Alkanes as fuels and pollutionEdexcel A-Level Chemistry: Revision notes
Section 1
Alkanes: formula and saturation
Alkanes are hydrocarbons (compounds of hydrogen and carbon only) with the general formula CₙH₂ₙ₊₂, e.g. C₃H₈ propane, C₇H₁₆ heptane. Cycloalkanes such as cyclohexane, C₆H₁₂, form a ring and have the general formula CₙH₂ₙ.
Both are saturated: every carbon–carbon bond is a single bond, so each carbon holds the maximum possible number of hydrogen atoms and there is no C=C double bond. Alkenes, by contrast, are unsaturated.
Cyclohexane, C₆H₁₂, fits CₙH₂ₙ, the same as an alkene, yet it is saturated. Check the structure, not just the formula.
Section 2
Fuels from crude oil: distillation, cracking and reforming
Fractional distillation. Crude oil is heated and the vapour enters a column that is hot at the bottom and cooler at the top. Hydrocarbons separate into fractions by boiling point: short chains (low boiling point) leave at the top, long chains (high boiling point) near the bottom.
Cracking. Demand for short-chain fuels is greater than the supply from distillation, so long-chain alkanes are broken into shorter alkanes and alkenes by heating with a zeolite catalyst (catalytic cracking) or at high temperature and pressure (thermal cracking). Example: C₁₂H₂₆ → C₈H₁₈ + 2C₂H₄.
Reforming. Straight-chain hydrocarbons are processed into branched-chain alkanes and cyclic hydrocarbons. These burn more efficiently and smoothly in engines, so they are better petrol components. The number of carbon atoms in the molecule stays the same.
Reforming does not shorten the chain (that is cracking). It rearranges the carbon skeleton.
Section 3
Combustion of alkanes in air
In a plentiful supply of oxygen alkanes undergo complete combustion to carbon dioxide and water: C₈H₁₈ + 12½O₂ → 8CO₂ + 9H₂O, or 2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O.
In a limited supply of oxygen incomplete combustion gives carbon monoxide and/or carbon (soot) as well as water: e.g. CH₄ + 1½O₂ → CO + 2H₂O. Some fuel may also leave the engine unburned.
Balance carbon first, then hydrogen, then oxygen last. Use halves if you must, then double the whole equation.
Section 4
Pollutants from burning fuels
Burning fuels in engines and power stations releases pollutants:
- Carbon monoxide, CO: incomplete combustion. It is toxic because it binds strongly to haemoglobin in the blood, so less oxygen is carried round the body.
- Oxides of nitrogen, NOₓ: at the very high temperatures in engines, N₂ + O₂ → 2NO; NO is oxidised to NO₂. They dissolve in rain water to form acidic solutions (acid rain).
- Oxides of sulfur, SO₂: from sulfur impurities in the fuel, S + O₂ → SO₂. Also dissolve in water to form acids, damaging buildings, plants and aquatic life.
- Carbon particulates: unburned carbon (soot) from incomplete combustion.
- Unburned hydrocarbons: fuel that escapes the engine without burning.
Nitrogen oxides come from the nitrogen in the AIR reacting at high temperature, not from the hydrocarbon fuel.
Section 5
Catalytic converters
A catalytic converter in the exhaust contains platinum, palladium and rhodium spread thinly over a ceramic honeycomb, giving a large surface area. On the surface the pollutants react to form less harmful gases:
2CO + 2NO → 2CO₂ + N₂
Unburned hydrocarbons are oxidised to carbon dioxide and water, e.g. C₈H₁₈ + 12½O₂ → 8CO₂ + 9H₂O.
So toxic CO becomes CO₂ and acid-forming NO becomes N₂. The converter does not remove carbon dioxide, which is still released.
Section 6
Alternative fuels
Biodiesel is made from plant oils and alcohols such as ethanol are made by fermenting plant sugars. They are renewable because the plants can be regrown, unlike non-renewable fossil fuels.
Advantages: close to carbon neutral (plants absorb CO₂ as they grow), and little sulfur so less SO₂.
Disadvantages: farmland is used for fuel rather than food, and energy is needed to grow and process the crops.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Alkanes as fuels and pollution
- A refinery separates crude oil by fractional distillation. One fraction contains mainly heptane, C₇H₁₆, a straight-chain alkane. Part of this fraction is processed further so that it can be blended into petrol for use in car engines.Explain why the refinery reforms some of the straight-chain alkanes in this fraction before blending them into petrol.2 marks
- A petrol car is driven on a motorway. Air enters the engine and the fuel burns at a very high temperature in a limited supply of oxygen, so the exhaust gases contain carbon monoxide, nitrogen monoxide, unburned hydrocarbons and carbon particulates before they pass through the exhaust system.Write an equation for the formation of nitrogen monoxide in the engine and explain why it forms in the engine but not in the cold air outside the car.2 marks
- The exhaust system of the car contains a catalytic converter. This is a honeycomb of ceramic coated with a thin layer of platinum, palladium and rhodium. The exhaust gases pass through it before leaving the car.The converter helps to solve two pollution problems when carbon monoxide reacts with nitrogen monoxide. Write an equation for this reaction and explain how it reduces each problem.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).