An important step in the synthesis of nitric acid is the conversion of ammonia to nitric oxide according to the following balanced chemical equation. What is ΔH°_rxn for this reaction? 4NH₃(g) + 5O₂(g) → 4NO(g) + 6H₂O(g) A) -1274.8 kJ/mol B) -904.4 kJ/mol C) -240.2 kJ/mol D) -197.8kJ E) 197.8 kJ/mol

Question

Quizplus · Accepted Answer

To calculate ΔH° for the reaction, we need to use Hess's Law and the enthalpy changes of the individual steps involved in the reaction.

The given equation is the overall reaction, which involves the following steps:

Step 1: NH3(g) + O2(g) → NO(g) + H2O(g)   ΔH° = -906.3 kJ/mol
Step 2: 2 NO(g) + O2(g) → 2 NO2(g)             ΔH° = -114.0 kJ/mol
Step 3: 3 NO2(g) + H2O(l) → 2 HNO3(l)         ΔH° = -173.2 kJ/mol

Adding the enthalpy changes of these steps gives the overall enthalpy change for the reaction:

ΔH° = [4 NH3(g) + 5 O2(g) → 4 NO(g) + 6 H2O(l)] 
         = [4 NO(g) + 6 H2O(l) → 4 HNO3(l)] 
         = [Step 1] + [2 × Step 2] + [3 × Step 3] 
         = (-906.3 kJ/mol) + 2(-114.0 kJ/mol) + 3(-173.2 kJ/mol) 
         = -1802.2 kJ/mol

Finally, we need to calculate ΔH° for the reverse reaction (since the question asks for ΔH° of formation):

ΔH°rxn = -ΔH°f°(HNO3) + ΔH°f°(NO) 
-1802.2 kJ/mol = -[ΔH°f°(HNO3)] + [ΔH°f°(NO)]

ΔH°f°(NO) = -1802.2 kJ/mol + ΔH°f°(HNO3)

Substituting the standard enthalpies of formation for HNO3 and NO (which are given as -207.5 kJ/mol and +90.4 kJ/mol, respectively):

ΔH°f°(NO) = -1802.2 kJ/mol + [-207.5 kJ/mol + 4(90.4 kJ/mol)] 
ΔH°f°(NO) = -904.4 kJ/mol

Therefore, the answer is B, -904.4 kJ/mol.

An Important Step in the Synthesis of Nitric Acid Is