CHEM 104 Lecture Chapter 7 Chemical Reactions and Quantities Part 1

hey everybody welcome back to chem 104 lecture we are starting chapter 7. for this chapter there are actually three short videos the reason did this is because it's kind of like three mini chapters the chapter 7 in your textbook it kind of covers three topics that are over spread out over more like two or three chapters in like traditional chemistry textbook this is more of brief skimming of each of these topics so wanted to put it in more bite-sized pieces so you can kind of watch get what you need to get and if you need to revisit something it's little bit easier for you to revisit you'll understand why say that when we get to parts two and three so part one should be pretty easy peasy we're just talking about the types of chemical reactions evidence of chemical reactions things of that nature no math but there will be math in chapter seven so make sure that you watch all three parts not just two three so like said in this chapter we're covering chemical reactions and quantities we'll start with the chemical reactions part first and then in parts two and three we'll get more to the quantitative stuff first section we're looking at the equations for chemical reactions we'll remind you of chemical change remember that chemical change occurs when substance is converted into one or more new substances those new substances have to have different chemical formula and different properties we're going to work on balancing equations and we're also going to identify the number of atoms in the reactants and the products so again chemical change which is what this chapter is about chemical change means chemical reaction there are lots of different ways that you can observe chemical reaction the ones that we're going to cover are the formation of bubbles changing color production of solid or change in energy so changing color the example here is when iron rests rust forms when you have metal interacting with oxygen okay that iron if you ever lived in northern climate in the u.s like michigan then you will see some rusted out cars that is because oxygen does doozy on the body of that car that is chemical reaction it's also change in color because you can see how you have that shiny metallic luster on those new nails then the old ones are kind of brownish red changing color that means chemistry has happened another type of evidence which should mention you don't have to see all of these pieces of evidence to suggest chemical change has occurred you really only need to see one so you might see change in color you might see formation of gas but you may not see both of them so if you mix two things together two aqueous solutions and all of sudden you see gas forming those bubbles that shows you that something's going on there or if you take piece of metal an active metal like sodium and put it in water there's going to be lot of bubbling going on you're forming hydrogen gas so formation of gas where you see lots of bubbling and fizzing or it could be little bit of bubbling that's chemical reaction we've also got the formation of solid which is called precipitate when you take two aqueous solutions combine them and then you have solid form here we've got potassium iodide and lead nitrate added together and then you see yellow solid form that yellow solid is actually led to iodide the last one we've got heat being produced or absorbed that's like change in energy right so we can talk about endothermic versus exothermic reactions and we'll talk about those later on in the chapter but if you're touching reaction vessel like flask or something which don't necessarily recommend but sometimes it happens you brush against something you may observe that the flask is colder than your environment or warmer than your environment that's change in energy where you're producing heat or absorbing heat we have to get together some kind of shorthand language to be able to write these chemical equations so we learned in chapter 6 about how to write names and formulas of compounds now we need to take that knowledge and apply it to writing chemical equations here what you see is some carbon being burned with oxygen you need some heat and you're forming carbon dioxide if you like to grill if you are lover of barbecue or smoked foods then you are familiar with this reaction so there's some symbols that you need to know when you're writing chemical equations or when you're interpreting chemical equation to write chemical equation you're going to use an arrow to separate the reactants from the products so what that looks like is you have your reactants on one side then an arrow finger products reactants are on the left always products on the right always think of the reactants as your ingredients and your products that's the result if you have multiple reactants or products you separate them by plus sign so if plus come together and make you separate and the two different reactants with plus sign the delta sign which is little triangle indicates that you use heat to start the reaction so in that example where we have the carbon and the oxygen and it had the triangle over the yield sign you got to had add heat so if you're trying to grill something you have some charcoal you can't just put the charcoal in the grill and just put the food on it you're going to look silly you have to add some heat the physical states of compounds are denoted in parentheses following each compound so you need to let the reader know is this solid liquid gas or aqueous aqueous just means dissolved in water so all of that information needs to make it into the chemical equation this chart summarizes the symbols that we use to write equations good thing to put in your notebook it's an