Crude Oil and Hydrocarbons

Crude oil and hydrocarbons

Crude oil is a finite resource formed over millions of years from the remains of ancient sea creatures (plankton). It is a mixture of many compounds, most of which are hydrocarbons.

A hydrocarbon is a compound containing only hydrogen and carbon atoms.

Alkanes

The alkanes are a family (homologous series) of saturated hydrocarbons — they contain only single C–C bonds.

• General formula: CₙH₂ₙ₊₂
• First four: methane (CH₄), ethane (C₂H₆), propane (C₃H₈), butane (C₄H₁₀).
• A homologous series = compounds with the same general formula and similar chemical properties, differing by CH₂.

Fractional distillation

Crude oil is separated into useful fractions (groups of similar-sized molecules) by fractional distillation:

1. The oil is heated and evaporates.

2. Vapours rise up a column that is hotter at the bottom, cooler at the top.

3. Each fraction condenses at a different height depending on its boiling point.

• Small molecules (low boiling point) rise high → fuels like petrol/LPG.
• Large molecules (high boiling point) condense low down → bitumen for roads.

Trends with molecule size

As hydrocarbon chains get longer:

• Boiling point increases (stronger intermolecular forces).
• They become more viscous (thicker) and less flammable.

Combustion

Burning hydrocarbons is exothermic:

• Complete combustion (plenty of oxygen): hydrocarbon → carbon dioxide + water.
• Incomplete combustion (limited oxygen): produces carbon monoxide (a toxic gas) and soot (carbon).

Cracking

Long-chain hydrocarbons are less useful, so they are cracked — broken into smaller, more useful molecules using heat and a catalyst (or steam). Cracking produces:

• Shorter-chain alkanes (useful fuels like petrol), and
• Alkenes (used to make polymers/plastics).

Alkenes

Alkenes are unsaturated hydrocarbons containing a C=C double bond (general formula CₙH₂ₙ, e.g. ethene C₂H₄). They are more reactive than alkanes. Test: alkenes decolourise bromine water (orange → colourless); alkanes do not.

Worked example

Why is a long-chain hydrocarbon cracked?

• There is more demand for short-chain molecules (fuels like petrol) and for alkenes to make polymers than there is for long, less useful chains. Cracking converts the surplus long chains into these useful products. ✓

Common mistakes

• Saying crude oil is a compound — it's a mixture.
• Mixing up saturated (alkanes, single bonds) and unsaturated (alkenes, C=C).
• Forgetting incomplete combustion makes carbon monoxide (toxic) and soot.

Exam tips

• Learn the alkane general formula CₙH₂ₙ₊₂ and the alkene CₙH₂ₙ.
• Describe fractional distillation using the temperature gradient and boiling points.
• Remember the bromine water test distinguishes alkenes from alkanes.

Key facts to remember

• Crude oil = finite mixture of hydrocarbons; separated by fractional distillation (by boiling point).
• Alkanes = saturated, CₙH₂ₙ₊₂; alkenes = unsaturated (C=C), CₙH₂ₙ, decolourise bromine water.
• Cracking turns long chains into shorter alkanes (fuels) + alkenes (for polymers); complete combustion → CO₂ + water, incomplete → CO + soot.