Thermodynamic data at 298 K for several species are given below.
Species (state) ΔHf∘ (kJ mol⁻¹) S∘ (J mol⁻¹ K⁻¹) ΔGf∘ (kJ mol⁻¹)
CH₄(g) −74.8 186.3 —
CO₂(g) −393.5 213.7 −394.4
H₂O(l) −285.8 69.9 −237.1
O₂(g) 0 205.0 0
Consider the combustion of methane:
CH₄(g) + 2 O₂(g) → CO₂(g) + 2 H₂O(l)
(1) Determine the standard Gibbs free energy of formation of CH₄(g) at 298 K using the tabulated enthalpy and entropy values.
(2) Using your result from part (a) and the other tabulated data, calculate ΔGrxn∘ for methane combustion.
(3) The standard molar entropy of CH₄(g) is subject to an estimated uncertainty of ±5 %. Re-evaluate ΔGrxn∘ using this upper-bound entropy value and compute the percentage change relative to the value obtained in part (b). Discuss whether the thermodynamic favorability of the combustion reaction is sensitive to this level of uncertainty.