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Three ways to find the enthalpy of combustion of ethanol

Hess's law, calorimetry and bond enthalpies

A student wants ΔcH\Delta_cH for ethanol, CX2HX5OH\ce{C2H5OH}. Data: ΔfH\Delta_fH (kJ mol−1^{-1}): CX2HX5OH(l)\ce{C2H5OH(l)} −277, COX2(g)\ce{CO2(g)} −394, HX2O(l)\ce{H2O(l)} −286, HX2O(g)\ce{H2O(g)} −242. Bond enthalpies (kJ mol−1^{-1}): C–H 414, C–C 346, C–O 358, O–H 463, O=O 498, C=O 804. c(water)=4.18c(\text{water}) = 4.18 J g−1^{-1} °C−1^{-1}. M(CX2HX5OH)=46.0M(\ce{C2H5OH}) = 46.0 g mol−1^{-1}.
Facts you may need
  • ΔrH = ΣΔfH(products) − ΣΔfH(reactants)
  • q = mcΔT
  • bonds broken − bonds formed

(a)

Write the balanced equation for the complete combustion of ethanol and use the ΔfH\Delta_fH data to calculate ΔcH\Delta_cH.

(b)

Burning 1.15 g of ethanol in a spirit burner heated 250 g of water from 18.5 °C to 36.2 °C. Calculate the energy released and the experimental ΔcH\Delta_cH.

(c)

Use the bond enthalpy data to estimate ΔcH\Delta_cH, assuming all species are gases.

(d)

The three values (−1369, −740, −1263 kJ mol−1^{-1}) differ. Explain the reasons for the differences.
Three ways to find the enthalpy of combustion of ethanol — Scholarship Chemistry | Xametric