0:02 hi everyone we're going to continue our
0:04 conversation with thermodynamics and
0:07 enthalpy so these are our learning
0:11 outcomes for the day um to First State
0:13 the first law of thermodynamics you
0:16 should be able to Define enthalpy and
0:18 explain why it's a state function and
0:20 we're going to write some thermochemical
0:21 equations so we're going to use some
0:24 more of our sto gometry then we should
0:26 be able to calculate enthalpy changes
0:28 for a bunch of different chemical
0:31 reactions and then explain hess's law
0:33 using and use it to uh compute
0:35 enthalpies and the last thing uh you
0:38 know throughout this uh activity we are
0:41 going to apply enthalpy to calorimetry
0:44 experiments so the first law of
0:46 thermodynamics is thinking about the total
0:47 total
0:50 energy um of a
0:59 system and this isolated system is the
1:02 energy is
1:04 constant that means the
1:13 transferred and it can't be created or
1:15 destroyed so this is one of the ways
1:18 your book uses it uh we call the energy
1:20 of the system U which is the internal
1:23 internal
1:26 energy and we can calculate the U uh the
1:29 change in internal energy as the heat
1:32 plus the work of the system and based on
1:34 the system and how the internal energy
1:38 is changing you can only put heat in or
1:40 heat can come out and you can only put
1:44 work in or work can be done by the
1:46 system so these terms are pretty
1:50 important these terms like bu on out and
1:54 in um are the kind of ways that we think
1:57 about our internal energy so this goes
1:59 back to our conservation of energy
2:02 energy can't be destroyed or created it
2:04 can only be transferred and for internal
2:08 energy it is only about heat and
2:12 work let's first think about uh U so U
2:15 is what we call a state
2:18 function and then a state function only
2:21 depends on its
2:33 so it doesn't matter the path of it so
2:35 the best way to describe this is trying
2:37 to get to a destination if you're trying
2:40 to drive to school it doesn't matter
2:42 that you you drove all through the
2:44 mountains or you drove to the shore and
2:47 then you came back it only matters that
2:49 you started at your home and you ended
2:51 up at
2:54 College um and then the big thing about
2:57 this is that the U is a state function
3:02 but q and w uh so your heat and the work does
3:07 depend on the
3:11 path so U uh q and W are not State
3:16 function but U is a state
3:20 function now the chemists think about
3:23 energy and and all these other kinds of
3:28 uh uh uh content through enthalpy so we
3:31 use enthalpy and we Define
3:35 enthalpy equals the internal energy plus
3:40 a PV kind of work so PV is our work that
3:45 we're thinking about um so we consider
3:49 PV um the negative of work equals P Delta
3:50 Delta
3:53 V so what we can have for internal
3:58 energy are U internal energy equals Q + w
4:01 w
4:05 so this Q + W can be put into our Delta
4:11 H equals Delta U plus P Delta V so we
4:14 can take this and dump this into the U
4:17 so that is now q and we are calling this constant
4:19 constant
4:21 pressure you see that the P doesn't
4:25 change over here then we can take this
4:29 W over here and we can import it into to
4:34 the P Delta V so therefore we end up
4:38 getting our Delta H equals Q at constant
4:42 pressure plus W minus W this ends up
4:46 being zero so therefore our Delta H
4:50 equals Q under constant pressure so this
4:53 is thinking back to our bomb calorimetry
4:55 uh kind of experiments and our coffee
4:58 cup calorimeter experiments so our
5:00 coffee cup calorimeters are under constant
5:01 constant
5:05 pressure and our bomb calorimeter is
5:08 actually under constant
5:11 volume so therefore we can start to
5:14 think about our Q of the reaction equals
5:17 our enthalpy change of the reaction in our
5:19 our
5:21 calorimetry so our next step is to go
5:24 ahead think about q and how we can use
5:26 enthalpy to actually
