Revision notes · Organic chemistry
Carbon compounds as fuels and feedstock
Crude oil, hydrocarbons and alkanes4.7.1.1
Definition: Crude oil is a finite resource found in rocks that consists mainly of hydrocarbons, which are molecules made up of only hydrogen and carbon atoms.
Crude oil is a complex mixture of many different compounds, mostly saturated hydrocarbons called alkanes.
- •Alkanes are saturated hydrocarbons containing only single carbon-carbon bonds.
- •The general formula for the homologous series of alkanes is CₙH₂ₙ₊₂.
- •Crude oil is formed over millions of years from the remains of ancient biomass, mainly plankton, buried in mud.
| Name | Formula |
|---|---|
| Methane | CH₄ |
| Ethane | C₂H₆ |
| Propane | C₃H₈ |
| Butane | C₄H₁₀ |
⚠️ Common mistake: Thinking that hydrocarbons contain elements other than hydrogen and carbon, such as oxygen.
🧠 Remember: Monkeys Eat Peanut Butter (Methane, Ethane, Propane, Butane).
Fractional distillation and petrochemicals4.7.1.2
Definition: Fractional distillation is the process used to separate crude oil into groups of hydrocarbons with similar numbers of carbon atoms, called fractions.
Crude oil is separated into useful fractions based on the different boiling points of its hydrocarbons.
- •The fractionating column is hotter at the bottom and cooler at the top.
- •Vapours rise up the column, cool, and condense at their different boiling points.
- •Hydrocarbons with long chains have high boiling points and condense near the bottom.
- •Hydrocarbons with short chains have low boiling points and condense near the top or leave as gases.
- •Fractions are used as fuels and as feedstock for the petrochemical industry to make polymers, solvents, lubricants, and detergents.
⚠️ Common mistake: Stating that fractional distillation breaks chemical bonds, when it only breaks weak intermolecular forces.
🧠 Remember: Hot at the bottom, cool at the top—long chains condense low, short chains condense high.
Properties of hydrocarbons4.7.1.3
Definition: The properties of hydrocarbons depend directly on the length of their carbon chain.
As the carbon chain length of a hydrocarbon changes, its physical properties and flammability change in a predictable way.
| As chain length increases... | Effect |
|---|---|
| Viscosity | Increases — thicker, flows less easily |
| Boiling point | Increases — stronger intermolecular forces |
| Flammability | Decreases — harder to ignite |
- •The complete combustion of any hydrocarbon in sufficient oxygen produces carbon dioxide and water, releasing energy.
⚠️ Common mistake: Writing that complete combustion produces carbon monoxide or soot instead of carbon dioxide and water.
🧠 Remember: Longer chain = thicker (more viscous), higher boiling point, less flammable.
Cracking and alkenes4.7.1.4
Definition: Cracking is a thermal decomposition reaction that breaks down long-chain hydrocarbons into shorter, more useful alkanes and alkenes.
Cracking matches the supply of short-chain hydrocarbons with their high demand as fuels and feedstocks.
- •There are two main methods: catalytic cracking (high temperature and a catalyst) and steam cracking (high temperature and steam).
- •Cracking produces at least one alkene, an unsaturated hydrocarbon containing a C=C double bond.
- •Alkenes are much more reactive than alkanes and are used to make polymers and other chemicals.
- •Bromine water turns from orange to colourless when mixed with an alkene, but stays orange with an alkane.
⚠️ Common mistake: Describing the bromine water test result as becoming 'clear' instead of colourless.
🧠 Remember: AlkEnes decolourise bromine water; alkAnes leave it As it was (orange).
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