easy one to reproduce too so if you don't have access to printer you can just kind of recreate it won't take you that long but you'll need to know these you will not get access to them on the exam so make sure you know it we're working towards balancing chemical equations so you know the symbols and kind of where things are now we need to talk about what balanced chemical equation is all that means is that you've got the same number of atoms on the reactant side as you do the product side for each element you're not gaining any atoms or losing any atoms they're just being shuffled around so in this image to the right you've got four hydrogen atoms and two oxygen atoms you have the same thing on both sides so if we were looking at reaction here we could look at this and say okay there are four hydrogen atoms and two oxygen atoms that's zero that's not zero that's an know it's little confusing and on the product side it's the same thing two times two is four hydrogen atoms and we've got two times one for our oxygen that is balanced equation while we're gearing up for it this is the goal so keep that in mind the goal for balancing balancing an equation is to have the same number of atoms on the reactant side as you do the product side for each element right now what we're going to do is little bit of practice we need to figure out the number of atoms of each element on the reactant side and the product side for this balanced equation i'm going to show you the chart that make and this is the method that use when i'm balancing equations too not just checking for whether an equation is balanced or not put each element that see and see for phosphorus and br for bromine across the top put an for reactants and for products that's my reactant side and my product side i'm going to count up all of my atoms of each element on the reactant side and do the same on the product side remember that subscript right there means that have four phosphorous atoms so that's what i'm going to write on my reactant side for phosphorus for bromine each bromine molecule has two bromine atoms this six is coefficient aka multiplier so however many bromines have in my molecule i'm going to then multiply that by six because have six bromine molecules so six times the number of bromines in each molecule is two that gives me 12 bromine atoms now we go over to the product side we've got 4 in front which is coefficient and then we've got subscript of three for the bromine that subscript is only for the bromine so we've got four phosphorus atoms because we have to multiply everything in the compound by four it's just like with math for bromine i've got four times 3 because of that subscript so have total of 12 bromine atoms so this is balanced it says that it should be balanced in the eq in the question so if you got two different numbers that would be wrong okay when we balance an equation our last chart that we make like this our check should be balanced just like this is so let's try balancing chemical equation this is the formation of aluminum sulfide first thing you want to do is take stock of what you have take lay of the land if you will remember that the left hand side is the reactant side the right hand side is the product side right hand side is pretty reactant side is pretty easy we've got one and one product side al has subscript of two so that means there's two aluminum atoms sulfur has subscript of three there are three sulfur atoms clearly not balanced right so this is our first chart just to take lay of the land and see what we have now we've got to start balancing what we do here is take the lowest common denominator we'll start with the aluminum the lowest common denominator between 1 and 2 is going to be 2. we don't need to touch the product side but we need to multiply the reactant side wherever we see aluminum we need to put 2 in front there we go now we do our second little chart and this will show us what happened after we added that to now you may be saying to yourself come on dr hefner do we really have to do table for this well probably not for this one but when you have compounds that have multiple elements and you're putting coefficient in front you need to keep track of what's happening to each of those because you may have something that was balanced then you put coefficient in front and now something comes out of balance so the way you practice it is the way that you'll do it recommend doing this way if you want to be 100 sure that you catch every little detail so we multiplied our aluminum by two now we've got two on the reactant side and two on the product side we haven't touched the sulfur so we've still got one on the reactant side and three on the product side we do the same trick with our lowest common denominator we've got one and three lowest common denominator is going to be three multiply the reactant side by three to get there put three in front of everywhere you see that sulfur which should only be one compound if you have the same element over two different compounds it's whole different ball game much more difficult to balance not going to throw that at you we do our final table just to check and make sure that we are actually balanced like our chart suggests we should be on the reactant side have two aluminums and three sulfurs on the product side have the same thing so check and check on your exam you'll see questions about what are the coefficients in the balanced equation or what is the coefficient for sulfur in the balanced equation what that means is you're going to say what these numbers are in front if you don't see number like with the aluminum sulfide it means that you have one okay we always assume one if were going to write the coefficients in order it would be two three