3.3.2 Alkanes Flashcards
(45 cards)
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?
Non-polar - carbon and hydrogen have similar electronegativities.
Which intermolecular forces do they have? Why?
Only van der Waals forces of attraction as bonds are non-polar
Are they soluble 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 and is 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 and 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 which leads to global warming.
Why are alkanes cracked?
To turn a long chain alkane into a shorter chain alkane (which are more economically valuable as can be used as 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 temperature (720 K)
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
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 (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
