Using the data below

CHEMISTRY · Subjective · A Level · Chemistry · Energetics · Energetics

Problem

Using the data below, calculate the standard enthalpy change ΔH∘\Delta H^\circ for each combustion reaction and then rank the three reactions from most exothermic to least exothermic per mole of fuel consumed. Standard enthalpies of formation (kJ·mol−1^{-1}): CH4(g)  −75\text{CH}_4(g)\; -75, C2H6(g)  −85\text{C}_2\text{H}_6(g)\; -85, C3H8(g)  −104\text{C}_3\text{H}_8(g)\; -104, CO2(g)  −394\text{CO}_2(g)\; -394, H2O(l)  −286\text{H}_2\text{O}(l)\; -286, O2(g)  0\text{O}_2(g)\; 0 Reactions (balanced for complete combustion): I. CH4(g)+2 O2(g)→CO2(g)+2 H2O(l)\text{CH}_4(g)+2\,\text{O}_2(g)\rightarrow \text{CO}_2(g)+2\,\text{H}_2\text{O}(l) II. 2 C2H6(g)+7 O2(g)→4 CO2(g)+6 H2O(l)2\,\text{C}_2\text{H}_6(g)+7\,\text{O}_2(g)\rightarrow 4\,\text{CO}_2(g)+6\,\text{H}_2\text{O}(l) III. C3H8(g)+5 O2(g)→3 CO2(g)+4 H2O(l)\text{C}_3\text{H}_8(g)+5\,\text{O}_2(g)\rightarrow 3\,\text{CO}_2(g)+4\,\text{H}_2\text{O}(l) Provide (a) the calculated ΔH∘\Delta H^\circ for each reaction and (b) the ranking (e.g., I > II > III).

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