0:02 so when we think about enthalpy we have
0:05 some definitions that we use and one of
0:07 them is what we call the standard state
0:09 so the standard State we are going to
0:13 indicate our Delta H with a not sign
0:15 above it so this indicates standard
0:19 State what does that actually mean
0:21 according to
0:25 IUPAC standard state is one bar and one molar
0:27 molar
0:29 concentrations however if you look at
0:32 most Labs at the end of your chap at
0:33 your end of your book there's actually
0:37 an appendix G they actually list um all
0:40 of their information um in a slightly
0:43 different way so most tables with
0:46 thermodynamic uh information they list
0:49 everything as one atmosphere and this is
0:51 really really close to a
0:53 bar and since we're doing things under
0:56 constant pressure typically it usually
0:58 doesn't matter so this is approximately
1:07 concentrations and our temperature is it
1:10 always listed but we usually do all of
1:13 our temperature all of our reactions at room
1:15 room
1:24 or
1:28 298.15 kelvin when we do our conversion
1:31 so we will see these Nots in a lot of
1:34 our thermodynamic kinds of calculations
1:36 so that just means standard state so you
1:39 can assume these values but if you don't
1:42 see a not sign if you don't see the zero
1:46 that means these are non standard
1:48 conditions so you'll have to read the
1:50 problem very
1:52 carefully so we also have some of the
1:54 other values that we will see in our
1:57 tables that are very common so our
1:59 standard enthalpy of combustion is going
2:02 to be indicated with a Delta H knot so
2:04 we're at standard conditions and you'll
2:07 see a lowercase C to it so that just
2:10 indicates that we are thinking about a
2:13 combustion so remember when we are
2:15 talking about reactions a combustion
2:18 reaction reacts oxygen with some sort of
2:21 organic compound that contains
2:24 CH uh kind of elements in it and we
2:30 always produce carbon dioxide and water
2:33 so we can use this to actually calculate
2:36 some of our enthalpies and if we look up
2:39 in the uh appendix G uh we find out the
2:44 enthalpy of combustion is uh 13668
2:46 13668
2:51 K so what if we had 1.0 lers of
2:56 ethanol so e is ethanol um and then how
2:59 many kles of energy is going to be
3:03 created by burning ethanol so our kogs
3:05 are going to be trying to calculate this
3:07 we have to go ahead and convert this to
3:09 liter uh
3:12 milliliters for one liter and then if we
3:15 look up the density of
3:17 ethanol uh it's
3:20 789 grams per
3:23 milliliter and then we multiply that or
3:25 divide that by the molecular weight so
3:29 it has um 46.0
3:34 7 G for every one mole and then we can
3:37 now convert this uh to the to the
3:40 ethanol because this value over here
3:42 indicates a one for the ethanol so
3:47 that's that many kles for one mole of of
3:50 ethanol so that's minus
3:55 13668 k for every one mole of
3:59 e and we can again a value and remember
4:01 this is a combustion so we should be
4:04 releasing heat or heat is produced so
4:07 exothermic reaction we should get a
4:11 negative Q for this or negative enthalpy
4:16 uh for this reaction so it's uh minus 2
4:20 23,000 408 K if we go ahead and do it to
4:23 the right sig figs minus
4:30 2.34 * 10 4 K of energy
4:32 and that's to compare to something like
4:42 octane um has an energy per liter is
4:45 about minus
4:49 3.31 * 10 4
4:53 K in ISO octane so you might uh
4:55 recognize the word octane because that
4:58 is gasoline so this is one of the reason
5:01 why our gasoline is made of a
5:03 hydrocarbon with eight different uh
5:06 carbon atoms in there because it has
5:09 more energy density per liter than
5:11 something like
5:14 ethanol we can also look into our
5:17 standard kind of uh values our standard
5:20 enthalpy of formation and in your book
5:22 this is in
5:25 appendix G so you can look this up all
5:29 these values are in there so you'll see
5:32 this as a Delta H knot and with a
5:36 lowercase f and our main concern for
5:38 enthalpy of formation is that we are
5:40 looking for one
5:44 mole of compound
5:51 formed um and it is formed from free elements
6:02 and what we'll do is something we'll
6:04 think about something like carbon
6:07 dioxide gas and looking at the enthalpy
6:09 of formation you look this book up in
6:11 the back of the book you'll see your Del
6:15 Delta H knot uh formation equals
6:20 -31 3.5 K but how do you actually write
6:23 a reaction for that so we are going to
6:27 form one mole of that material so we
6:29 have to form it from the element so we
6:33 have have carbon and this is in the
6:37 solid state at standard conditions plus
6:41 oxygen so oxygen is diatomic and at its
6:45 standard State oxygen is a gas and that
6:48 will give us our equation for the
6:51 enthalpy of formation and one more thing
6:55 is typically we will look at
6:58 elements your Delta
7:03 HF is is going to equal zero so for free
7:11 State your enthalpy of formation is zero
7:14 so keep that in mind for what we need to
7:15 do uh when you're writing these equations
