Hydrogen and Methanol have both been proposed as alternatives to hydrocarbon fue

Question

Hydrogen and Methanol have both been proposed as alternatives to hydrocarbon fuels. Write balanced reactions for the complete combustion of hydrogen and methanol (Identify phases) and use standard enthalpies of formation to calculate the amount of heat released per kilogram of the fuel (kJ/kg). Which fuel contains the most energy in the least mass? How does the energy of these fuels compare to that of octane (C8H18) (amount of heat released by octane (kJ/kg))?

Explanation / Answer

Hydrogen's combustion reaction is: 2H2 (g) + O2 (g) ---> 2H20 (l) + 572 kJ    Enthalpy of -286 kJ/mole Converting from kJ/mole to kJ/kg requires the molar mass ofmolecular hydrogen, which is 2.016 g/mole. So Enthalpy of combustion is also 0.141865 kJ/kg of hydrogengas. Methanol's combustion reaction is: 2CH3OH (l) + 3 (O2) ----> 2CO2 (g) + 4H20 (l)   -715kJ/mole Methanol's molar mass is 32.041g/mole. Converting theenthalpy of combustion to kJ/kg gives us a value of .02231 kJ/kg of methanol. Comparing these two, we see that hydrogen by far has the mostenergy released per weight. This is what we would expect, asthe stability of the pure product (water) is so high. Calculate octane's heat of combustion and then convert to thenecessary units, just as we did for the other two above. Thencompare.

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