Thermodynamics

本站添加: 2026-05-09

[00:00:02]all righty guys so we are starting our last unit of ap chemistry um hopefully we can make it go pretty quickly for you guys um this unit is applications of thermochemistry so it does have some of the thermo stuff that we have done in a previous unit looking at like delta h and things like that but we're going to be looking more at energy of the overall reaction versus just specifically looking at heat where we did in thermo so looking at thermodynamics there are three laws of thermodynamics first law of thermodynamics we've actually talked about before that's where we talked about energy is conserved we can't like create energy or destroy energy so if energy is added to a system whatever the energy change of a system is or a reaction is the opposite of that energy has to be released or absorbed from the surroundings so energy is just transferred from the surroundings to the system or vice versa so the overall amount of energy is the same but if our reaction is releasing energy that energy is being added to the syst to the surroundings if our reaction is gaining energy that energy had to come from the surroundings the second law of thermodynamics talks more about entropy um entropy looks more at the disorder in a system so it looks at do you have atoms splitting apart or not atoms but do you have compounds splitting apart or coming together um it looks at if a reaction as a whole is also insp also spontaneous so the second law of thermodynamics is the entropy of the universe increases in a spontaneous reaction so what that means is a spontaneous reaction you'll also hear these referred to as thermodynamically favorable reactions this means that if something is spontaneous it does not need something to jump start the reaction you don't have to have to flip a switch for a reaction to occur or it doesn't necessarily need a catalyst to promote the reaction to occur and anytime we refer to entropy you'll see this abbreviated as a capital s this is looking at the measure of disorder of a system so a lot of times we'll look at like gas particles are further apart and more disorderly than liquid or solid particles so we're going to look at this in terms of if there's an increase in entropy or an increase in disorder or a decrease in entropy a decrease in disorder and then the third law of thermodynamics states that the entropy the disorder of a perfect crystalline solid at absolute zero is zero so if we have a solid at absolute zero we consider this to have no disorder so we will come back to that at a later time so here's looking at that third law of thermodynamics so we said the entropy of a perfect crystal at zero kelvin which is absolute zero is zero so if you look at the image a on the left all of the particles are orderly aligned we would consider that to have no disorder whereas the picture on the right those particles some of them are turned different directions we might have i don't think we have any like negatives negatives together there may be in a couple of places but that's going to be more disorderly so entropy would not be zero in that case so in general as the temperature is raised entropy increases so if the temperature increases disorder tends to decrea too excuse me disorder tends to increase because the particles are going to have more energy they're going to move around more it's going to be more disorderly so remember as temperature goes up entropy or disorder is going to increase overall so looking at what we're going to figure out with entropy is we'll look at the change in entropy we'll look at is it increasing in disorder or decreasing in disorder um so an example would be melting a solid if we are melting a solid we are going from a solid to a liquid there's less intermolecular forces after melting the particles are able to move around more so that would be more disorderly so the melting is solid we would say has a positive delta s excuse me because we're increasing disorder same kind of thing if we're boiling a liquid we're going from liquid to gas remember we're completely breaking apart those intermolecular forces gas particles are going to be very high energy very far apart so we are increasing disorders that would be a positive delta s so in general the entropy of a solid is going to be very low the entropy of a liquid will be higher the entropy of a gas will be even higher than that so entropy of gases are going to be higher than entropies of liquids or solids now if we're dissolving a solid in solution so remember we went from having say we had like salt as a solid versus salt that is dissolved so if it's dissolved it will be aqueous so remember whenever something is aqueous it splits apart into the ions so the aqueous one is going to have more ions it's going to have more particles so that will have more entropy same thing is raising the temperature we said if you increase temperature it increases entropy it increases your delta s the particles are going to have more kinetic energy they're going to move more be more disorderly with entropy so in a chemical reaction how we actually look at this our delta s our change in entropy is related to the change in number of moles of gas anytime we produce a gas a lot of times it will increase that entropy so here if we're looking at we have calcium carbonate solid producing calcium oxide which is a solid and carbon dioxide notice that here we have one reactant splitting into two products so we're making more products so it would be an increase in entropy just based off of that but we went from having zero moles of gas to having one mole of gas so this was an increase in entropy increase in delta s with the bottom reaction we have the two sulfur dioxide and oxygen producing two sulfur trioxides so notice here that we are combining um so we're actually making less products so that tends to be a decrease in entropy but you can look at the number of moles we had two moles of the sulfur dioxide plus one mole of oxygen so that's a total of three moles of gas on the reactant side and it changed into two moles of gas on the product side so that would be a negative decrease in entropy or negative change in entropy so it decreased in this order so change in entropy the change in delta s can be calculated the same way as whenever we were doing the change in h or using the heat of formation values so this is actually a formula that is going to be on your reference sheet it's on the second page of formulas in the section under thermodynamics and electrochemistry so our disorder in our reaction is going to be equal to the sum of the entropy of our products minus the sum of the entropy of our reactants so this one just like whenever we did heat of formation it's the sum of your products minus the sum of the reactants is going to tell us the overall entropy of our reaction and that will also tell us if it is increasing entropy or decreasing entropy whenever we calculate this is also going to relate to spontaneity and if a reaction is spontaneous so for a spontaneous reaction it's a reaction that once started can occur without outside assistance so i said you do not need like a power source or a battery or something like that to make the reaction occur it will occur on its own once it started we say that spontaneous reactions are favored or most common when we have a positive delta s so we like to have an increase in disorder and a