Alkanes Flashcards
Boiling points of alkanes
Boiling point increases as length of chain increases-because of increasing van der Waals forces
Branches chains have lower boiling points (can’t pack as close together)-vdW forces are not so effective
Solubility of alkanes
They are insoluble in water - because water molecules are held together by hydrogen bonds which are much stronger than the van der Waals forces, however they mix with other relatively non-polar liquids
How alkanes react
Relatively unreactive
Strong C-C bond and C-H bond.
Don’t react with acids, based, oxidising agents, and reducing agents.
They do burn
Will react with halogens under suitable conditions.
What is crude oil
A mixture mostly of alkanes, both unbranched and branched- (contained small amounts of other compounds dissolved in it)
Fractional distillation
•crude oil is heated in a furnace
•a mixture of liquid and vapour passes into a tower that is cooler at the top than the bottom
•vapours rise via series of trays containing bubble caps until arriving to a tray cool enough for them and condense into liquids
•condensed mixture is piped off
Shorter chains are at the top as they have lower boiling points
Why is fractional distillation a physical process
Because no covalent bonds are broken. It’s the van der Waals forces that are broken during vaporisation and reform of condensation
What is cracking
Breaking the C-C bond in alkanes to form shorter alkane and alkane molecules
why are shorter chain products more economically valuable that the longer chain material
- shorter more useful chains are produced, especially petrol
* some of the products are alkanes, which are more reactive than alkanes
What is thermal cracking and specify temp and pressure
Heating alkanes to high temp, 700-1200k, under high pressure, up to 7000kPa
What happens in the process of thermal cracking
C-C bond breaks and one e- from the covalently bonded pair goes to each atom. As there not enough H atoms to produce two alkanes, one of the new chains must have a C=C and becomes an alkene
What does thermal cracking tend to produce
Tends to produce a high proportion of alkenes
What happens in catalytic cracking
Takes place at a lower temp (approx. 720k) and lower pressure using a zeolite catalyst -consists silicon dioxide and aluminium oxide.
What is generally produced in catalytic cracking
Mostly branched alkanes, cycloalkanes, and aromatic compounds
How are products obtained by cramping separated?
Fractional distillation
Polarity of alkanes
Almost non-polar (C and H have similar electronegativity)
Making weak van der Waals forces and stronger ones between larger molecules
