Reactions Serving Dual Role in Enthalpy Changes

Steps:

• Define standard enthalpy of formation (ΔHf°): heat change for forming 1 mol of compound from elements in standard states.
• Define standard enthalpy of combustion (ΔHc°): heat change for complete combustion of 1 mol of substance in O2 to CO2, H2O, etc., under standard conditions.
• Identify reactions where the process forms a compound from elements via combustion.
• Match options to both definitions simultaneously.

Why D is correct:

• D (H2(g) + ½O2(g) → H2O(l)) forms 1 mol H2O from elements (ΔHf°) and combusts 1 mol H2 completely (ΔHc°), per standard definitions.

Why the others are wrong:

• A: Forms CO2 (ΔHf°), but combusts C, not matching 1:1 mol basis for both.
• B: Forms CO (ΔHf°), but incomplete combustion, not standard ΔHc° to CO2.
• C: Combusts CO (ΔHc°), but starts from compound, not elements for ΔHf°.

Final answer: D