Unit 7: Crude Oil
Understanding the origin and processing of the world's primary source of organic compounds.
7.14 Fractional Distillation
Crude oil (petroleum) is a complex mixture of thousands of hydrocarbons, primarily alkanes, formed from the remains of ancient marine organisms over millions of years. To be useful, this mixture must be separated into its components. This is achieved by fractional distillation.
The process relies on the fact that different hydrocarbons have different boiling points: longer carbon chains have stronger intermolecular van der Waals forces and therefore higher boiling points.
The Process:
- Crude oil is heated to a high temperature (around 350-400°C), causing most of it to vaporise.
- The hot liquid and vapour mixture is pumped into the bottom of a tall fractionating column.
- The column has a temperature gradient: it is very hot at the bottom and gradually gets cooler towards the top.
- The hot vapours rise up the column. As they rise, they cool and condense at different levels (on trays) according to their boiling points.
- Fractions with high boiling points (long-chain hydrocarbons) condense on lower trays, while fractions with low boiling points (short-chain hydrocarbons) rise higher before condensing.
- The separated mixtures, called fractions, are piped off at their respective levels.
Solved Examples:
- What is crude oil a mixture of?
Solution: Mainly hydrocarbons, especially alkanes of various chain lengths. - What physical property is used to separate crude
oil?
Solution: Boiling point. - Why do long-chain alkanes have higher boiling points than short-chain
alkanes?
Solution: They have more electrons and a larger surface area, leading to stronger intermolecular van der Waals forces which require more energy to overcome. - Where in the fractionating column are the fractions with the lowest boiling
points collected?
Solution: At the top, where the column is coolest. - Name a fraction with a very high boiling
point.
Solution: Bitumen or fuel oil. - What is a "fraction" in this context?
Solution: A mixture of hydrocarbons with similar boiling points and chain lengths. - Is fractional distillation a chemical or physical
process?
Solution: A physical process. It separates the mixture without breaking any chemical bonds. - Which fraction is used as fuel for
cars?
Solution: The petrol (gasoline) fraction. - What is the state of the crude oil when it enters the fractionating
column?
Solution: A mixture of hot liquid and vapour. - What happens to the viscosity (thickness) of the fractions as you go down
the column?
Solution: The viscosity increases, as the hydrocarbon chains get longer.
7.15 Cracking
Fractional distillation often produces more long-chain hydrocarbons (like fuel oil) than are in demand, and not enough short-chain hydrocarbons (like petrol). Cracking is a process that breaks down large, less useful hydrocarbon molecules into smaller, more valuable ones.
Cracking is important for two main reasons:
- It helps match the supply of fractions with economic demand (e.g., making more petrol).
- It produces valuable alkenes, which are highly reactive and are the starting materials for making polymers (plastics), alcohols, and many other important chemicals.
The process involves passing hydrocarbon vapours over a hot catalyst (e.g., aluminium oxide, silicon dioxide) at high temperatures (around 600-700°C). This breaks the C-C bonds.
Example: Cracking of decane ($C_{10}H_{22}$)
$C_{10}H_{22} \rightarrow C_8H_{18} + C_2H_4$
(Decane → Octane + Ethene)
Note that cracking always produces at least one alkane and at least one alkene.
Solved Examples:
- What is the main purpose of cracking?
Solution: To break down large hydrocarbons into smaller, more useful ones like petrol and alkenes. - What are the typical conditions for catalytic
cracking?
Solution: High temperature (600-700°C) and a catalyst (e.g., alumina, silica). - What type of chemical bond is broken during
cracking?
Solution: Covalent C-C bonds. - Name two important products of
cracking.
Solution: Petrol (shorter-chain alkanes) and alkenes (e.g., ethene). - Why are alkenes so useful?
Solution: They are reactive due to the C=C double bond and are used as feedstock for the petrochemical industry to make polymers, alcohols, etc. - Write a possible cracking equation for dodecane ($C_{12}H_{26}$) that
produces ethene and one other product.
Solution: The other product must be an alkane. $C_{12}H_{26} \rightarrow C_{10}H_{22} + C_2H_4$. - Is cracking a physical or chemical
change?
Solution: A chemical change, as covalent bonds are broken and new molecules are formed. - A cracking reaction of $C_{15}H_{32}$ produces two molecules of propene
($C_3H_6$) and one other molecule. What is the other
molecule?
Solution: Reactants: 15 C, 32 H. Products: 2 x (3 C, 6 H) = 6 C, 12 H. Remaining: 15-6 = 9 C; 32-12 = 20 H. The other molecule is the alkane nonane, $C_9H_{20}$. - What is steam cracking?
Solution: A type of cracking that uses very high temperatures and steam, which produces a higher proportion of alkenes. - How does cracking help an oil refinery meet
demand?
Solution: It converts surplus long-chain fractions (like fuel oil) into high-demand short-chain fractions (like petrol).
7.16 Reforming
Reforming is a process that converts straight-chain alkanes into branched-chain alkanes and aromatic compounds (like benzene). It involves rearranging the atoms in the molecule without changing the number of carbon atoms.
The main purpose of reforming is to produce high-quality petrol. Branched-chain and aromatic hydrocarbons burn more efficiently in an engine and resist "knocking" (premature ignition), meaning they have a higher octane number.
Reforming is typically carried out by passing the naphtha fraction over a catalyst (e.g., platinum) at high temperature and pressure.
Examples:
- Isomerisation: Straight-chain to branched-chain.
Hexane → Methylpentane - Cyclisation/Aromatisation: Straight-chain to cyclic/aromatic.
$C_6H_{14}$ (Hexane) $\rightarrow C_6H_6$ (Benzene) + $4H_2$
Reforming is also an important source of hydrogen gas, which is used in the Haber process to make ammonia.
Solved Examples:
- What is the main goal of reforming?
Solution: To convert straight-chain hydrocarbons into branched-chain and aromatic hydrocarbons to improve the quality of petrol (increase its octane number). - What does a high octane number
signify?
Solution: A fuel with a high resistance to knocking or premature ignition in an engine. - What type of hydrocarbon burns most efficiently in a car
engine?
Solution: Branched-chain alkanes and aromatic compounds. - What is a valuable by-product of
reforming?
Solution: Hydrogen gas ($H_2$). - What is the process called when hexane is converted to
methylpentane?
Solution: Isomerisation (a type of reforming). - Does reforming change the number of carbon atoms in the
molecule?
Solution: No, it rearranges them. - What catalyst is often used in
reforming?
Solution: Platinum. - Write an equation for the reforming of heptane ($C_7H_{16}$) into
methylcyclohexane ($C_7H_{14}$).
Solution: $C_7H_{16} \rightarrow C_7H_{14} + H_2$. - Which fraction from crude oil is typically used for
reforming?
Solution: The naphtha fraction. - How is benzene produced from hexane?
Solution: Through reforming (aromatisation), which removes hydrogen atoms and forms a ring: $C_6H_{14} \rightarrow C_6H_6 + 4H_2$.