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

• straight = molecules 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 ability of a cation to distort the electron cloud of an anion

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 so separation depends on bpt

• boiling point depends on chain length / size / M_r

• 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 so condense at different levels.

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

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.

  • e.g. broken down to produce a mixture of alkanes and alkenes.

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

  • high temperatures used.

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.

conditions and products of catalytic cracking

• high temperature (and slight pressure).

• zeolite cataylst to compensate for less harsh conditions.

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

• produces aromatic hydrocarbons and motor fuels.

conditions and products of thermal cracking

• higher temperature.

• high pressure.

• higher % of alkenes.

  • alkenes are chemical feedstock.

chemical feedstock

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

uses of alkanes

• motor fuel

• jet fuel

• diesel

• lubricant

• petrochemicals

• kerosene

• fuel oil

• central heating fuel

• paraffin

uses of alkenes

chemical feedstock used in:

• making polymers

• making detergents

• making alcohols