• The discovery of how to light a fire is considered as one of the most important discoveries in human civilisations.
• Energy generated through combustion allows humans to chase away wild animals, cook food and control their own light sources and heat.
• Knowledge of exothermic and endothermic reactions enables human life to be more comfortable.
• Examples of applications you have learned in Form Three are the use of cold packs and hot packs.

• Fuels are substances that burn in the air to produce heat energy.
• Each fuel has different heat of combustion.
• The unit for fuel value is \(\text{kJ g}^{−1}\).
• The fuel value can also be determined with the heat of combustion of substances.

Question

The heat of combustion of octane, \(C_8H_{18}\) is \(-5\,500 \text{ kJ mol}^{−1}\).

Calculate the fuel value of octane, \(C_8H_{18}\).

[Relative atomic mass: H = 1, C = 12]

Solution

Molar mass of octane, \(C_8H_{18}\).

\(=(8 \times 12)+(18 \times1)\\ =96+18\\ =114\text{ g mol}^{−1}\)

Complete combustion of 114 g of octane, \(C_8H_{18}\) releases 5 500 kJ of heat. 

Therefore, combustion of 1 g of octane, \(C_8H_{18}\) is as follows;

\(=\dfrac{5\,500 \text{ kJ mol}^{−1}}{114 \text{ g mol}^{−1}}\\ =48.2\text{ kJ g}^{−1}\)

The fuel value of octane, \(C_8H_{18}\) is \(48.2\text{ kJ g}^{−1}\).

• The discovery of how to light a fire is considered as one of the most important discoveries in human civilisations.
• Energy generated through combustion allows humans to chase away wild animals, cook food and control their own light sources and heat.
• Knowledge of exothermic and endothermic reactions enables human life to be more comfortable.
• Examples of applications you have learned in Form Three are the use of cold packs and hot packs.

• Fuels are substances that burn in the air to produce heat energy.
• Each fuel has different heat of combustion.
• The unit for fuel value is \(\text{kJ g}^{−1}\).
• The fuel value can also be determined with the heat of combustion of substances.

Question

The heat of combustion of octane, \(C_8H_{18}\) is \(-5\,500 \text{ kJ mol}^{−1}\).

Calculate the fuel value of octane, \(C_8H_{18}\).

[Relative atomic mass: H = 1, C = 12]

Solution

Molar mass of octane, \(C_8H_{18}\).

\(=(8 \times 12)+(18 \times1)\\ =96+18\\ =114\text{ g mol}^{−1}\)

Complete combustion of 114 g of octane, \(C_8H_{18}\) releases 5 500 kJ of heat. 

Therefore, combustion of 1 g of octane, \(C_8H_{18}\) is as follows;

\(=\dfrac{5\,500 \text{ kJ mol}^{−1}}{114 \text{ g mol}^{−1}}\\ =48.2\text{ kJ g}^{−1}\)

The fuel value of octane, \(C_8H_{18}\) is \(48.2\text{ kJ g}^{−1}\).