- #1
gfd43tg
Gold Member
- 950
- 50
Hello,
Can the heat of combustion only be found experimentally? I am looking on this page at the heat of combustion of ethanol
http://www.ausetute.com.au/heatcomb.html
and I see that ##\Delta H^{o}_c = 1368 \hspace{0.05 in} kJ/mol##. But if you calculate from its constituents
##C_{2}H_{6}O (l) + 3O_{2} (g) \rightarrow 2CO_{2} (g) + 3H_{2}O (l)##
The enthalpy of formation (calculated the same way as enthalpy of combustion??) is using the equation
[tex]\Delta H^{o}_{rxn} = \sum_{products} \nu_{i} \Delta H_{i}^{o} - \sum_{reactants} \nu_{i} \Delta H_{i}^{o}[/tex]
where ##\nu_{i}## is the stoichiometric coefficient of a chemical species ##i##
##[\Delta H_{f,ethanol}^{o} + 3(0 \hspace{0.05 in} kJ/mol)] - [(2)(-393.5 \hspace{0.05 in} kJ/mol) + 3(-285.8 \hspace{0.05 in} kJ/mol)] = 1368 \hspace{0.05 in} kJ/mol##
##\Delta H_{f,ethanol}^{o} = -276.4 \hspace{0.05 in} kJ/mol##
But to calculate the heat of formation of ethanol assumes a priori that you know the heat of combustion of ethanol. So is there a mathematical or theoretical way to calculate the heat of combustion, or is it purely from experiment?
Can the heat of combustion only be found experimentally? I am looking on this page at the heat of combustion of ethanol
http://www.ausetute.com.au/heatcomb.html
and I see that ##\Delta H^{o}_c = 1368 \hspace{0.05 in} kJ/mol##. But if you calculate from its constituents
##C_{2}H_{6}O (l) + 3O_{2} (g) \rightarrow 2CO_{2} (g) + 3H_{2}O (l)##
The enthalpy of formation (calculated the same way as enthalpy of combustion??) is using the equation
[tex]\Delta H^{o}_{rxn} = \sum_{products} \nu_{i} \Delta H_{i}^{o} - \sum_{reactants} \nu_{i} \Delta H_{i}^{o}[/tex]
where ##\nu_{i}## is the stoichiometric coefficient of a chemical species ##i##
##[\Delta H_{f,ethanol}^{o} + 3(0 \hspace{0.05 in} kJ/mol)] - [(2)(-393.5 \hspace{0.05 in} kJ/mol) + 3(-285.8 \hspace{0.05 in} kJ/mol)] = 1368 \hspace{0.05 in} kJ/mol##
##\Delta H_{f,ethanol}^{o} = -276.4 \hspace{0.05 in} kJ/mol##
But to calculate the heat of formation of ethanol assumes a priori that you know the heat of combustion of ethanol. So is there a mathematical or theoretical way to calculate the heat of combustion, or is it purely from experiment?
Last edited: