Alkanes Flashcards
what is petroleum?
Petroleum is a mixture consisting mainly
of alkane hydrocarbons
what is a petroleum fraction?
Petroleum fraction: mixture of hydrocarbons with
a similar chain length and boiling point range
what are key points of fractional distillation?
- Oil is pre-heated
- then passed into column.
- The fractions condense at different heights
- The temperature of column decreases upwards
- The separation depends on boiling point
- Boiling point depends on size of molecules.
- The larger the molecule the larger the van der waals forces
- Similar molecules (size, bp, mass) condense together
- Small molecules condense at the top at lower temperatures
- and big molecules condense at the bottom at higher temperatures
what is the use of a vacuum distillation unit?
• Heavy residues from the fractionating column are distilled
again under a vacuum.
• Lowering the pressure over a liquid will lower its boiling
point.
what is the benefit of a vacuum distillation unit?
Vacuum distillation allows heavier fractions to be further separated without high temperatures which could break them down.
what is the process of fractional distillation in a laboratory?
- Heat the flask, with a Bunsen burner or electric mantle
- This causes vapours of all the components in the mixture to be produced.
- Vapours pass up the fractionating column. • The vapour of the substance with the lower boiling point reaches the top of the fractionating column first.
- The thermometer should be at or below the boiling point of the most volatile substance.
- The vapours with higher boiling points condense back into the flask.
- Only the most volatile vapour passes into the condenser.
- The condenser cools the vapours and condenses to a liquid and is collected.
define cracking
conversion of large hydrocarbons to smaller hydrocarbon molecules by breakage of C-C bonds
what is the formula for cracking?
High Mr alkanes -> smaller Mr alkanes+ alkenes + (hydrogen)
what are some economic reasons for cracking?
The petroleum fractions with shorter C chains (e.g. petrol and
naphtha) are in more demand than larger fractions.
• To make use of excess larger hydrocarbons and to supply demand
for shorter ones, longer hydrocarbons are cracked.
• The products of cracking are more valuable than the starting
materials (e.g. ethene used to make poly(ethene), branched alkanes
for motor fuels,
what does the process of cracking involve?
This is a chemical process
involving the splitting of
strong covalent bonds so
requires high temperatures.
what are the conditions of thermal cracking?
High pressure (7000 kPa) High temperature (400°C to 900°C)
what are the conditions of catalytic cracking?
Slight or moderate pressure
High temperature (450°C)
Zeolite catalyst
what are the products of thermal cracking?
produces mostly alkenes e.g. ethene used
for making polymers and ethanol
sometimes produces hydrogen used in the
Haber Process and in margarine manufacture
what are the products of catalytic cracking?
Produces branched and cyclic
alkanes and aromatic hydrocarbons
Used for making motor fuels
why is catalytic cracking better than thermal cracking?
-Branched and cyclic hydrocarbons burn more
cleanly and are used to give fuels a higher octane
number
-Cheaper than thermal cracking because it saves
energy as lower temperatures and pressures are used
