(for ethanol using method 1.3a2)

The enthalpy of combustion of anything is the enthalpy change when 1 mole of any combustible material is completely burned in oxygen (or air containing oxygen) equated to standard conditions (298K/25^oC, 1 atm/101kPa).

The combustible material is usually organic, so you could define the enthalpy of combustion of ethanol as:

ΔH^θ_comb. (ethanol) The enthalpy of combustion of ethanol (l) is the enthalpy change when 1 mole of ethanol is completely burned in oxygen (or air containing oxygen) to form carbon dioxide (g) and water (l) under standard conditions (298K/25^oC, 1 atm/101kPa).

• Method (more details using method 1.3a2)

  • 100 cm³ of water (100g) was measured into the calorimeter.

  • The spirit burner contained the fuel ethanol CH₃CH₂OH ('alcohol') and weighed 18.62g at the start.

  • After burning it weighed 17.14g and the temperature of the water rose from 18 to 89^oC.

• Calculation

  • The temperature rise = 89 – 18 = 71^oC (exothermic, heat energy given out).

  • Mass of fuel burned = 18.62–17.14 = 1.48g.

  • Heat given out to the water = mass of water x SHC_water x temperature change

    • q = m c ΔT = 100 x 4.18 x 71 = 29678 J (for 1.48g)

  • M_r(ethanol) = 46 (H=1, C=12, O=16)

  • Therefore 1.48g ethanol = 1.48/46 = 0.03217 mol

  • So, scaling up to 1 mole of ethanol burned gives 29678 x 1 / 0.03217 = 922536 J

  • Enthalpy of combustion of ethanol =

  • ΔH_c(ethanol) = –923 kJmol^–1  (only accurate to 3 sf at best)

  • for the reaction: CH₃CH₂OH_(l) + 3O_2(g) ===> 2CO_2(g) + 3H₂O_(l)

• Comments

  • The data book value for the heat of combustion of ethanol is –1367 kJmol^–1, showing lots of heat loss in the experiment!

  • It is possible to get more accurate values by calibrating the calorimeter with a substance whose energy release on combustion is known.

See also Chemistry of alcohols - combustion

and Chemistry of alkanes - combustion

thermochemistry of determining the enthalpy of combustion of ethanol, method, apparatus, data, calculation, copper calorimeter, thermochemistry for AQA, Edexcel, OCR, Salters, CIE, WJEC Eduqas & CCEA A-level chemistry exam students, US grades 11-12 K12 AP Honors chemistry courses