# Hess's Law Example Problem

so we need to calculate this enthalpy for this reaction and we're given three sub reactions to do that and to complete this we need to use

## Hess

's law which basically states that the enthalpies of the Constituent reactions add up to the enthalpy of the total reaction so let's do this first we need to manipulate these three equations so that they add up to this and I see here in this first equation that we have a CH a C 2 H 2 on this side on the left side but the total reaction requires 2 C 2 H 2 to be on the right side so let's just reverse this whole first equation and that will give us 2 co2 plus h2o yield C 2 H 2 plus 5 halves of co2 and as you can see I wrote the Delta H is on the side and when we reverse the equation we need to do the same thing to the Delta H or the enthalpy of the reaction so instead of a negative one thousand two hundred ninety nine point five kilo Joule per mole enthalpy change we're going to have a positive enthalpy change so this is going to be positive one thousand two hundred ninety nine point five kilojoules per mole of reaction and a mole of reaction is just this whole equation right here now let's go on to the next one I see that the carbon is in the same place on the left side of the equation but we need two moles of the carbon so we have to multiply this whole equation by two and what we do over here is going to be the same thing for the enthalpies so instead of negative three hundred ninety three point five we're...going to multiply that by two which gives us negative 787 kilojoules per mole of reaction kilojoules per mole of reaction and we now have two C plus two o2 yields two moles of co2 okay so the last one I see that we have each - it's on the left side of the equation like we need it to be over here we could just ignore these and ignore so it looks like we could just leave this third equation by itself and I also said that we could ignore these because they're not even in the occlusion here so that's gonna tell us that they're probably going to cancel out once we add up these three so let's see how that works I forgot to write B enthalpy for the sauce equation so let me just write that down and now let's add these all together so I see two co2 over here is there any other place where we have co2 and yes it's over here so these two cancel out because they're on opposite sides of the equation now I have h2o I have another H over here and since this one's on the left this one's on the right they cancel out C - H - I don't see any more so we're just going to leave that on the right side of the equation then we have five halfs go - now I see you O two over here and go - over here when we add two and one-half that's going to leave us with five halves so all of these oxygens cancel out and now we're left with two C plus h2 yields C - H - just like we needed over here so that's a sure sign that we're on the right track now...

all we have to do is add up these enthalpies so 1200 99.5 - 787 - two eighty five point eight which gives us two hundred twenty six point seven kilojoules per mole and that should be the answer