Exercise 2 | Thermochemistry

Calculate the value of ΔUrxn at 25.0°C for the following reaction:

4 NH3 (g) + 5 O2 (g) → 4 NO (g) + 6 H2O (l)                  [ΔHrxn = -1166 kJ.mol-1]

ΔHrxn = ΔUrxn + Δ(PV)rxn


Ideal-gas equation:

PV = nRT

⇒ ΔHrxn = ΔUrxn + Δ(nRT)rxn


T is constant in this exercise:

⇒ ΔHrxn = ΔUrxn + RT Δnrxn

⇒ ΔUrxn = ΔHrxn - RT Δnrxn


ΔHrxn = change of enthalpy (in J.mol-1)

R = ideal gas constant = 8.314 m3.Pa.K-1.mol-1

T = temperature in K ⇒ 25.0°C = 298 K

Δnrxn = change of number of moles of gases

Δnrxn = 4 moles of NO – 4 moles of NH3 – 5 moles of O2 = - 5 moles


ΔUrxn = -1166 x 103 – 8.314 x 298 x (-5)

⇒ ΔUrxn = -1.15 x 107 J.mol-1 = -1154 kJ.mol-1