3.3.2 Alkanes Flashcards
process of refining crude oil?
Fractional Distillation
what does petroleum consist of?
a mixture consisting mainly of alkane hydrocarbons
what’s a petroleum fraction?
mixture of hydrocarbons with similar chain length and boiling point range
Key Points of Fractional Distillation
-oil is preheated
-passed into a column
-the fractions condense at different heights
-temperature of column decreases upwards
-separated by boiling points
-the larger the molecule = larger the van der vaals forces
-similar molecules condense together
-small molecules condense at top (lower temperatures)
-big molecules condense at the bottom (higher temperatures)
what does the process of Fractional Distillation involve?
splitting of weak van der waals forces between molecules
Long Hydrocarbon Chain properties?
-high boiling points
-high viscosity
-low flammability
-burn w smokey flame
Short Hydrocarbon Chains properties?
-low boiling point
-low viscosity
-high flammability
-burns w clean flame
what does fractional distillation allow?
heavier fractions to be further separated without high temperatures which could break them down
cracking definition?
conversion of large hydrocarbons to smaller hydrocarbon molecules by breaking of C—C bonds
high Mr alkanes —> smaller Mr alkanes + alkenes + hydrogen
economic reasons for cracking?
-the petroleum fractions with shorter C chains are in more demand than larger fractions
-to make use of excess larger hydrocarbons and to supply demand for shorter ones, longer hydrocarbons are cracked
-the products of cracking are more valuable than the starting materials
two types of cracking?
-Thermal
-Catalytic
thermal cracking conditions?
-high pressure (7000kPa)
-high temperature (400°C to 900°C)
-produces mostly alkenes, sometimes hydrogen
catalytic cracking conditions?
-slight or moderate pressure
-high temperature (450°C)
-zeolite catalyst
-produces branched and cyclic alkanes and aromatic hydrocarbons
-used for making motor fuels
fuel
releases heat energy when burnt
complete combustion
in excess oxygen alkanes will burn with complete combustion
products = CO₂ & H₂O
incomplete combustion
limited amount of oxygen, incomplete combustion occurs
produces = CO and/or C (producing a sooty flame)
which combustion produces less energy per mole?
incomplete combustion
what do alkanes do in the presence of oxygen?
combustion, highly exothermic, used as fuels
what does carbon (soot) cause?
global dimming
Pollutants from combustion: SO₂
-SO₂ produced when burned, dissolves in atmospheric water produces acid rain
-can be removed from the waste gases from furnaces by flue gas desulfurisation.
-the gases pass through a scrubber containing basic calcium oxide which reacts with the acidic sulfur dioxide in a neutralisation reaction
Pollutants from combustion: NOx
nitrogen oxides, produced in car engines (N₂ and O₂)
-high temp and spark in engines provides sufficient energy to break strong N₂ bond
-it’s toxic and can form acidic gas NO₂, which forms acid rain
Pollutants from combustion: carbon monoxide
produced in incomplete, toxic
Pollutants from combustion: CO₂
contributes towards global warming
