Question

Ammonium nitrate is a potent fertilizer due to its high nitrogen content. However, it is also a powerful explosive, decomposing to nitrogen and oxygen gases and water vapor. One of the worst industrial accidents in the U.S. occurred in 1947 when a cargo ship loaded with this chemical exploded at the port, killing hundreds of people. Calculate the standard enthalpy of reaction for this decomposition using the standard enthalpy of formation values.

Answer 1

Consider reaction : 2 NH₄NO₃ (s)   2 N ₂(g) + O ₂(g) + 4 H₂O (g)

The standard enthalpy change of reaction is given as

r H ⁰ = H ⁰_f ( products ) -  H ⁰ _f ( reactants )

H ⁰ _f (products) = 2 H ⁰ _f N ₂ + H ⁰ _f O ₂+ 4 H ⁰ _f H₂O

= 2 ( 0) + 0 + 4 ( -241.82 kJ/mol)

= - 967.28 kJ / mol

H ⁰ _f ( reactants ) = 2 H ⁰ _f NH₄NO₃ = 2 (-356.56 kJ / mol) = - 713.12 kJ / mol

r H ⁰ = [ - 967.28 kJ / mol] -[ - 713.12 kJ / mol]

= - 254.16 kJ / mol

ANSWER : r H ⁰ = - 254.16 kJ / mol