Calculate ?_rH° for the combustion of ammonia,
4 NH₃(g) + 7 O₂(g) ? 4 NO₂(g) + 6 H₂O( $\ell$ )
Using standard molar enthalpies of formation.
$\begin{array} { l l } \text { molecule } & \Delta _ { f } H ^ { \circ } ( \mathrm { kJ } / \mathrm { mol } - \mathrm { rxn } ) \\\hline \mathrm { NH } _ { 3 } ( \mathrm {~g} ) & - 45.9 \\\mathrm { NO } _ { 2 } ( \mathrm {~g} ) & + 33.1 \\\mathrm { H } _ { 2 } \mathrm { O } ( \ell ) & - 285.8\end{array}$

Question

Calculate ?_rH° for the combustion of ammonia,
4 NH₃(g) + 7 O₂(g) ? 4 NO₂(g) + 6 H₂O(

\ell

)
Using standard molar enthalpies of formation.

\begin{array} { l l } \text { molecule } & \Delta _ { f } H ^ { \circ } ( \mathrm { kJ } / \mathrm { mol } - \mathrm { rxn } ) \\\hline \mathrm { NH } _ { 3 } ( \mathrm {~g} ) & - 45.9 \\\mathrm { NO } _ { 2 } ( \mathrm {~g} ) & + 33.1 \\\mathrm { H } _ { 2 } \mathrm { O } ( \ell ) & - 285.8\end{array}

Quizplus · Accepted Answer

The standard molar enthalpy of formation of a compound is the enthalpy change when 1 mol of the compound is formed from its constituent elements in their standard states. The standard state of an element is its most stable form at 298 K and 1 atm.The standard molar enthalpy of combustion of a compound is the enthalpy change when 1 mol of the compound is burned in oxygen to form the most stable oxides of its constituent elements.The standard molar enthalpy of formation of NH3(g) is -45.9 kJ/mol-rxn.The standard molar enthalpy of formation of NO2(g) is +33.1 kJ/mol-rxn.The standard molar enthalpy of formation of H2O(l) is -285.8 kJ/mol-rxn.The standard molar enthalpy of combustion of NH3(g) can be calculated using the following equation:ΔrHf∘=∑ΔfH∘(products)−∑ΔfH∘(reactants)ΔrHf∘=[4ΔfH∘(NO2(g))+6ΔfH∘(H2O(l))−4ΔfH∘(NH3(g))]ΔrHf∘=[4(+33.1 kJ/mol-rxn)+6(-285.8 kJ/mol-rxn)−4(-45.9 kJ/mol-rxn)]ΔrHf∘=-1398.8 kJ/mol-rxnTherefore, the standard molar enthalpy of combustion of NH3(g) is -1398.8 kJ/mol-rxn.

Calculate ?rH° for the Combustion of Ammonia, 4 NH3(g) ℓ\ellℓ

Calculate ?_rH° for the Combustion of Ammonia,
4 NH₃(g) $\ell$