022 · mccord Page 18Composition of AirOk, I'll spare you the sentences about taking a breath and thinking about it. Let's cut to the chase...
Nitrogen, N2 78% Oxygen, O2 21% Argon, Ar 1% Water, H2O 0% yeah, z-e-r-o that's why it's called DRY air That is it. The major components of dry air. I know, 1% for argon doesn't seem that major. I hear you, but there are so many other things in the air that are below 1%, it is worth putting argon there as a friendly reminder of the last of the major components. Hey, while we are here, the 78/21 thing with nitrogen and oxygen is really close to 80/20 which is really just a percentage way of saying 4/1 or 4:1 or 4-to-1 or there are 4 parts nitrogen to every 1 part oxygen. I'm ok with that. What About Water? Humidity?Well that is our big variable in the composition of air. If you have bone-dry air then the water content is truly 0%. This is what we call "dry" air. Now imagine air that is saturated with water - meaning water is free to evaporate and go to gas phase and reach its vapor pressure at a given temperature. Know this, the highest that will ever be under typical weather conditions is about 7% and that is for 100% relative humidity You're Wondering about CO2 - right?It's good to wonder about CO2 and our air. It gets a lot of news due to its acclaimed position as the number one greenhouse gas for earth (not counting water vapor). Let's just say up front that CO2 is only about 0.04% of our troposphere. CO2 even has its own website to tell the world what its level is - go to co2.earth and find out. What is IN and OUT of your breath?Well the answer to that is air, duh - but let's go a bit further than that. Let's say you are in Austin, TX on an 85 °F day with about 73% humidity. When you breathe in, you are pulling an air composition of about 76% N2, 20% O2, 3% H2O, and 1% Ar (a trace, 0.04% CO2). But, the air you are breathing back out is about 75% N2, 15% O2, 5% H2O, 4% CO2, and 1% Ar. There are a couple of things to notice about those exhaled vs inhaled numbers. First, notice that tiny amount of CO2 inhaled (0.04%) is now up at a full 4% which is a 100× jump in concentration. And second, there is still 15% O2 in your exhaled breath - which is good news for anyone on the receiving end of mouth-to-mouth resuscitation. Nice to know they are blowing 15% oxygen into your lungs to save you. Pretty much all mammals do this breathing thing... we pull in air and blow out air with a about 5% less oxygen and a jump up to about 4% CO2. Multiply this by all the living mammals on earth and integrate over a year and you get around 60 Gt (that's a gigatonne = 1012 kg) of CO2 pushed into the atmosphere per year by respiration. We will chat more about CO2 in chapter 4. Interesting "Trivia" page about Air: https://climate.nasa.gov/news/2491/10-interesting-things-about-air/.
back to top next © 2019-2022 · mccord Page 19 Electromagnetic radiation (EM) is light. It is characterized by its frequency and wavelength (next section). When you consider all the possibilities you have the entire electromagnetic spectrum. It is broken up into zones that each have a specific name. Here is a very nice wikipedia graphic showing this. Memorize...You need to memorize a few things here. First memorize the order of the seven various regions/bands/zones within the entire range. In order from highest energy down to lowest energy is gamma rays, x-rays, ultra violet, visible, infra-red, microwaves, radio waves. Second, memorize the two ends of the visible spectrum. We do the following to make this an easier thing... the highest energy end is the "blue" end at 400 nm, the lowest energy end is the "red" end at 700 nm. So we "say" visible light is from 400-700 nm, blue to red. 400 nm is actually more violet than blue, but we call it the blue end nonetheless. Roy G. BivMemorize the order of the colors of the visible spectrum as well. Roy G. Biv helps a lot here. Red, orange, yellow, green, blue, indigo, and violet. This is also the colors of the rainbow in the right order. We will further break up some other zones as well. We will learn the differences in UV-A, UV-B, and UV-C in section 3.4 of this chapter. For now, get the visible straight and the ordering of the various bands.
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The better name is Lewis Electron Dot formulas - then the lines come in later. We shall tackle all it. As shown in the last section, we find it extremely helpful to actually show the valence shell electrons (the s and p ones) as dots surrounding the element symbol. Put your new found knowledge of electron configurations to work for you. Count those valence electrons - yes, the outermost s and p ones. I'll give you a big hint, the number of valence electrons EQUALS the ones-digit of the group number of the periodic table (skipping the d-transition metals). So group 1 and group 2 have 1 and 2 valence electrons. Now jump over to the p-block and you have groups 13-18. That will be 3 valence electrons all the way up to 8 valence electrons. You just put the dots symmetrically around the symbol and you've got it. Here is the electron dot "formula" for the 2nd row elements. Easy breezy. There is really no wrong way to surround the element - however, there ARE some assumed rules that most everyone follows - so let's just pretend its a real rule (although it's not). Think up, down, left, right for positioning of the electron dots. On a clock face that is 12, 6, 9, and 3. Pretend those positions are orbitals and then fill them in order... right, left, up, down for the first 4 dots, then again with pairing in the positions. You don't have to go in any order, but it looks nice when you try to be somewhat symmetric in your choices. Covalent Bonds with DotsSo let's ignore Li, Be, and B - they're a bit odd in their rules and we don't need them at all (sorry). Much much more useful is hydrogen and it's one dot, like this H· Ok? see one dot, one electron. Now lets remember how all those non-metals would just love to have a full valence shell which is s2p6 electron configuration around it. Neon (see above) is already there which is why neon does NOT ever react with anyone else. Neon is happy and content being neon - aloof and distant, one could even say noble. So how do the other elements come up with 8 e– ? By sharing! So carbon, needs to gain another 4 electrons via sharing and it will share 4 electrons in return. Nitrogren needs another 3 electrons and will share 3 in return. Oxygen needs 2 more electrons and will share 2 in return. Hopefully you get the idea here. Below are several dot structures for some common compounds. Lines!Now lets convert those same structures above into line/dot formulas. We swap out any bonding pairs of electrons with lines. A single shared pair will be a single line or single bond. Two shared pairs will be a double line or double bond. And, three shared pairs will be a triple line or triple bond. All this is shown below. Recognizing Lone Pair and Bonding Pair ElectronsBelow is a line/dot structure of the amino acid glycine (H2NCH2COOH). Note the structure and how to identify the lone pair electrons and the bonding pair electrons. Be able to both create the structure (draw it) and analyze the structure (count features within the structure).
The S = N – A rule is a simple "formula" that helps you at least identify how many electrons will be in chemical bonds (bonding pairs of electrons) - which also means you know how many are not in chemical bonds (lone pair electrons). Let's tell you what each of those letters stands for:
When organic structures start to get larger and larger - showing every atom and bond can get very cluttered. A skeletal line structure is basically a "shorthand" for doing structures. Many of the carbons are implied by simply drawing lines and bends. If there is not an explicit symbol there, it is intended to be a carbon. In addition, all implied carbons are assumed to be complete with a full octet and any missing bonds are just assumed/implied to be hydrogens. So the hydrogens aren't shown at all but are known to be there. And, again, to streamline the skeletal structure, lone pair electrons are often not show as well. Below is a skeletal structure for aspirin (acetylsalicylic acid, C9H8O4). It's an animated gif that first shows the plain skeletal structure (its from Wikipedia) then the carbons show up in red, then the hydrogens show up in green, and finally, the lone pair electrons on all the oxygens.
You will want to develop the skill to know and "see" all those implied atoms and lone pairs (this is the kind of thing we would ask on an exam). Like - check this out, here is the skeletal structure for a very popular and consumed substance. Can you come up with the correct empirical formula? Q1: How many C's? H's? lone pairs? What's the empirical formula? What is the substance? answers▴ You DO need at least 3 or more atoms in a chain in order to show a skeletal structure. Too few carbons will just look silly. Like — is ethane. Yes, that's a short little single line, technically ethane, but don't do it. At least with propane you can put a bend in the line to imply 3 carbons. Skeletal structures make much more sense when things get much much bigger. Like this. Q2: How many C's? H's? lone pairs? What's the empirical formula? What is this substance? answers▴
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Fire - we all know it when we see it. Look! there's a fire over there... Let's have a campfire and roast marshmallows. Well, to a chemist, fire is combustion which is a very specific reaction. 🔥 A combustion is reaction with oxygen - specifically oxygen gas or O2. When we ask the question "does it burn?" we are asking does it react with oxygen and then self sustain the reaction.
