Posted by hannah on .
Consider the following equations.
N2H4(l) + O2(g) N2(g) + 2 H2O(l) ÄH = 622.2 kJ
H2(g) + 1/2 O2(g) H2O(l) ÄH = 258.5 kJ
H2(g) + O2(g) H2O2(l) ÄH = 187.8 kJ
Use this information to calculate the enthalpy change for the reaction shown below.
N2H4(l) + 2 H2O2(l) N2(g) + 4 H2O(l) ÄH = ?
i really do0nt know how to do these

chem i reallyneed help 
DrBob222,
Add equn 1 as is, to 2x equation 2, to 2x the reverse of equation 3 to obtain the equation you want. Then add the delta H values. When you multiply an equation you must multiply delta H values too. When reversing an equation, change the sign of delta H.

chem i reallyneed help 
rebekah,
so is it  764.2?

chem i reallyneed help 
hannah,
oops that's me ^

chem i reallyneed help 
DrBob222,
763.6 is what I get.