Alkanes and Alkenes

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Sprinkle Butterfly

Cards (23)

Hydrocarbon molecules vary in size, affecting their properties and how they can be used as fuels
The larger the hydrocarbon:
the more viscous it is (flows less easily)
the higher its boiling point
the less volatile it is and the less easily it ignites
Each fraction contains hydrocarbon molecules with a similar number of carbon atoms and similar boiling points
Most hydrocarbons obtained from fractional distillation are alkanes
During fractional distillation, the crude oil is heated until it evaporates
The vapour moves up the fractionating column, with the top being much colder than the bottom
Shorter hydrocarbon molecules reach the top of the column before condensing and are collected
Longer hydrocarbon molecules condense at higher temperatures and are collected lower down the column
Hydrogen can be added to alkenes to produce alkanes, using a nickel catalyst
Example: propene + hydrogen -> propane
This is an addition reaction
Ethanol can be produced by reacting ethene with steam in the presence of phosphoric acid catalyst
Example: ethene + steam -> ethanol
Bromine Water test
Alkenes react with bromine water, turning it from orange to colourless
This reaction can be used to differentiate between alkanes and alkenes
Example: ethene (colourless) + bromine water (orange brown) -> colourless solution
Example: ethane (colourless) + bromine water (orange brown) -> remains orange brown solution
Cracking is the process of breaking longer-chain hydrocarbons into shorter, more useful hydrocarbons
During catalytic cracking:
Hydrocarbons are heated until they vaporise
The vapour is passed over a hot catalyst
A thermal decomposition reaction takes place
The products include alkanes and alkenes
During steam cracking, hydrocarbons are mixed with steam and heated to a high temperature
Products of cracking are useful as fuels
There is a high demand for fuels with small chains of carbon atoms because they are easy to ignite and have low boiling points