Heat of Combustion

Heat of Combustion

1. Heat of combustion – the heat change when one mole of a substance is completely burnt in oxygen under standard conditions.
2. Combustion – redox reaction between substance (fuel) reacts rapidly with oxygen with the production of heat energy.
3. Combustion reaction gives out heat and always an exothermic reaction.
4. Heat evolved in combustion of fuel = Heat absorbed by water.
5. Bomb calorimeter is used to determine the heat of combustion.
6. The more carbon and hydrogen atoms per molecules in a fuel, the more heat that is released when 1 mol of fuel combusts.

There are differences in heats of combustion:

Chemical equation	ΔH (kJ mol-1)
H2(g) + ½ O2(g) –> H2O(l)	-286
C(s) + O2(g) –> CO2(g)	-392
CH4(g) + 2O2(g) –> CO2(g) + 2H2O(l)	-890
CH3OH(l) + 3/2 O2(g) –> CO2(g) + 2H2O(l)	-728
C2H5OH(l) + 3O2(g) –> 2CO2(g) + 3H2O(l)	-1376
C3H7OH(l) + 9/2 O2(g) –> 3CO2(g) + 4H2O(l)	-2016

The selection of suitable fuel:

• Fuel value (the amount of heat energy given out when one gram of the fuel is completely burnt in excess of oxygen): The higher the fuel value, the more energy is released.
• Effect on the environment: Production of soot which caused air pollution. Hydrogen fuel is known as clean fuels (no soot or poisonous gases).
• Cost per gram of fuel.

Fuel values of common fuels

Substances	Fuel value (kJ g-1)
Fruits	2
Egg	6
Coal	14
Glucose	15.5
Dry cow dung	15.5
Sugars	17
Wood	18
Gasoline	34
Butanol	36.6
Kerosene	37
Biodiesel (Vegetable oil)	42.2
Diesel fuel	46
Natural gas	53.6
Hydrogen	143

Qualities of a fuel are based on the following:

• Easily available
• Cheap in cost
• High fuel value
• Do not pollute the environment
• Less storage space

The Existence of Various Energy Sources

1. The Sun: solar cell (still expensive and inefficient)
2. Fossil Fuels: relatively high fuel value and convenient to use (non-renewable and cause greenhouse effect and acid rain)
3. Water: hydroelectric power. It is clean, renewable, convenient and economical to use (high cost of construction and destruction of the surrounding environment)
4. Biomass: plants and droppings of animals – biodiesel (large areas of land to grow plants)
5. Radioactive substances: uranium and plutonium (non-renewable and very destructive if an accident occurs)