A bomb-calorimetry experiment is conducted to determine the standard enthalpy of formation of ethanol, C₂H₅OH(l).
Combustion reaction: C₂H₅OH(l) + 3 O₂(g) → 2 CO₂(g) + 3 H₂O(l)
Experimental data
• Mass of ethanol combusted: 1.282 g (molar mass = 46.1 g mol⁻¹)
• Temperature rise of 2000 g of surrounding water: 3.50 °C
• Heat capacity of the steel calorimeter canister: 2.13 kJ °C⁻¹
• Specific heat capacity of water: 4.18 J g⁻¹ °C⁻¹
Standard molar enthalpies of formation
CO₂(g): –393.5 kJ mol⁻¹
H₂O(l): –285.8 kJ mol⁻¹
(a) Calculate the total heat released during combustion (in kJ) by accounting for both the water and the calorimeter canister.
(b) Convert this heat value to the molar enthalpy of combustion ΔH_comb (kJ mol⁻¹) for ethanol, including the correct sign.
(c) Using ΔH_comb and the given standard enthalpies of formation for CO₂(g) and H₂O(l), determine the standard molar enthalpy of formation, ΔH_f°, of C₂H₅OH(l). Show all algebraic steps.
(d) The literature value for ΔH_f°[C₂H₅OH(l)] is –277 kJ mol⁻¹. Calculate the percent error of the experimental result and discuss two potential systematic errors specific to bomb calorimetry that could account for the discrepancy.