In: Chemistry
A. Do you think the enthalpy of a methane combustion is more or
less negative than the enthalpy of a reaction for methanol
combustion? Why?
B. Do you think the enthalpy of an ethanol combustion is more or
less negative than the enthalpy of a reaction for a methanol
combution? Why?
A.The enthalpy of methane combustion is more negative than the enthalpy of a reaction for methanol combustion. Molecular formula for methane is CH4 and for methanol is CH3OH. Both have same carbon number but the values ΔH combustion for alcohols are slightly smaller than those for alkanes because the alcohols are already partially oxidized (the presence of a single oxygen atom in each alcohol molecule.)
2CH4 +2O2 ------> 2CO2 + 2H2O ΔH combustion –890KJ/mol
2CH3OH +3 O2 ------> 2CO2 + 4H2O ΔH combustion for methanol is -726 kJ/mol
B. Molecular formula of and ethanol is CH3 CH2OH. When the number of Carbon atoms in an alcohol increases; the enthalpy of combustion will become more negative. Ethanol has more carbon atom, so the enthalpy of ethanol combustion is more negative than the enthalpy of a reaction for methanol combustion. As we move down the homologous group, a CH2 molecule is being added into the alcohol chain.
When ethanol is burnt, there will be one extra C-C bond and two extra C-H bonds to be broken. Therefore, much more energy is required for combustion. ΔH combustion for methanol is -726 kJ/mol and ΔH combustion for ethanol –1367 KJ/mol.