Most of the stuff we burn, we burn it as a fuel with the purpose of extracting the energy from the fuel and transferring it to something else. There are lots of things we can extract energy to. Sometimes we just want the heat from the fire directly - as in "I'm cold, let's have a fire to warm up.". Or we take the heat from the fuel and use it to cook with - as Alton Brown has said and written "Food + Heat = Cooking". We also use that energy to provide energy in the form of torque from a combustion engine to power a motor to propel a vehicle. And of course we take all that heat from combustion and convert it (at least some of it) into a distributable energy - electricity. So there is a multitude of ways in which we tap into the power of combustion. Let's have a look at some rather prevalent combustion reactions which are listed below according to the fuel that is used. In general, the fuel is a hydrocarbon and there are literally 100s of possibilities for fuels. Some fuels also have some oxygen already in there as well, e.g. wood, alcohol. natural gas is mostly methane CH4 + 2O2 → CO2 + 2H2O gasoline is a mixture of many hydrocarbons one is octane C8H18 + 12½O2 → 8CO2 + 9H2O kerosine is also a mixture of many hydrocarbons (10-16 carbons) - one is dodecane C12H26 + 18½O2 → 12CO2 + 13H2O wood is mostly cellulose which is a polymer of glucose - just glucose C6H12O6 + 6O2 → 6CO2 + 6H2O Then there are some fuels and fuel additives that can also have some nitrogen and sulfur as a component whether by chance or on purpose. If any nitrogen or sulfur is in the fuel, we will get NO2 and SO2 gases as products - which, as we know is not good because those are big time air polluters. The thing is that no matter what the fuel source, we can write out a fairly accurate chemical reaction for the combustion of that fuel. We can account for every bit of carbon that gets burned, and we know just how much CO2 is produced. This is just simple stoichiometry. Energy is Stoichiometric AlsoNot only can we do the accounting for CO2, but we also can do the accounting for all the energy that is released during combustion as well. Accounting for and quantifying the energy of chemical reactions is the science and practice of thermochemistry. Thermochemistry just adds one more layer on top of our stoichiometry prowess. If we can count moles of reaction, we can count joules of energy. That is the essence of thermochemistry... and here is a quick example: Propane has a heat of combustion of 2220 kJ/mol. Therefore the fully balanced reaction that also shows energy output is C3H8 + 5O2 → 3CO2 + 4H2O + 2220 kJSo this can be scaled up or down depending on how much actual propane you burn. That equation is for one mole of propane which is 44.1 grams of propane. So how much heat can we get if we burn 250 grams of propane? Just use the unit factor of 2220 kJ/44.1 g and you'll have your answer: \(\require{cancel} \newcommand\ccancel[2][black]{\color{#1}{\bcancel{\color{black}{#2}}}} \left({250\,\ccancel[red]{\rm g}\over 1}\right) \left({2220\,{\rm kJ}\over 44.1\,\ccancel[red]{\rm g}}\right) = 12585\,{\rm kJ}\) So there's your answer: 12585 kJ of heat is released when you burn 250 grams of propane. Heck, if you wanted to you could even change that somewhat clunky 2220 kJ/ 44.1 g into a much nicer (do the math) 50.34 kJ/g (kJ per gram of propane)... now you only have the one number to use for ANY gram amount. If you only burn 3 grams of propane, you'll release 151.0 kJ of heat. Burn 5 grams and get 251.7 kJ... and so on! Stoichiometry works for both amounts (moles and grams) AND energy (joules and kilojoules). HANG ON! Hold up...This is all great and we could just keep doing more stoichiometry problems and the like except for one rather big issue. We really haven't addressed the whole conceptual aspect of this energy transferal/dissipation and how the study of that phenomena is a really BIG part of chemistry and physics. That big issue is thermodynamics and we need a little taste of it in order for us to move deeper into energy as a topic. So let's move on to learning some thermo speak if you will. Next section! →