negative delta h so we like them to be exothermic reactions that become more disorderly those are most likely to be spontaneous there are other cases where we can have a spontaneous reaction but those are our main ones so how we determine if a reaction spontaneous or not is by finding the gibbs free energy which is going to be abbreviated as delta g so free energy is the energy available to do useful work so it looks at like is it the energy available to make products is it releasing energy to the surroundings and things like that so if delta g is less than zero so if it is a negative number the reaction will be spontaneous if delta g is above zero if it is a positive number it will be a non-spontaneous reaction non-spontaneous reactions need some sort of power source or battery to fuel the reaction they are not going to occur on their own if they end up with that positive delta g value if the delta g is equal to 0 that means that our reaction is completely at equilibrium so the forward and reverse reactions occur at the same rate so we're not noticing a change in the overall reaction at equilibrium so gibbs free energy can be calculated just like delta h and delta s this is another formula on your formula sheet we can find the free energy of the overall reaction by taking the sum of the free energy of the products and subtracting the sum of the free energy of the reactants all of these values are in appendix 2 in your textbook there is a chart that it has every substance its delta h value delta g value and delta s value so these are numbers that will be given to you you just need to know how to plug them into the equation appropriately gibbs free energy can also be calculating or calculated using another formula which compares gibbs free energy with entropy h and enthalpy s as well as the temperature in kelvin this equation is also on your reference sheet so our delta g value is equal to our enthalpy delta h minus the temperature in kelvin times the entropy value the delta s value and yet again this would be a case where you're given certain values and you're solving for that unknown so looking at that equation for gibbs free energy there are certain signs of delta h and delta s that we can use to figure out what the value of delta g would be that we can use to tell if the reaction would be spontaneous or non-spontaneous so this is a chart that you are going to see and need to be familiar with you'll also see a problem like this in the practice work just filling out a chart like this so if we have an exothermic reaction a negative delta h and a positive entropy positive disorder this we said favors a spontaneous reaction and spontaneous reactions have a negative delta g value so if delta h is negative delta s is positive that will always be spontaneous we will always end up with a negative delta g value because you're going to have a negative minus a positive which becomes more negative so that's where we can plug in those signs into this equation if we have it where it's reversed we have an endothermic reaction so it's absorbing heat and losing disorder if we have a positive minus a negative a positive plus a positive delta g will be positive so that will be non-spontaneous at all temperatures other cases are where those signs are the same say that we have an endothermic reaction a positive delta h and a positive delta s if we have a positive minus a positive it may end up negative if the second value is large enough but it may stay positive so this is going to be one that is going to be temperature dependent so if that t value if that temperature is high enough this will end up being a negative delta g so these reactions are going to be spontaneous at high temperatures and the opposite is true if both values are negative if we have a negative minus a negative so a negative plus a positive yet again depending on the magnitude of that second value the reaction could be spontaneous or non-spontaneous so in order for that delta g to be negative we have to have a fairly low temperature that way that second number is small enough to stay negative so this is one that i would definitely familiarize yourself with this chart overall how i remember this is whenever the signs are opposite delta h tells you what delta g is when the signs are the same the sine of delta h tells you what the temperature needs to be whether it's high or low in order to be spontaneous a lot of this will be calculation based though so notice that if delta g was equal to zero we said the reaction's at equilibrium so neither the forward nor the reverse reaction is favored so we're at equilibrium we're not increasing the amount of products or increasing the amount of reactants at that point in time however if we change the temperature the reaction will shift so this goes back to equilibrium where we shift our reaction one reaction will end up being spontaneous one reaction will be non-spontaneous so it's either gonna favor the forward reaction or the reverse reaction whichever one is gonna produce more moles of gas all right so going back to our formula the delta g equals delta h minus t delta s if g is equal 0 this is going to be at equilibrium we can rearrange this to solve for temperature so this is going to give us what temperature we are going to be at equilibrium where the two equalize so the reaction is going to be spontaneous above or below that specific temperature so if it's one of our reactions where it's the delta h is positive and delta s is positive it's going to be spontaneous above that temperature because it needs to be the high temperature if both of them are negative that's where it will be spontaneous at the lower temperatures so below that set temperature so we can use this equation to figure out what the temperature needs to be for the reaction to be spontaneous so let's look at a problem i'm actually plugging in so here we have for the decomposition mercury ii oxide it tells us delta h is 90.8 kilojoules and delta s is 108 joules per kelvin what is the temperature range for which this reaction is spontaneous so one thing to note is here our delta h is positive and our delta s is positive so based off of our chart this is where we want high temperatures for the reaction to be spontaneous now we said that we can take the delta g equation and rearrange it so we said our temperature is going to be equal to excuse me it's doing the weird line thing it's to delta h divided by delta s plugging in our values and one thing to be careful of is look at your units delta h is in kilojoules delta s is in joules you have to put them in the same unit to be able to divide them i tend to go ahead and change them both into kilojoules so remember that we'll move our decimal over three spaces and that will give us a temperature in kelvin but make sure that you change to where your units match so our delta h is 90.8 our delta s is.108 so when we do that it gives us 841 kelvin so like we said because h and s were positive we want higher temperatures so this will be spontaneous whenever it is above 841 kelvin anything below 841 will be a non-spontaneous reaction based off of our data given um definitely check out the practice problems in the homework there are practice for finding delta h delta s finding delta g a little bit everything and we will pick up with this tomorrow and talk a little bit more about free energy and how it relates a little bit more to equilibrium