R1.3 Energy from fuels

Every time you fill a car with petrol, cook over a gas flame, or ride a bus, you are harnessing energy stored in chemical bonds. The fuels we use , from ancient fossil fuels to modern biofuels and hydrogen , all release energy through chemical reactions, primarily combustion. Understanding how these reactions work, what products they form, and what impact those products have on human health and the environment is central to this subtopic.

This subtopic covers:

• The chemistry of combustion reactions (metals, non-metals, and organic compounds)
• Complete vs. incomplete combustion and their consequences
• The enthalpy of combustion ($\Delta H_c^{\circ }$) as a measure of energy released by fuels
• The advantages and disadvantages of fossil fuels, biofuels, and hydrogen fuel cells
• The environmental challenges associated with our current energy systems

Combustion: Combustion is a chemical reaction in which a fuel reacts with oxygen ($O_2$), releasing energy in the form of heat and light.

Every combustion reaction requires three components , sometimes called the fire triangle:

1. Fuel , the substance being burned
2. Oxygen , the reactant that enables the reaction
3. Heat/ignition source , an initial energy input (spark or flame) to start the reaction; this provides the activation energy needed to initiate bond breaking

The products of combustion depend on the type of fuel and how much oxygen is available:

Complete combustion: Complete combustion occurs when there is an excess of oxygen. Hydrocarbons are fully oxidised, producing only carbon dioxide ($C{O}_2$) and water ($H_{2}O$).

Incomplete combustion: Incomplete combustion occurs when oxygen is limited. It produces carbon monoxide ($CO$) and/or soot (elemental carbon, $C$) in addition to water.

Combustion is fundamentally a redox reaction:

• The fuel is the reducing agent , it loses electrons (is oxidised)
• Oxygen is the oxidising agent , it gains electrons (is reduced)