Ammonium nitrate decomposes to n2o and water at temperatures between 250 c and 300 c. write a balanced chemical reaction describing the decomposition of ammonium nitrate, and calculate the standard heat of reaction using the appropriate enthalpies of formation. include phases. include the steps that you took

The formula of Ammonium nitrate is (NH4)(NO3).

The decomposition of ammonium nitrate is shown in the following reaction:

(NH4)(NO3) ---> N2O + 2H2O 

To calculate for the standard heat of ammonium nitrate, add the enthalpies of formation of the products, (N2O, H2O)

delta H = H_N2O + 2*(H_H2O)

The enthalpy of formation table can be found in several reference books, like Perry's Chemical Engineering Handbook or Thermodynamics books.