Standard molar enthalpies of formation at 298 K for selected species are provided in the table.
Species ΔHf∘ (kJ mol−1)
CH₄(g) –74.8
O₂(g) 0.0
CO₂(g) –393.5
H₂O(l) –285.8
Consider the combustion reaction
CH₄(g) + 2 O₂(g) → CO₂(g) + 2 H₂O(l) ΔHrxn∘=?
Use the information above to address the following.
(1) Part 1: (a) Write an explicit algebraic expression for ΔHrxn∘ in terms of the tabulated ΔHf∘ values. Do not perform a numerical calculation.
(2) Part 2: (b) Explain, with reference to the definition of a standard state, why the value of ΔHf∘ for O₂(g) is zero and describe how this influences the expression written in part (a).
(3) Part 3: (c) Using the concept that enthalpy is a state function, justify qualitatively why the operation performed in part (a) yields the enthalpy change of combustion, even though no direct experimental measurements for that reaction were presented in the table.