this video is part two of a three-partthermodynamics video series and what we want to talk about in this video isgibbs energy or free energy after today's video hopefully you'llunderstand what gibbs energy is and know symbols and units you'll know how to calculate it you knowhow to interpret it when you see a free energy value you know how to evaluatespontaneity of reaction based on gibbs free energy and when a spontaneity istemperature dependent find where that what we call the crossover attempt whichis the temperature above or below which things spontaneous
so let's go back to the reaction wetalked about in the last video where you're basically just burning math aimscommon natural gas we used to heat our water to cook on thestove and we know that as things happen the gas burns creates heat it's an exothermic reaction and we knowthat from the last video this spontaneous that basically theentropy that's created an environment makes up for the entropy lost in thesystem and basically in thermodynamics you really want to know two things hebetter have practical application so one of the things you want to know iswhether the reaction is spontaneous or
not and the other thing you would like toknow is how much energy you can use from reaction based on the chemical potentialenergy that's stored in the bond breaking bond forming events in the inthe past we talked about you know utilizing the heat but i don't want youto misunderstand that usable energy or free energy can be used in differentways so in the case of methane in the lastyear we've actually increased the amount of method we've used to createelectricity so we're lowering our dependence on coal and were increasingdemand electricity generated with
natural gas but of the chemicalpotential energy that's stored in every mole of methane the question is is how much of that energy can i use toactually create electricity or maybe we use of transportation i know here in denver we actually drivesome of our shuttle buses oven and the sixteenth street mall on methane gas andyou see if you have a vehicle is powered by compressed natural gas or methaneyou'll see this little symbol on that side of the vehicle
the cmg's sticker this is a little sidenote that's a little unrelated to energy but just of interest in regards tomethane it's also chemical feedstock to choose for a moment making ammoniafertilizer methanol and hydrogen but once again we're back to thatfundamental question of the chemical energy that's available in methane howmuch electricity or transportation movement can we actually generate what'swhat's actually usable and that's what gives free energy comes in and givesenergy or free energy as this referred to because again gives a standard energy orreaction is a measure of the energy is
available from the chemical potentialenergy that can be released in the reaction or that can be used in theseways the unit's ford are commonly in kilojoules per mole some sort of energyper mole using when you're looking at a table you'll see gibbs free energymeasured but it tends to be assumed that is that a given temperature and pressureand you see that temperatures 298 k in general you can start with your reactionbut you're going to lose a little bit of the potential from the heat of reactionbecause you have to pay an entropy decks and entropy is what we talked about thelast section gibbs free energy can also be useddirectly to measure spontaneity as we've
talked about in the last video you cando this from the entropy standpoint say well theater entropy of the universe isincreasing according to the second law ofthermodynamics and that also would give rise to spontaneous reaction gibbs freeenergy is also a direct message that allows you to determine whetherreaction is spontaneous and just a little bit of the history there the thefree energy is assigned with the unit g after josiah gibbs who discovered it andand just i always tried to describe like pressures really the true effects offorce if you really want to know how much of up damaging effects that forsays you would actually want to measure
the pressure in the same way of all theenergies you might measure gibbs energy or free energy is the proper measurethat will actually tell you usable energy that comes from chemicalreaction so here we go here's the ears of the formula the gibbs equation i'vegot it rearranged from the way it's traditionally written just so you canunderstand it a little bit and that is here reaction which we stayed in get onchemistry one basically which accounts for the energy required to bond breakingevents in the energy released in bond forming events that net energy actuallyis available for use but you have the gibbs energy which is really what'savailable to use for you know running a
battery running an engine doing chemicalreactions but you always have to pee in addition the entropy tax which iswhat is dispersed to the university so we've learned in the last videothat's actually temperature-dependent what is the gibbs energy telling me so in terms of spontaneity here's howyou interpret a gibbs measurement if the gibbs energy is less than zeroyou have a negative sign on it then that is automatically a spontaneous processif the gibbs energy is greater than 0 then that indicates that it's a nonspontaneous process and if the gibbs energy is exactly equal to 0 then atthat point you can assume that the
reaction is and until it's reached equilibrium so ican no longer harvest energy from them now as far as the useful for free energy anytime the delta g is less than zerothat is actually a measure of the useful energy and remember according to thefirst law of thermodynamics that whatever energy is a boss to the gibbsfree energy is negative that's given off by the reaction that is the exact sameamount of energy that be that can be utilized by another reaction it's a sign change but remember what theconvention means if it's negative it
