# Hess's Law

so one way to find the enthalpy change for an equation uses what's known as

## Hess

's law and the way you use## Hess

's law isn't that unlike a kind of thing you do in algebra say for example you got X plus y equal 36 and you got another equation equal we have y equals Z plus X early on an algebra you know you were taught how to add equations together all right in this case you see the X's are on opposite sides so they drop out so when you add them together you get 2y equals Z plus 36## Hess

's law does the same sort of thing but instead of wearing with algebra equations we kind of use that trick with chemical equations like for example you can take these two equations and if you add them together like the X's did a minute ago carbon monoxide is going to cancel out because it's a product in one or reactant in the other you add everything else down you get carbon half and a half give you 102 equals or producers co2 what## Hess

's law says that when you add two equations together like we did here the enthalpy change for that reaction is the sum of the enthalpy change for the equations that were added together a lot of times though when you're given equations sometimes you're not giving them where you automatically can add them add them together for example all right a difficult## Hess

's law problem might be set up like this where you're trying to find the enthalpy change of this equation and you're given these three equations here now...if you just try to add them up like we did a minute ago they're not going to come anywhere close to the one they're asking for so a lot of times you have to modify the equation so they will add it properly but when you modify the equation you've got to modify the enthalpy change as well so for example look at this first equation here in this equation we have hydrogen fluoride shown and if you look this is the only place we see hydrogen fluoride for these three given equations so whatever we do to hydrogen fluoride it whatever we do this equation it better be set up where we have the proper coefficient and HF is on the proper side of the equation so if you look it's on the proper side because it's a product here and it's a product in the equation we want but you only have to in the given equation and we need four so we can fix that by multiplying this equation by two now we're changing the coefficients of hydrogen and fluorine but since we took care of HF then hopefully these will work themselves out and you'll see later on they do but one thing about working with tests is law whatever you do to the equation you're going to have to do the same thing to your enthalpy change and we're doubling the coefficient so we got to double the Delta H look at the second equation again try to focus on a substance that's you only see in this one equation alright when you compare it to the equation you want so in this case we kind of want to focus...

on c2 h4 better known as ethylene so you have one ethylene on the right but in the equation that we want we have one on the left so we're going to flip this equation around so when you flip the equation you effectively have to flip the sign around which means pop plus 52 turns into negative 52 and for this last equation we want to focus on CF for because carbon is not even in the equation we want all right fluorines actually in a couple of places so it might be a headache trying to fix fluorine so again whatever we do to a CF 4 and this or whatever we do to this equation we have to fix it where CF 4 is in the right place with the right coefficient these other two will hopefully work themselves out so here we have 1 on the reactant side but we want to on the product side so we're going to multiply it by 2 and we're going to flip it around to put it on the product side of the equation so we've got to take our enthalpy change we got to multiply by two and we're going to flip the sign so when we look at our modified equations and there's our first equation modified we're now the coefficients have been multiplied by two although I left off the four design there we go and we've gone ahead and multiplied the Delta H we flip this equation around and again we flip the sign and for our last equation we had to do both flip it and double so again we flip the sign and we doubled it so when you add these three equations together notice that the things that...

weren't part of the overall equation are the things we didn't want in the overall equation our hydrogen's those carbons they cancel out because aren't opposite sides of our arrow so it leaves you with 1 C 2 H 4 plus 6 fluorines say the fluorines did work themselves out gives you 2 CF fours and 4 h FS and that is the equation that we want and so like