Heat of Combustion

Combustion

  1. Combustion is a reaction when a substance burns completely in the excess oxygen.
  2. Example, combustion of methane in excess of oxygen produces carbon dioxide and water.
    CH4 + 2O2 → CO2 + 2H2O
  3. All fuel combustion are exothermic reactions.

Heat of Combustion

The Heat of Combustion of a substance is the heat energy released when 1 mole of the substance is completely burnt in excess oxygen in a standard condition

Finding Heat of Combustion

Figure Below shows a simple method for obtaining an approximate value for the heat of combustion of a fuel.

A fuel lamp is lighted and the heat energy released is used to heat a fixed volume of water. The mass of the fuel is weighed and the maximum increase of temperature  is recorded.
The heat of combustion can be calculated by using the formula

It is assumed that all heat energy released from the combustion of fuel is absorbed by water.

Precaution Steps
  1. Use wind shield to block the air flow. This can reduce the heat loss to the surrounding.
  2. Do not use wire gauze, to reduce the heat loss.
  3. Copper container is used because copper is a good heat conductor,
  4. The flame must be put near the base of the container,
  5. The water is always stirred with the thermometer.
  6. The increase of water temperature is kept below 30°C to prevent over heating and energy loss to the surroundings,
  7. The fuel is weighed frequently to prevent condensation
Example 1
Combustion of 1.71 g heptane (C7H16) in excees oxygen will relase 75kJ of heat.
[RAM: H = l; C = 12]
(a) Write the equation of the reaction.

(b) Find the heat of combustion of heptane.
Relative molecular mass of heptane
= 7(12) + 16(1) = 100

Number of mole of heptane

Heat of Combustion

Comparison of Combustion Heat of Difference Fuel

The table below shows the heat of combustion of a few fuel. It shows that when the number of carbon in a molecule increase, the heat of combustion will increase.

AlcoholMolecularFormula
DH (kJ mol-1)
Methanol
CH3OH
‑715
Ethanol
C2H5OH
‑1376
Propan-1-ol
C3H7OH
‑2017
Butan-1-ol
C4H9OH
‑2675

If a graph is plotted.

We will find that the heat of combustion is directly proportional to the number of carbon in the fuel.

Example 1
AlcoholCombustion heat (kJ mol-1)
Methanol
Ethanol
Propanol
Pentanol
-710
-1370
-2015
-3300

Table above shows the heat of combustion for different alcohol. Estimate the heat of combustion of butanol. By referring to the table above, find the heat release if 6g propanol is combusted. (Relative molecular mass: Carbon = 12; Hydrogen = 1, Oxygen = 16)

Answer
Heat of combustion of butanol
= -(2015+3300)/2 ≈ 2660 kJ mol-1

Relative molecular mass of propanol (C3H7OH)
= 3(12)+7(1)+16+1
= 60

Number of mol of propanol,
n = 6g/50gmol-1
n = 0.1 mol

Amount of heat released
= 0.1 x 2015kJ
= 201.5kJ