3.3.2 Alkanes
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what is the general formula of an alkane?
CnH2n+2
what is the general formula of cycloalkanes?
CnH2n
What is an alkane?
saturated hydrocarbon containing C-H bonds only
Are alkanes’ bonds polar? why/why not?
non-polar - carbon and hydrogen have similar electronegativities
Which intermolecular forces do alkanes have? Why?
Only Van der Waals forces of attraction - bonds are non-polar
Are alkanes soluble in water? Why?
No, hydrogen bonds in water are stronger than alkanes’ Van der Waals forces
How reactive are alkanes?
Very unreactive
Which reactions will alkanes undergo?
Combustion Reaction with halogens
What is crude oil? How is it formed? Is it renewable? Why?
Mixture of fractions (hydrocarbons with similar boiling points and properties) Formed at high temperatures and pressures deep below earth’s surface over millions of years Non renewable
What is fractional distillation? How does it work?
Crude oil heated until it is vaporised Passed into a fractioning tower that is cooler at the top than the bottom Vapours rise up the column and - via trays and bubble caps - condense when temperature is lower than their boiling point Shorter chain alkanes condense at the top - they have the lowest boiling points
Why are alkanes cracked?
to turn a long chain alkane into shorter ones and an alkene. Shorter chains are more useful
What are the conditions for Thermal cracking?
700-1200K temp Up to 7000kPa pressure
What is the intermediate for the thermal cracking reaction?
Free radicals
What are the main products of thermal cracking?
Alkenes
What are the conditions for catalytic cracking?
Lower temperature Lower pressure Zeolite catalyst with a honeycomb structured give a large SA
What are the main products of catalytic cracking?
Cycloalkanes, aromatic hydrocarbons, branched alkanes
Write an equation for the combustion of propane.
C3H8 + 5O2 —> 3CO2 + 4H2O
What is a fuel?
Something which releases heat energy when combusted
What are the five main fuels comprising of alkanes?
Methane, butane, propane, petrol, paraffin
What is incomplete combustion and what products are formed in the case of alkanes?
Combustion in a limited supply of oxygen CO - carbon monoxide - poisonous C - carbon particulates - soot - global dimming
Which type of hydrocarbons are most likely to undergo incomplete combustion?
Longer chains
What is the environmental impact of carbon monoxide?
Toxic / poisonous
What is the environmental impact of nitrogen oxides?
Form nitric acid —> acid rain, smog
What is the environmental impact of sulphur impurities/sulphur dioxide?
Form sulphuric acid —> acid rain