Three oxidation reactions are shown below

CHEMISTRY · Multi-part · A Level · Chemistry · Energetics · Energetics

Problem

Three oxidation reactions are shown below. All substances are at 298 K298\,\text{K} and 1 atm1\,\text{atm}. Reaction 1: CH4_4(g) + 2 O2_2(g) →\to CO2_2(g) + 2 H2_2O(g) Reaction 2: CH4_4(g) + 2 O2_2(g) →\to CO2_2(g) + 2 H2_2O(l) Reaction 3: CO(g) + 12\frac{1}{2} O2_2(g) →\to CO2_2(g) Standard enthalpies of formation, ΔHf∘\Delta H_f^\circ (kJ mol−1)\text{(kJ mol}^{-1}\text{)}: • CH4_4(g) = −74.8-74.8  • CO(g) = −110.5-110.5  • CO2_2(g) = −393.5-393.5  • H2_2O(g) = −241.8-241.8  • H2_2O(l) = −285.8-285.8  • O2_2(g) = 0.00.0 (a) Calculate the standard enthalpy change, ΔH∘\Delta H^\circ, for each of the three reactions. Show all work. (b) Rank the reactions from most exothermic to least exothermic in terms of their ΔH∘\Delta H^\circ magnitude. Clearly indicate your ordering. (c) Provide a chemical justification for the differences in exothermicity you observed, focusing on the role of physical state and bond energies.

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