The enthalpy of the reaction:
2NO(g)+O₂(g)→2NO₂(g) ΔH = -114 kJ/mol
Further explanation
The change in enthalpy in the formation of 1 mole of the elements is called enthalpy of formation
The enthalpy of formation measured in standard conditions (25 ° C, 1 atm) is called the standard enthalpy of formation (ΔHf °)
Determination of the enthalpy of formation of a reaction can be through a calorimetric experiment, based on the principle of Hess's Law, enthalpy of formation table, or from bond energy data
(ΔH) can be positive (endothermic = requires heat) or negative (exothermic = releasing heat)
The value of ° H ° can be calculated from the change in enthalpy of standard formation:
∆H ° rxn = ∑n ∆Hf ° (product) - ∑n ∆Hf ° (reactants)
The elements in standard conditions are not included in the enthalpy calculations because the enthalpy of those elements under the standard conditions is zero.
From the problem can be known
∆Hf ° A: NO₂ = 33.2 kJ / mol
∆Hf ° B: NO = 90.2 kJ / mol,
In reaction:
2NO (g) + O₂ (g) → 2NO₂ (g)
Since the reaction coefficients of NO₂ and NO are 2, then ΔHf ° is also multiplied by 2
∆H ° rxn = ∑n ∆Hf ° (product) - ∑n ∆Hf ° (reactants)
∆H ° rxn = 2.∆Hf ° NO₂ - 2. ∆Hf ° NO (O₂ not included)
\(\rm \Delta H^orxn=2\times 33.2-2\times 90.2\\\\\Delta H^orxn=66.4-180.4\\\\\Delta H^orxn=\boxed{-114\:kJ/mol}}\)
Learn more
Delta H solution
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an exothermic reaction
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as endothermic or exothermic
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