3.3.2 Alkanes Flashcards
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
What is the environmental impact of soot (carbon)?
Asthma, cancer, global dimming
What is the environmental impact of unburnt hydrocarbons?
Smog
What is the environmental impact of carbon dioxide?
Greenhouse gas —> global warming
What is the environmental impact of water vapour?
Greenhouse gas —> global warming
What are flue gases?
Gases given out by power stations
What can we use to desulphurise flue gases?
CaO
CaCO3
What are catalytic converters made up of?
Ceramic honeycomb coated with platinum, palladium and rhodium metals
What are greenhouse gases?
Gases which trap infrared radiation, making earth like a greenhouse
What is the greenhouse effect? How does it contribute to global warming?
Greenhouse gases trap infrared radiation in the atmosphere, atmosphere heats up
Define carbon neutral activities?
Produce no net / overall carbon dioxide emissions
How are halogenoalkanes formed from alkanes?
Free radical substitution reactions
What are the three stages of free radical substitution?
Initiation - breaking halogen bond to form free radicals
Propagation - chain part of the reaction where products are formed but free radical remains
Termination - free radicals removed, stable products formed
What are the conditions needed for the formation of a free radical chlorine atom?
Presence of UV light
What is the ozone layer’s function?
Protects the earth from harmful exposure to too many UV rays
How do CFCs break the ozone layer down?
Free radical substitution
