Liquid fuels A and B have the following properties at 298 K

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

Problem

Liquid fuels A and B have the following properties at 298 K. Fuel A (CH3OH(l)\text{CH}_3\text{OH}(l)): ΔHf∘=−239 kJ mol−1\Delta H_f^{\circ} = -239\,\text{kJ mol}^{-1}, molar mass = 32.0 g mol−132.0\,\text{g mol}^{-1}, density = 0.79 g cm−30.79\,\text{g cm}^{-3} Fuel B (C8H18(l)\text{C}_8\text{H}_{18}(l)): ΔHf∘=−250 kJ mol−1\Delta H_f^{\circ} = -250\,\text{kJ mol}^{-1}, molar mass = 114.2 g mol−1114.2\,\text{g mol}^{-1}, density = 0.70 g cm−30.70\,\text{g cm}^{-3} Standard enthalpies of formation for other species (kJ mol−1^{-1}): CO2(g)=−394\text{CO}_2(\text{g}) = -394, H2O(l)=−286\text{H}_2\text{O}(\text{l}) = -286, O2(g)=0\text{O}_2(\text{g}) = 0. Balanced combustion reactions: A: CH3OH(l)+1.5 O2(g)→CO2(g)+2 H2O(l)\text{CH}_3\text{OH}(l) + 1.5\,\text{O}_2(\text{g}) \to \text{CO}_2(\text{g}) + 2\,\text{H}_2\text{O}(l) B: C8H18(l)+12.5 O2(g)→8 CO2(g)+9 H2O(l)\text{C}_8\text{H}_{18}(l) + 12.5\,\text{O}_2(\text{g}) \to 8\,\text{CO}_2(\text{g}) + 9\,\text{H}_2\text{O}(l) (a) Calculate the enthalpy change per liter of each fuel when completely combusted at 298 K. (b) Indicate which fuel delivers more energy per liter.

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