means it's leaving the system or in thiscase the chemicals where the reactions taking place and it's been picked up bythe surroundings so how do you calculate the delta g thegibbs equation basically shows you that you can take the enthalpy of thereaction the heater reaction you subtracted by the temperature and onceagain this is the temperature which the reaction is taking place in essence isthe temperature of the surroundings of the reaction and then you multiply thatby the entropy of the reaction which we've learned in the last section youcan calculate from entropy tables as you can calculate the heat of reaction fromheat of formation tables all contained
in a thermodynamic table another way to do this is to calculatedirectly from the thermodynamic table using the gibbs energy information whichis analogous to heat of formation this is the gibbs energy informationit's there three energy required to make thereactants from the standard state and analogous manner your elements and yourdiatomics are considered have a gibbs energy of 0 but once again pay attention that you dothe product - the reactants so whatever the reactions are you change the sign on
those so let's let's do an example herethis is where tascam chlorate is converted to passingperchlorate with heat this is a chemical reaction or potassiumchloride is used in explosives and fireworks it's just more stable than the potassiumchlorate so a lot of times it's converted over and this is actdisproportionation reaction which the chlorine is both oxidized and reduced inthe same reaction but once again you go back to your thermodynamic tablesomething i like to do is just basically build the table
use this reaction like columns yourreactants or products just build columns underneath there and then what i like todo is i i figure out the heat of reaction the gibbs free energy in the delta s i just basically filling in the columnsand so remember that we do this in terms of products minus reactants so whateveri put the reactant side i change the sign and the other thing is is i wantyou to notice it enthalpy and the free energy or both inkilojoules per mole but when you look at the entropy it's actually in joules permole
that's a thousand times different insize so what you want to do is make a conversion either make these kilojoulesand jewels or jewels in kilojoules and i tend to always just move the decimalover three times on my i entropy i'm going to go over there and startfilling in the blanks and once again this is a reaction so i need chain needto change sign on it the other thing is you need to multiplyby the coefficient in front because you have four moles in this balancedchemical equation here you have three moles of the potassium perchlorate soyou multiply by three keep the sign the same and then one more potassiumchloride
then you can just some across once youhave it set up in this way this is what i like to do it like a table then i canjust take these multiplied together and adam 2 b's and add across and and some all three of thesetogether and get me my answer so the heat of reaction is that - ahundred forty four kilojoules is exothermic it's losing energy i go in and use the free energy do thesame thing and notice that it's less and we wouldexpect it to be less because remember you have to give up some free energy taxbut in terms of spontaneity
this is a favorable picture losingenergy and we know that that tends two of you know that's favorable to be inspontaneous but last but not least we have entropy and we can look at thisbeforehand and kind of make a prediction these are both solid state and i haven'treally gain that many more pieces i got four solid pieces here and i got foursolid pieces here so this one up and complexity this one down and complexityso i this probably should be very close to wash of very low change in entropyjust based on what we learned in the last video about quantitativelyanalyzing entropy so go through the calculations and sureenough it's a very low actually a loss
of entropy a point zero three six eightone kilojoules per k which is temperature and i just want to remindyou again that the unit's here i change the decimal over three times for eachone of these calculations and i just simplifies it so these can all becompared together so just by looking at the three energybecause of the fact that is negative we know that it's a spontaneous reaction okay you don't have to look at anythingelse when you have that but for comparison's sake if you want to do itthe other way right you can take the enthalpy of reactionhere which is minus a hundred forty-four
you - the temperature now the standardtables are all set up at 298 kelvin so that's going to be the temperature andthen you multiply that by the entropy when you do this calculation you came upwith the same answer to three significant figures is negative . 133kilojoules and once again this reaction isspontaneous because you see a negative sign on the delta g if it came out 0 it would not be spontaneous would be anequilibrium that have come out positive you would know that it's non spontaneous so this reaction has a hundred fortyfour kilojoules of chemical potential
and that's what that tells me the bondbreaking a bond forming events release hundred forty four kilojoules but that is not all usable because youhave to pay the entropy decks that's this entropy that's actually lost timesthe temperature that's 11 kilojoules once you subtract that out though thatyou get the free energy and that is what's actually usable and if you wantedyou could run this reaction to power a battery or you could use it to heat aspace or maybe lift away whatever you want to do with it the free energy is actually the propermeasurement to use to figure out how
much usable energy is is there after youpay the entropy tax so now comes the next question and this is where givesequation is helpful because most of these tables are set up for 298 k but as you know a lot of reactions arenot going to be ran a 298 k and according to gives equation as youchange the temperature is going to change that free energy outcome so it's going tochange the spontaneity so just to prove the point we're goingto look at this reaction and kind of change the temperature a little bit sothe first reaction we did you basically
