Calculating enthalpy using thermochemical equations !?

Question

Calculate the enthalpy change of the following reaction between nitrogen gas and oxygen gas, given thermochemical equations 1, 2 ,and 3 .

2N(2) + 5 0(2) ----->2H(2)0(5) AH = ?

1. 2H(2) + 0(2) ------>2H(2)0 AH = -572 kj/mol
2. N(2)0(5) + H(2)0 ---> 2HNO(3) AH = -77 kj/mol
3. 1/2 N(2) + 3/2 0(2) + 1/2 H(2) ---->HNO(3) AH = -174 kj/mol

Ok , thanks for answering , and I follow u up to this point ... but I'm not sure what to do now ...help ! thanks

I.m not sure what to do with equation 1 or why ?..I'm think ing I should multiply it by 5 .. but it doesnt sound right ..Thanks

Answer 1

I'm guessing you meant "2 N2O5" in the equation your trying to calculate DH for.

This problem is just a puzzle. You have to combine the various equations you are given so that when added together, you end up with the top equation. You can multiply equations (multiply DH by the same thing) and flip equations over (change the sign on DH).

So, since you're trying to form 2 N2O5, I'd start with the second equation, reverse it and multiply it by 2. That will give you the 2 N2O5 on the product side.

Then, you'll have to get rid of 4 HNO3, so next take your 3rd equation and multiply it by 4.

Finally, you'll have to figure out how many water molecules you need to get rid of, so you'll have to figure out what to do with equation #1. Add everything together, and you should have what you're after...

Just think of these problems as games and puzzles...