so one of the advantages thinking about our enthalpy as a state function is that we can start to add things together uh between the different reaction so that concept is from hess's law and this is totally due to the fact that it's a state function for enthalpy so it the process of enthalpies can be written as a series of independent stepwise reactions and the enthalpy change for all of it is the sum of all the enthalpy changes in all of the reactions so here's an example of thinking about hess's law so we want to determine the enthalpy of formation of iron Tri chloride so thinking about how we want to calculate that and we are given these two steps and what we can do is we can do some sort of manipulations of this so first of all we need to go ahead and WR the reaction for the formation of iron chloride so we know that the enthalpy of formation for the reaction is we are going to form one mole of iron chloride and we are going to form this from the elements at its standard state so we know that we have iron as a solid in here and we know chlorine uh exists as a datomic gas but how many chlorines do we actually need so if we think about this we only need three so if we think about our leish structure looks something like this we only need three out of the four so that's three Hales of a chlorine of chlorine molecules so how do we actually calculate this so this is now our Delta H knot this is written as a formation we are given these two fundamental steps um uh up here so one and two what can we do to actually manipulate them and add them together and in this case it's actually relatively straightforward we notice that we have a product of fe2 over here and we have one of them on this side of the equation and lo and behold if we take both of these equations and we just add them up we see that we have an iron on this side we have have an iron over here we have one chloride plus a half of a chloride this gives us our three Hales chlorine and um our iron over here and if we look on the product side if we add this up this iron uh D chloride ends up being canceled on both sides and we have iron Tri chloride so for this we can take our hess's law that our heat of formation is going to be the sum of these two over there and if we add up the some of these we end up getting -3 99.5 K so that is the final answer for this reaction so that is a fairly straightforward kind of hess's law where you can just kind of add it up the only tricky part was go was writing this kind of um enthalpy of formation and that was just by the definition of what a formation reaction actually is and the next part is we will go through something a little bit more complicated