Question

What is the heat of combustion of ethane, C2H6, in kilojoules per mole of ethane? Enthalpies of formation can be found in the table of thermodynamic properties.

Answer 1

Balanced chemical reaction for combustion of ethane is as shown

$C_2H_6(g)+\frac{7}{2}O_2(g)\rightarrow 2CO_2(g)+3H_2O(g)$

Heat of combustion for this reaction=Enthalpy of formation of products-Enthalpy of formation of reactants=

$2\Delta H_f^o{(CO_2(g))}+3\Delta H_f^o{(H_2O(g))}-\frac{7}{2}\Delta H_f^o{(O_2(g))}-\Delta H_f^o{(C_2H_6(g))}$

=2x-393.5 kJ/mol+3x-241.8 kJ/mol-(7/2)x0 kJ/mol-(-84.7 kJ/mol)

=-787 kJ/mol-725.4 kJ/mol - 0 kJ/mol-(-84.7 kJ/mol)

=-1427.7 kJ/mol

So heat of combustion of ethane=-1427.7 kJ/mol

Answer 2

The above answer is correct aside from the use of H2O as a gas - H2O in its liquid form must be used instead. 

H2O (l) standard formation is -285.8

3(-285.8) = -857.4

Thus the answer is -1559.7