so when we think about enthalpy we have some definitions that we use and one of them is what we call the standard state so the standard State we are going to indicate our Delta H with a not sign above it so this indicates standard State what does that actually mean according to IUPAC standard state is one bar and one molar concentrations however if you look at most Labs at the end of your chap at your end of your book there's actually an appendix G they actually list um all of their information um in a slightly different way so most tables with thermodynamic uh information they list everything as one atmosphere and this is really really close to a bar and since we're doing things under constant pressure typically it usually doesn't matter so this is approximately one bar and we also have one molar concentrations and our temperature is it always listed but we usually do all of our temperature all of our reactions at room temperature so this is 25 Dees C or 298.15 kelvin when we do our conversion so we will see these Nots in a lot of our thermodynamic kinds of calculations so that just means standard state so you can assume these values but if you don't see a not sign if you don't see the zero that means these are non standard conditions so you'll have to read the problem very carefully so we also have some of the other values that we will see in our tables that are very common so our standard enthalpy of combustion is going to be indicated with a Delta H knot so we're at standard conditions and you'll see a lowercase C to it so that just indicates that we are thinking about a combustion so remember when we are talking about reactions a combustion reaction reacts oxygen with some sort of organic compound that contains CH uh kind of elements in it and we always produce carbon dioxide and water so we can use this to actually calculate some of our enthalpies and if we look up in the uh appendix G uh we find out the enthalpy of combustion is uh 13668 K so what if we had 1.0 lers of ethanol so e is ethanol um and then how many kles of energy is going to be created by burning ethanol so our kogs are going to be trying to calculate this we have to go ahead and convert this to liter uh milliliters for one liter and then if we look up the density of ethanol uh it's 789 grams per milliliter and then we multiply that or divide that by the molecular weight so it has um 46.0 7 G for every one mole and then we can now convert this uh to the to the ethanol because this value over here indicates a one for the ethanol so that's that many kles for one mole of of ethanol so that's minus 13668 k for every one mole of e and we can again a value and remember this is a combustion so we should be releasing heat or heat is produced so exothermic reaction we should get a negative Q for this or negative enthalpy uh for this reaction so it's uh minus 2 23,000 408 K if we go ahead and do it to the right sig figs minus 2.34 * 10 4 K of energy and that's to compare to something like ISO octane ISO octane um has an energy per liter is about minus 3.31 * 10 4 K in ISO octane so you might uh recognize the word octane because that is gasoline so this is one of the reason why our gasoline is made of a hydrocarbon with eight different uh carbon atoms in there because it has more energy density per liter than something like ethanol we can also look into our standard kind of uh values our standard enthalpy of formation and in your book this is in appendix G so you can look this up all these values are in there so you'll see this as a Delta H knot and with a lowercase f and our main concern for enthalpy of formation is that we are looking for one mole of compound formed um and it is formed from free elements and what we'll do is something we'll think about something like carbon dioxide gas and looking at the enthalpy of formation you look this book up in the back of the book you'll see your Del Delta H knot uh formation equals -31 3.5 K but how do you actually write a reaction for that so we are going to form one mole of that material so we have to form it from the element so we have have carbon and this is in the solid state at standard conditions plus oxygen so oxygen is diatomic and at its standard State oxygen is a gas and that will give us our equation for the enthalpy of formation and one more thing is typically we will look at elements your Delta HF is is going to equal zero so for free elements at standard State your enthalpy of formation is zero so keep that in mind for what we need to do uh when you're writing these equations