hello bobcats so in this video we are going to discuss enthalpy but in order to understand enthalpy first i need to talk about state functions okay and so what is a state function the definition of state function is so a state function describes the current state of a system and is independent of the path taken to achieve its value now what does this mean so i'm going to give you an example i'm going to use what's called the displacement versus distance analogy so displacement versus distance analogy so if i have let's say i have two points on that i need to get to so let's say i have point a and i'm going to go to point b okay so if i go straight from point a to point b so from point a to point b this is displacement okay now but um distance on the other hand is the path that i may have taken so if i start at point a and i go around and i go over here and i go over there and i come back over here and then i come there that path is the distance that i took to get from a to b and so this would be the distance and in this case the direct path straight from a to b um which does not depend on the path it's independent so displacement is a function a state function whereas distance is a path function okay so it's important to understand this concept and the way we see it in chemistry is we have these what are called energy diagrams in chemistry and i'll make one so this is a the same same type of an example of a state function but we're going to use an energy diagram to discuss it now so if i have a diagram here let's say a a graph and energy increases going this direction and then we have the reaction coordinate or when i say reaction coordinate i mean what is going on the reaction uh sometimes you may also see this as the time it takes to do the reaction so a lot of times i'll call it the reaction coordinate and so what happens is let's say we start here with the reactant so i have my reactants and as i go through this and then my products end up way down here so here's my products so in reactants and products this has got a high potential energy this has got low potential energy and so in this case the distance between or what happens the energy from the reactant to the product so let's see if this is the product down here this right here from here to here is the change in energy that is my state function um but that reaction probably went through a couple of transition states so you had to give it some energy give it some activation energy and it goes through one transition state and then maybe it needed to go through another transition state at a higher energy and then it goes down so this here is the path that the reaction had to take and it has different levels of energy you know it goes up in energy then down and back up and then finally goes down but the state function all we really are interested in is between the the energy between the reactants and the products or the change in energy in this case so uh delta e or the internal energy is a state function well we also know that enthalpy is a state function so let's talk about enthalpy now and define enthalpy now enthalpy is um how do i say this is uh the transfer of heat specifically so enthalpy is the transfer of heat to or from a system um at constant pressure oops const let me write that again constant pressure which is what you do most the time because if you think about it you have open uh systems in your lab and so the pressure is the atmospheric pressure it's constant pressure so enthalpy is the transfer of heat to or from a system at constant pressure and again enthalpy i forgot to mention up here is represented with a capital h okay that's enthalpy capital h so most reactions are performed in open systems with exchange between uh both heat and work between the systems so most reactions will occur in open systems with exchange of both matter eh matter and energy or heat we'll say energy between system and surroundings and open systems are at constant pressure uh one atmosphere of pressure because you've got it open to the atmosphere the other thing is um chemists are really most more often interested in the flow of heat between systems at constant pressure so chemists really are concerned about this flow of heat at constant pressure now enthalpy to get to enthalpy and understand a little bit more about it i'm going to derive enthalpy from delta e so enthalpy is a state function that relates internal energy with pressure and volume so internal energy pressure and volume so this is the equation to start out with so enthalpy is equal to the internal energy of a system plus pressure times volume okay but really what we look at is we want to find we don't really know internal energy what we really look at is the change in energy so we would say the delta h or change in enthalpy is equal to the change in internal energy plus pressure times the change in volume now there's a couple equations we need to remember so don't forget remember you need to you need to know the equation that delta e or change in internal energy is equal to q plus w and for chemistry w is equal to a negative p delta v okay so i'm going to use those so here where i have delta e i'm going to substitute q plus w so change in enthalpy is equal to q plus w [Music] plus p delta v okay so i've i've substitute delta e with this now um i'm also going to substitute delta p delta v right here with w but because it's a plus here and it's a negative here i'm going to write it this way so delta h is equal to q plus work minus so those that was from delta e minus work which is from delta p right here we see this and so work minus work so delta h is equal to heat energy so we don't have to worry about work and i'm going to put a p here so that's constant pressure so this is the key concept is that my enthalpy is equal to the heat energy remember we have heat energy and work that's how we transfer energy so delta h is just the heat energy at constant pressure and so i'll just make sure you write that at constant pressure now what makes this so nice is that really i don't have to worry about work calculating work i don't have to worry about doing a negative p delta v most of the cases all i have to worry about is the q so let me state this again is that a change in enthalpy a change in enthalpy which is delta h is equal to the flow of heat at constant pressure so in a lot of cases what you'll see is sometimes somebody'll ask you what is the change in energy of the system so if i have to do change in energy of a system i have to do the heat and the work so i have to conser i have to uh take into account both the heat transfer and the work transfer of energy but if i'm looking at just delta h all i have to worry about is the heat and it eliminates work so i have to worry about the work part of the equation when i'm dealing with enthalpy and again it's at at constant pressure so that is enthalpy so really what we look at in chemistry is enthalpy that