Crude Oil, Hydrocarbons & Alkanes
What Is Crude Oil?
Crude oil is a naturally occurring mixture found deep underground. It formed over millions of years from the remains of ancient marine organisms buried under layers of sediment. Heat and pressure converted this organic matter into a complex mixture of hydrocarbons — compounds made of carbon and hydrogen atoms only.
Crude oil is a complex mixture of hydrocarbons. It contains molecules in which carbon atoms are joined in chains or rings. Crude oil is an important source of useful substances (fuels and feedstock for the petrochemical industry) and is a finite (non-renewable) resource.
Fractional Distillation
Crude oil is not useful as it comes out of the ground. It must be separated into simpler, more useful fractions by the process of fractional distillation.
In a fractionating column:
- Crude oil is heated until most of it evaporates.
- The vapour rises up the column, which is hotter at the bottom and cooler at the top.
- Hydrocarbons with high boiling points condense near the bottom; those with low boiling points rise further before condensing near the top.
- Each fraction is collected at a different level.
You need to know the names and uses of these six fractions (from top to bottom of the column):
- Gases (e.g. methane, propane, butane) — used in domestic heating and cooking.
- Petrol — used as fuel for cars.
- Kerosene — used as fuel for aircraft.
- Diesel oil — used as fuel for some cars and trains.
- Fuel oil — used as fuel for large ships and in some power stations.
- Bitumen — used to surface roads and roofs.
How Fractions Differ
As you go down the fractionating column (from gases to bitumen), the hydrocarbon molecules get larger. This causes a pattern in their physical properties:
- Number of carbon and hydrogen atoms increases (longer chains).
- Boiling point increases (stronger intermolecular forces between larger molecules need more energy to overcome).
- Viscosity increases (the liquid becomes thicker and harder to pour).
- Ease of ignition decreases (larger molecules are harder to ignite).
Longer hydrocarbon molecules have higher boiling points, higher viscosity and are harder to ignite. Shorter molecules have lower boiling points, are less viscous and ignite more easily. This is because larger molecules have stronger intermolecular forces.
Alkanes — A Homologous Series
Most hydrocarbons in crude oil belong to a family called the alkanes. Alkanes are a homologous series — a family of compounds with a pattern. Every homologous series has four defining features:
- They share the same general formula. For alkanes, this is \(C_nH_{2n+2}\).
- Each member differs from the next by \(CH_2\) in their molecular formula.
- They show a gradual variation in physical properties (e.g. boiling point increases steadily as chain length grows).
- They have similar chemical properties because they share the same functional group (or in the case of alkanes, the same type of bonding).
Using the general formula \(C_nH_{2n+2}\):
- Methane \(CH_4\) — 1 carbon. Found in natural gas.
- Ethane \(C_2H_6\) — 2 carbons.
- Propane \(C_3H_8\) — 3 carbons. Used in camping gas.
- Butane \(C_4H_{10}\) — 4 carbons. Used in lighters.
Alkanes are saturated hydrocarbons — all carbon–carbon bonds are single bonds, so they contain the maximum possible number of hydrogen atoms.
Crude oil is a finite resource. Petrol, kerosene and diesel oil are non-renewable fossil fuels obtained from crude oil. Methane is a non-renewable fossil fuel found in natural gas. Once used, these cannot be replaced on a human timescale.