Standard enthalpy of combustion of carbon equals its formation as CO2 Steps: - Define standard enthalpy of formation (ΔH_f°): heat change for forming 1 mol of compound from elements in standard states. - Define standard enthalpy of combustion (ΔH_c°): heat change for burning 1 mol of substance completely in oxygen under standard conditions. - For graphite, ΔH_c° is C(graphite) + O₂(g) → CO₂(g). - This reaction matches exactly the ΔH_f° for CO₂(g), so values are identical. Why C is correct: - By definition, ΔH_c° of graphite is the enthalpy change for C(graphite) + O₂(g) → CO₂(g), identical to ΔH_f° of CO₂(g) from elements. Why the others are wrong: - A: Atomisation of CH₄(g) (endothermic, ~+1660 kJ/mol) breaks C-H bonds to gaseous atoms; formation of CH₄(g) is exothermic (-75 kJ/mol). - B: Combustion of CH₄(g) (-890 kJ/mol) ≠ sum of combustion of graphite (-394 kJ/mol) + 2×combustion of H₂(g) (-572 kJ total), per Hess's law relation to ΔH_f CH₄. - D: Neutralisation of HCl(aq) + NaOH(aq) (-57 kJ/mol for H⁺ + OH⁻ → H₂O) ≠ formation of H₂O(l) from H₂ + …