3.2.1-2 fractional distillation and cracking

a saturated hydrocarbon

contains only single bonds between C atoms

• alkanes

an unsaturated hydrocarbon

has at least one double or triple bond between C atoms

• alkenes (double bond)

• alkynes (triple bond)

explain the difference between the boiling points of branched chain isomers and straight chain isomers

• branched = cannot pack as closely together = less surface contact = weaker van der Waals between molecules = lower boiling point.

• straight = can pack closely together = more surface contact = stronger van der Waals between molecules = higher boiling point.

why are branched chain isomers less polarisable molecules?

because they have less surface contact/less points of contact.

• polarisability: the tendency of molecules to generate induced electric dipole moments when subjected to an electric field.

fractional distillation

separating a mixture of substances into its fractions according to their different boiling points

raw materials that can be separated using fractional distillation

• crude oil - a mixture consisting mainly of hydrocarbons.

  • it is a finite resource found in rocks.

  • it is made from the remains of ancient biomass, consisting mainly of plankton, that was buried in mud and compressed by sediment.

• petroleum - a mixture consisting mainly of alkanes.

  • it includes crude oil and other substances.

  • it is found in rocks and resevoirs.

what is meant by the term ‘fraction’ ?

a mixture of compounds/substances with similar boiling points

essential features of fractional distillation

• negative temperature gradient in the fractionating column: hotter at the bottom, cooler at the top.

• fractions have different boiling points/separarion depends on bp.

• boiling point depends on chain length / size / Mr.

• heavier molecules/longer chain molecules have higher boiling points so cool at the bottom of the column.

process of fractional distillation of crude oil

1. boil/vaporise crude oil and pass into fractionating column.

2. vapours rise, cool and condense: the fractions have different boiling ranges and condense at different levels.

3. hotter at the bottom; heavier molecules/longer chain molecules have higher boiling points so cool at the bottom of the column.

4. products are siphoned off for different uses.

5. compounds collected can also be broken down further by cracking.

cracking

• large molecules are broken into smaller molecules.

• it is thermal decomposition, as heat is used to break C-C bonds (in alkanes).

• high temperatures used.

• a mixture of alkanes and alkenes produced.

the main economic reason for cracking alkanes

• to break down long chain alkanes into smaller, more useful molecules

• thus, producing substances which are more in demand and have a high value

properties of both catalytic cracking and thermal cracking

• both use high temperatures/both are thermal decomposition.

• both break long chains into shorter chains.

• both produce alkanes and alkenes.

• both do not require a mechanism.

conditions and products of catalytic cracking

• high temperature (and slight pressure).

• uses zeolite cataylst to compensate for less harsh conditions.

• produces a higher % of branched alkanes and hydrocarbons between 5-10 C atoms.

• produces aromatic hydrocarbons (has a carbon ring/a benzene ring structure) and motor fuels.

conditions and products of thermal cracking

• higher temperature.

• high pressure.

• higher % of alkenes.

  • alkenes are chemical feedstock used to make polymers, alcohols and detergents.

chemical feedstock

raw materials, usually organic substances, that are used as fuel or as starting chemicals in manufacturing other chemicals