How fuels release energy by combustion, why incomplete burning is dangerous, and how we compare fossil fuels, biofuels and hydrogen fuel cells.
This sub-topic is examined identically at SL and HL. A fuel is a substance that releases useful energy when it reacts — almost always by combustion with oxygen. Here you will balance combustion equations, see why a limited oxygen supply is hazardous, and weigh up fossil fuels, biofuels and fuel cells.
In complete combustion, a fuel burns in plenty of oxygen. Hydrocarbons and alcohols give carbon dioxide and water only, releasing a lot of energy (all combustion is exothermic). Pick a fuel — the balanced equation and products update:
When oxygen is limited, combustion is incomplete. Instead of CO₂ you get carbon monoxide (CO) and/or carbon (C, soot), plus water. Two big problems follow: less energy is released per mole of fuel, and carbon monoxide is a toxic, odourless gas — it binds to haemoglobin far more strongly than oxygen, so it stops the blood carrying O₂. Soot causes sooty flames, blocked burners and respiratory harm. (Use the toggle above to compare the equations for the same fuel.)
Fossil fuels — coal, crude oil and natural gas — formed from the remains of organisms buried over millions of years. They are energy-dense and convenient, but non-renewable and their combustion releases CO₂, a greenhouse gas that enhances the greenhouse effect and drives climate change. Coal also releases sulfur oxides (acid rain); incomplete burning adds CO and particulates.
Biofuels (e.g. bioethanol from fermented sugar, biodiesel from plant oils) come from recently living biomass, fixing carbon by photosynthesis over months, not aeons — so they are renewable. They are often described as approximately carbon-neutral because the CO₂ released on burning was recently absorbed from the air as the plants grew. In practice they are not perfectly neutral: growing, harvesting and processing the crop uses energy and land, and can compete with food production.
Two measures help compare fuels. Specific energy is the energy released per unit mass (kJ g⁻¹ or MJ kg⁻¹); energy density is the energy released per unit volume (kJ cm⁻³ or MJ L⁻¹). Hydrogen has by far the highest specific energy per kilogram, yet a very low energy density as a gas — which is the practical challenge of storing it. Typical specific-energy values:
A fuel cell converts the chemical energy of a fuel directly into electrical energy, without burning it — so it avoids the efficiency losses of a heat engine. Fuel and oxygen are fed in continuously; unlike a battery it does not run down as long as fuel is supplied. In the hydrogen–oxygen fuel cell, hydrogen is oxidised at the negative electrode (anode) and oxygen is reduced at the positive electrode (cathode); the only product is water. Toggle the electrolyte to see how the half-equations change:
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