Alkanes (Chapter 12) Flashcards
What is petroleum fraction?
Mixture of hydrocarbons with a similar chain length and boiling point range.
What forces are affected by fractional distillation?
Weak van der Waals forces are split.
Describe the process of fractional distillation in an oil refinery
1) Oil is pre-heated in a furnace and passed into the column.
2) The temperature of the column decreases upwards.
3) The fractions are condensed at different heights depending on their boiling points.
4) The boiling point depends on the size of the molecules as the larger the molecules then the larger the van der Waals forces.
5) Similar molecules condense together with the smaller molecules condensing at the top and the longer molecules condensing at the bottom.
Describe a vacuum distillation unit
Heavy residues from the fractioning column are distilled again under a vacuum.
Lowering the pressure over a liquid will lower its boiling point.
Why is vacuum distillation useful?
Allows heavier fractions to be further separated without high temperatures which could break them.
Describe fractional distillation in a lab
1) Heat the flask with a Bunsen burner or electric mantle.
2) This causes vapours of all the components in the mixture to be produced and pass up the fractioning column.
3) The vapour of the substance with the lower boiling point reaches the top if the fractioning column first.
4) The thermometer should be at or below the boiling point of the most volatile substance.
5) The vapours with the highest boiling points condense back into the flask.
6) Only the most volatile vapour passes into the condenser which cools the vapours and condenses to a liquid and is collected.
What is fractional distillation used for?
To separate liquids with different boiling points.
What is cracking?
Conversion of large hydrocarbons to smaller hydrocarbon molecules by breakage of c-c bonds.
What is the equation for cracking?
High Mr alkane»_space;> smaller Mr alkanes + alkene + (hydrogen)
Why does cracking require a high temperature?
Chemical process which involves the splitting of strong covalent bonds.
What are economic reasons for cracking?
- The petroleum fractions with shorter c chains are in more demand than larger fractions.
- To make use of excess larger hydrocarbons and to supply demand for shorter ones, longer ones 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 etc.)
What are the two types of cracking?
Thermal and catalytic
What are the conditions for thermal cracking?
High pressure (7000kpa) and high temperature (400°C to 900°C).
What are the conditions for catalytic cracking?
Slight or moderate pressure, high temperature (450°C) and zeolite catalyst.
What are the products of thermal cracking?
Mostly alkanes and sometimes produces hydrogen.
What are the products of catalytic cracking?
Branched and cyclic alkanes and aromatic hydrocarbons.
Why is catalytic cracking advantageous to thermal cracking?
- Branched and cyclic hydrocarbons burns more quickly and are used to give fuels with a higher octane number.
- Cheaper than thermal cracking because it saves energy as lower temperature and pressure are used.
What is a fuel?
Substance which releases heat energy when burnt.
What are the products of complete combustion?
Carbon dioxide and water
