[petstar21]

Ok so I attempted these problems but I just dnt get it, if someone would clarify it It would clear soo much up!

The heat of combustion for solid urea (NH2)2CO to CO2, N2,and H20(liquid) is -632.2KJ per mole.
1. Write a balances eaution for the reaction (but how do you do it if its for combustion)?
2. How much heat is given off per mole oxygen, just how do you solve for that?
3. Calculate the standard heat of urea using ref table, what are you doing, hess's law?

thanks!

[english]

1. Write a balances eaution for the reaction (but how do you do it if its for combustion)?

Any combustible process involves the reaction of the substance with oxygen, by definition.

2. How much heat is given off per mole oxygen, just how do you solve for that?

Think stoichiometrically.  After balancing the equation, how much energy corresponds to 1 mol of O2 if the heat released is 632.2 kJ per mole of urea?  It's essentially the same procedure you're used to (i.e., calculating molar amounts given grams, and vice versa...only this time you're calculating heat given molar amount). A hint...you will use the molar ratio of O2 to urea.

3. Calculate the standard heat of urea using ref table, what are you doing, hess's law?

If there are two formation reaction steps involved, then you will need to balance the steps and add the heat energies appropriately.