3 NO2 (g) + 1 H2O (ℓ) → 2 HNO3 (aq) + NO (g)

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Stoichiometry

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NO2            Mass: g  or Gas Volume: L
H2O            Mass: g
HNO3           Mass: g  or Solution Volume: mL of Concentration: mol/L
NO             Mass: g  or Gas Volume: L

Enthalpy of Reaction

[2ΔHf(HNO3 (aq)) + 1ΔHf(NO (g))] - [3ΔHf(NO2 (g)) + 1ΔHf(H2O (ℓ))]
[2(-207.36) + 1(90.25)] - [3(33.18) + 1(-285.83)] = -138.18 kJ
-138.18 kJ     (exothermic)

Entropy Change

[2ΔSf(HNO3 (aq)) + 1ΔSf(NO (g))] - [3ΔSf(NO2 (g)) + 1ΔSf(H2O (ℓ))]
[2(146.44) + 1(210.65)] - [3(239.95) + 1(69.91)] = -286.23 J/K
-286.23 J/K     (decrease in entropy)

Free Energy of Reaction (at 298.15 K)

From ΔGf° values:
[2ΔGf(HNO3 (aq)) + 1ΔGf(NO (g))] - [3ΔGf(NO2 (g)) + 1ΔGf(H2O (ℓ))]
[2(-111.34) + 1(86.57)] - [3(51.3) + 1(-237.18)] = -52.83 kJ
-52.83 kJ     (spontaneous)

From ΔG = ΔH - TΔS:
-52.84 kJ     (spontaneous)

Equilibrium Constant, K (at 298.15 K)

1802729507.73687
This process is favorable at 25°C.

Reference(s):

Ebbing, Darrell D. General Chemistry 3rd ed.; Houghton Mifflin Company: Boston, MA, 1990; pp 211, 219.

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