Organic Chemistry and Fuels
This domain tests hydrocarbons, their homologous series and the fuels obtained from crude oil. You must distinguish alkanes from alkenes and describe combustion, cracking, fractional distillation and polymerisation.
Alkanes and alkenes
- Alkanes have the general formula CnH2n+2 and contain only single bonds. They are saturated and generally unreactive; they burn and undergo substitution with halogens in ultraviolet light.
- Alkenes have the general formula CnH2n and contain a carbon–carbon double bond. They are unsaturated and more reactive. They decolourise orange bromine water, which is the standard test that distinguishes an alkene from an alkane.
Crude oil and fuels
Crude oil is a mixture of hydrocarbons separated by fractional distillation. Fractions with lower boiling points are collected near the top of the column and have smaller molecules; larger molecules condense lower down. Cracking breaks large molecules into smaller, more useful alkanes and alkenes. Complete combustion gives carbon dioxide and water; incomplete combustion gives carbon monoxide and soot, both harmful.
Polymers
Addition polymerisation joins many alkene monomers through the double bond, producing a polymer with the same empirical formula as the monomer. The repeat unit has two carbon atoms in the backbone.
Worked example
A colourless liquid is shaken with bromine water. The orange colour disappears. What does this show, and what would you observe with hexane? The colour disappears because the substance is unsaturated and contains a C=C double bond, which adds bromine. With hexane, an alkane, the orange colour remains because no addition reaction occurs under these conditions.
Traps that cost marks
- Saying alkanes decolourise bromine water; they do not in the dark.
- Calling crude oil a compound; it is a mixture.
- Confusing the repeating unit with the monomer.
- Saying incomplete combustion produces carbon dioxide; it produces carbon monoxide and soot.