Crude Oil and Separation Into Useful Products
In 1859, Edwin Drake drilled the first oil well in Pennsylvania and struck crude at 21 metres deep, within 10 years, fractional distillation towers were separating it into kerosene, lubricating oil, and paraffin wax.
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Q1 · If crude oil is a single black liquid pumped from the ground, how do you think a refinery could possibly turn it into very different products like petrol, diesel, jet fuel, and candle wax?
Q2 · Why do you think different hydrocarbon fractions separated from crude oil boil at different temperatures?
● Know
- What crude oil is and how it formed
- The principle of fractional distillation
- The main fractions and their uses (gas, petrol, kerosene, diesel, lubricants, bitumen)
● Understand
- Why shorter-chain hydrocarbons collect at the top of the distillation column
- How fractional distillation exploits differences in boiling point
- Why crude oil must be separated before it is useful
● Can do
- Describe the process of fractional distillation
- Explain which fractions collect at which levels and why
- Rank fractions by boiling point and chain length
Strike a match near a natural gas stove: the flame appears instantly and burns with a clean blue colour, because methane, a single carbon bonded to four hydrogens, ignites easily and combusts completely. Alkanes are the simplest family of organic compounds, they contain only carbon and hydrogen, with all single bonds between carbon atoms. The general formula is $C_nH_{2n+2}$, where n is the number of carbon atoms. A molecule with all single bonds is called saturatedthe carbons cannot accommodate any additional hydrogen atoms. Each carbon in an alkane is tetrahedral, with bond angles of approximately 109.5°. The chain can be straight (unbranched) or branched, both are alkanes.
The simplest alkane is methane (CH₄, n=1): 1 carbon, 4 hydrogens. Then ethane (C₂H₆), propane (C₃H₈), butane (C₄H₁₀), pentane (C₅H₁₂). The formula $C_nH_{2n+2}$ predicts them all. Each successive member adds one CH₂ unit to the chain, this regular pattern defines a homologous series: a family of compounds with the same general formula, differing by CH₂, and showing a gradual trend in physical properties as chain length increases.
Checking the formula for pentane (n=5): $C_nH_{2n+2}$ = C₅H₁₂. Count: 5 carbons, 12 hydrogens. Pentane is the main component of petroleum naphtha, used as a blowing agent in expanded polystyrene foam cups. At room temperature and pressure it is a volatile colourless liquid (boiling point 36 °C).
Natural gas piped to Australian homes (Origin Energy, AGL, Jemena networks across NSW and Victoria) is approximately 90% methane (CH₄), the smallest alkane. Households use it for heating and cooking. Australia exports about 80 million tonnes of LNG (liquefied natural gas = mostly methane) per year from the NW Shelf, the world's largest LNG export sector.
Alkanes are named using IUPAC (International Union of Pure and Applied Chemistry) systematic nomenclature. The name has two parts: a prefix indicating the number of carbons in the longest chain, and the suffix '-ane' indicating it is an alkane (all single bonds). The prefixes for 1–8 carbons are: meth- (1), eth- (2), prop- (3), but- (4), pent- (5), hex- (6), hept- (7), oct- (8). These prefixes are universal across all organic chemistry, they also appear in alcohols (methanol, ethanol), acids (methanoic acid), and polymers (polyethylene = poly-eth-ylene).
Drawing structural formulas: methane has a central carbon with four H atoms radiating out. Ethane is H₃C–CH₃, two carbons each with three H atoms and bonded to each other. Propane is H₃C–CH₂–CH₃, three carbons in a chain. Butane adds one more CH₂ unit. For each additional carbon in the chain, two more H atoms are added to satisfy the tetravalency of carbon, this is where the '+2' in CₙH₂ₙ₊₂ comes from: the two end carbon atoms each have one extra H compared to the interior CH₂ units.
Heptane (C₇H₁₆): 7 carbons in a chain. Formula check: C₇H₂×7+2 = C₇H₁₆. Heptane is a key component of petrol, its combustion in car engines is used as a reference point (0 on the octane scale) to measure how resistant petrol is to premature ignition (knock).
Ampol's fuel specifications for Australian petrol are defined by the chain length and branching of alkane components, isooctane (a branched C₈H₁₈ alkane) rates 100 on the octane scale; heptane (straight-chain C₇H₁₆) rates 0. The '95 RON' printed on premium petrol pumps at Australian service stations means the fuel behaves like a 95:5 mixture of isooctane and heptane.
As chain length increases across the alkane homologous series, three physical properties change in a smooth, predictable trend. (1) Boiling point rises: methane (−162 °C) → ethane (−89 °C) → propane (−42 °C) → butane (−1 °C) → pentane (+36 °C). Longer chains have more surface area for van der Waals (intermolecular) forces, more energy is required to separate the molecules. (2) Viscosity increases: short alkanes are thin, runny liquids; long alkanes are thick, oily; very long chains form waxes and solids. (3) Flammability decreases: short-chain alkanes (methane, ethane) are highly flammable gases; long-chain waxes are much harder to ignite.
These trends are not coincidences, they all follow from the single underlying cause: longer chains have stronger intermolecular forces. Understanding this cause-and-effect relationship is more powerful than memorising each individual trend, because it lets you predict properties for any alkane you've never encountered. An alkane with n=20 must have a higher boiling point than pentane, be more viscous, and less flammable, no measurement needed.
Paraffin wax (C₂₀–C₄₀ alkanes) has a melting point of 46–68 °C and is solid at room temperature, the same family as methane gas. The only difference is chain length (1 carbon vs 20–40), which creates intermolecular forces strong enough to make the substance solid rather than gaseous at 25 °C.
Lubricating oils for Australian mining machinery (Caterpillar excavators, haul trucks) use long-chain alkane-based mineral oils (C₂₀–C₃₅) precisely because their high viscosity and high boiling points make them stable at the operating temperatures of heavy diesel engines in the Pilbara heat, where summer temperatures exceed 45 °C.
As alkane chain length increases, boiling point in a smooth, predictable trend. Longer chains have more surface area for van der Waals, or , forces. More energy is therefore needed to the molecules. also increases, so short alkanes are runny while long ones are thick and oily. Flammability , so long-chain waxes are much harder to ignite than methane.
At the start of this lesson, you heard that methane is an invisible, odourless gas while candle wax is solid at room temperature, yet both are alkanes, differing only in how many carbon atoms they chain together. The general formula CₙH₂ₙ₊₂ predicts the properties of every alkane from a single equation.
Now that you've worked through the lesson, can you explain how increasing the chain length changes the boiling point and physical state of alkanes? How does that pattern make fractional distillation of crude oil work?
Q1. Explain why crude oil must be separated by fractional distillation before it can be used as petrol or diesel.
Q2. A student thinks petrol and bitumen are collected at the same level of the distillation column. Explain why this is incorrect, referring to chain length and boiling point.
Q3. Describe the process of fractional distillation from start to finish. Explain the role of temperature gradient in the column and why different fractions condense at different heights.