One chemistry question. 10 points to best answer.?

Question

Calculate the change in enthalpy when ammonium nitrate decomposes and show whether it is endothermic or exothermic.

NH4 NO3 -->N2 +2H2 O

given H NH4 NO3 = -87.4 kcal/mole

N2 O = 19.5

H2 O = -57.8

Answer 1

I think there is a type-o in your question, you mean N2O (not N2) in your decomposition reaction right? (I can give you answer if you did not have a type-o too, but its more complicated and I doubt that its your question)

If this is the case, its really an easy problem.  The chemical equation is already balanced.  The change in enthalpy is just a state function so all you need to do is add up the enthalpies of formation of the products and subtract the enthalpies of formation of the reactant.

So we get [19.5 + (2*-57.8] - (-87.4) = -8.7 kcal/mole.  

Also, the negative sign indicates that the products had less energy than the reactants, thus the reaction is exothermic.