An important step in the synthesis of nitric acid is the conversion of ammonia to nitric oxide. $\Delta$ H°_f [NH₃(g) ] = -45.9 kJ/mol; $\Delta$ H°_f [NO(g) ] = 90.3 kJ/mol; $\Delta$ H°_f [H₂O(g) ] = -241.8 kJ/mol
4NH₃(g) + 5O₂(g) $\to$ 4NO(g) + 6H₂O(g)
Calculate $\Delta$ H°_rxn for this reaction.

Question

An important step in the synthesis of nitric acid is the conversion of ammonia to nitric oxide.

\Delta

H°_f [NH₃(g) ] = -45.9 kJ/mol;

\Delta

H°_f [NO(g) ] = 90.3 kJ/mol;

\Delta

H°_f [H₂O(g) ] = -241.8 kJ/mol
4NH₃(g) + 5O₂(g)

\to

4NO(g) + 6H₂O(g)
Calculate

\Delta

H°_rxn for this reaction.

Quizplus · Accepted Answer

To calculate the enthalpy change for the reaction, we need to use Hess's Law, which states that the total enthalpy change for a reaction is independent of the pathway taken. We can break the reaction into three steps:
Step 1: 4NH3(g) + 5O2(g) → 4NO(g) + 6H2O(g)      ΔH° = ?
Step 2: 2NO(g) + O2(g)→ 2NO2(g)         ΔH° = -114.1 kJ
Step 3: 4NH3(g) + 6NO2(g)→ 4NO(g) + 6H2O(g)         ΔH° = ?

Step 1 is the reverse of the formation of ammonia from its elements and the formation of water from its elements. Therefore, ΔH° for step 1 can be found by adding the enthalpy changes for the formation of ammonia and water:

ΔH° = [4ΔH°f(H2O) + 4ΔH°f(NH3)] - [5ΔH°f(O2)]
ΔH° = [-241.8 kJ/mol + (-46.1 kJ/mol)] - [0 kJ/mol]
ΔH° = -287.9 kJ/mol

Now, we add the ΔH° values for the three steps to get the overall ΔH°:

ΔH° = ΔH°1 + ΔH°2 + ΔH°3
ΔH° = -287.9 kJ/mol + (-114.1 kJ/mol) + 0 kJ/mol
ΔH° = -402 kJ/mol

However, we need the enthalpy change for the reaction as written, which is the reverse of this value:

ΔH°rxn = -(-402 kJ/mol) = 402 kJ/mol

Now, we can use the given enthalpies of formation to find the enthalpy change for the reaction as written:

ΔH°rxn = ΣnΔH°f(products) - ΣnΔH°f(reactants)
ΔH°rxn = [4ΔH°f(NO) + 6ΔH°f(H2O)] - [4ΔH°f(NH3) + 5ΔH°f(O2)]
ΔH°rxn = [(4)(90.3 kJ/mol) + (6)(-241.8 kJ/mol)] - [(4)(-45.9 kJ/mol) + (5)(0 kJ/mol)]
ΔH°rxn = -906.0 kJ/mol

Therefore, the answer is (A) -906.0 kJ.

An Important Step in the Synthesis of Nitric Acid Is Δ\DeltaΔ

An Important Step in the Synthesis of Nitric Acid Is $\Delta$