Crude oil is a finite mixture

Crude oil is a finite resource found in rocks — the remains of an ancient biomass (mainly plankton) buried in mud over millions of years. "Finite" means it will one day run out.

• Most of the compounds in crude oil are hydrocarbons — molecules made of hydrogen and carbon only.
• Crude oil is a mixture of many hydrocarbons, so it can be separated by physical means (fractional distillation).
• The most common hydrocarbons in crude oil are the alkanes.

Watch out: a hydrocarbon contains only C and H. A molecule with oxygen in it (like ethanol or a carboxylic acid) is not a hydrocarbon.

Alkanes — a saturated homologous series

The alkanes form a homologous series: a family with the same general formula and similar chemical properties. They are saturated — every carbon–carbon bond is a single bond, so the molecule holds as many hydrogen atoms as possible.

The first four members are methane (CH₄), ethane (C₂H₆), propane (C₃H₈) and butane (C₄H₁₀) — each differs from the next by CH₂.

Your turn — counting hydrogens

Separating crude oil

The hydrocarbons in crude oil have different boiling points, so they can be split in a fractionating column that is hot at the bottom and cooler at the top. The oil is heated to a vapour; each fraction rises until it cools to its boiling point and condenses.

Which fraction?

How chain length changes properties

As the hydrocarbon chains get longer (moving down the column), the intermolecular forces grow stronger, so the properties change in a predictable way:

• Boiling point — increases with chain length.
• Viscosity (how thick/gloopy it is) — increases; long-chain fractions flow less easily.
• Ease of ignition / flammability — decreases; short chains catch fire most easily.

Reading the trend

Complete vs incomplete combustion

Hydrocarbon fuels burn (oxidise) in oxygen. The products depend on how much oxygen is available:

• Complete combustion (plenty of O₂): produces carbon dioxide and water only.
• Incomplete combustion (limited O₂): produces carbon monoxide (CO), solid carbon (soot / particulates) and water as well.
• Carbon monoxide is a toxic, colourless, odourless gas — it binds to haemoglobin so the blood carries less oxygen.
• Soot (particulates) blackens buildings and can cause breathing problems.

Watch out: incomplete combustion does not mean "no products" — it means a limited oxygen supply, giving CO and carbon (soot) instead of all CO₂.

Predict the products

Sulfur dioxide and oxides of nitrogen

Burning fuels can release other pollutants beyond CO and soot:

• Sulfur dioxide (SO₂) — forms when sulfur impurities in the fuel burn.
• Oxides of nitrogen (NOₓ) — form when the high temperature of combustion makes nitrogen and oxygen in the air react.
• In the atmosphere these dissolve in rainwater: SO₂ → sulfurous/sulfuric acid, NOₓ → nitric acid, giving acid rain.

Acid rain erodes stonework and metal, damages trees and makes lakes too acidic for fish. Removing sulfur before burning, or scrubbing flue gases, reduces SO₂.

Breaking long chains into useful ones

Fractional distillation gives too much of the long-chain fractions and not enough petrol. Cracking breaks larger hydrocarbons into smaller, more useful molecules using heat and a catalyst (catalytic cracking) or steam (steam cracking).

• One product is a shorter alkane (e.g. more petrol).
• The other is an alkene — a reactive molecule used to make polymers (plastics) and other chemicals.

Watch out: cracking always makes smaller molecules, and at least one of them is an alkene. It does not build bigger molecules.

Alkenes — unsaturated, with a C=C bond

Alkenes are hydrocarbons with a carbon–carbon double bond (C=C). This makes them unsaturated — they do not hold the maximum number of hydrogens, and the reactive C=C lets them take part in addition reactions.

The first four are ethene (C₂H₄), propene (C₃H₆), butene (C₄H₈) and pentene (C₅H₁₀).

The bromine water test

The reactive C=C bond gives a simple test to tell an alkene from an alkane: shake the sample with orange bromine water.

Watch out: it is the alkene that turns bromine water colourless. "Decolourised" = the colour disappears = unsaturated = C=C present.

The bromine test

Alkane or alkene?

Use the general formulae — alkanes are C_nH_2n+2 (saturated); alkenes are C_nH_2n (a C=C). Tap the correct family for each molecule.

Alcohols and fermentation

Alcohols are a homologous series with the functional group –OH. The first four are methanol, ethanol, propanol, butanol. Ethanol (C₂H₅OH) is the alcohol in drinks and a useful fuel/solvent.

• Fermentation: yeast converts sugar (glucose) → ethanol + carbon dioxide, in warm (~30–37 °C), anaerobic conditions.
• Alcohols are flammable and dissolve in water.
• Alcohols can be oxidised to carboxylic acids (e.g. ethanol → ethanoic acid, the acid in vinegar).

Carboxylic acids

Carboxylic acids are a homologous series with the functional group –COOH. The first four are methanoic, ethanoic, propanoic and butanoic acid. Ethanoic acid (CH₃COOH) is the acid in vinegar.

• They show typical acidic properties — they are weak acids that react with metals, carbonates and alkalis.
• With a carbonate they fizz, giving a salt, water and carbon dioxide.
• They are formed when an alcohol is oxidised.

Link it up: ethanol (–OH) → oxidised → ethanoic acid (–COOH). The functional group is what decides how each family reacts.

Name the functional group

Alkenes make addition polymers

Because alkenes have a reactive C=C, many monomers can join together with no other product — this is addition polymerisation. Many ethene monomers make poly(ethene).

The repeating unit

Condensation polymers and the plastics problem

In condensation polymerisation, monomers each with two functional groups join up and lose a small molecule (often water) at each link — e.g. polyester. Naturally occurring polymers include DNA (made of nucleotide monomers).

• Addition polymers: monomers join with no other product.
• Condensation polymers: monomers join and release a small molecule (e.g. water).
• Disposal problem: most addition polymers are non-biodegradable — they don't rot, so they fill landfill and pollute oceans. Burning them can release toxic gases; recycling helps but is limited.

Tell them apart: if a small molecule (like water) is given off, it is condensation; if nothing else is produced, it is addition.

Fraction → its use

Tap a fraction on the left, then its correct use on the right.

Why crack?

The facts to know

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