Alkanes Flashcards
What is an alkane?
A saturated hydrocarbons containing C-H bonds only
What is the general formula of an alkane?
Cn H2n+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.