one those are the coefficients of the compounds in order of the balanced chemical equation notice that when we were doing the balancing there's one thing that never did never touched the subscripts and that's because you shouldn't so do not change subscripts that would change the compound itself that's not what we do we add coefficients which are the numbers in front it may be tempting to change all the numbers don't only change the coefficient which is the number in the very front of the compound here's another balancing question and i'd recommend at this point that you pause and try it out so i'll help you get to the point of having chemical reaction but this is good example of what you may have to do for your homework or for an exam you have chemical equation that's written in words and you have to write it out as chemical equation with formulas balance the chemical equation when you've got this solid iron compound reacting with hydrogen gas to produce solid iron and water so we have this solid reacts with hydrogen gas that's hydrogen gas okay hydrogen exists as diatomic molecule which we're going to talk about but for now just trust me that's how you write hydrogen gas then we have our yield sign and it produces solid iron and water water remember is liquid so you need to put that in parentheses now that we have our unbalanced equation we need to take lay of the land see what we've got here we have iron oxygen and hydrogen on the reactant side there are three irons four oxygens two hydrogens product side one iron two hydrogens one oxygen clearly not balanced so we need to just pick place to start let's start with our oxygen the oxygen starts off with the iron and ends up with the hydrogen so if we make change to the oxygen it's going to affect both of those let's just get that done at the forefront and then we'll figure out how everything shakes out we've got four and one the lowest common denominator is four so where we see oxygen on the product side we're going to put coefficient of 4 out in front that's going to change some things let's see what we changed react inside we didn't change anything so we'll just copy that over product side we should take our time iron we didn't touch oxygen we've got four times one which gives us four oxygen atoms hydrogen we absolutely change that to because it's in the same compound as the oxygen on the product side so that's four times two gives us eight hydrogen atoms well now we've got little bit of problem we went from having our hydrogen balanced to now we've got two on the reactant side and eight on the product side we've got to fix that the lowest common denominator with 2 and 8 is going to be eight we multiply the reactant side by four so we put that coefficient out there we do another chart and know it may seem little bit tedious but if you really want to make sure that you don't miss any details you need to keep track of each change that you make versus just kind of throwing some numbers into the equation and just doing it in your head if you have record of what you changed and you can see wait minute i'm not balanced what did do you can undo the last thing that you did and start afresh if you're just doing it in your head you might kind of get lost and do some things that are that are unallowable like changing subscript okay we don't want to do that so we press on we just did four times our hydrogen on the reactant side so that gives us eight then we still have the four oxygens and the three irons products we didn't change anything so we've got one four and eight last thing we need to do is balance that iron the lowest common denominator between three and one is three so we're going to add three in front of where we see iron this should be our last table everything should be balanced at this point if it's not then we can go back look at our math but we've got detailed analysis of what we did and where we may have messed up so on the reactant side we didn't change anything so that's three four and eight for iron oxygen and hydrogen product side we multiply the iron by three so that gives us three and then everything else remain the same now we're balanced the coefficients for this would be one four three and four but keep that in mind we don't have coefficient in front of this solid iron compound but if you were doing homework question or quiz or anything like that the coefficient is one now we need to make it little bit more complicated because it wouldn't be chemistry without it right remember those polyatomic ions the compounds that you had to learn how to name and write formulas for you can see those in chemical equations the good news is if you observe polyatomic ion on both sides of your equation you can treat it as one unit po4 is phosphate and we see it intact on both sides of the equation when we make our charts and we do our balancing we can actually keep that po4 all together just like we do with the elements we treat it as one unit first table we take lay of the land we've got sodium phosphate magnesium and chlorine let's count them up three sodiums one phosphate one magnesium two chlorines remember to look at those subscripts and for the po4 we're considering the whole thing as unit we're counting that as one product side we've got three magnesiums two phosphates uh-oh still one sodium one chlorine wow nelly very much not balanced what do we start with start with balancing the polyatomic ion the lowest common denominator between 1 and 2 is going to be 2. we put 2 in front of the compound that contains phosphate now we check and see what we made mess of because it gets little bit messy before it gets better it's kind of like life nothing wrong with little bit of mess so we put 2 in front of that sodium phosphate compound that