Answer
a) $-973.3 \text{ kJ.mol^{-1}}$; Ethanol releases $21.12 \text{ kJ.g^{-1}}$
b) $-1166.9 \text{ kJ.mol}^{-1}$; Ethane releases $38.89 \text{ kJ.g}^{-1}$
Ethane releases $17.77 \text{ kJ}$ of energy more per one gram of substance than ethanol
Work Step by Step
The $\Delta H^{\circ}_{rxn}$ value can be taken by subtracting initial enthalpy from the final using the given values,
$\Delta H^{\circ}_{rxn} = H^{\circ}_{f} Initial-H^{\circ}_{f}final$
When the sign of the enthalpy change is negative reaction is exothermic as energy is released, when sign is positive reaction is endothermic as energy is absorbed.
The energy release per gram of substance can be taken as by dividing the molar enthalpy of the reaction by the molar mass of the reactant substance.
a) $-973.3 \text{ kJ.mol^{-1}}$; Ethanol releases $21.12 \text{ kJ.g^{-1}}$
b) $-1166.9 \text{ kJ.mol}^{-1}$; Ethane releases $38.89 \text{ kJ.g}^{-1}$
Ethane releases $17.77 \text{ kJ}$ of energy more per one gram of substance than ethanol
