Practice question

Given: The enthalpy change for the reaction H₂O(l) -> H₂O(g) at 373 K is 40.79 kJ/mol. What is the internal energy change if Δ n_g = 1 and R = 8.314 J/mol · K? These values define the system as per NCERT data. Formula: Δ H = Δ U + Δ n_g RT. This is the standard NCERT relation for this phenomenon. Substitution & Calculation: Given Δ H = 40.79 kJ/mol, Δ n_g = 1, T = 373 K, RT = 8.314 × 373 / 1000 = 3.1 kJ. So, Δ U = 40.79 - 3.1 = 37.69 kJ/mol. Result: The computed value matches the expected outcome and confirms the correct choice. Units and powers like J kg⁻¹ K⁻¹, m/s², 10⁻⁵ are properly used as per NCERT.