means that instead of just three sodium atoms we've got two times three because we've got two of these now so there's six sodiums we have two phosphates and we didn't do anything to the magnesium chloride so those numbers just stay the same we also didn't do anything to the product side so those numbers are still the same next thing to tackle let's finish balancing the sodiums we did the phosphates we started messing with the sodium let's mess with the sodium some more and get that squared away lowest common denominator is going to be six between one and six so we're putting six in front of the compound that has sodium let's see what we did sodiums and everything on the reactant side we didn't do anything so we should be able to just copy it the product side however we made some changes let's see what we messed up so magnesium still the same we've got three of those phosphate still the same two of those sodium we now have six we also now have six chlorine atoms so the last two things that need to be balanced are the magnesium and the chlorine and thankfully on the reactant side they're in the same compound so whatever we do to one it's going to happen to the other and that better make them match up we'll do the chlorine since that's little bit messier and we'll make sure that that lines up with the magnesium we've got two and six lowest common denominator is going to be six we're going to put three in front of that magnesium chloride and that makes sense because if we looked at the lowest common denominator for the magnesium between the reactant and product side it would be three so putting 3 in front of that should make us right as rain but we need to double check and make sure that we really are done just said right as rain don't even know where that turner phrase comes from why is rain right let's see what we did sodiums we didn't touch them so it's still six phosphate didn't touch them should still be two the magnesium and the chlorine we did touch it's three times one which gives us three for magnesium and three times two which gives us six for the chlorine the other side should be the same but always double check and make sure we've got three magnesiums two phosphates six sodiums and six chlorines everything matches up so we're good to go that means our coefficients in order from left to right are two three one and six should mention here before we move on to the next section that all of these coefficients they're small whole numbers and we shouldn't have we should also have the simplest ratios so if doubled this to say 4 6 2 and 12 yeah that's balanced but it's not the lowest or the smallest coefficients that you can have so make sure that your coefficients are whole numbers and that they're as small as they can be can't reduce them any further moving on we did balancing equations we're pros at that now let's talk about types of chemical reactions these are the types of chemical reactions that you will be responsible for we're going to go through each one in detail so i'm not going to list them off here we're just going to go through one by one first one will attack combination reactions in combination reaction you've got two or more elements forming one product you're taking simple ingredients putting them together to form one complex thing your example here is taking sulfur and oxygen gas you're creating sulfur dioxide two simple compounds combining form one more complex product you've got some other examples here where you're taking another metal plus oxygen and making an oxide you've got sodium and chlorine so metal and non-metal many types of examples all of these are combination reactions you're combining things together plopping out one product it's kind of like if you're making cookies or brownies or anything like that where you kind of mix it all up in bowl oversimplifying how you make good cookies by the way so don't don't just plop everything in bowl but you're taking all these ingredients to form one thing cookie in decomposition reaction you've got one reactant that is split into more simpler sublime substances so two or more simpler substances i'm struggling with the word substances today you have something that's complex like in this example you've got mercury two oxide and it decomposes to make liquid mercury and oxygen gas so you've got one complex thing two or more simpler things you can think about the decomposition reaction as the reverse of the combination so if you can remember one just know the other is the opposite next type is single replacement reaction in these kinds of reactions one element takes the place of different element and another reactant compound so let's say that you've got and you're placing it into solution of bc okay so is solid of some kind bc is an aqueous solution kicks out and says look you didn't pay your rent you've been bad roommate i've been called in because is needing some help okay has been footing the bill for the rent for the past four months you need to get out i've already got my rent and security deposit so by it becomes part of that aqueous solution and is like well dang i'm out here reckless lost it again with chemicals some examples would be zinc solid added to hydrochloric acid the zinc is active enough to replace the hydrogen in the acid you end up forming zinc chloride and you liberate hydrogen gas this can also happen between two metals if you have copper two sulfate solution which is nice pretty blue color and you add some source of solid iron what you'll see is the copper in the copper ii sulfate will actually start to come out of solution and kind of form it'll plate onto that iron so you'll coat the source of iron and the iron will become part of the aqueous solution and you'll have iron ii sulfate in double replacement