3.2 Alkanes
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What is an alkane ?
A saturated hydrocarbons containing C-H bonds only
What is the general formula of an alkane
CnH2n+2
Are their bonds polar? Why / why not?
Nonpolar- carbon and hydrogen have similar electronegativities
Which intermolecular forces do they have?
Only Van de Waals forces of attraction - bonds are non polar
Are they soluble in water? Why?
Insoluble because hydrogen bonds in water are stronger than Van de Waals forces of attraction
How reactive are alkanes?
Very unreactive
Which reactions will alkanes undergo?
Combustion and reactions 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 the earths surface over millions of years —> therefore non-renewable
Name the fractions from high to low boiling points
Gases - fuels on sight Gasoline/ petroleum/ naphtha- cars Kerosene/ paraffin - jet fuel / lighting Diesel fuel - lorries / taxis Lubricating oils/ waxes - candles/ engine oil Fuel oil- ships/ power stations Tar / bitumen- roads / roofing
What is fractional distillation and how does it work?
Crude oil heated til mostly vaporised Passed into a fractioning tower that is cooler at the top than at the bottom Liquid fractions are piped off at the bottom Vapours rise up the column and - via trays and bubble caps - condense when temperature < their boiling point Shortest chain hydrocarbons condense at the top as they have the lowest boiling points
What is fracking and how is it done?
Natural gas held within a shale rock Drill into shale, force pressurised water and sand in the rock to fracture it, collect gas HCl and methanol added to break up shale and prevent corrosion
Pros/cons of fracking
Advantages - gas supply for many years, reduces imported gas and electricity Disadvantages - lots of traffic to local area, concern about amount of water used, chemical additives can pollute water supplies, can cause small earthquakes, combustion of CH3 —> CO2 —> global warming
Why are alkanes cracked?
To turn a long chain alkane, which is not very economically valuable, into a shorter chain alkane chain (more economically valuable as used as fuels) and an alkene (more reactive, starting point for many products)
What are the conditions for thermal cracking?
700-1200K temperature Up to 7000kPa pressure
What is the intermediate for the reaction?
Free radicals
What are the main products of thermal cracking?
Alkenes
What are the conditions for catalytic cracking
Lower temperature (720K) Lower pressure (but above atmospheric) Zeolite catalyst (SiO2 and Al2O3) with a honey comb structure to give a large surface area
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 5 main fuels comprising of alkanes ?
Methane, Butane, propane, petrol (about C8) and paraffin (C10-C18)
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 CO
Toxic/poisonous