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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, _cH is negative.
By definition, the heat of combustion is minus the enthalpy change for the combustion reaction, ie, -_cH.
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)	Combustion Reaction	H_reaction (kJ/mol)
methane	890	CH_4(g) + 2O_2(g) ---> CO_2(g) + 2H₂O_(l)	H = -890
ethane	1560	C₂H_6(g) + ⁷/₂O_2(g) ---> 2CO_2(g) + 3H₂O_(l)	H = -1560
propane	2220	C₃H_8(g) + 5O_2(g) ---> 3CO_2(g) + 4H₂O_(l)	H = -2220
butane	2874	C₄H_10(g) + ¹³/₂O_2(g) ---> 4CO_2(g) + 5H₂O_(l)	H = -2874
octane	5460	C₈H_18(g) + ²⁵/₂O_2(g) ---> 8CO_2(g) + 9H₂O_(l)	H = -5460
methanol	726	CH₃OH_(l) + ³/₂O_2(g) ---> CO_2(g) + 2H₂O_(l)	H = -726
ethanol	1368	C₂H₅OH_(l) + 3O_2(g) ---> 2CO_2(g) + 3H₂O_(l)	H = -1368
propan-1-ol	2021	C₃H₇OH_(l) + ⁹/₂O_2(g) ---> 3CO_2(g) + 4H₂O_(l)	H = -2021
butan-1-ol	2671	C₄H₉OH_(l) + 6O_2(g) ---> 4CO_2(g) + 5H₂O_(l)	H = -2671

From the table we see that 1 mole of methane gas, CH_4(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

A known quantity of water is placed in a flask or beaker.
A thermometer is positioned with bulb near the middle of the volume of water.
A known quantity of fuel, such as an alcohol (alkanol), is placed in the spirit burner.
The initial temperature of the water is measured and recorded (T_i).
The wick on the spirit burner is lit, burning the fuel, and heating the water.
When the temperature has risen an appreciable amount, the spirit burner is extinguished and the final temperature recorded (T_f).
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 (T_i) = 20^oC intial mass burner + ethanol = 37.25 g

final water temperature (T_f)= 75^oC final mass burner + ethanol = 35.50 g

change in temperature = T_f - T_i = 55^oC mass ethanol used = 1.75g

Calculation of Heat of Combustion of ethanol:

Calculate moles (n) of fuel used
molecular mass (MM) of ethanol = 46.1g/mol
mass ethanol used = 1.75g
n = mass ÷ MM = 1.75 ÷ 46.1 = 0.0380 mol
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 JK^-1g^-1 x 200g x 55^oC = 46024 J = 46.024 kJ
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
The heat of combustion of ethanol is 1211 kJ/mol

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