reaction you've got the positive ions in the reactant compound switching places so and are roommates in their own little aqueous apartment known as flask and they've got their own place across the hall then they swap roommates and become friends maybe that's still aqueous but and you might make precipitate some kind of solid or you might liberate gas but you make two different compounds because you took two aqueous solutions mixed them up and you made something different you could also have solid but as long as you've got two different ions in each compound then you can have double replacement reaction now some real world examples would be silver nitrate and sodium chloride you mix those two you make silver chloride which is insoluble so you form solid and then you have sodium nitrate which is aqueous so new aqueous solution and solid forms called precipitate you could have zinc sulfide which is solid and add that to hydrochloric acid you're going to make aqueous zinc chloride and liberate hydrogen sulfide combustion reactions this is reaction type that you're likely familiar with even if you don't know it if you drive car or have ever ridden in car bus motorcycle anything like that combustion reaction is what gets you going you're taking your fuel source which is gasoline or diesel combining it with oxygen and that burns you form carbon dioxide and water you also release lot of energy and that energy is what's used to propel your vehicle forward to define combustion reaction you have to take something that's carbon containing compound so gasoline and diesel are examples of carbon containing compounds you could also have ch4 which is methane you could have propane any of those are carbon containing compounds you burn it in the presence of oxygen and you form carbon dioxide and water this table table 7.3 gives you examples of each of the reaction types we just covered great to reference when you're doing your homework if you enjoy having this kind of reference in your notebook go for the gold otherwise the notes that you have whether you just print out this presentation or you write it down in your notebook should be sufficient so would recommend if you haven't while taking notes go back and write an example for each of them so that you have something to compare your homework and quiz and whatever problems too time for learning check we've talked about all these reaction types let's actually try to identify each reaction recommend pausing it here and trying to label each of these through then rejoin me and see how you did so in part we are taking two simple reactants forming one complex product we're combining things to make one product which means it's combination reaction abbreviate reaction rxn second one we've got silver and some kind of an aqueous solution and notice look at where that silver ends up it has swapped partners it was alone then it booted out the hydrogen so that must mean it's single replacement part we've got carbon containing reactant here plus oxygen we're making carbon dioxide and water and lot of energy has to be combustion part once you start seeing all of those polyatomic ions and stuff it looks like ionic compounds be on the lookout for replacements like to use colors to demonstrate whether or not something has switched partners so if give each element or ion its own color then can see what happens much more easily and it absolutely looks like we have double replacement the lead is now with the sulf the sulfate the potassium is now with the chlorine final reaction letter we've got something that's complex and we've got two simple products this looks like the reverse of letter so it must be decomposition get comfortable with that kind of logic that's what you need to identify reactions and don't be afraid to use the color pencils or markers or highlighters all those things are allowed on exams so go go crazy still in the realm of talking about reactions now we're going to talk about one specific reaction type which is oxidation reduction reactions will very likely abbreviate that redox same thing an example of redox reaction is rust forming what happens with redox reactions well redox reactions are great because they provide us with energy from food so all not all but lot of the chemistry that happens in your body has to do with redox reactions it's also the reason why your phone your laptop and all the things that you have that have lithium ion battery in them can produce electrical energy like said before it's also why things with iron rest that is redox reaction what happens in redox reactions is that you're transferring electrons from one substance to another if has an electron and it transfers that electron over to then that means that is going to receive an electron and be reduced reduction is the gain of electrons however just gave up its electron it lost it oxidation is the loss of electrons so one way that you can remember oxidation versus reduction is using this mnemonic here oil rig oxidation is loss of electrons reduction is gain of electrons oil rig now when was coming up through chemistry like general chemistry and stuff like that we learned leo the lion says gur lose electrons oxidize gain electrons reduce whichever one helps you or if there's another one out there on the interwebs use it if you can just straight up remember the difference that's great too but think that these little mnemonics are silly and they kind of help make chemistry little bit more light and fun have young children so we make animal sounds all the time and going grrrr every once in while it's got to be healthy right kids are happy so why not try it back to redox statue of liberty if you've ever seen it it's green didn't start off that way it's actually green because the