A hydrocarbon contains only carbon and hydrogen. Methane, CH₄, is a hydrocarbon; ethanol is not, because it also contains oxygen.
Crude oil is a complex mixture of hydrocarbons with different molecular sizes and chain or ring structures. It is not one pure compound and most constituents are not polymers.
Crude oil supplies fuels and starting materials (feedstock) for the petrochemical industry. These are used to make other chemicals and materials, including polymers.
Crude oil and fossil natural gas are finite, non-renewable resources on human timescales. Natural gas is mainly methane and is used in cooking and heating.
Petrol, kerosene and diesel are fossil fuels obtained from crude oil. Hydrocarbons can also be produced by other routes, so not every hydrocarbon molecule is necessarily fossil-derived.
A fraction is a mixture of hydrocarbons with a similar range of boiling points, not normally a single chemically pure substance.
Fractional distillation of crude oil
Heat crude oil so much of it vaporises, and pass vapour into a fractionating column. The column is hot at the bottom and cooler at the top.
Hydrocarbons condense at different levels according to their boiling points: longer-chain, higher-boiling molecules condense lower down; shorter-chain, lower-boiling ones travel higher.
Very low-boiling refinery gases can leave the top as gases rather than condensing as liquids there. High-boiling residue such as bitumen is collected at the bottom.
Fractional distillation is a physical separation. It does not break carbon chains or change the identity of the hydrocarbons; cracking does that separately.
The main fractions in order from top towards bottom are gases, petrol, kerosene, diesel oil, fuel oil and bitumen.Schematic column; top gases need not condense there and bitumen is bottom residue.
Gases are used for domestic heating and cooking; petrol for cars; kerosene for aircraft; diesel for some cars and trains; fuel oil for large ships and some power stations; bitumen for roads and roofs.Fractions contain mixtures, not one pure alkane each.
Chain length and alkane patterns
As chain length generally increases, boiling point and liquid viscosity increase and ease of ignition decreases. Larger molecules have stronger intermolecular attractions.Compare similar hydrocarbons under the same conditions.
Viscosity describes resistance to flow: a more viscous liquid flows less readily under comparable conditions. Compare temperature as well as composition.
Alkanes are saturated hydrocarbons: they have only single carbon–carbon bonds. Alkanes without rings (acyclic alkanes) have general formula CₙH₂ₙ₊₂. Ring-shaped hydrocarbons do not necessarily follow this formula.
The first four alkanes are methane CH₄, ethane C₂H₆, propane C₃H₈ and butane C₄H₁₀.Acyclic saturated alkanes follow CₙH₂ₙ₊₂.
A homologous series has the same general formula, similar chemical properties and a gradual change in physical properties. Neighbouring members differ by CH₂.
For an acyclic alkane with five carbons, 2n + 2 = 12 hydrogen atoms, giving C₅H₁₂. Check that you are using the correct series before applying the formula.
Complete and incomplete combustion
Complete combustion with sufficient oxygen produces carbon dioxide and water and releases energy: hydrocarbon + oxygen → carbon dioxide + water.
For methane: CH₄ + 2O₂ → CO₂ + 2H₂O. Balance carbon, hydrogen and oxygen atoms without changing the substance formulae.
Incomplete combustion occurs with insufficient oxygen and can produce carbon monoxide and/or carbon particles (soot), together with water and sometimes carbon dioxide.
Example CO-producing equation: 2CH₄ + 3O₂ → 2CO + 4H₂O. A soot-producing example is CH₄ + O₂ → C + 2H₂O. Real incomplete combustion can produce mixtures.Incomplete combustion can yield carbon monoxide and/or carbon, rather than one fixed product mix.
Incomplete combustion releases less useful energy from a given amount of fuel than full oxidation to CO₂, and creates hazards and pollution.
Carbon monoxide is colourless, odourless and toxic. It binds to haemoglobin, reducing the blood's ability to transport oxygen.
Soot can dirty appliances and obstruct air flow or flues, increasing problems with fuel-burning appliances. Carbon particles are also harmful air pollution.
Complete combustion still releases carbon dioxide, a greenhouse gas. Absence of soot does not mean the fuel has no environmental impact.
Sulfur dioxide, nitrogen oxides and acid rain
Sulfur impurities in some fuels burn to form sulfur dioxide: S + O₂ → SO₂. Removing sulfur from fuels reduces this source of pollution.
Sulfur dioxide dissolves in atmospheric moisture to produce acidic solutions; further oxidation can contribute to sulfuric acid formation. Do not treat SO₂ + water alone as directly making H₂SO₄ without extra oxygen.
Acid rain can damage plants, acidify lakes and harm aquatic organisms, and react with limestone buildings and monuments.
An acid-rain carbonate equation is H₂SO₄ + CaCO₃ → CaSO₄ + H₂O + CO₂. With a suitable reactive metal: Fe + H₂SO₄ → FeSO₄ + H₂ for dilute acid.
At high engine temperatures, nitrogen and oxygen from air can react to produce nitrogen oxides, NOₓ. Their formation does not require nitrogen to be part of the hydrocarbon fuel.Their effects differ: toxic gases, particles, acid rain and greenhouse warming.
Nitrogen oxides contribute to acidic precipitation and air pollution. Catalytic converters reduce several harmful exhaust gases, including converting nitrogen oxides towards nitrogen under suitable conditions.
Removing sulfur, treating emissions and reducing combustion can help, but different pollutants need different measures. CO₂ climate effects differ from SO₂ acid-rain effects.
Cracking and useful products
Cracking breaks larger saturated hydrocarbon molecules into smaller molecules, including shorter alkanes and unsaturated alkenes, using suitable heating and sometimes catalysts.
Cracking is useful because demand for short-chain fuels and alkene feedstocks can exceed the supply from simple distillation; longer fractions can be converted into more useful products.
Alkenes are unsaturated hydrocarbons with a carbon–carbon double bond. Alkenes without rings and with one C=C bond have general formula CₙH₂ₙ.
A balanced cracking example is C₁₀H₂₂ → C₈H₁₈ + C₂H₄. Check totals: ten carbons and twenty-two hydrogens on each side.Cracking is chemical change; fractional distillation is physical separation.
Ethene is a useful feedstock for poly(ethene): monomers join into long-chain molecules. Distillation only separates existing molecules; cracking changes them chemically.
Specific demand and profitability vary by market and time. Do not assume petrol is always more profitable than every other fraction.
Hydrogen versus petrol for cars
Petrol is energy-dense, easy to transport as a liquid and supported by widespread refuelling infrastructure. Its combustion emits CO₂ and can produce other pollutants.
Hydrogen reacts with oxygen to form water: 2H₂ + O₂ → 2H₂O. The hydrogen fuel itself contains no carbon, so this reaction produces no fuel-derived CO₂.
Hydrogen is difficult to store and transport compactly and needs suitable tanks or other storage methods and refuelling infrastructure. It is flammable and needs appropriate engineering controls.
Hydrogen production can use fossil feedstocks or electricity, so overall emissions depend on the production method and energy source. Tailpipe emissions alone do not describe the full life cycle.
Burning hydrogen in air at high temperature can form nitrogen oxides; a hydrogen fuel cell operates differently and produces electricity without ordinary combustion.No fuel-derived CO₂ at use does not establish zero whole-life emissions.
Evaluate fuel choices using storage, energy supply, costs, safety and whole-life environmental impacts. Avoid saying hydrogen is automatically emission-free in every sense.
Revise fuels with this narrated video. Use the player controls to pause, seek, adjust the volume or mute. Turn English captions on or off using the captions menu.
Mr Kidman's Classroom