ran it 298 k you do the calculations yousee that it's actually spontaneous but just for grins us raise the temperaturea lot let's raise it up to 5000 k and see ifthis is still true and so you now do the calculation andyou see that the free energy that comes out of this calculation is actuallypositive which means non spontaneous so what this means is that at sometemperature this reaction runs spontaneously but then anothertemperature at higher temperatures apparently changes spontaneity so this gives rise to the concept ofwhat we call crossover temperature
there's obviously some temperature whichchanges from a spontaneous to a non-spontaneous reaction so where is the crossover temperaturehow do you find that so we're we're basing it on gibbs equation and now whati'm i've done here is i i put up a graph where we basically going to figure outthe the gibbs energy at different temperatures were going to start it andthen and remember this is in kelvin so thelowest you could go this is theoretically 0 which is not reallyobtainable but if there was such a thing and youdid the calculation at that point you
literally would harvest all of the bondbreaking bond forming from it and then as you start to raise the temperatureyou can see that the gibbs energy falls off so now we're approaching zero but atthis point one just point something out any time thatthe free energy is less than zero that's usable energy so anytime that you have adelta g that is less than zero you at literally have at these temperaturesdown here you have free energy available and the reaction is spontaneous but thenwhen you get to around 4,000 degrees k the free energy is zero so that's whatwe call the crossover point we go from
where free energy is available or where it'sspontaneous to where it's at equilibrium and there is no free energy availableand then if you just keep raising the temperature about 4,000 k now all of asudden the delta g is positive which means there's no free energy availableand it is no longer spontaneous if you wanted to around the reaction at thesetemperatures you would have to pump in energy fromthe outside so let's figure out where that crossovertemperature is the idea it happens at equilibrium and we knowthat that actually occurs when the free
energy equals 0 so we're going to plug in 0 for freeenergy and what we want to do is solve for t the temperature which it occurs toalgebraic manipulation bring the t delta s to the other side and then divide outby the entropy so this is the equation you want to usewhenever you want to find the crossover temperature basically just divide yourdelta h by d delta a so now if i take my enthalpy which is minus a hundredforty-four and i / the entropy which is a minus . of 368 one kilojoules per know i'm dividing by a fraction i'd invert multiply my kilojoules cancel andas a result i get 3912 k and that is the
crossover temperature and that is theequation to find the crossover temperature now is there always acrossover temperature you can imagine that according to that equation there's notalways going to be processed the temperature so we're going to base you know figure this out based on usingthe gifts equation so to understand this let's just think about this whatscenarios are favorable for spontaneity in terms of the enthalpy and the entropy well we know that when the enthalpy theheater reaction is actually exotherm
make or the sign would actually benegative so i'm put in a negative here right we know that that is favorable firstbeing a spontaneous reaction and we also know that when the entropy is increasingwhich means that the sign on that would be positive than in that condition itwould be spontaneous at all temperatures so anytime you have a reaction when youhave a heat of reaction this negative and then entropy that's positive nomatter what the temperature is going to be spontaneous because in that case no matter whatnumbers you plug in here if they have
these signs of positive and negativehear the positive x them- would make this whole piece of the equationnegative or hemp plants plus and minuses essence would give you a free energythat's always negative so it's always a spontaneous well what scenarios don't ledaspontaneity well that's when your heat of reactionis positive it's an endothermic reaction and the other scenario is when theentropy the reaction is actually decreasing which would make it negativeand when you see signs of positive on your reaction and the negative on yourentropy that can to condition your never
spontaneous because the delta g wouldalways be positive so let's take scenarios where it's mixed where you canactually have a situation where it's endothermic which is not favorable boththe spontaneity but it is increasing entropy and so youcan imagine that if the temperature is low enough in this particular factor isnot influential in determining whether spontaneous or not until the temperaturegets large enough so that this factor overall increases and overwhelms thefact that the enthalpy is not favorable to spontaneity that high temperaturesthe negative factor would override the positive factor and then the otherscenarios
now i have a heat of reaction that'sfavorable spontaneity it's exothermic but then i have aentropy that's decreasing which is not favorable and so if this temperature islow then this factor all together has very little influence in the negativesign comes from the entropy enthalpy but whenever the temperature gets high then all the sudden entropy starts tooverride and so that would be spontaneous and low temperatures you need to study this chart and putsome numbers in there until you understand that so you can look at thesigns on that enthalpy and the entropy
and decide whether there's a crossovertemperature and whether it occurs the spontaneity occurs at high temperaturesbelow above the crossover temperature low temp below the cross of the temperature nowwhat i'd like for you to do is just kind of practice all these things we'velearned here i'm going to do a reaction where you take copper to oxide and youdecompose it into actual copper solid copper and oxygen is given off and takeyour thermodynamic table so you'll have to take one out of your book or one thati've had you in class and see figure out you're dealt
