R1.3 Energy from fuelsIB Chemistry SL: Revision notes
Section 1
Combustion reactions
Combustion is a reaction with oxygen that releases energy. Reactive metals burn to form oxides (2Mg + O₂ → 2MgO), non-metals burn to form oxides (S + O₂ → SO₂; C + O₂ → CO₂), and organic compounds burn to form carbon dioxide and water.
Complete combustion of a hydrocarbon or alcohol gives only CO₂ and H₂O:
- C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
- C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
Balance C first, then H, then O last. For alcohols, subtract the O already in the molecule before counting O₂.
Section 2
Incomplete combustion
With a limited oxygen supply, incomplete combustion produces carbon monoxide and/or carbon (soot) as well as water:
- CH₄ + 1½O₂ → CO + 2H₂O
- CH₄ + O₂ → C + 2H₂O
- C₂H₅OH + 2O₂ → 2CO + 3H₂O
CO is toxic because it binds to haemoglobin and reduces oxygen transport; soot causes respiratory problems and a yellow, smoky flame.
Leaving the water out of an incomplete combustion equation: hydrogen is still oxidised to water.
Section 3
Fossil fuels and carbon dioxide
Fossil fuels (coal, crude oil, natural gas) formed over millions of years and are non-renewable.
- Coal: plentiful and cheap, but gives the most CO₂ per unit of energy plus SO₂ and particulates.
- Crude oil: liquid fuels with high energy density, easy to transport; limited reserves.
- Natural gas: cleanest burning, least CO₂ per unit of energy; leaks of methane are a potent greenhouse gas.
To compare fuels, calculate the mass of CO₂ per unit of energy: mol of fuel = energy ÷ ΔH per mole; mol CO₂ = mol fuel × carbon atoms per molecule. Carbon ≈ 112 g MJ⁻¹, octane ≈ 64 g MJ⁻¹, methane ≈ 49 g MJ⁻¹.
Section 4
Carbon dioxide and the greenhouse effect
The Earth absorbs solar radiation and re-emits infrared radiation. Greenhouse gases (CO₂, H₂O, CH₄) absorb infrared because it makes their bonds vibrate, then re-radiate it in all directions, warming the lower atmosphere. Burning fossil fuels releases long-stored carbon faster than natural sinks can absorb it, so atmospheric CO₂ rises and the enhanced greenhouse effect drives global warming.
Section 5
Biofuels and photosynthesis
Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (light energy, chlorophyll).
Biofuels such as bioethanol and biodiesel come from carbon fixed by photosynthesis over a short period, so they are renewable.
- Advantages: renewable; lower net CO₂ addition; can be produced locally.
- Disadvantages: not fully carbon neutral (fossil energy for farming, fertiliser, distillation); compete with food for land; deforestation; lower energy per gram (ethanol 29.7 kJ g⁻¹ against octane 47.9 kJ g⁻¹).
Section 6
Fuel cells
A fuel cell converts the chemical energy of a fuel directly into electrical energy: oxidation at the anode and reduction at the cathode, with a continuous supply of fuel and oxygen.
Hydrogen, acidic electrolyte: anode H₂ → 2H⁺ + 2e⁻; cathode O₂ + 4H⁺ + 4e⁻ → 2H₂O. Hydrogen, alkaline electrolyte: anode H₂ + 2OH⁻ → 2H₂O + 2e⁻; cathode O₂ + 2H₂O + 4e⁻ → 4OH⁻. Methanol, acidic: anode CH₃OH + H₂O → CO₂ + 6H⁺ + 6e⁻.
Fuel cells are more efficient than burning the fuel in an engine, but hydrogen is hard to store and is often made from fossil fuels.
Putting the fuel at the cathode. The fuel is always oxidised, so it reacts at the anode.
Must know
- Complete combustion → CO₂ + H₂O; incomplete → CO and/or C + H₂O.
- Fossil fuels are non-renewable; compare them by CO₂ per unit of energy.
- CO₂ absorbs and re-emits infrared: the greenhouse effect.
- Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
- Biofuels are renewable but not fully carbon neutral.
- Fuel cell: fuel oxidised at the anode, O₂ reduced at the cathode.
That's the notes covered.
Carry on to the next subtopic.