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Heat (enthalpy) of Combustion

Key Concepts

  • Heat of Combustion of a substance is the heat liberated when 1 mole of the substance undergoes complete combustion with oxygen at constant pressure.

  • Combustion is always exothermic, the enthalpy change for the reaction is negative, ΔH is negative.

  • By definition, the heat of combustion is minus the enthalpy change for the combustion reaction, ie, -ΔreactionH.

  • By definition, the heat of combustion is a positive value.

  • Heat of Combustion can be measured experimentally.

Heat of Combustion of Some Common Substances

Substance Heat of
Combustion ( kJ mol-1)
Combustion Reaction ΔHreaction ( kJ mol-1)
methane 890 CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) ΔH = -890
ethane 1560 C2H6(g) + 7/2O2(g) → 2CO2(g) + 3H2O(l) ΔH = -1560
propane 2220 C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(l) ΔH = -2220
butane 2874 C4H10(g) + 13/2O2(g) → 4CO2(g) + 5H2O(l) ΔH = -2874
octane 5460 C8H18(g) + 25/2O2(g) → 8CO2(g) + 9H2O(l) ΔH = -5460
methanol 726 CH3OH(l) + 3/2O2(g) → CO2(g) + 2H2O(l) ΔH = -726
ethanol 1368 C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(l) ΔH = -1368
propan-1-ol 2021 C3H7OH(l) + 9/2O2(g) → 3CO2(g) + 4H2O(l) ΔH = -2021
butan-1-ol 2671 C4H9OH(l) + 6O2(g) → 4CO2(g) + 5H2O(l) ΔH = -2671

From the table we see that 1 mole of methane gas, CH4(g), undergoes complete combustion in excess oxygen gas releasing 890 kJ of heat.

2 moles of methane would combust to release 2 x 890 = 1780 kJ of heat.

½ mole of methane would combust to release ½ x 890 = 445 kJ of heat.

Measuring Heat of Combustion

  1. A known quantity of water is placed in a flask or beaker.

  2. A thermometer is positioned with bulb near the middle of the volume of water.

  3. A known quantity of fuel, such as an alcohol (alkanol), is placed in the spirit burner.

  4. The initial temperature of the water is measured and recorded (Ti).

  5. The wick on the spirit burner is lit, burning the fuel, and heating the water.

  6. When the temperature has risen an appreciable amount, the spirit burner is extinguished and the final temperature recorded (Tf).

  7. The final quantity of fuel is measured and recorded.
Typical Results for an experiment where ethanol is used to heat 200g of water:

initial water temperature (Ti) = 20oC intial mass burner + ethanol = 37.25 g
final water temperature (Tf)= 75oC final mass burner + ethanol = 35.50 g

change in temperature = Tf - Ti = 55oC mass ethanol used = 1.75 g

Calculation of Heat of Combustion of ethanol:

  1. Calculate moles (n) of fuel used

    molar mass (M) of ethanol = 46.1 g mol-1
    mass ethanol used = 1.75 g

    n = mass ÷ molar mass = 1.75 ÷ 46.1 = 0.0380 mol

  2. Calculate energy required to change temperature of water

    energy = specific heat capacity of water x mass of water x change in water temperature

    energy = 4.184 J K-1g-1 x 200 g x 55oC = 46024 J = 46.024 kJ

  3. Calculate the heat of combustion of ethanol

    Assume all the heat produced from burning ethanol has gone into heating the water, ie, no heat has been wasted.

    0.0380 mole ethanol produced 46.024 kJ of heat.

    Therefore 1 mole of ethanol would produce 46.024 kJ ÷ 0.0380 mol = 1211 kJ mol-1

    The heat of combustion of ethanol is 1211 kJ mol-1

The experimentally determined value for the heat of combustion of ethanol is usually less than the accepted value of 1368 kJ mol-1 because some heat is always lost to the atmosphere and in heating the vessel.

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