transfer of heat from one system from a system to the surroundings at constant pressure so let me do a couple of more things for this lecture and that would be let's look at energy diagrams potential energy diagrams so we're going to look at a couple of potential energy diagrams and it's something you need to be able to read when you see them potential energy diagrams okay so let's do an uh first just an exothermic process and actually i'll do it all in red so that you know it's exothermic and i'll do endothermic and blue so exothermic processes okay now in an exothermic process remember we have a negative delta h exothermic is negative delta h and if i were to write it in an equation we would say it'd be something like two moles of hydrogen plus one mole of oxygen will produce two moles of water plus 510 kilojoules of energy so this is if we have an exothermic process we have a negative delta h and i'll put a square around that to make sure you know that that's important to know the equation the thermochemical equation would be the chemical equation plus the energy and on exothermic processes energy would be written as a product okay or i could write it as two moles of hydrogen plus one mole of oxygen goes to two moles of water and then separately over here right delta h is equal to a negative 510 kilojoules okay so i can write it this way or this way but this is an exothermic process and we have a negative delta h and then we would have a energy diagram potential energy diagram that looks like this with energy increasing going up and then we have the reaction coordinate and in this case our initial energy the reactants start at a high potential energy so we have the reactants at a high potential energy and the products at a low potential energy so when the bonds break of the reactants so as you go through the reaction and the bonds break and then reform products if they reform products that are very low in potential energy they're much more stable so that is there's a uh an energy that has to be released to go down to these more stable products so this from here so from here to here is the delta h and remember it's a negative delta h because if our initial this is the initial and this is the final and remember final minus initial so i take a small number minus a larger number i will get a negative value this is exothermic so if we see it this way here also we might see it like written like this where we just have energy again reaction coordinate again but in this case you might just see simply two things here where we have the reactants on top at the higher potential energy and we can say that that's the initial energy and then the react products on bottom and that's the energy final and we went from from there to there and so that was a negative delta h and remember delta h in this case would be equal to e final minus e initial now remember we're talking about energy at a constant pressure so i know i'm writing e here but i'm just trying to say that that's energy but we're looking at the delta h so reactants must be higher in potential energy than the products to get an exothermic process the other process that we will talk about oh and by the way in this case energy is released so the next thing we're going to talk about is endothermic so b process so again with an endothermic process we have a positive delta h value and if i were to write it as a chemical equation a thermochemical equation we would say something like 510 kilojoules of energy plus two moles of water [Music] will decompose to produce two moles of hydrogen gas plus one mole of oxygen gas and in this case under endothermic we would write the energy as reactant or i can write it as simply as two moles of water will decompose to produce two moles of hydrogen plus one mole of oxygen and then separately go delta h is equal to a positive 510 kilojoules so these are two ways that we can write a thermal chemical equation now to look at it again let's look at the graphs for these we have energy and then we have reaction coordinate whoops rxn coordinate and move this up coordinate so there's my reaction coordinate and in this case my initial energy or potential energy for the reactant starts low and we have to go up a hill and then we may end up right here up here for products but if you'll notice to get from reactant to product we actually let me draw a little dotted line here we actually have from here to here and that's my delta h and it's a positive delta h because if you think about it if this is the initial so final which is a higher number minus initial smaller number that's a positive value and so you have to absorb energy here so energy is absorbed i'll write it over here another way to look at it is if we have e and then reaction coordinate again but simply we just write product here reactant here and remember we got to go from the reactants up to the product so this is my e initial this is e final and so um we have a positive delta h and if you remember that positive comes from e final minus e initial equals a positive delta h and in this case energy is absorbed so for an exothermic process it's a negative delta h and we would write a thermal chemical reaction where the energy is the product or you can just write the reaction and write delta h is equal to a negative value on a graph it would look like this where you have the reactants are higher in potential energy than the product and so you uh the state function is just going from reactants to products or it's a negative delta h energy is released you can see it written this way or you can see it simply as reactants at the initial energy products at the final energy and i'm going to just go ahead and draw the little arrow right here so you can see and so we're going from reactants to products and it's a negative value and again delta h the negative delta h in this case comes from the fact that we have a lower uh potential energy for the final energy and a higher initial energy and so final minus initial gives you a negative value same thing for endothermic except backwards or opposite it's a positive delta h energy is written as a reactant in a thermochemical reaction or you can write the reaction and then write delta h is equal to a positive value in this case the reactants are lower than the products this here i didn't forget i forgot to mention this but this point right here from here to here it's called activation energy and we don't include that in the delta h delta h is just from product to reactant it doesn't matter what the path was this is the path to get here it's just from this to this and its energy is absorbed and because the product is higher in potential energy than the reactant you have to go up and in add energy to it and so it's a positive delta h and so that is enthalpy and that's what we are looking at mostly in this class for thermal chemistry and the next a few videos are going to be on how to calculate enthalpy