hi everyone we're going to continue our conversation with thermodynamics and enthalpy so these are our learning outcomes for the day um to First State the first law of thermodynamics you should be able to Define enthalpy and explain why it's a state function and we're going to write some thermochemical equations so we're going to use some more of our sto gometry then we should be able to calculate enthalpy changes for a bunch of different chemical reactions and then explain hess's law using and use it to uh compute enthalpies and the last thing uh you know throughout this uh activity we are going to apply enthalpy to calorimetry experiments so the first law of thermodynamics is thinking about the total energy um of a system of an isolated system and this isolated system is the energy is constant that means the energy uh can only be transferred and it can't be created or destroyed so this is one of the ways your book uses it uh we call the energy of the system U which is the internal energy and we can calculate the U uh the change in internal energy as the heat plus the work of the system and based on the system and how the internal energy is changing you can only put heat in or heat can come out and you can only put work in or work can be done by the system so these terms are pretty important these terms like bu on out and in um are the kind of ways that we think about our internal energy so this goes back to our conservation of energy energy can't be destroyed or created it can only be transferred and for internal energy it is only about heat and work let's first think about uh U so U is what we call a state function and then a state function only depends on its initial and final State uh only matter so it doesn't matter the path of it so the best way to describe this is trying to get to a destination if you're trying to drive to school it doesn't matter that you you drove all through the mountains or you drove to the shore and then you came back it only matters that you started at your home and you ended up at College um and then the big thing about this is that the U is a state function but q and w uh so your heat and the work does depend on the path so U uh q and W are not State function but U is a state function now the chemists think about energy and and all these other kinds of uh uh uh content through enthalpy so we use enthalpy and we Define enthalpy equals the internal energy plus a PV kind of work so PV is our work that we're thinking about um so we consider PV um the negative of work equals P Delta V so what we can have for internal energy are U internal energy equals Q + w so this Q + W can be put into our Delta H equals Delta U plus P Delta V so we can take this and dump this into the U so that is now q and we are calling this constant pressure you see that the P doesn't change over here then we can take this W over here and we can import it into to the P Delta V so therefore we end up getting our Delta H equals Q at constant pressure plus W minus W this ends up being zero so therefore our Delta H equals Q under constant pressure so this is thinking back to our bomb calorimetry uh kind of experiments and our coffee cup calorimeter experiments so our coffee cup calorimeters are under constant pressure and our bomb calorimeter is actually under constant volume so therefore we can start to think about our Q of the reaction equals our enthalpy change of the reaction in our calorimetry so our next step is to go ahead think about q and how we can use enthalpy to actually calculate um our heat exchange ES