your heat of reaction you're free energyyour reaction and your entropy of reaction and then answer the followingquestion is the reaction spontaneous at 298 k based on the delta g canqualitatively look at the reaction and tell me hey this should be in terms of entropybe increasing entropy of decreasing entropy is the reaction is exothermic orendothermic based on course your heater reaction is and is there a crossovertemperature based on the chart we just looked at and if so can you solve for it andkelvin and then if you understand that
do you know what it's going to bespontaneous above or below the crossover temperature so this . stop the videosolve these problems when you're done you can restart the video and check youranswers so let's see how you did so let's startoff by getting your enthalpy your free energy in your entropy quickcalculations like i said i usually just build this into a table use the each of the reactants andproducts as a calm i find my table from the book or whati've been handed to you in class take your thermodynamics table
use your heat of formation okay andremember that on the table if you look there's a reference point using 298 kand also i'm going to take my entropy in particular and converted to kilojouleson them all the same units and i remind you once again and i'm using the freeenergy calculation that its products minus reactants so you gotta change signand all your reactants which in this case is copper to walk side so i go tothe reactants or products side first and fundamental elements are 0 heat ofreaction the same thing with free energy or gibbsenergy is zero the entropy for making copper solid remember the referencepoint there is where at 298 k it would
be zero if you were at 0 k just kind ofa theoretical component so now i have oxygen which once again as a diatomicfundamental element that's zero on that the enthalpy and 0 on the free energy orgives energy i remember to multiply by thecoefficient which is one half now we're going to the copper to oxide it has a measurement of the formation of- honor 57.3 kilojoules per mole make sure you change the sign there's nocoefficient in here do the gibbs energy and finally do theentropy and don't forget to move the decimal over three times so everythingis expressed in kilojoules
now you can just some across with yourcalculator you see that the heat of reaction is a hundred 57.3 kilojoulesthe free energy is a hundred 29.7 kilojoules and this should be a a check that makessense that whatever my heat of reaction is you lose a little bit in terms ofpaying the entropy tax so is the reaction spontaneous that thun a 298 kbased on the delta t g reaction well the delta g reaction is positive sothe answer to that is no it is not spontaneous ok if it had been negative beenspontaneous
can you qualitative but look at thereaction and protect the sign of entropy so but you know sign is that it'sgaining the entropy and let's see if that makes sense i'm starting off with one solid reactantand i'm making an solid product but i'm also making it a half more of gaseousproduct and we know that this is definitely a higher entropy in the solidstate so it makes sense that you're increasing entropy a little bit ok this is confirmed by the positivenumber question 3 is the reaction is exothermicor endothermic well you look at your
free energy or heat of reaction andthat's positive so it's actually endothermic okay and that is notfavorable spontaneity by the way so now is there a crossover temperaturefor this reaction and what is it ok well let's evaluate if there is acrossover temperature first off i have a human reaction that's not favorable buti do have an entropy that is favorable so i know that if i take my term entropytimes temperature it will increase this so yeah there must be a crossovertemperature okay and how do you find that where youtake your mp / your entropy remember your kilojoules cancel your left kelvin
this occurs at one thousand seven and 10k alright next question so if we understand there's a cross upthe temperature about 1,700 k is it spontaneous below the crossovertemperature or love across the temperature so let's let's walk throughthis again remember asking stand right from thebeginning at 298 we know that it's non spontaneous so that would kind of answerthe question already but we can walk through this and take it out and go okaywell my my heater reaction is not favorable spontaneity its end of theirmake but my and entropy is so i have to have temperature multiplied by thisfactor and increase the temperature so
that this becomes the overriding factorright did you you only multiply temperaturetimes entropy and is this factor gets large enough it will start to overridethis factor make the overall equation negative which is favorable tospontaneity for free energy and then last but not least this is one of thequestions but you can always do this is good to double check your work you figured out the free energy you didit directly off the table but we know according to gives equation that ifyou'll take your delta h - temperature times your entropy youshould still get the same gives free
energy so if i plug in my numbers and doublecheck i come out a spot on to the significant figures i've been given ofhundred 29.7 kilojoules so i hope this has helped you understandgibbs free energy to know its symbols to know its units using some sort of jewelsor kilojoules per mole know how to calculate it using both the gibbs equation and also to use products minusreactants using the gibbs energy formation i know how to interpret it if it's anegative sign on it we know that a
spontaneous if it's positive it's nonspontaneous if it's zero it's an equilibrium then know how to interpretthe spot an ad based on the size of age and ass in other words know when there'scrossover and when it is temperature dependent can you find where thecrossmember temperature is enthalpy / entropy so i hope this has helped you understand gibbs free energy and and to understandtheir when i dynamics better
Post a Comment for "crossover suv denver"