3.2 Alkanes
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What is an alkane ?
A saturated hydrocarbon containing only carbon-hydrogen bonds.
What is the general formula of an alkane?
CnH2n+2.
Do alkenes have polar bonds ? Why / why not ?
They don’t, this is because they have similar electronegativities.
What intermolecular forces do alkanes have ? Why ?
They only have van der waals, they are non polar.
Are alkanes soluble in water ?
They are insoluble because the hydrogen bonds in water are stronger than alkanes’ van der waals forces.
How reactive are alkanes ?
Very unreactive.
Which reactions will alkanes undergo ?
Combustion and reactions with halogens.
What is crude oil? Is it renewable?
• It is a mixture of fractions (hydrocarbons with different boiling points) • They are formed from biological matter at high temperatures of pressure over millions of years - unrenewable.
What is fractional distillation and how does it work with crude oil ?
• Crude oil is heated until mostly vaporised • It is then passed into a fractionating column (cooler at top) • Vapours rise up the column and condense when temperature < their bp • Shorter chains will condense at the top as their bp is lowest.
Why are alkanes cracked ?
Long chain alkenes can be cracked into a shorter chain alkane and shorter chain alkene, which are both more valuable.
What are the conditions for thermal cracking ?
700-1200K Up to 7000kPa.
What is the intermediate for the thermal cracking ?
Free radicals.
What are the main products of thermal cracking ?
Shorter chain alkenes.
What are the conditions for catalytic cracking?
Lower temp (720K) Lower pressure but higher than atmospheric Zeolite catalyst with a honeycomb structure for high SA.
What are the main products of catalytic cracking ?
Cycloalkanes, aromatic hydrocarbons and branched alkanes.
Write an equation for the complete combustion of propane
C3H8 + 5O2 -> 3CO2 + 4H2O
What is a fuel ?
Something that releases heat energy when combusted.
What are the 5 main alkane fuels ?
Methane, butane, propane, diesel (~C8), paraffin (C10-18).
What are the products for the incomplete combustion of alkanes ? What are their harmful effects ?
CO - poisonous C - soot, carbon dimming.
What type of hydrocarbons are more likely to undergo incomplete combustion ?
Longer chain alkanes.
What is the environmental impacts of CO being released ?
It is toxic / poisonous.
What is the environmental impact of nitrous oxides being released ?
They form nitric acid -> acid rain.
What is the environmental impact of sulfur impurities being released ?
Form sulphuric acid -> acid rain.
What is the environmental impact of C being released into the atmosphere ?
Asthma, cancer, global dimming.