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ORGANIC CHEMISTRY: ALKANES (FRACTIONAL DISTILLATION (crude oil can be…
ORGANIC CHEMISTRY: ALKANES
CRUDE OIL AND HYDROCARBONS
crude oil
formed over millions of years from the fossilised remains of plankton
found in porous rocks in the Earth's crust
a finite (non-renewable) resource that is used to produce fuels and other chemicals
most of the compounds in crude oil are hydrocarbons (molecules made of only carbon and hydrogen atoms)
hydrocarbon molecules vary in size, which affects their properties and how they can be used as fuels
the larger the hydrocarbon:
the more viscous
the higher its boiling point
the less volatile
the less easily it ignites
FRACTIONAL DISTILLATION
crude oil can be separated into different fractions (parts) by fractional distillation
each fraction contains hydrocarbon molecules with a similar number of carbon atoms
most of the hydrocarbons obtained are alkanes
the crude oil is heated until it evaporates
the vapour moves up the fractionating column
the top of the column is much colder than the bottom
shorter hydrocarbon molecules can reach the top of the fractionating column before they condense and are collected
longer hydrocarbon molecules condense at higher temperatures and are collected lower down the column
ALKANES
carbon atoms are linked to four other atoms by single bonds
alkanes only contain single bond (saturated hydrocarbons)
alkanes are fairly unreactive, but they burn well
general formula: CnH2n+2
alkanes can be drawn with a single line between atoms which represents a single covalent bond
alkanes
methane, CH4
ethane, C2H6
propane, C3H8
butane, C4H10
shorter-chain alkanes release energy more quickly by burning, so there is greater demand for them as fuels
BURNING FUELS
most fuels are compounds of carbon and hydrogen, many also contain sulfur
during the combustion of hydrocarbon fuels:
both carbon and hydrogen are oxidised
energy is released
waste products are produced and released into the atmosphere
if combustion is not complete, then carbon monoxide, unburnt fuels and solid particles containing soot (carbon) may be released
carbon monoxide is a colourless, odourless and toxic gas
solid particles in the air, called particulates, can cause global dimming by reducing the amount of sunlight reaching the Earth's surface and cause damage to people's lungs
due to the high temperature reached when fuels burn, nitrogen in the air can react with oxygen to form nitrogen oxides
these gases can cause respiratory problems in people and react with rain water (in the same way as sulfur dioxide) to form acid rain, which can damage plants and buildings
sulfur can be removed from fuels before burning (in motor vehicles) and removed from the waste gases after combustion (in power stations)
KEY WORDS