For the reaction C₂H₂(g) + (5/2)O₂(g) → 2CO₂(g) + H₂O(l) , Δ H = -1299.5 kJ/mol at 298 K. What is Δ U ? ( R = 8.314 J/mo
For Δ ng = 2 - (1 + 2.5) = -1.5 , RT = 8.314 × 298 × 10⁻³ = 2.4776 kJ . Using Δ H = Δ U + Δ ng RT , Δ U = -1299.50 - (-1.5 × 2.4776) = -1299.50 + 3.7164 = -1295.78 kJ/mol .
Ref: NCERT Class 11 Chemistry > Chapter 5: Thermodynamics > Topic: Third Law of Thermodynamics and Applications
