Crude Oil
Crude oil is a mixture of hydrocarbons which is formed from the remains of dead animal and plants matter which fossilised millions of years ago. It is considered to be a finite, non-renewable resource as we will eventually run out of crude oil supplies.
Fractional distillation
Fractional distillation is used to separate crude oil into fractions, containing a group of hydrocarbons of similar lengths. The crude oil is vaporised and passed into a fractionating column which is hot at the bottom and cooler at the top. Vapours from the oil rise through the column and condense when the temperature of their boiling point is reached. Longer hydrocarbons have higher boiling points than shorter hydrocarbons because they form stronger intermolecular forces between hydrocarbon molecules.
Use the mneumonic: Robot Pirate Killers Don’t Have Belly Buttons to help you remember the order of the fractions.
Properties of short-chain vs long-chain hydrocarbons
Longer hydrocarbons:
Have higher boiling points (condense towards the bottom of the column)
Are less volatile (do not evaporate easily)
Are less flammable (do not ignite easily)
Are more viscous (do not flow easily and have a thick, treacly appearance)
Have a darker colour
Shorter hydrocarbons:
Have lower boiling points (condense at the top of the column)
Are volatile (evaporate easily)
Are flammable (ignite easily)
Are less viscous (they flow easily)
Are either colourless or pale in colour
These properties make short-chain hydrocarbons really useful for car fuels, which means there is a much bigger demand for the shorter hydrocarbons compared to the less flammable long chain hydrocarbons. Since crude oil consists of a large proportion of the longer hydrocarbons, we use a process called cracking to snip longer hydrocarbons into shorter, more useful ones.
Cracking
Cracking converts long alkanes into shorter alkanes and alkenes. Temperatures of 600-700oC and a zeolite catalyst are used. Zeolite is a crystal made up of silicon, aluminium and oxygen.
Shorter saturated hydrocarbons (alkanes) are mostly used for fuels. Alkenes can be used in the production of plastics in polymerisation reactions.
Addition polymerisation
Addition polymerisation involves the breaking of double carbon bonds within alkene molecules and stitching lots alkene molecules together to form a long polymer. In the polymerisation of ethene for example, the double bond between the two carbon atoms breaks and many ethene molecules bond together to form a long chain of ethene molecules known as poly(ethene) or ‘polythene’. Polythene is used to make thin plastics and can be found in plastic bags and shampoo bottles.
Rather than drawing out hundreds of ethene molecules connected together in one long chain, it’s far easier to draw a single monomer with brackets around it. We write a ‘n’ in the bottom right corner to show that there are lots of the same molecule joined together in a chain.