Hydrocarbons and alkanesIB MYP Chemistry: Revision notes

Crude oil is a thick liquid formed over millions of years from the remains of tiny sea organisms. It is a mixture of many different hydrocarbons. Because it is a mixture, the hydrocarbons are not chemically bonded to one another and can be separated by physical methods.

A hydrocarbon is a compound made of hydrogen and carbon atoms only. Most hydrocarbons in crude oil are alkanes. Crude oil is a finite, non-renewable resource.

Crude oil is separated into useful fractions by fractional distillation:

1. The oil is heated until it vaporises.
2. The vapours enter a fractionating column that is hot at the bottom and cooler at the top.
3. Each hydrocarbon rises until it reaches a height where the temperature is below its boiling point, then it condenses and is drawn off.

Hydrocarbons with similar boiling points end up in the same fraction.

• Top: short chains, low boiling points, very runny, easy to ignite (refinery gases, petrol).
• Bottom: long chains, high boiling points, thick and sticky, harder to ignite (fuel oil, bitumen).

The order from top to bottom is: refinery gases, petrol, kerosene, diesel, fuel oil, bitumen.

Alkanes are hydrocarbons in which all the bonds between carbon atoms are single bonds. They are saturated: they hold the maximum possible number of hydrogen atoms, so they cannot add any more atoms.

The general formula is CₙH₂ₙ₊₂, where n is the number of carbon atoms. Worked example: for 8 carbon atoms, n = 8, so hydrogen atoms = 2 × 8 + 2 = 18, giving C₈H₁₈.

Alkanes are a homologous series: each member differs from the next by CH₂, and they have similar chemical properties. As the chain gets longer, the boiling point and viscosity increase and flammability decreases.

The first ten alkanes are:

• 1 carbon: methane CH₄
• 2: ethane C₂H₆
• 3: propane C₃H₈
• 4: butane C₄H₁₀
• 5: pentane C₅H₁₂
• 6: hexane C₆H₁₄
• 7: heptane C₇H₁₆
• 8: octane C₈H₁₈
• 9: nonane C₉H₂₀
• 10: decane C₁₀H₂₂

A structural formula shows the atoms in each group, for example CH₃CH₂CH₃ for propane. A displayed formula shows every atom and every bond as lines: for propane, a chain of three carbon atoms, each joined to the next by a single line, with enough single lines to hydrogen atoms so that every carbon atom has four bonds.

Alkanes burn in oxygen and release energy, so they are useful fuels.

Complete combustion (plenty of oxygen) makes carbon dioxide and water:

alkane + oxygen → carbon dioxide + water

Example: CH₄ + 2O₂ → CO₂ + 2H₂O

Balancing propane, step by step: C₃H₈ + O₂ → CO₂ + H₂O. Three carbon atoms give 3CO₂. Eight hydrogen atoms give 4H₂O. Oxygen atoms on the right are 6 + 4 = 10, so 5O₂ are needed: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.

Incomplete combustion (limited oxygen) makes carbon monoxide (a toxic gas) and/or carbon (soot), plus water. It gives a yellow, sooty flame and releases less energy.

Fractions are used as fuels: refinery gases (heating and cooking, for example propane and butane), petrol (cars), kerosene (aircraft), diesel (lorries, trains) and fuel oil (ships and power stations). Bitumen is used to surface roads.

Using fuels from crude oil has advantages (high energy release, easy to store and transport, existing infrastructure) and disadvantages: non-renewable, burning releases carbon dioxide (climate change), incomplete combustion makes carbon monoxide and soot, and sulfur impurities make sulfur dioxide, which causes acid rain.