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Time to think about water and just how important it is to pretty much everything here on earth. I need it, you need it, plants need it, animals, the whole earth needs it to "operate" correctly. I'll spare you more drama on it in that respect. I'm really after the whole chemistry of aqueous solutions thing since this is a chemistry class. Pure Water? Is there really such a thing as pure water? Well yes... but it is really really difficult to just crank out ultra-pure water. It certainly doesn't come from nature pure. Not even rain falling from the sky is pure water. So when I say pure water I'm referring to a rather special reference standard that we often approach, but rarely truly achieve. Truly pure water is nothing but H2O and that's it. But there is more to this than meets the eye... Water is a very polar molecule with a good partial positive charge (δ+) on the hydrogen atoms and a partial negative charge (δ–) on the oxygen atoms. This in and of itself is why H-bonding is such a big deal and why one of the smallest molecules in the universe has a relatively high melting point and boiling point. Water is only 18 g/mol for its molar mass and is a liquid at room temperature with a boiling point of 100 °C. This same polarity makes water an excellent solvent for hundreds of polar compounds and salts. Salt dissolves in water by dissociating into its separate cations and anions: NaCl(s) → Na+(aq) + Cl–(aq) Let's dissect that equation: Solid NaCl (salt crystals like you sprinkle on food), when put into water, will completely break up into separate aqueous ions. The (aq) in the above equation is the aqueous phase of matter. It really isn't a true independent phase like solid, liquid, or gas are for pure substances - but it is a very very important homogeneous mixture (a solution) in all of science and therefore gets its own (aq) designation. The aqueous phase is really a species (cation, anion, or neutral species) that is completely surrounded by molecular H2Os Also know that I've only shown the water molecules that are participating in the dissolution. There would actually be way more molecules present - so much so that our illustration would be totally buried in them. Let's be Clear About the Ion-Dipole InteractionWe make a really big deal about H-bonding in certain sets of molecules. It is like the strongest possible dipole-dipole interaction there is. So much so, it got its own fancy name of H-bonding. Still, even the best H-bonding interaction is small compared to actual ion-dipole interactions. A full blown charge of +1 or -1 is going to have far more pull than any partial charge of δ+ or δ–. So once an ion is in aqueous solution, it typically stays that way until something far stronger comes along to disrupt it. Check out the Wikipedia link for "aqueous solution" in the nav-menu (margin). There is a lovely illustration of a solvated (hydrated) sodium ion on that page. Water-Specific InteractionsRemember, back in section 4.7 I pointed out the details of three important interactions between molecules. These intermolecular forces (IMFs) are present pretty much everywhere. Dipole-dipole interaction is the main interaction occurring between polar molecules, while dispersion forces are the primary ones for most large molecules, and is the only force for non-polar molecules. H-bonding is a special case of dipole-dipole that happens to have relatively strong interactions compared to other IMFs and is the major IMF in water. So that's my little recap of that chapter 4 section. Because water is such a major molecule here on earth, we often have terminology just for it. The attractive/repulsive forces within water are a major player in the properties of aqueous solutions and any water-like species. For this reason, we often categorize forces of attraction with water as being either hydrophobic (water-fearing) or hydrophilic (water-loving). These are water-specific terms but are really just more IMFs when you really look deeper. But sometimes it is far easier to talk about aqueous interactions as being either dominated by hydrophobic interactions or hydrophilic interactions. I'll explain each below. Hydrophilic InteractionsThe general rule-of-thumb for any hydrophilic species is that is tends to have very favorable (attractive) interactions with water. Simply put, the species is most likely a polar species. Water loves polar compounds. The partial postive (δ+) and partial negative (δ-) regions of water will line up with a considerable amount of attractions with polar species. This certainly increases the solubility of any polar compound or species. As long as there is nothing even more attractive around, most polar substances will dissolve in water. The water molecules will completely hydrate each and every molecule of the species and pull the species into the aqueous phase. I showed this in detail with NaCl in section 6.1 through the process of dissolution. Even though I showed full blown cations and anions with NaCl, the same does occur with any polar molecule. Maybe not