topic 6b: alkanes Flashcards
general formula alkane
CnH2n+2
what is an alkane?
saturated hydrocarbon
alkane main use
fuels
how do u obtain alkane fuels
fractional distillation , cracking, then reforming of crude oil
how does fractional distillation work
- oil heated and passed into column
- temperature of column DECREASES upwards
- fractions condense at different heights according to boiling point
- bp depends on size of molecule
- larger molecule = stronger LF so higher boiling point
- molecules of similar length condense together
- small molecules condense at the low temps at the top
- bigger molecules condense at high temps at the bottom
cracking
conversion of large hydrocarbons to smaller molecules by breakage of C-C bonds
reasons for cracking?
- shorter C chains in higher demand than longer
- more useful- eg alkenes used to make polymers. smaller alkanes for motor fuels (burn more efficiently)
reforming
processing of straight-chain hydrocarbons into
branched-chain alkanes and cyclic hydrocarbons for efficient combustion
purpose of reforming
- branched and cyclic hydrocarbons burn more cleanly
- give fuels a higher octane number
what is a fuel
releases heat energy when burnt
why are alkanes good fuels
combustion is highly exothermic
complete combustion alkane
co2+h2o
incomplete combustion alkanes
- limited amount of oxygen
- less energy per mole produced
- CO(toxic) /C (c gives soot)
carbon monoxide
- highly toxic
- odourless
- reduces the o2 carrying capacity of blood
carbon production problem
particulates in atmosphere
sulfur compounds from combustion
- sulfur impurities in fuel
- produces SO2 when burnt
- acidic
- dissolves in atmospheric water to produce acid rain
- harful to aquatic life
NOx compounds from combustion
- high temp and spark in engine provides sufficient energy to break strong N2 bond, causing N2 and O2 to react
NOx compounds problem
- NO = toxic, forms smog
- NO2 = acidic, forms acid rain
CO2 problems
-greenhouse gas
-global warming
- traps IR radiation from earthand reflects it back to earth
catalytic converters
- remove CO, NOx and unburned hcs from the exhaust gases
- turn them into CO2, N2, H2O
remaining unburned hydrocarbons problem
Toxic
fossil fuels
- from crude oil
- finite , non renewable
advantages of alcohol and biodiesel as fuel alternatives
- renewable
- carbon neutral
- less pollution
disadvtantage of biofuels and ethanol as fuel alternative
- less food crops grown
- rainforests cut down to provide land
- shortage of fertile soils
how are the alcohol fuels produced
fermentation of sugars from plants
how are the biodiesel fuels produced
reacting veg oils with alkali and methanol
