3.3 - Application of Exothermic and Endothermic Reactions in Daily Life

Daily Applications

Reaction type	Application
Exothermic	Combustion of fuel for cooking; thermite reaction for welding railway tracks; a Hot Can that warms food or drinks without external heating.
Endothermic	Gel pads to relieve fever; self-cooling canned beverages; sorbet that makes the tongue feel cold.

Hot packs use exothermic processes; cold packs use endothermic processes.

Fuel Value

• A fuel is a substance that burns in air to produce heat energy.
• Fuel value is the amount of heat energy released when one gram of fuel is completely burnt in excess oxygen.
• Its unit is $\mathrm{kJ}{\mathrm{g}}^{-1}$.

For octane, ${\mathrm{C}}_8{\mathrm{H}}_{18}$, $\Delta H_c=-5500\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$ and its molar mass is $114\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$:

• RON means Research Octane Number; vehicle fuels include RON 95 and RON 97. A higher RON makes fuel more difficult to burn, whereas a lower RON makes it easier to burn.

Comparing and Choosing Fuels

Fuel	Fuel value ($\mathrm{kJ}{\mathrm{g}}^{-1}$)
Wood	20
Methane	27
Charcoal	30
Ethanol	30
Petrol	34
Kerosene	37
Natural gas	50
Hydrogen	143

A suitable fuel is not chosen by fuel value alone. Consider:

• fuel value;
• cost;
• availability and source; and
• effects on the environment.

Octane has a higher fuel value than methane, but produces more soot when burnt.