as strong as NaCl, but the interaction is very significant. Hydrophobic InteractionsNon-polar molecules (hydrocarbon chains and the like) really have nothing substantial for water to grab hold of. Water is far more attractive to itself than to non-polar molecules or regions of non-polarity. This generally causes the non-polar molecule to just stick to itself or other non-polar species instead of water. Water will generally "stay outta the way" of these non-polar interactions. This is why it is called a hydrophobic interaction. Typically, this just leads to the general rule that non-polar compounds are insoluble in water. Greases and oils (mainly long chain hydrocarbons) will bead up on water and float on the surface (less dense than water). As they say... "oil and water don't mix" and that is true because the oil is a very hydrophobic substance. There is even a hydrophobic effect which is when these hydrophobic species find each other in aqueous solutions and congregate/aggregate building little islands and globs of non-polarity. This is further discussed in the emulsification section further down this page. All of this leads up to the chemists rule-of-thumb about solubility in general: "Likes dissolve likes" So simple, and so true. If you want to dissolve greases and oils, get yourself a non-polar solvent like carbon tetrachloride or hexane. Many sticky adhesives are hydrophobic which makes cleaning off the sticky residue difficult with any aqueous solution. But using a more non-polar solvent like original version of Goof-Off or Goo Gone will dissolve that sticky stuff pretty quick. But if your sticky stuff is aqueous based - like a sugary mess (maple syrup) - then by all means, use water. Even better, use warm or hot water because the dissolving process for most solids in water is endothermic which means a little heat will help things get pushed more forward (dissolving). EmulsificationSo oil and water don't mix. Right? Yes, oil will completely separate out and float as a separate layer ontop of water. You can mix, stir, shake, whatever... but you will not get them to stay mixed because of the very real set of hydrophilic (water-to-water) and hydrophobic (water-avoiding-oil) forces at play. But we can get the two to mix in a very clever way. Add an emulsifier. An emulsifier is a large molecule with two distint regions on each end. One end is a long-chain hydrocarbon and is known as the hydrophobic "tail". The other end is typically a deprotonated carboxylic acid or a sulfate group. That means those ends are highly polar and even fully charged negative (not just a partial charge). This makes the end very hydrophilic and is known as the "head" group. So the "head" end loves water and will have favorable interactions while the "tail" portion will flee from water and just try to find more hydrophobic tail groups to strengthen those dispersion forces. This is the building block for making micelles. Below is an example using stearate ion (the deprotonated form of stearic acid) showing how the actual line structure is depicted in a schematic drawing with a circle for the head group and a long line for the tail group.
A micelle is a relatively large molecular super-structure that is generally spherical and is a bunch of these emulsifier molecules like stearate making the sphere. The entire surface of the sphere is the head ends of the molecules - all pulled together with a full water layer reinforcing the structure. At the same time, the tail groups are all maximizing their interactions with each other to build the interior of the micelle. So the overall picture of a micelle is a little (super little as in you can see them) micro-droplet of "oil" (the tails) with an outer shell of water-loving charged heads. Below is a schematic drawing/rendering of a typical micelle (a cross-sectional view). The same schematic of the stearate ion shown above is now used 24 times to create an entire circle. Realize that it actually makes and entire sphere and not just a ring.
If you add enough emulsifier such as stearate, micelles will form and any oil floating on the water can easily be emulsified which means the oil gets broken into very very small oil droplets (smaller than your eye can see!) that are in the "cozy" hydrophobic environment of the center of the micelle. So when you shake/stir/mix up the oil and water, you get a stable emulsion which to the eye looks like a homogeneous solution. It looks as though everything completely mixes and is soluble. In reality, the oil is just broken up into tiny-ass droplets encased by the emulsifier. This is show below.