copper metal in it oxidized what does that mean copper metal which you see this zero up here that's just charge charge of zero means it's neutral when you get into electrochemistry it is very commonplace to see zero for neutral substance just because it's easier to track when something gains or loses electrons so we've got two half reactions here what happens to the copper and what happens to the oxygen copper starts off it's neutral it's got no charge protons equals two electrons okay in terms of the number of them but the copper is oxidized which means it loses electrons notice how now it's positive ion and we have lost four electrons with the oxygen o2 gas neutral it's going to lap up those four electrons because oxygen is very electronegative since we have the electrons on the reactant side and we're adding to the oxygen that means that it's going to gain electrons so if you're losing electrons you'll see something positive being formed and you'll also see some electrons on the product side if you are gaining electrons then you'll see those electrons on the reactant side this is another example of redox reaction it's single replacement reaction you've got zinc being oxidized to zinc two plus if you have solution of copper ii sulfate which is very pretty blue and you add zinc strip wait little bit and what you'll see is the copper will start to plate onto that zinc strip and you will have released some zinc ions into solution so that's an example of real world reaction that you could do if you had all of these components at your disposal like alluded to redox reactions are really important in biological systems one example of molecule that does redox in your body is coenzyme called fad coenzyme just means that it's helper to an enzyme the enzyme needs this molecule to do the chemistry that it does now i'm biochemist so this kind of thing is exciting to me talk about it forever it's great but won't do that fad which stands for flavin adenine dinucleotide can be reduced to form fadh2 by the transfer of two hydrogen atoms and we're talking about the proton and the electron okay so we're transferring couple of electrons to fad and reducing it that can be reversed too fadh2 can give up two hydrogens and be oxidized so this is very handy biological molecule that can do some really interesting chemistry lots of enzymes in the body use it very very important this table summarizes the characteristics of oxidation and reduction oxidation always means that you're losing electrons ways that you can see this is the addition of an oxygen or the loss of hydrogen so those are some examples reduction always means gain of electrons it may involve the loss of an oxygen or the gain of hydrogens this is another summary slide which gives you more information about oxidation and reduction if you're chart person you got your chart if you're word person you got all the words so it's the same thing as the chart there are more biochemical redox reactions that we could talk about if you're metabolizing alcohol so methyl alcohol is poisonous don't test that it is poisonous your body can metabolize little bit of it but it is poisonous one of the reasons it's poisonous is because you're forming intermediates that are poisonous they're toxic to your body so you're making formaldehyde and formic acid before you finally make carbon dioxide and water formaldehyde and formic acid are toxic to you they'll cause headaches they interfere with reactions bad bad news so please don't try it but the body is amazing and it can sometimes take the things that we do to it the bad things that we do to it and figure out how to solve it that is beautiful chemistry right there let's do learning check you need to identify when something is being oxidized and when something's being reduced so this should help you with that take second and try it on your own and then join me and we'll talk it through we need to figure out which reactant is oxidized and which one is reduced let's look at what happens to the silver we're starting with plus and we're going to which you can kind of write in zero here if you don't see any kind of charge that means zero so we're going from positive to zero the only way that we can do that is if we add an electron to get rid of that charge if we're adding an electron to the silver that means it's gaining an electron if leo the lion says ger then gaining electrons means reduced so ag plus is reduced the chlorine we're starting with cl minus and going to cl2 which again doesn't have charge there's zero there you just don't always see it you can write it in if that helps you to go from negative charge and we'll have two of these to zero charge it means that we must have gotten rid of some electrons along the way we lost electrons remember losing it means that it's on the product side so that means the chloride ion is being oxidized if this was confusing to you then you should revisit what it means to gain electrons and lose electrons and the first place that we started talking about that was in chapter four so revisit chapter four make sure that you're comfortable with gaining electrons and losing electrons and what that does in terms of making ions we also talked about that in chapter six when we were writing formulas and writing names so those are two places you can look for more information about forming ions and what that means that's it for the first part of chapter seven please make sure that you watch the next two videos know it's lot of material two videos and we already hour deep but promise you want all this information because chapter seven is really big make sure you come to live lecture we always do practice problems you get all the details regarding assignments and exams so come join until then be safe