Question

Hydrocarbon fuel can be produced from methanol by reactions such as the following, which yields 1-hexene: 6 CH₃OH _(g) -> C₆H_12(g) + 6 H₂O _(g)

Compare the standard heat of combustion at 25^oC of 6 mol CH₃OH_(g) with the standard heat of combustion at 25^oC of 1 mol C₆H_12(g), reaction products in both cases being CO_2(g) and H₂O_(g). Note that here the comparison is made on the basis of equal number of C atoms.

For a given amount of heat generated from combustion, which fuel, methanol or 1-hexene, will produce more CO₂ in the exhaust gas?

Answer 1

CH3OH + 3/2O2 --> CO2 + 2H2O

Heat of combustion of methanol = Enthalpy of CO2 + 2X enthalpy of H2O - Enthalpy of formation of Methanol

? H_f⁰ in kJ/mol of H2O = -285.8

? H_f⁰ in kJ/mol of CO2 = -393.5

? H_f⁰ in kJ/mol of CH3OH = -238.7

So heat of combustion = -726.4 kJ/mol

C6H12 + 9O2 --> 6CO2 + 6H2O

-43.5 kJ/mol = heat of formation of hexene

Heat of combustion of hexene = 6 X Enthalpy of CO2 + 6X enthalpy of H2O - Enthalpy of formation of Methanol

Heat of combustion of hexene = -4032.3 KJ / mol

however in methanol only one carbon is present so on that basis the heat of combustion per carbon is more for hexene.

Hexene will produce more carbon dioxide