BTW... if you are in mostly oil and just a little water, the micelle will invert which results in the hydrophilic heads all congregating to the center (avoiding the non-polar oil) and the hydrophobic tails pointing out into the oil. Neato. Have a look...
What is one of the most common emulsified substances of oil and water? Mayonnaise. Oil is emulsified with an egg. The egg provides the molecules that act as emulsifiers. Use lemon juice for your "water" to get the right taste and mix thoroughly. Bam - mayonnaise. Keep in mind, not all emulsifications end up thick and spreadable. But many do. Lotions and creams are examples of cosmetics that are emulsions (they have micelles in them). There are many other examples of getting hydrophobic substances together with hydrophilic substances. Soaps and detergents are excellent emulsifiers and help you get clothes and yourself clean. Ok - that's it for now. There is so much more on this but I'm taking a rest. Hydrophobic/hydrophilic interactions are important across many disciplines. Read up on some.
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Electrochemistry is a division of chemistry that depends on the electrochemical behavior of substances. Specifically, the nature of a substance to push or pull electrons from another substance. Any reaction that involves the transfer of electrons from one species to another is called a redox reaction. Redox is an abbreviated form of saying reduction-oxidation. Electrochemistry is built on the shoulders of the combination of reduction reactions in tandem with oxidation reactions. Here are the specific definitions.
reduction
oxidation
The first definition is the most important one. The others are very helpful at times but not always. For instance, a redox reaction can take place with no H or O in the formulas, so those last two definitions are only helpful with certain compounds - but helpful none-the-less, so I included them. Below are two "classic" helpful mnemonics to assist you in remembering. Leo says ger... loss electrons oxidation / gain electrons reduction. Or...oil rig... oxidation is loss / reduction is gain. Take your pick - draw your own.
The Daniell Cell is a classic electrochemical cell used as an example of an electrochemical reaction. It happens to be one of the easiest redox reactions to see and understand.
Study it - get the "lingo" down...The Cu2+ ion gains 2 electrons and is therefore reduced. It also facilitates the removal of 2 electrons from zinc, thus making the Cu2+ the oxidizing agent. The Zn loses 2 electrons and is therefore oxidized. It facilitates the gain of electrons for copper(II) ion, thus making Zn the reducing agent. Know the way we define and use the word agent here. Like it says "get the lingo down".
reducing agent
oxidizing agent
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We chemistry will usually think in terms of moles of this or that when talking amounts of stuff. For a redox reaction that is providing electric current, we can get at those moles with some math. We learned earlier that you can calculate the total number of moles of electrons being pumped around a circuit by using current times time divided by the faraday. \[{I\cdot t\over F} = {\rm mol\;e^-}\] And as much as I'd love a world that counted in moles, it just doesn't work that way. The real world only uses the top part of that equation (the numerator) and calculates coulombs of charge passed which is just \[I\cdot t = {\rm coulombs}\] And we don't even say coulombs either (sigh). We say amp-hours which is just straight up current units times time units in hours, an amp-hr or Ahr. And, most "little" batteries that we use (the AAA, AA, C, D) only pass a wee bit of current usually in their devices which means you'll get better numbers if you use milliamp-hours or mA·hr (commonly shown as just mAh). So multiply the current in milliamps by the time in hours and you'll get mAhr which IS the go-to standand for battery capacities in this size of application (small electronics/toys/devices). Here is a typical AA alkaline battery capacity written out for you: AA capacity = 2800 mAh Which is true only IF you are only pulling 25 mA or less during the operation. Yes, the drain-rate is important and it affects the actual capacity of the battery. Bump that drain-rate up to 500 mA and you'll only get 1300 mAh which is less than half the capacity at the smaller 25 mA rate. So when you are comparing bang for your buck with batteries, you need to know the current drain of your device and then check the capacity of the battery under those conditions. Alkaline batteries are NOT so great in high-drain devices like digital cameras because you'll only get about 1/2 or less of the advertised