3.2 Alkanes
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What is the general formula of an alkane?
CnH2n+2
Are their bonds polar? Why/why not?
Nonpolar- carbon and hydrogen have similar electronegativities
Intermolecular forces? Why?
Only van der Waals forces of attraction - bonds are non-polar
Solubility in water? why?
Insoluble because hydrogen bonds in water are stronger than alkanes’ van der Waals forces of attraction
How reactive are alkanes?
Very unreactive
Which reactions will alkanes undergo?
Combustion and 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 → therefore non-renewable
Name the fractions from high to low boiling point.
Gases - fuel on site Gasoline/petrol/naphtha - cars Kerosene/paraffin - jet fuel, lighting Diesel oil - lorries/taxis Lubricating oil/waxes - candles, engine oil Fuel oil - ships, power stations Tar/bitumen - roads/roofing
What is fractional distillation/how does it work?
Crude oil heated until mostly vapourised Passed into a fractionating tower that is cooler at the top than 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 shale rock Drill into shale, force pressurised water and sand into 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, combust CH4 → CO2 → global warming
Why are alkanes cracked?
To turn a long chain alkane, with is not very economically valuable, into a shorter chain alkane (more economically valuable as can be used as a fuel) and an alkene (more reactive, starting point for many products)
What are the conditions for thermal cracking?
700-1200 K temperature Up to 7000 kPa 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 temp (720K) Lower pressure (but above atmospheric) Zeolite catalyst (SiO2 and Al2O3) with a honeycomb structure to give a large surface area
What are the main products of catalytic cracking?
Cycloalkanes, aromatic hydrocarbons, branched alkanes
What is a fuel?
Something which releases heat energy when combusted
What are the five main fuels comprising of alkanes?
Methane, butane, propane, petrol (about C8), 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 carbon monoxide?
It is toxic/poisonous
What is the environmental impact of nitrogen oxides?
form nitric acid → acid rain, photochemical smog
What is the environmental impact of sulfur impurities/ sulfur dioxide?
form sulphuric acid → acid rain