capacity. Size MattersAlso remember that those electrons zipping around your circuit have to come from somewhere. Yes, from your reactants in the redox reaction. Do some quick chemistry math here... a 2000 mAh AA battery means you are running 25 mA for 80 hr. If we use our alkaline cell redox reaction (see last section) we know that 2 moles of electrons are passed for the overall reaction. So using our old conversion: \[{0.025\cdot 80(3600)\over 2(96485)} = 0.037\;{\rm mol\;rxn}\] Now we can convert that into the amount of Zn and MnO2 which is 2.4 g of Zn and 6.5 g of MnO2. So only about 9 grams of actual material is being converted to products in a AA battery. A AA battery weighs about 23 g total so the other 14 g is the materials holding it all together, plastic, stainless steel, more plastic + the electrolyte, water, and a little excess Zn and MnO2 for good measure. Now say you want more current (capacity) in your set up... you can either add more AA batteries OR use a higher capacity battery like a C or D cell. If you go with a D alkaline, you'll get 16000 mAh. Wow, that is 8× that of a AA battery. You DO pay the cost in size and weight though, now you are at 139 grams for the weight which is about 6× more than the AA. Max Current scales with SizeIt is important to remember that ALL the electrons entering and leaving a battery HAVE to go to and from the two reactants via the electrode SURFACE. It is the surface area of a battery that ultimately limits the maximum current possible from the battery. Yes yes fine, there are other factors too but the main one is surface area of the electrode. You could have a big-ass battery with lots and lots of material, but if the electrode surface is still tiny, then you will NOT pass any high currents at all. It's like draining a bathtub full of water. The water is going to drain much faster if you have a very wide drain and pipe to drain the water. Put a tiny drain and narrow drain pipe on and you are going to wait a long time to drain the tub. The analogy is good for thinking about electrons moving in and out of the cathode and anode in an electrochemical cell. Find ways to increase the surface area of the electrode, and you will increase the maximum current possible. Let me write that really big for you. max current \(\propto\) electrode surface area So bigger batteries generally have larger electrodes and surface areas which means that more current can be delivered. Plus, if you still just want small currents, big batteries deliver that as well AND last a lot longer due to their much larger capacity. Design Matters TooThe classic "can" type batteries are great a cylinder electrode centered in a can electrode. This is what alkaline cells are like. The electrode sizes in a can type housing are limited by the size of the battery (the radius and height). A very clever way to get even more surface area out of the same basic size is to roll the battery out of 2 sheets of electrode materials. NiCads, NiMH's, lithium, and some Li-ion batteries are made this way. They are called spiral bound or jelly rolled. You can get about 20× more surface this way and therefore 20× more current. This can actually be a bad thing if a battery like this gets short circuited. The high current causes a tremendous heat rise and the battery can catch on fire and even explode. All the spiral bound batteries I mentioned above have been known to do this... so don't short circiut a battery. Good idea to not short circuit a battery of any type while you're at it. Performance vs TemperatureAll batteries have much lower performance at low temperatures. As the diagram below shows, cold temperatures decrease run time considerably. Less of the battery's energy is available at low temperatures. Best performance is at room temperature or a bit warmer (not hot, just warm). Plot of AA alkaline battery potential vs time during a 250 mA dischargeBottomline: batteries hate the cold and they perform poorly in it. Manufacturers will promote how great their battery performs in cold weather. A car battery has a performance rating known as CCA which stands for cold cranking amps. This tells how many amps can be delivered from a battery at freezing temperature (0 °F, or –18 °C) for 30 seconds and keep the voltage above 7.2 volts.
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no nav 7.1 Redox Reactions 7.2 Oxidation Numbers 7.3 Balancing Redox Reactions 7.4 Electrochemical Cells 7.5 Stranger Things / Electrodes 7.6 Standard Potential 7.7 Conc and Potential 7.8 Counting Coulombs, Moles, and Joules 7.9 Batteries 7.10 Battery Facts 7.42 Learning Outcomes ❮ previous chapter next chapter ❯ |
Two types of electrochemical cells
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